Homing in on consciousness in the nervous system: An action-based synthesis

1.1. The scientific approach should focus on the most basic form of consciousness

We believe that, to advance the study of consciousness, one should focus not on high forms of consciousness (e.g., “self-consciousness”), but on the most basic forms of consciousness (e.g., the experience of a smell, visual afterimages, tooth pain, or urges to scratch an itch). This form of consciousness has fallen under the rubrics of “sentience” (Pinker Reference Pinker1997), “primary consciousness” (Edelman Reference Edelman1989), “phenomenal consciousness” (Block Reference Block1995b), “qualia” (J. A. Gray Reference Gray2004), “phenomenal states” (Tye Reference Tye1999), and “subjective experience.” In our framework, we refer to a thing of which one is conscious (e.g., an afterimage) as a conscious content (Merker Reference Merker2007; Seth Reference Seth2007). All of the contents of which one is conscious at one time can be construed as composing the conscious field (Freeman Reference Freeman2004; Köhler Reference Köhler1947; Searle Reference Searle2000). The contents of the conscious field change over time.

1.2. The approach should be descriptive, non-normative

We believe that the approach to consciousness should be a descriptive, naturalistically based one (which describes the products of nature as they evolved to be) rather than a normative one (which construes processes in terms of how they should function). Nervous mechanisms have been fashioned by the happenstance and tinkering process of evolution, whose products can be counterintuitive and suboptimal (de Waal Reference de Waal2002; Gould Reference Gould1977; Lorenz Reference Lorenz1963; Marcus Reference Marcus2008; Roe & Simpson Reference Roe and Simpson1958; Simpson Reference Simpson1949), far unlike the kinds of things humans design into machines (Arkin Reference Arkin1998). Hence, the ethologist Konrad Lorenz (Reference Lorenz1963) cautions, “To the biologist who knows the ways in which selection works and who is also aware of its limitations it is in no way surprising to find, in its constructions, some details which are unnecessary or even detrimental to survival” (p. 260). Similarly, when speaking about the reverse engineering of biological products, the roboticist Ronald Arkin concludes, “Biological systems bring a large amount of evolutionary baggage unnecessary to support intelligent behavior in their silicon based counterparts” (Arkin Reference Arkin1998, p. 32). The difference between the products of evolution and human artifacts is obvious when one considers the stark contrast between human locomotion and artificial locomotion – legs versus wheels (Morsella & Poehlman Reference Morsella and Poehlman2013).

When adopting a descriptive standpoint, even the most cursory examination of the brain reveals a contrast between conscious and unconscious processes (see Bleuler Reference Bleuler and Brill1924). Thus, in every field of inquiry, there is the de facto distinction between the two kinds of processes, though often without mention of the taboo term “consciousness.” For example, in perception research, there exists the distinction between supra- versus subliminal. In memory research, there is the distinction between “declarative” (explicit) processes and “procedural” (implicit) processes (Schacter Reference Schacter1996; Squire Reference Squire1987). In motor and language research, the conscious aspects of voluntary action or of speech production are contrasted with the unconscious aspects of, say, motor programming (Levelt Reference Levelt1989; Rosenbaum Reference Rosenbaum, Yantis and Yantis2002; J. A. Taylor & Ivry Reference Taylor, Ivry, Prinz, Beisert and Herwig2013). Various fields also contrast “controlled” processing (which tends to be conscious) and “automatic” processing (which is often unconscious; Lieberman Reference Lieberman, Harmon-Jones and Winkielman2007). In summary, from a descriptive approach, the contrast between conscious and unconscious processes in the brain is somewhat inevitable (Morsella & Poehlman Reference Morsella and Poehlman2013).

1.3. The approach should be minimalistic, focusing on simple cases

When attempting to unravel a phenomenon as perplexing as consciousness, it is important to adopt a strategy in which scientific inquiry begins with the examination of the most basic, elemental instantiation of the phenomenon of interest (Panksepp Reference Panksepp2007). Such a strategy proved fruitful in the development of physics (Einstein & Infeld Reference Einstein and Infeld1938/1967). Hence, in our approach, we focus on the actions of a hypothetical, simplified, human-like mammal that, though conscious (for a treatment of consciousness in mammals, see J. A. Gray [2004]), is not equipped with many of the complicated abilities/states associated with adult humans (e.g., syntax and music appreciation). Capable of having approach-avoidance conflicts (Lewin Reference Lewin1935; N. E. Miller Reference Miller and Koch1959), this hypothetical organism is occupied only with basic operations (e.g., breathing, locomoting, and avoiding tissue damage) rather than with higher-level phenomena (e.g., mental rotation and sustained, directed thinking). This organism is also incapable of indirect cognitive control (Morsella et al. Reference Morsella, Lanska, Berger and Gazzaley2009b), whereby one can, for instance, make oneself hungry or scared by deliberately imagining the kinds of things that would induce these states. Such higher-level phenomena are more likely to be predicated upon (a) extensive learning, (b) cultural influences, (c) intricate interactions among more elemental conscious processes, and (d) adaptations that are less phylogenetically primitive than those of the basic operations of interest (Morsella Reference Morsella2005).

In our “simple case,” this hypothetical organism is resting in a warm enclosure (e.g., a cave). It can consciously perceive an opening from which it could exit. For hours, the organism performs no notable locomotive act toward the opening nor to anything else, but then it perceives a noxious smell (e.g., smoke) from within the enclosure. Because of this new conscious content, it now exits hesitantly through the opening, even though it was inclined to remain within the warm enclosure. To illuminate the nature of consciousness, we will revisit this “creature in the cave” scenario throughout our treatise. We refer to the first events composing the scenario (e.g., the percept of the opening and the warmth) as Stage 1, and the subsequent events (e.g., the smell and the inclination to stay in the cave) as Stage 2.

In contrast to our strategy, descriptive accounts of consciousness have tended to focus on high-level functions, leading to conclusions such as that the function of consciousness pertains to social interaction (Frith Reference Frith2010; Prinz Reference Prinz2012), language (Banks Reference Banks1995; Carlson Reference Carlson1994; Macphail Reference Macphail1998), “theory of mind” (Stuss & Anderson Reference Stuss and Anderson2004), the formation of the self (Greenwald & Pratkanis Reference Greenwald, Pratkanis, Wyer and Srull1984), semantic processing (Kouider & Dupoux Reference Kouider and Dupoux2004; Mudrik et al. Reference Mudrik, Faivre and Koch2014; Thagard & Stewart Reference Thagard and Stewart2014), the meaningful interpretation of situations (Roser & Gazzaniga Reference Roser and Gazzaniga2004), and simulations of behavior and perception (Hesslow Reference Hesslow2002). (It is worth noting that, for good reasons, it has also been proposed that, contrary to the present account, consciousness does not contribute to ongoing action; see Hommel Reference Hommel2013; Jackson Reference Jackson1986; Kinsbourne Reference Kinsbourne, Cohen and Schooler1996; Reference Kinsbourne2000; Masicampo & Baumeister Reference Masicampo and Baumeister2013; Pinker Reference Pinker1997.)

1.4. Overview of present, untraditional approach

Our approach is untraditional in several ways. First, instead of focusing on the relationship between consciousness and perception (which has been the dominant approach; Crick & Koch Reference Crick and Koch2003; Rosenbaum Reference Rosenbaum2005), we focus on the relationship between consciousness and overt action. Second, unlike traditional stimulus→response approaches, we “work backward” from overt action to the underlying processes responsible for it (Sperry Reference Sperry1952). Thus, from our untraditional, action-based approach, we subscribe to an uncommon theoretical position – that the nature of consciousness is best understood by examining the requirements of adaptive (efferent) action control rather than the needs of perceptual analysis. From this unconventional approach to consciousness, one can appreciate that the requirements of adaptive skeletomotor action reveal much about the nature of both the conscious field and the generation of conscious contents. Third, building on Morsella and Bargh (Reference Morsella and Bargh2007), instead of focusing on vision to understand consciousness (which has been the prevalent approach; Crick & Koch Reference Crick and Koch2003), we focus on the (possibly) more tractable system of olfaction, as illustrated in our “creature in the cave” example. The olfactory system possesses several features that render it a fruitful system in which to study consciousness.Footnote ¹

To summarize, our approach is elemental, action-based, simple, and evolutionary-based (or, for short, “EASE,” meaning “to make something less difficult”). We believe that an EASE perspective provides the most fruitful approach to the perplexing problem of consciousness and the brain. Whenever in our enterprise we encountered an obstacle for theoretical progress (e.g., the neural regions associated with consciousness), it was through our EASE perspective that progress was made. In the next three sections, we discuss from an EASE perspective the empirically supported hypotheses that serve as the tenets of passive frame theory. Through the process, we begin to isolate the neuroanatomical, cognitive-mechanistic, and representational (e.g., conscious contents; sect. 3) processes associated with consciousness.

2. The circumscribed role of consciousness in the nervous system

2.1. Tenet: Consciousness is associated with only a subset of nervous function

Based on developments of the past four decades, there is a growing subset consensus – that consciousness is associated with only a subset of all of the processes and regions of the nervous systemFootnote ² (Aru et al. Reference Aru, Bachmann, Singer and Melloni2012; Crick & Koch Reference Crick and Koch1995; Reference Crick and Koch2003; Dehaene & Naccache Reference Dehaene and Naccache2001; J. A. Gray Reference Gray2004; Grossberg Reference Grossberg1999; Koch Reference Koch2004; Koch & Greenfield Reference Koch and Greenfield2007; Logothetis & Schall Reference Logothetis and Schall1989; Merker Reference Merker2007; Reference Merker2013c; Penfield & Jasper Reference Penfield and Jasper1954; Weiskrantz Reference Weiskrantz1992; Zeki & Bartels Reference Zeki and Bartels1999). This subset seems to be qualitatively distinct – in terms of its functioning, physical makeup/organization, or mode of activity – from that of its unconscious counterparts in the brain (Bleuler Reference Bleuler and Brill1924; Coenen Reference Coenen1998; Edelman & Tononi Reference Edelman and Tononi2000; Goodale & Milner Reference Goodale and Milner2004; J. A. Gray Reference Gray2004; Llinás et al. Reference Llinás, Ribary, Contreras and Pedroarena1998; Merker Reference Merker2007; Ojemann Reference Ojemann1986).

Consistent with the subset consensus, many aspects of nervous function are unconscious.Footnote ³ Complex processes of an unconscious nature can be found at all stages of processing (Velmans Reference Velmans1991), including low-level perceptual analysis (e.g., motion detection, color detection, auditory analysis; Zeki & Bartels Reference Zeki and Bartels1999), semantic-conceptual processing (Harley Reference Harley1993; Lucas Reference Lucas2000), and motor programming (discussed in sect. 3.1). Evidence for the complexity of unconscious processing is found in cases in which the entire stimulus-response arc is mediated unconsciously, as in the case of unconsciously mediated actions (e.g., automatisms). There is a plethora of evidence that action plans can be activated, selected, and even expressed unconsciously.Footnote ⁴ In summary, it seems that much in the nervous system is achieved unconsciously. This insight from the subset consensus leads one to the following question: What does consciousness contribute to nervous function?

2.2. Tenet: The conscious field serves an integrative role

The integration consensus (Baars Reference Baars1988; Reference Baars1998; Reference Baars2002; Reference Baars2005; Boly et al. Reference Boly, Garrido, Gosseries, Bruno, Boveroux, Schnakers, Massimini, Litvak, Laureys and Friston2011; Clark Reference Clark2002; Damasio Reference Damasio1989; Dehaene & Naccache Reference Dehaene and Naccache2001; Del Cul et al. Reference Del Cul, Baillet and Dehaene2007; Doesburg et al. Reference Doesburg, Green, McDonald and Ward2009; Edelman & Tononi Reference Edelman and Tononi2000; Freeman Reference Freeman1991; Koch Reference Koch2012; Kriegel Reference Kriegel2007; Llinás & Ribary Reference Llinás and Ribary2001; Merker Reference Merker2007; Ortinski & Meador Reference Ortinski and Meador2004; Sergent & Dehaene Reference Sergent and Dahaene2004; Srinivasan et al. Reference Srinivasan, Russell, Edelman and Tononi1999; Tallon-Baudry Reference Tallon-Baudry2012; Tononi Reference Tononi2012; Tononi & Edelman Reference Tononi and Edelman1998; Uhlhaas et al. Reference Uhlhaas, Pipa, Lima, Melloni, Neuenschwander, Nikolic and Singer2009; Varela et al. Reference Varela, Lachaux, Rodriguez and Martinerie2001; Zeki & Bartels Reference Zeki and Bartels1999) proposes that consciousness integrates neural activities and information-processing structures that would otherwise be independent. Most of the hypotheses comprising this consensus speak of conscious information as being available “globally,” in some kind of workspace, as in Baars's (Reference Baars1988) influential global workspace theory. For present purposes, we construe the contents occupying such a workspace as composing the conscious field (defined in sect. 1.1 above).

Consistent with the integration consensus, the conscious processing of a percept involves a wider and more diverse network of regions than does the subliminal (unconscious) processing of the same percept (Singer Reference Singer, Laureys and Tononi2011; Uhlhaas et al. Reference Uhlhaas, Pipa, Lima, Melloni, Neuenschwander, Nikolic and Singer2009). The latter is subjected only to “local” processing. This evidence stemmed initially from research on perception (Del Cul et al. Reference Del Cul, Baillet and Dehaene2007; Uhlhaas et al. Reference Uhlhaas, Pipa, Lima, Melloni, Neuenschwander, Nikolic and Singer2009), anesthesia (Alkire et al. Reference Alkire, Hudetz and Tononi2008; Boveroux et al. Reference Boveroux, Vanhaudenhuyse, Bruno, Noirhomme, Lauwick, Luxen, Degueldre, Plenevaux, Schnakers, Phillips, Brichant, Bonhomme, Maquet, Greicius, Laureys and Boly2010; Långsjö et al. Reference Långsjö, Alkire, Kaskinoro, Hayama, Maksimow, Kaisti, Aalto, Aantaa, Jääskeläinen, Revonsuo and Scheinin2012; Lee et al. Reference Lee, Kim, Noh, Choi, Hwang and Mashour2009; Lewis et al. Reference Lewis, Weiner, Mukamel, Donoghue, Eskandar, Madsen, Anderson, Hochberg, Cash, Brown and Purdon2012; Schroter et al. Reference Schroter, Spoormaker, Schorer, Wohlschlager, Czish, Kochs, Zimmer, Hemmer, Schneider, Jordan and Ilg2012; Schrouff et al. Reference Schrouff, Perlbarg, Boly, Marrelec, Boveroux, Vanhaudenhuyse, Bruno, Laureys, Phillips, Pélégrini-Isaac, Maquet and Benali2011), and unresponsive states (e.g., coma or vegetative state; Laureys Reference Laureys2005). Regarding perception research, it has been proposed that, during binocular rivalry,Footnote ⁵ the neural processing of the conscious percept requires special interactions between both perceptual regions and other, traditionally non-perceptual regions (e.g., frontal cortex; Doesburg et al. Reference Doesburg, Green, McDonald and Ward2009). This supports the view that some mode of interaction between widespread brain areas is important for consciousness (Buzsáki Reference Buzsáki2006; Doesburg et al. Reference Doesburg, Green, McDonald and Ward2009; Fries Reference Fries2005; Hummel & Gerloff Reference Hummel and Gerloff2005).

Evidence for the integration consensus is found also in action-based research. Conscious actions involve more widespread activations in the brain than do similar but unconscious actions (Kern et al. Reference Kern, Jaradeh, Arndorfer and Shaker2001; McKay et al. Reference McKay, Evans, Frackowiak and Corfield2003; Ortinski & Meador Reference Ortinski and Meador2004). Moreover, when actions are decoupled from consciousness (e.g., in neurological disorders), the actions often appear impulsive or inappropriate, as if they are not adequately influenced by the kinds of information by which they should be influenced (Morsella & Bargh Reference Morsella, Bargh, Cacioppo and Decety2011).

2.3. Advances regarding the physiological processes engendering consciousness depend on advances regarding the neuroanatomy of consciousness

The nature of the neuroanatomical network engendering the physiological processes (e.g., neural oscillations) proposed to be associated with consciousness remains controversial.Footnote ⁶ Progress regarding the neurophysiology of consciousness depends on advances regarding the identification of the neuroanatomical substrates of this state (Aru et al. Reference Aru, Bachmann, Singer and Melloni2012). Regarding neuroanatomy, when attempting to isolate the anatomical underpinnings of consciousness, investigators have followed Crick and Koch's (Reference Crick and Koch2003) recommendation and have focused on vision. (See reviews of neural correlates of visual consciousness in Blake and Logothetis [Reference Blake and Logothetis2002], Dehaene [2014], Koch [2004], Lamme and Spekreijse [2000], Metzinger [Reference Metzinger2000], and Tong [2003].) In vision research, controversy remains regarding whether consciousness depends on higher-order perceptual regions (Crick & Koch Reference Crick and Koch1995; Reference Crick and Koch1998; Panagiotaropoulos et al. 2012; Reference Panagiotaropoulos, Kapoor and Logothetis2013) or lower-order regions (Aru et al. Reference Aru, Bachmann, Singer and Melloni2012; Damasio, Reference Damasio2010; Friedman-Hill et al. Reference Friedman-Hill, Robertson and Treisman1995; Lamme Reference Lamme2001; Liu et al. Reference Liu, Paradis, Yahia-Cherif and Tallon-Baudry2012; Robertson Reference Robertson2003; Tallon-Baudry Reference Tallon-Baudry2012; Tong Reference Tong2003). Moreover, as noted in Note 2, whether cortical matter is necessary for consciousness remains controversial.

Theorists focusing on vision have proposed that, although the cortex may elaborate the contents of consciousness, consciousness is primarily a function of subcortical structures (Merker Reference Merker2007; Penfield & Jasper Reference Penfield and Jasper1954; Ward Reference Ward2011). Penfield and Jasper (Reference Penfield and Jasper1954) based this hypothesis on their studies involving both the direct stimulation of, and ablation of, cortical regions. Based on these and other findings (e.g., observations of patients with anencephaly; Merker Reference Merker2007), it has been proposed that consciousness is associated with subcortical areas (e.g., Merker Reference Merker2007; Reference Merker2013c). This has led to the cortical-subcortical controversy (Morsella et al. Reference Morsella, Berger and Krieger2011). While data from studies on patients with profound disorders of consciousness (e.g., vegetative state) suggest that signals from the frontal cortex may be critical for the instantiation of any form of consciousness (Boly et al. Reference Boly, Garrido, Gosseries, Bruno, Boveroux, Schnakers, Massimini, Litvak, Laureys and Friston2011; Dehaene & Naccache Reference Dehaene and Naccache2001; Lau Reference Lau2008; Panagiotaropoulos et al. 2012; Velly et al. Reference Velly, Rey, Bruder, Gouvitsos, Witjas, Regis, Peragut and Gouin2007), research on the psychophysiology of dream consciousness, which involves prefrontal deactivations (Muzur et al. Reference Muzur, Pace-Schott and Hobson2002), suggests that, although the prefrontal lobes are involved in cognitive control, they may not be essential for the generation of basic consciousness (Aru et al. Reference Aru, Bachmann, Singer and Melloni2012; Merker Reference Merker2007; Ward Reference Ward2011). Regarding the necessity of the integrity of the frontal lobes for consciousness, it is important to consider that the surgical procedure of frontal lobotomy, once a common neurosurgical intervention for the treatment of psychiatric disorders, was never reported to render patients incapable of sustaining consciousness (see also Aleman & Merker Reference Aleman and Merker2014).

The role of subcortical structures in the production of consciousness, and the amount of cortex that may be necessary for the production of consciousness, remains to be elucidated (see further discussion in sect. 3.5). Clearly, more investigation is needed regarding the neural correlates of consciousness, because controversy continues to surround not only the neurophysiological processes underlying consciousness, but even the identification of the gross, neuroanatomical regions that are responsible for this peculiar form of processing (see treatment in Merker Reference Merker2007; Reference Merker2013b).

Faced with this challenge, we propose that, because of the intimate liaison between function and structure in the nervous system (Cohen & Dennett Reference Cohen and Dennett2011; Merker Reference Merker2013c), progress can be made regarding the neural underpinnings of consciousness by having a more precise understanding of the role of consciousness in nervous function (Lamme & Spekreijse Reference Lamme and Spekreijse2000). With this in mind, one can reason as follows: If the consensus is that consciousness serves an integrative role, then, from an EASE perspective, what is the most basic form of integration that requires consciousness? Addressing this question allows one to better isolate consciousness within the nervous system, which could, in turn, resolve controversies regarding the neural correlates of consciousness.

2.4. Tenet: The conscious field is for a specific kind of integration, involving the skeletal muscle output system

One limitation of the integration consensus is that it fails to specify which kinds of integrations require consciousness and which kinds do not. Consciousness seems unnecessary for various kinds of integrations in the nervous system. For example, integrations across different sensory modalities, as in the case of afference binding (Morsella & Bargh Reference Morsella, Bargh, Cacioppo and Decety2011), can occur unconsciously. This form of integration occurs in feature binding (e.g., the binding of shape to color; Zeki & Bartels Reference Zeki and Bartels1999) and in intersensory binding (Vroomen & de Gelder Reference Vroomen and de Gelder2003), as in the ventriloquism and McGurk effects (McGurk & MacDonald Reference McGurk and MacDonald1976). The latter, for instance, involves interactions between visual and auditory processes: An observer views a speaker mouthing “ga” while presented with the sound “ba.” Surprisingly, the observer is unaware of any intersensory interaction, perceiving only “da.” (See list of many kinds of unconscious afference binding in Morsella [2005], Appendix A.) Integrations involving smooth muscle effectors (e.g., in peristalsis or in the pupillary reflex), too, can occur unconsciously (Morsella et al. Reference Morsella, Gray, Krieger and Bargh2009a), as can another form of integration known as efference binding (Haggard et al. 2002).

Efference binding links perceptual processing to action/motor processing. This kind of stimulus-response binding is mediated unconsciously in actions such as reflexive pain withdrawal or reflexive inhalation. In learned behavior, efference binding allows one to press a button when presented with an arbitrary cue. Such a form of binding can be learned quickly (e.g., from a few trials of stimulus-response mapping; Hommel & Elsner Reference Hommel, Elsner, Morsella, Bargh and Gollwitzer2009) and with little effort (Cohen-Kdoshay & Meiran Reference Cohen-Kdoshay and Meiran2009; Melcher et al. Reference Melcher, Weidema, Eenshuistra, Hommel and Gruber2008). Learned forms of efference binding can be expressed unconsciously (Fehrer & Biederman Reference Fehrer and Biederman1962; Fehrer & Raab Reference Fehrer and Raab1962; Hallett Reference Hallett2007; J. L. Taylor & McCloskey Reference Taylor and McCloskey1990; Reference Taylor and McCloskey1996). For example, subjects can select the correct motor response (one of two button presses) when confronted with subliminal stimuli, suggesting that “appropriate programs for two separate movements can be simultaneously held ready for use, and that either one can be executed when triggered by specific stimuli without subjective awareness” (Taylor & McCloskey Reference Taylor and McCloskey1996, p. 62; see review in Hallett Reference Hallett2007). We return to the topic of efference binding when discussing how conscious contents influence action (sect. 3.2).

In contrast to these unconscious forms of integration, people tend to be very much aware of some integrations, as when one holds one's breath while underwater or experiences an approach-avoidance conflict (Lewin Reference Lewin1935; N. E. Miller Reference Miller and Koch1959). In the former, one experiences the inclinations to both inhale and to not inhale. Similarly, when carrying a hot dish of food, one experiences the inclinations to drop the dish and to not drop the dish (Morsella Reference Morsella2005). Unlike unconscious integrations, such conscious conflicts (Morsella Reference Morsella2005) reflect a form of integration that is associated not with perceptual processing, but rather with action selection.Footnote ⁷ This form of integration has been distinguished from unconscious integrations/conflicts, such as the McGurk effect and smooth muscle conflicts (e.g., in the pupillary reflex). In short, conflicts at the stage of processing of action selection are experienced consciously, whereas conflicts at perceptual stages of processing are unconscious. It has been proposed that, unlike unconscious integrations, these integrations involve competition for control of the skeletal muscle (“skeletomotor,” for short) output system (Morsella Reference Morsella2005). The skeletomotor output system contains the unconscious motor plans that are necessary to enact one skeletomotor act versus another (Bizzi & Mussa-Ivaldi Reference Bizzi, Mussa-Ivaldi and Gazzaniga2004; Rizzolatti et al. Reference Rizzolatti, Fogassi, Gallese and Gazzaniga2004; Rosenbaum Reference Rosenbaum, Yantis and Yantis2002). It stores, for example, the unconscious articulatory plans that are necessary for speech production (Buchsbaum Reference Buchsbaum2013) and the plans for blinking (Graziano Reference Graziano2008). When these plans are stimulated sufficiently, overt actions arise.

Involving urges and other action-related inclinations, conscious conflicts occur when two streams of efference binding are trying to influence skeletomotor action simultaneously (Morsella & Bargh Reference Morsella, Bargh, Cacioppo and Decety2011). For example, conscious conflicts occur when one holds one's breath, suppresses uttering something, suppresses a prepotent response in a response interference paradigm, or voluntarily breathes faster for some reward. (The last example illustrates that not all cases of this kind of integration involve suppression.) These conscious conflicts appear to be triggered into existence by the activation of incompatible skeletomotor plans.Footnote ⁸ In our “creature in the cave” scenario, this form of integration occurs when the organism is inclined to both exit the enclosure (because of the smoke) and remain within it (because of the warmth).

Thus, Morsella (Reference Morsella2005) proposes that the primary function of consciousness is to integrate information, but only certain kinds of information – the kinds involving incompatible skeletal muscle intentions for adaptive action (e.g., holding one's breath while underwater).Footnote ⁹ From this standpoint, the conscious field is unnecessary to integrate perceptual-level processes (as in feature binding or intersensory conflicts), smooth muscle processes (e.g., pupillary reflex; Morsella et al. Reference Morsella, Gray, Krieger and Bargh2009a), or processes associated with motor control (discussed in sect. 3.1 below). Instead, the conscious field is necessary to integrate what appear to be multiple inclinations toward the skeletomotor output system, as captured by the principle of P arallel Responses into Skeletal Muscle (PRISM; Morsella Reference Morsella2005). From this perspective, and as fleshed out in the next section, it is this third kind of binding that is the most basic form of integration that requires consciousness. PRISM explains why, phenomenologically, a wink is different from a reflexive blink and from the dilation of a pupil.

2.5. Tenet: The conscious field is for adaptive voluntary action

In colloquial terms, one can conclude that consciousness is for adaptive “voluntary” action. Scientifically, consciousness can be construed as the medium that allows action processes to influence skeletomotor action collectively, leading to integrated actions (Morsella & Bargh Reference Morsella, Bargh, Cacioppo and Decety2011), such as holding one's breath. Just as a prism combines different colors to yield a single hue, the conscious field permits for multiple response tendencies to yield a single, integrated action. Absent consciousness, skeletomotor behavior can be influenced by only one of the efference streams, leading to unintegrated actions (Morsella & Bargh Reference Morsella, Bargh, Cacioppo and Decety2011), such as unconsciously inhaling while underwater or reflexively removing one's hand from a hot object. Reflecting a lack of integration, unintegrated actions appear as if they are not influenced by all of the kinds of information by which they should be influenced. If a conscious content is not in the field, then it cannot influence voluntary action. For example, if the knowledge representations necessary for, say, “reality monitoring,” are not in the field (e.g., due to fever), then nothing else can assume the functional influence of these contents. (This is evident in action selection in dreams, which are often irrational, and in disorders of awareness, such as sensory neglect and anosognosia.) Therefore, in voluntary action, when the appropriate contents are absent, there is no independent system or repository of knowledge that can step in to fill their role. Thus, the conscious field wholly and exclusively determines what in everyday life is called voluntary behavior. Conversely, for every voluntary action, the organism can report a conscious content responsible for that action, regardless of the veracity of the introspection (Poehlman et al. Reference Poehlman, Jantz and Morsella2012).

These conclusions also reveal that it is no accident that, historically, skeletal muscle has been described as “voluntary” muscle. Since at least the nineteenth century, it has been known that, though often functioning unconsciously (as in the frequent actions of breathing and blinking), skeletal muscle is the only bodily effector that can be consciously controlled, but why this is so has never been addressed theoretically. PRISM introduces a systematic reinterpretation of this age-old fact (Morsella Reference Morsella2005): Skeletomotor actions are at times “consciously mediated” because they are directed by multiple systems that require consciousness to influence action collectively – what we refer to as collective influence.

Regarding the skeletomotor output system, one must consider that all processes trying to influence skeletomotor behavior must, in a sense, “go through it.” Each system giving rise to inclinations has its peculiar operating principles and phylogenetic origins (Allman Reference Allman2000): One system “protests” an exploratory act while another system reinforces that act (Morsella Reference Morsella2005). Because each skeletomotor effector can usually perform only one act at a time (e.g., one can utter only one word at a time; Lashley Reference Lashley and Jeffress1951; Wundt 1900), there must be a way in which the inclinations from the many heterogeneous systems can be “understood” and processed collectively by the skeletomotor output system. To yield adaptive action, this process must also integrate information about other things (e.g., the physical environment). To a degree greater than that of any other effector system (e.g., smooth muscle), distinct regions/systems of the brain are trying to control the skeletomotor output system in different and often opposing ways. All inclinations toward it, from primitive plans about basic needs to complex plans associated with language, must engage this system. Thus, the skeletomotor output system is the “final common path” for processes capable of influencing skeletomotor function (McFarland & Sibly Reference McFarland and Sibly1975; Sherrington Reference Sherrington1906). Figuratively speaking, the skeletomotor output system is akin to a single steering wheel that is controlled by multiple drivers (Morsella Reference Morsella2005).

3.1. Tenet: Conscious contents must be “perceptual-like” in nature

We propose that the cognitive and neural processes associated with the contents of our “creature in the cave” should be perceptual-like in nature. When making this claim, we acknowledge that conscious contents are neither purely sensorial nor purely motor related; instead, they represent well-crafted representations occurring at a stage of processing between sensory analysis and motor programming (Jackendoff Reference Jackendoff1990; Lashley Reference Lashley1956; Merker Reference Merker2013c; J. Prinz Reference Prinz, Velmans and Schneider2007; W. Prinz Reference Prinz, Maasen, Prinz and Roth2003b). In everyday life, when speaking about this level of representation of external objects, we use the term percept (J. A. Gray Reference Gray1995); however, this level of representation is more precisely construed as an intermediate representational format (e.g., the color red or the illusion of “da” in the McGurk effect) that links perception to action (W. Prinz Reference Prinz, Maasen, Prinz and Roth2003b). (To not introduce more jargon, we will continue to use the term percept to refer to conscious contents about the external world or the body, but we do so mindful that the term, because of its sensory connotation, can be misleading.)

The proposal that contents are perceptual-like is based on the synthesis of conclusions from diverse areas of study. First, according to the age-old sensorium hypothesis (Godwin et al. Reference Godwin, Gazzaley, Morsella, Pereira and Lehmann2013; Goodale & Milner Reference Goodale and Milner2004; J. A. Gray Reference Gray2004; Grossberg Reference Grossberg1999; Harleß Reference Harleß1861; James Reference James1890; Müller Reference Müller1843; Woodworth Reference Woodworth1915), the contents of consciousness are influenced primarily by perceptual-based (and not motor-based) events and processes, because motor processes are largely unconscious. There is substantial phenomenological evidence for this hypothesis. During action, for example, one is unconscious of the efference to the muscles that dictates which fibers should be activated at which time (Rosenbaum Reference Rosenbaum, Yantis and Yantis2002). Although one is unconscious of these complex programs (Johnson & Haggard Reference Johnson and Haggard2005), one is often aware of their proprioceptive and perceptual consequences (e.g., perceiving the hand grasping; Fecteau et al. Reference Fecteau, Chua, Franks and Enns2001; Fourneret & Jeannerod Reference Fourneret and Jeannerod1998; Gottlieb & Mazzoni Reference Gottlieb and Mazzoni2004; J. A. Gray Reference Gray2004; Heath et al. Reference Heath, Neely, Yakimishyn and Binsted2008; Liu et al. Reference Liu, Chua and Enns2008; Rossetti Reference Rossetti and Grossenbacher2001). These images tend to be perceptual-like images of action outcomes (Hommel Reference Hommel2009; Jeannerod Reference Jeannerod2006; Pacherie Reference Pacherie2008): “In perfectly simple voluntary acts there is nothing else in the mind but the kinesthetic idea … of what the act is to be” (James Reference James1890, p. 771).Footnote ¹¹ It seems that we do not have direct, conscious access to motor programs, to syntax, to aspects of executive control (Crick Reference Crick1995; Suhler & Churchland Reference Suhler and Churchland2009; Tallon-Baudry Reference Tallon-Baudry2012; van Gaal et al. Reference van Gaal, Ridderinkhof, Fahrenfort, Scholte and Lamme2008), or to other kinds of “efference generators” (Grossberg Reference Grossberg1999; Morsella & Bargh Reference Morsella and Bargh2010b; Rosenbaum Reference Rosenbaum, Yantis and Yantis2002), including those for emotional systems (e.g., the amygdala; Anderson & Phelps Reference Anderson and Phelps2002; LeDoux Reference LeDoux1996; Öhman et al. Reference Öhman, Carlsson, Lundqvist and Ingvar2007; Olsson & Phelps Reference Olsson and Phelps2004). (Unconscious executive control from activated action sets exemplifies what has been historically referred to as “imageless,” determining tendencies; Ach Reference Ach and Rapaport1905/1951.)

In line with the sensorium hypothesis, examination of the liaison between action and consciousness reveals an isomorphism regarding what one is conscious of when one is (a) observing one's own action, (b) anticipating an action effect, (c) dreaming, and (d) observing the behaviors of others (Graziano Reference Graziano2010). In every case, it is the same, perceptual-like dimension of the experience that constitutes what is consciously available (Farrer et al. Reference Farrer, Frey, Van Horn, Tunik, Turk, Inati and Grafton2008; Melcher et al. Reference Melcher, Winter, Hommel, Pfister, Dechent and Gruber2013; Morsella & Bargh Reference Morsella and Bargh2010b; Rizzolatti et al. Reference Rizzolatti, Sinigaglia and Anderson2008; Sperry Reference Sperry1952). Speech processing provides a compelling example. Consider the argument by Levelt (Reference Levelt1989) that, of all of the processes involved in language production, one is conscious of only a subset of the processes, whether when speaking aloud or subvocalizing. (Language reveals that mechanisms in action production can be complex but unconscious, as in the case of syntax.) It is for this reason that, when speaking, one often does not know exactly which words one will utter next until the words are uttered or subvocalized following lexical retrieval (Levelt Reference Levelt1989; Slevc & Ferreira Reference Slevc and Ferreira2006). For instance, in the phonological loop, it is the phonological representation and not, say, the motor-related, “articulatory code” (Ford et al. Reference Ford, Gray, Faustman, Heinks and Mathalon2005) that one is conscious of during spoken or subvocalized speech (Buchsbaum & D'Esposito Reference Buchsbaum and D'Esposito2008; Fodor Reference Fodor1998; Rizzolatti et al. Reference Rizzolatti, Sinigaglia and Anderson2008). It is for this reason that Buchsbaum (Reference Buchsbaum2013) concluded that, in the phonological loop, the “inner voice” (i.e., the articulatory code) cannot hear itself. Although there has been substantial debate regarding the nature of conscious representations (e.g., whether they are “analogical” or “propositional”; Markman Reference Markman1999), few would argue about the isomorphism among the conscious contents experienced while acting (e.g., saying “hello”), dreaming (e.g., saying “hello” in a dream), or observing the action of another (e.g., hearing “hello”).

3.2. Tenet: Conscious contents can directly activate action processes in the skeletal muscle output system

According to ideomotor theory (Greenwald Reference Greenwald1970; Harleß Reference Harleß1861; Hommel Reference Hommel2009; Hommel et al. Reference Hommel, Müsseler, Aschersleben and Prinz2001; James Reference James1890; Lotze Reference Lotze1852), the perceptual representations identified by the sensorium hypothesis provide a mechanism for goal-directed action control. In this theory, the mental image of the (perceptual-like) action effects (in the body or in the world) of an instrumental action leads to the execution of that action, with the motor programming involved being unconscious. (It is noteworthy that contemporary ideomotor accounts are agnostic regarding the role of consciousness in action control [e.g., Hommel Reference Hommel2013].)

In ideomotor accounts, action selection is thus driven by the selection of the representation of the perceptual consequences of a motoric act. Hence, the many conscious contents about the world and the body can be construed as “action options” for the skeletomotor output system. From this standpoint, the urge to move the arm leftward is isomorphic to the perceptual consequences of what would be observed if the act were performed. This is also the case for the “higher” abilities, such as language. For example, before making an important toast (or making a toast in an unmastered language), a person has conscious imagery regarding the words to be uttered. Thus, action selection is concerned with achieving a final end state (e.g., flicking a switch or saying “hello”), which can be realized in multiple ways, as in the case of motor equivalence (Lashley Reference Lashley and Kluver1942), in which several different behaviors can lead to the same end state. The unconscious motor programs realizing these end states are complex and context sensitive, as in the case of co-articulation in speech (Levelt Reference Levelt1989; see also Zhang & Rosenbaum Reference Zhang and Rosenbaum2008).

According to ideomotor theory, there is a direct link between activation of action-related perceptual processes and (unconscious) action systems. Such a link is consistent with overwhelming evidence demonstrating that the presentation of action-related perceptual stimuli automatically and systematically influences action processing (see reviews of evidence in: Ellis Reference Ellis, Morsella, Bargh and Gollwitzer2009; Hommel & Elsner Reference Hommel, Elsner, Morsella, Bargh and Gollwitzer2009). This is evident in classic paradigms such as the flanker (Eriksen & Eriksen Reference Eriksen and Eriksen1974) and Stroop tasks (Stroop Reference Stroop1935). In the latter, participants must name the color in which words are written. When the color and word name do not match (e.g., RED in blue font), response interference arises because the automatic (and unintentional) word-reading plan competes with the weaker (and intended) color-naming plan (Cohen et al. Reference Cohen, Dunbar and McClelland1990). Behavioral and psychophysiological evidence reveals that, during such response interference, competition involves simultaneous activation of the brain processes associated with both the target- and distracter-related responses (Coles et al. Reference Coles, Gratton, Bashore, Eriksen and Donchin1985; DeSoto et al. Reference DeSoto, Fabiani, Geary and Gratton2001; Eriksen & Schultz Reference Eriksen and Schultz1979; Mattler Reference Mattler2005; McClelland Reference McClelland1979; van Veen et al. Reference van Veen, Cohen, Botvinick, Stenger and Carter2001). Additional evidence stems from neurological conditions (see review in Morsella & Bargh Reference Morsella, Bargh, Cacioppo and Decety2011) and in the aforementioned research on unconscious efference binding, in which subliminal stimuli influence motor responses (Hallett Reference Hallett2007).

3.3. Tenet: Action selection as the result of inter-representational dynamics

From an ideomotor standpoint, once an action goal (e.g., pressing a button) is selected, unconscious motor efference enacts the action directly.Footnote ¹³ From this standpoint, that which prevents the activation of an action goal representation from directly influencing overt action is only the activation of an incompatible action goal (James Reference James1890; W. Prinz et al. Reference Prinz, Aschersleben, Koch, Morsella, Bargh and Gollwitzer2009). In this framework, conscious representations of one's finger flexing, for instance, automatically lead to the flexing of one's finger, unless representations of incompatible action effects (e.g., the finger not flexing; James Reference James1890) happen to be activated. It is important to note that the incompatibility regarding these two action effects resides, not in the conscious field, in which both action effects could be represented simultaneously, but rather in the simultaneous execution of the two action plans.

Consistent with this view of action conflicts, in one scenario, a conflict may involve representations A and B (associated with neural correlates A _NC and B _NC), and then, at a later time and in a different context, a conflict may involve representations C and D (associated with neural correlates C _NC and D _NC; Curtis & D'Esposito Reference Curtis, D'Esposito, Morsella, Bargh and Gollwitzer2009). Importantly, the two conflicts involve separate cognitive and neural processes, suggesting that “no single area of the brain is specialized for inhibiting all unwanted actions” (Curtis & D'Esposito Reference Curtis, D'Esposito, Morsella, Bargh and Gollwitzer2009, p. 72). Instead, representations, including those of action sets (Fuster Reference Fuster2003; Grafman & Krueger Reference Grafman, Krueger, Morsella, Bargh and Gollwitzer2009) and rules (E. K. Miller Reference Miller2000) compete for the control of action. Such competition between action-related representations is evident in the aforementioned Stroop Task (Stroop Reference Stroop1935).

In our approach, this arrangement in which the contents of the conscious field lead to the activation of multiple (and often competing) action plans causes one to appreciate that, in the skeletomotor output system, there must be a (unconscious) mechanism by which one action plan can influence behavior more than other activated action plans. Such a mechanism would ensure that when holding one's breath while underwater, for example, the action plan to refrain from inhaling would influence behavior more than that of inhaling, although the conscious field would represent both inclinations. Appreciation of such potential “bottlenecks” in action selection can serve as a valuable constraint on theorizing regarding the neural structures underlying consciousness.

Importantly, in the perception-to-action loop, consciousness represents conflicts and not necessarily the representations associated with the resolution of such conflicts, should such representations exist (Morsella Reference Morsella2005). This peculiar property of consciousness arises because consciousness is about a stage of processing reflecting action options and not the mechanisms that, should they exist, represent conflict resolution. This illuminates why Chomsky (Reference Chomsky1988) observes that humans, unlike machines, are not only compelled to act one way or another but also can be inclined to act a certain way. Again, such inclinations could be construed as action options. The resolution of conflict depends not on some general property of consciousness, but on the peculiarities (e.g., relative strengths) of the systems that happen to be in conflict (Skinner Reference Skinner1953). Consciousness only permits that conflicts occur; it does not aim to resolve them (Morsella Reference Morsella2005). Each conflict is idiosyncratic and, if it is to be resolved, must require post-conscious, content-specific algorithms (e.g., one in which overt behavior is influenced most by prepotent action plans; Gold & Shadlen Reference Gold and Shadlen2007; Logan et al. Reference Logan, Yamaguchi, Schall and Palmeri2015). Hence, it is challenging to arrive at general principles for predicting the outcomes of conflicts involving different systems (Campbell & Misanin Reference Campbell and Misanin1969; Krauzlis et al. Reference Krauzlis, Bollimunta, Arcizet and Wang2014; see model of countermanding in Logan et al. [2015]). The Internet provides a good analogy for the role of consciousness in conflict: The Internet permits two people from different cities to debate, but it cannot resolve conflicts between them. Another analogy would be an interpreter that translates for two parties which are in conflict about some issue. The interpreter is necessary for the instantiation of the conflict and for its potential resolution; the interpreter, however, cannot resolve the conflict.

In summary, to advance the identification of the neural substrates of consciousness, it is essential to keep in mind that consciousness is a phenomenon associated with perceptual-like processing and interfacing with the somatic nervous system (Fig. 1).

Figure 1. The divisions of the nervous system and place of consciousness within the system (based on Poehlman et al. 2012), including the major divisions of the Somatic and Autonomic systems. Afference binding within systems can be unconscious. Although response systems can influence action directly, as in the case of unintegrated actions, only in virtue of consciousness can multiple response systems influence action collectively, as when one holds one's breath while underwater.

3.4. Neural evidence supports the sensorium hypothesis

The sensorium hypothesis and ideomotor theory reveal that, in terms of stages of processing, that which characterizes conscious content is the notion of perceptual afference (information arising from the world that affects sensory-perceptual systems; Sherrington Reference Sherrington1906) and corollary discharges (e.g., when subvocalizing; cf. Chambon et al. Reference Chambon, Wenke, Fleming, Prinz and Haggard2013; Christensen et al. Reference Christensen, Lundbye-Jensen, Geertsen, Petersen, Paulson and Nielsen2007; Jordan Reference Jordan2009; Obhi et al. Reference Obhi, Planetta and Scantlebury2009; Scott Reference Scott2013), both of which are cases of perceptual-like content. This hypothesizing is consistent with the idea that, insofar as consciousness must always contain some content (Brentano Reference Brentano1874; Fodor Reference Fodor1980; Reference Fodor1998; J. A. Gray Reference Gray1995; Reference Gray2004; Hume Reference Hume and Selby-Bigge1739/1888; Koch Reference Koch2004; Schopenhauer Reference Schopenhauer1818/1819; Sergent & Dehaene Reference Sergent and Dahaene2004), then it is parsimonious to propose that the neural regions responsible for processing that content must be part of the neural correlate of consciousness for that content. Thus, if content X is in consciousness, then the circuits processing content X must be part of a neural correlate of consciousness (e.g., at least of X). (Of course, within such an arrangement, it may be that the region[s] processing the particular content need not be the region[s] in which that content becomes associated with the conscious field; content processing could arise in one locus of the network, but the participation of contents in the conscious field could arise at another locus of the network.) With this notion in mind, we turn to the neural evidence regarding conscious contents.

Consistent with the sensorium hypothesis, there is evidence implicating perceptual brain regions as the primary regions responsible for consciousness. For example, direct electrical stimulation of parietal areas gives rise to the conscious urge to perform an action, and increased activation makes subjects believe that they actually executed the corresponding action, even though no action was performed (Desmurget et al. Reference Desmurget, Reilly, Richard, Szathmari, Mottolese and Sirigu2009; Desmurget & Sirigu Reference Desmurget and Sirigu2010; see also Farrer et al. Reference Farrer, Frey, Van Horn, Tunik, Turk, Inati and Grafton2008). However, activating motor areas (e.g., premotor regions) leads to the expression of the actual action, but subjects believe that they did not perform any action whatsoever (see also Fried et al. Reference Fried, Katz, McCarthy, Sass, Williamson, Spencer and Spencer1991). Importantly, consistent with our foregoing conclusions, the urge to perform a motor act is associated with activation of perceptual regions.

In accord with the sensorium hypothesis, the majority of studies involving brain stimulation and consciousness have found that stimulation of perceptual (e.g., posterior) brain areas leads to changes in consciousness (e.g., haptic hallucinations). This should not be surprising given that these regions were identified as “perceptual” in the first place by the use of self-report during brain stimulation (e.g., Penfield & Roberts Reference Penfield and Roberts1959). Self-report usually involves consciousness (see discussion in Bayne Reference Bayne, Clark, Vierkant and Kiverstein2013). In the literature, we found only one datum in which brain stimulation of a frontal (non-olfactory) area led to a conscious content. In this study (Fried et al. Reference Fried, Katz, McCarthy, Sass, Williamson, Spencer and Spencer1991, cited in Haggard Reference Haggard2008), weak electrical stimulation of the pre-supplementary motor area led to the experience of the urge to move a body part, with stronger stimulation leading to movement of the same body part. It has been proposed that such activation led to corollary discharge that was then “perceived” by perceptual areas (Chambon et al. Reference Chambon, Wenke, Fleming, Prinz and Haggard2013; Farrer et al. Reference Farrer, Frey, Van Horn, Tunik, Turk, Inati and Grafton2008; Iacoboni Reference Iacoboni, Hurley and Chater2005; Iacoboni & Dapretto Reference Iacoboni and Dapretto2006; Lau et al. Reference Lau, Rogers and Passingham2007; Melcher et al. Reference Melcher, Winter, Hommel, Pfister, Dechent and Gruber2013; Scott Reference Scott2013), which would be consistent with the sensorium hypothesis. One strong hypothesis from this line of theorizing is that activations in regions that are non-perceptual or motor should never (independent of corollary discharge) influence the conscious field.

Consistent with the sensorium hypothesis and ideomotor theory, research reveals that a key component of the control of intentional action is feedback about ongoing action plans to perceptual areas of the brain, such as post-central cortex (Berti & Pia Reference Berti and Pia2006; Chambon et al. Reference Chambon, Wenke, Fleming, Prinz and Haggard2013; Desmurget et al. Reference Desmurget, Reilly, Richard, Szathmari, Mottolese and Sirigu2009; Farrer et al. Reference Farrer, Frey, Van Horn, Tunik, Turk, Inati and Grafton2008; Iacoboni Reference Iacoboni, Hurley and Chater2005; Miall Reference Miall2003). With this information in mind, it has been proposed that consciousness is associated not with frontal or higher-order perceptual areas, but with lower-order perceptual areas (J. R. Gray et al. Reference Gray, Bargh and Morsella2013; Liu et al. Reference Liu, Paradis, Yahia-Cherif and Tallon-Baudry2012; Tallon-Baudry Reference Tallon-Baudry2012). However, it is important to qualify that though the sensorium hypothesis specifies that consciousness involves neural circuits that, traditionally, have been associated with perception, such circuits are widespread throughout the brain and exist within both cortical and subcortical regions (Merker Reference Merker, Shimon, Tomer and Zach2012). Hence, the sensorium hypothesis is consistent with several neuroanatomical accounts of consciousness, including cortical, subcortical (e.g., thalamic), and thalamocortical accounts of consciousness. Thus, on the basis of the sensorium hypothesis alone, it is premature to dismiss subcortical accounts of consciousness (e.g., Merker Reference Merker2007; Penfield & Jasper Reference Penfield and Jasper1954; Ward Reference Ward2011).

In conclusion, at the present stage of understanding, the literature provides no clear answer regarding the neural substrates of any kind of conscious content (see treatment in Merker Reference Merker2013b; Reference Merker2013c). Based on the foregoing conclusions about conscious contents, we believe that, to illuminate this issue further, progress can be made by adopting an EASE perspective and focusing on a (relatively) tractable perceptual region – namely, that of the understudied olfactory system.

3.5. Tenet: The olfactory system provides clues regarding the neural correlates of conscious perceptual content in the sensorium

Our EASE perspective led us to the sensorium hypothesis. Now, with the same perspective, we focus on one kind of content in the sensorium. As noted in section 1.4, when attempting to isolate the substrates of a conscious content, researchers have followed Crick and Koch's (Reference Crick and Koch2003) recommendation and focused on vision. It is clear that isolating the neuroanatomical substrate of a visual conscious content remains controversial. From an EASE perspective, and based on previous research (Merrick et al. Reference Merrick, Godwin, Geisler and Morsella2014; Morsella & Bargh Reference Morsella and Bargh2007), we focus our attention instead on olfaction (see also Keller Reference Keller2011), a phylogenetically old system whose circuitry appears to be more tractable and less widespread in the brain than that of vision or higher-level processing such as music perception. As Shepherd (Reference Shepherd2007) concludes, “the basic architecture of the neural basis of consciousness in mammals, including primates, should be sought in the olfactory system, with adaptations for the other sensory pathways reflecting their relative importance in the different species” (p. 93).

Several features of this system render it a fruitful arena in which to isolate the substrates of consciousness. First, olfaction involves a primary processing area that consists of paleocortex (which contains only half of the number of layers of neocortex) and primarily only one brain region (the frontal cortex; Shepherd Reference Shepherd2007). In contrast, vision and audition often involve large-scale interactions between frontal cortex and parietal cortices. These observations reveal the relative simplicity of the anatomy of the olfactory system compared to that of other systems. Second, regarding the cortical-subcortical controversy, olfaction can reveal much about the contribution of thalamic nuclei in the generation of consciousness: Unlike most sensory modalities, afferents from the olfactory sensory system bypass the first-order, relay thalamus and directly target the cortex ipsilaterally (Shepherd & Greer Reference Shepherd, Greer and Shepherd1998; Tham et al. Reference Tham, Stevenson and Miller2009). This minimizes spread of circuitry, permitting one to draw conclusions about the necessity of first-order thalamic relays in (at least) this form of consciousness.

By studying olfaction, one can also draw some conclusions about second-order thalamic relays (e.g., the mediodorsal thalamic nucleus [MDNT]). After cortical processing, the MDNT receives inputs from olfactory cortical regions (Haberly Reference Haberly and Shepherd1998). Although it is likely that the MDNT plays a significant role in olfactory discrimination (Eichenbaum et al. Reference Eichenbaum, Shedlack and Eckmann1980; Slotnick & Risser Reference Slotnick and Risser1990; Tham et al. Reference Tham, Stevenson and Miller2011), olfactory identification, and olfactory hedonics (Sela et al. Reference Sela, Sacher, Serfaty, Yeshurun, Soroker and Sobel2009), as well as in more general cognitive processes including memory (Markowitsch Reference Markowitsch1982), learning (Mitchell et al. Reference Mitchell, Baxter and Gaffan2007), and attentional processes (Tham et al. Reference Tham, Stevenson and Miller2009; Reference Tham, Stevenson and Miller2011), we have found no evidence that a lack of olfactory consciousness results from lesions of any kind to the MDNT (see theorizing about this possibility in Plailly et al. [Reference Plailly, Howard, Gitelman and Gottfried2008]). Regarding second-order thalamic relays such as the MDNT, one must keep in mind that, in terms of circuitry, these nuclei are similar in nature to first-order relays (Sherman & Guillery Reference Sherman and Guillery2006), which are quite simple compared to, say, a cortical column.

Consistent with “cortical” theories of consciousness, Cicerone and Tanenbaum (Reference Cicerone and Tanenbaum1997) observed complete anosmia (the loss of the sense of smell) in a patient with a lesion to the left orbital gyrus of the frontal lobe. In addition, a patient with a right orbitofrontal cortex (OFC) lesion experienced complete anosmia (Li et al. Reference Li, Lopez, Osher, Howard, Parrish and Gottfried2010), suggesting that the OFC is necessary for olfactory consciousness. (It is worth mentioning that we are speaking of the OFC with respect to, not the high-level executive processes with which it has been associated, but, consistent with the sensorium hypothesis, its perceptual processing [i.e., olfactory perception].) Moreover, conscious aspects of odor discrimination have been attributed to the activities of the frontal and orbitofrontal cortices (Buck Reference Buck, Kandel, Schwartz and Jessell2000). Keller (Reference Keller2011) concludes, “There are reasons to assume that the phenomenal neural correlate of olfactory consciousness is found in the neocortical orbitofrontal cortex” (p. 6; see also Mizobuchi et al. Reference Mizobuchi, Ito, Tanaka, Sako, Sumi and Sasaki1999). (According to Barr and Kiernan [Reference Barr and Kiernan1993], olfactory consciousness depends on the piriform cortex.) However, not all lesions of the OFC have resulted in anosmia: Zatorre and Jones-Gotman (Reference Zatorre and Jones-Gotman1991) reported a study in which OFC lesions yielded severe deficits, yet all patients demonstrated normal olfactory detection.

Another output pathway from the piriform cortex projects to the insular cortex (Haberly Reference Haberly and Shepherd1998; Schoenbaum & Eichenbaum Reference Schoenbaum and Eichenbaum1995), a structure that has anatomical connections to the ventral posteromedial (VPM) nucleus of the thalamus (Price et al. Reference Price, Carmichael, Carnes, Clugnet, Kuroda, Ray, Davis and Eichenbaum1991). In light of (a) this information, (b) the conclusions presented above about the MDNT, and (c) theories in which thalamic structures play an important role in consciousness (e.g., Joliot et al. Reference Joliot, Ribary and Llinás1994; Llinás & Ribary Reference Llinás and Ribary2001; Llinás et al. Reference Llinás, Ribary, Contreras and Pedroarena1998; Ward Reference Ward2011), one could propose that olfactory consciousness depends on the integrity of the insula and thalamus. However, regarding the thalamus, it has been observed that, though thalamic lesions can impair olfactory discrimination and complex olfactory learning (Eichenbaum et al. Reference Eichenbaum, Shedlack and Eckmann1980; Martin Reference Martin2013), such lesions, including those of the VPM, never result in anosmia (Martin Reference Martin2013; Price Reference Price1985; Price et al. Reference Price, Carmichael, Carnes, Clugnet, Kuroda, Ray, Davis and Eichenbaum1991; Sela et al. Reference Sela, Sacher, Serfaty, Yeshurun, Soroker and Sobel2009). The lesion literature also reveals an additional important fact about olfactory consciousness. Olfactory consciousness does not require the involvement of any transthalamic pathway. In addition, for corticocortical connections, the olfactory system requires no “higher-order” (Sherman & Guillery Reference Sherman and Guillery2006) thalamic relays (e.g., the MDNT or VPM; Gottfried Reference Gottfried, Hummel and Welge-Lüssen2006; Price Reference Price1985; Price et al. Reference Price, Carmichael, Carnes, Clugnet, Kuroda, Ray, Davis and Eichenbaum1991). Considering these characteristics, Gottfried (Reference Gottfried, Hummel and Welge-Lüssen2006) concludes, “The most parsimonious explanation for this anatomical variation is an evolutionary one: As primitive paleocortex, the olfactory circuitry simply developed long before the emergence of a thalamic module” (p. 53). These peculiar neuroanatomical characteristics are unique to olfactory consciousness.

Regarding the role of the insula in olfactory consciousness, after reviewing the literature, we concur with Mak et al. (Reference Mak, Simmons, Gitelman and Small2005) that there is no evidence that anosmia results from damage of any kind (e.g., unilateral or bilateral lesions) to the insular cortex: “There are no reports of olfactory deficits resulting from damage to the insula” (p. 1693; see also Damasio et al. Reference Damasio, Damasio and Tranel2012; Philippi et al. Reference Philippi, Feinstein, Khalsa, Damasio, Tranel, Landini, Williford and Rudrauf2012; Tranel & Welsh-Bohmer Reference Tranel and Welsh-Bohmer2012).

Taken together, the neuroanatomical evidence presented above leads one to conclude that, in order to advance the current understanding of the neural underpinnings of consciousness, the next hypothesis to falsify is that olfactory consciousness requires cortical processes. This hypothesis is far from obvious, and it is falsifiable, because there are strong empirically based frameworks (e.g., Damasio Reference Damasio1999; Merker Reference Merker2007; Panksepp Reference Panksepp1998) proposing that consciousness is a function of subcortical processes. When these frameworks are integrated with our present treatment of the liaison between consciousness and olfactory circuits, our hypothesis could be proven to be inaccurate. For example, it might be that olfactory percepts are elaborated at a cortical level but become conscious only at some subcortical level (e.g., in the brainstem). Such a falsification of our hypothesis would advance our understanding of consciousness and the brain. Figuratively speaking, falsifying this particular “cortical” hypothesis provides the “lowest hanging fruit” for identifying the neural substrates of consciousness. In this way, the olfactory system can be used as a test-bed for hypotheses stemming from the cortical-subcortical controversy.

Third, from an EASE perspective, there are phenomenological and cognitive/mechanistic properties that render this system a fruitful network in which to investigate consciousness. Regarding phenomenological properties, unlike what occurs with other modalities, olfaction regularly yields no subjective experience of any kind when the system is under-stimulated, as when odorants are in low concentration, or during sensory habituation. This “experiential nothingness” (Morsella et al. Reference Morsella, Krieger and Bargh2010) is more akin to the phenomenology of the blind spot than to what one experiences when visual stimulation is absent (darkness). In the latter case, there still exists a conscious, visual experience (e.g., that of a black field). The experiential nothingness associated with olfaction yields no conscious contents of any kind to such an extent that, absent memory, one in such a circumstance would not know that one possessed an olfactory system. Hence, for our purposes, the creation of a conscious olfactory content is a true “addition” to the conscious field in that, not only does it involve the introduction of information about a particular stimulus, but also it involves the addition, from one moment to the next, of an entire modality. (See additional advantages of studying olfactory consciousness in Note 1.)

For these reasons, olfaction provides the best portal for understanding the neural correlates of additions to the conscious field. In our “creature in the cave” example, the smell of smoke is an addition to the conscious field that influences skeletomotor responses toward other conscious contents (e.g., the visual percept of the opening). Examining the neural correlates of such an addition might provide more evidence for the integration consensus. For example, it has been hypothesized that one becomes conscious of an olfactory percept only when the representation is part of a wider network involving other systems (Cooney & Gazzaniga Reference Cooney and Gazzaniga2003), such as motor (Mainland & Sobel Reference Mainland and Sobel2006) or semantic-linguistic (Herz Reference Herz2003) systems. (See review of the relationship between neural oscillations and olfactory consciousness in Merrick et al. [2014].)

In conclusion, regarding neuroanatomy, our primary hypothesis is that consciousness is associated with what has traditionally been regarded as “perceptual” regions of the brain, a hypothesis that challenges some accounts of consciousness in which consciousness is associated with executive processes in frontal cortex (e.g., Boly et al. Reference Boly, Garrido, Gosseries, Bruno, Boveroux, Schnakers, Massimini, Litvak, Laureys and Friston2011; Dehaene & Naccache Reference Dehaene and Naccache2001; Lau Reference Lau2008; Panagiotaropoulos et al. 2012; Safavi et al. Reference Safavi, Kapoor, Logothetis and Panagiotaropoulos2014; Velly et al. Reference Velly, Rey, Bruder, Gouvitsos, Witjas, Regis, Peragut and Gouin2007). Our secondary hypothesis is that olfactory consciousness can be constituted entirely by cortical circuits.

4.1. Tenet: Content generation is encapsulated

In our “creature in the cave” example, the addition of an olfactory content to the conscious field just “happens,” without any noteworthy effort on the part of the organism (Mainland & Sobel Reference Mainland and Sobel2006). The content arises from a particular configuration of afference (e.g., the unconscious visual and auditory afference in the McGurk effect) to what can be construed as a content generator (associated with a perceptual region). Traditionally, these content generators (e.g., for color) have been construed as “modules” (Fodor Reference Fodor1983). Such a configuration of afference may include not only bottom-up afference, but also afference from unconscious top-down processes from knowledge systems and from frontal control regions (Suhler & Churchland Reference Suhler and Churchland2009; Tallon-Baudry Reference Tallon-Baudry2012). Importantly, these generative processes that create conscious content are themselves context sensitive and unconscious (e.g., as in the McGurk effect; Lamme & Spekreijse Reference Lamme and Spekreijse2000). Regarding context sensitivity, consider that the image of a snake on a television screen triggers little if any fear, but such is not the case in a natural context.

Usually, contents enter consciousness in a manner that is more automatic, and less driven by intentions of the experiencing “agent,” than appears to be the case in the everyday life of us pensive humans (Tallon-Baudry Reference Tallon-Baudry2012; Vierkant Reference Vierkant, Clark, Kiverstein and Vierkant2013). Often, contents tend to “just happen” (Vierkant Reference Vierkant, Clark, Kiverstein and Vierkant2013). In line with these views, Helmholtz (Reference Helmholtz and Shipley1856/1961) proposed that unconscious processes can generate conscious content in a manner that resembles reflexes and other unintentional actions. When speaking about such “unconscious inferences,” Helmholtz was referring not only to the generation of the conscious contents associated with low-level perceptual processes such as depth perception, but also to higher-level, non-perceptual processes such as automatic word reading – an unnatural, intellectual process that requires years of training. Helmholtz noted that when one is confronted with an orthographic stimulus (e.g., HOUSE), the stimulus automatically triggers a conscious representation of the phonological form of the word (i.e., /haus/). Seldom in everyday life is it appreciated that, in this situation, the visual stimulus triggers a conscious content that is very different in nature from that of the environmental stimulation that brought the content into existence: The conscious representation of the phonological form of the word is associated not with the visual modality, but with audition (Levelt Reference Levelt1989).

Conscious content can be generated by unconscious inferences also in the case of action-related urges (e.g., from unconsciously generated corollary discharge). These urges are often triggered in a predictable and insuppressible manner. For example, when one holds one's breath while underwater or runs barefoot across the hot desert sand in order to reach water, one cannot help but consciously experience the inclinations to inhale or to avoid touching the hot sand, respectively (Morsella Reference Morsella2005). Regardless of the adaptiveness of the expressed actions, the conscious strife triggered by the external stimuli cannot be turned off voluntarily (Morsella Reference Morsella2005; Öhman & Mineka Reference Öhman and Mineka2001). In these cases, the externally activated action-related urges are, in a sense, insulated, or “encapsulated” (Fodor Reference Fodor1983), from voluntary control. Thus, although inclinations triggered by external stimuli can be behaviorally suppressed, they often cannot be mentally suppressed (Bargh & Morsella Reference Bargh and Morsella2008). One can think of many cases in which externally triggered conscious contents are more difficult to control than overt behavior (Allen et al. Reference Allen, Wilkins, Gazzaley and Morsella2013).

It has been argued that it is adaptive for content generation to be encapsulated in this way and for conscious contents to be incapable of directly influencing each other in the conscious field (Firestone & Scholl Reference Firestone and Scholl2014; Merrick et al. Reference Merrick, Godwin, Geisler and Morsella2014; Rolls et al. Reference Rolls, Judge and Sanghera1977). From this standpoint, the conscious, perceptual representations for instrumental action should be unaffected by the organism's beliefs or motivational states (Bindra Reference Bindra1974; Reference Bindra1978). As Rolls and Treves (Reference Rolls and Treves1998) conclude, “It would not be adaptive, for example, to become blind to the sight of food after we have eaten it to satiety” (p. 144). Similarly, it would not be adaptive for contents pertaining to incentive/motivational states to be influenced directly by other contents, such as desires and beliefs (Baumeister et al. Reference Baumeister, Vohs, DeWall and Zhang2007). For example, if one's beliefs could lead one to voluntarily “turn off” pain, guilt, or hunger, then these negative states would lose their adaptive value. Although motivation and beliefs may contaminate higher-order processes such as memory, they should have little influence over perceptual contents (Cooper et al. Reference Cooper, Sterling, Bacon and Bridgeman2012; Firestone & Scholl Reference Firestone and Scholl2014; Pylyshyn Reference Pylyshyn1984; Reference Pylyshyn1999). Such “cross-contamination” across contents would compromise the critical influence of such incentive/motivational states on behavior.

Thus, each content is independent of other contents in the conscious field, whether the contents arise from environmental stimulation or from memory. Specifically, a conscious content (e.g., “da” in the McGurk effect) cannot directly influence the nature of other contents already in the conscious field (e.g., the smell of a rose, a toothache; Morsella Reference Morsella2005). (Of course, this is not to mean that the configuration of afference engendering one content cannot influence the generation of other contents – a form of context sensitivity in afference processing that occurs unconsciously [Lamme & Spekreijse Reference Lamme and Spekreijse2000; Merker Reference Merker, Shimon, Tomer and Zach2012].) Because of encapsulation, illusions persist despite one's knowledge regarding the actual nature of the stimuli (Firestone & Scholl Reference Firestone and Scholl2014; Pylyshyn Reference Pylyshyn1984).

It could be said that a given content does not “know” about its relevance to other contents (including high-level, knowledge-based contents) or to current action. When representing a food object, for example, the content does not know whether the food item will be eaten or, instead, be thrown as a weapon. This view stands in contrast to several influential theoretical frameworks in which both the activation of, and nature of, conscious contents are influenced by what can be regarded as overarching goals or current task demands (e.g., Banerjee et al. Reference Banerjee, Chatterjee and Sinha2012; Bhalla & Proffitt Reference Bhalla and Proffitt1999; Bruner Reference Bruner1973; Bruner & Postman Reference Bruner and Postman1949; Dehaene Reference Dehaene2014; Meier et al. Reference Meier, Robinson, Crawford and Ahlvers2007; Stefanucci & Geuss Reference Stefanucci and Geuss2009). Because of the principle of encapsulation, conscious contents cannot influence each other either at the same time nor across time, which counters the everyday notion that one conscious thought can lead to another conscious thought.

In the present framework, not only do contents not influence each other in the conscious field, but also as Merker (personal communication, June 30, Reference Merker, Shimon, Tomer and Zach2012) concludes, content generators cannot communicate the content they generate to another content generator. For example, the generator charged with generating the color orange cannot communicate “orange” to any other content generator, because only this generator (a perceptual module) can, in a sense, understand and instantiate “orange.” Hence, if the module charged with a particular content is compromised, then that content is gone from the conscious field, and no other module can “step in” to supplant that content (Kosslyn et al. Reference Kosslyn, Ganis and Thompson2001). As Merker notes, in constructing the conscious field, modules can send, not messages with content, but only “activation” to each other (see also Lamme & Spekreijse Reference Lamme and Spekreijse2000). This activation, in turn, influences whether the receiver module will generate, not the kind of content generated by the module from which it received activation, but rather its own kind of content (e.g., a sound). Because messages of content cannot be transmitted to other content generators, the neural correlates of the content for X must include activation of the module that generates X, because a given content cannot be segregated from the process by which it was engendered, as stated previously.

4.2. Tenet: Field contents must meet multiple-constraint satisfaction, be unambiguous, and appear as if apprehended from a first-person perspective

From an EASE perspective, one can ask: What does a conscious content require if it is to lead to adaptive action? To answer this question, one must first consider that, in the case of object perception (such as the opening in our “creature in the cave” example), representations must be veridical to some extent in order to render action adaptive (Firestone & Scholl Reference Firestone and Scholl2014). For example, during action selection, it would not be adaptive for a circular object to be represented with, say, straight lines and corners. Similarly, it would not be adaptive for an object on the left to be represented as if it were on the right. Therefore, for the conscious field to afford adaptive action, it must represent with some veracity the spatial relation of those objects to the organism (Gibson Reference Gibson1979). Under normal circumstances, the contents of the conscious field at each moment are complete and unambiguous. Accordingly, Merker (Reference Merker, Shimon, Tomer and Zach2012) concludes that, because of the very nature of the constitution of the systems giving rise to conscious sensory representations, these systems are incapable of representing stimulus ambiguity (e.g., as in the Necker cube), at least at one moment in time. (However, such ambiguity could exist in unconscious perceptual processing [Merker Reference Merker, Shimon, Tomer and Zach2012].) Thus, a given content emerges from polysensory configurations of afference, as in the McGurk effect, leading to the “global best estimate” of what that content should be (Helmholtz Reference Helmholtz and Shipley1856/1961; Merker Reference Merker, Shimon, Tomer and Zach2012).

Such well-constructed contents could stem from (a) the proposed, unconscious mechanisms of “multiple drafts” (Dennett Reference Dennett1991); (b) the interpretative processes of “apperception” (Wundt Reference Wundt and Titchener1902/1904); or (c) “reentrant processing,” in which a module, in order to give rise to a conscious representation, must receive feedback activation from other modules about that representation (Lamme Reference Lamme2001; Pascual-Leone & Walsh Reference Pascual-Leone and Walsh2001; Tong Reference Tong2003). For example, if visual modules X and Y construct a representation which leads to the activation of other modules, then that representation becomes conscious only after feedback activation from the other modules returns to X and Y (Di Lollo et al. Reference Di Lollo, Enns and Rensink2000; Fahrenfort et al. Reference Fahrenfort, Scholte and Lamme2007; Goodhew et al. Reference Goodhew, Dux, Lipp and Visser2012; Grossberg Reference Grossberg1999; Hamker Reference Hamker2003; Kriegel Reference Kriegel2007; Lamme Reference Lamme2001; Lee et al. Reference Lee, Kim, Noh, Choi, Hwang and Mashour2009; Llinás et al. Reference Llinás, Ribary, Contreras and Pedroarena1998; Pascual-Leone & Walsh Reference Pascual-Leone and Walsh2001; Tong Reference Tong2003). Re-entrant processing may instantiate a kind of “checks-and-balances” system for constructing accurate conscious contents that satisfy the criteria of multiple modules, a form of multiple-constraint satisfaction (Dennett Reference Dennett1991; Merker Reference Merker, Shimon, Tomer and Zach2012). In addition, feedback of this sort may underlie the phenomenon of “contextual modulation” (e.g., in figure-ground effects; Lamme & Spekreijse Reference Lamme and Spekreijse2000). More simply, this feedback may be necessary because conscious contents may require (a) high levels of activation (Kinsbourne Reference Kinsbourne, Cohen and Schooler1996) or (b) sustained activation for a prolonged period (Lau Reference Lau and Gazzaniga2009), both of which can be furnished by sustained reverberation (Hebb Reference Hebb1949). In summary, for a content generator to contribute to the conscious field and for its contents to be well-crafted, it may require the concurrent activation from both feed-forward and feedback mechanisms (Lamme & Spekreijse Reference Lamme and Spekreijse2000).

The conscious field of our “creature in the cave” includes representations of urges and external objects, which incorporate the relation between such things and the organism itself (Lehar Reference Lehar2003; Merker Reference Merker, Shimon, Tomer and Zach2012; Yates Reference Yates1985). More generally, contents are usually sensed to be different from, and separate from, the observing agent (Brentano Reference Brentano1874; Merker Reference Merker, Shimon, Tomer and Zach2012; Schopenhauer Reference Schopenhauer1818/1819). Insofar as the action selection process of the skeletomotor output system must take into account spatial distance from the organism as one of the many factors in adaptive selection, then all contents about the external world (including the body) must have a common, egocentric reference (Merker Reference Merker2013c). It would be disadvantageous for this rule to be violated and for, again, an object on the left to be represented as if on the right. Hence, most conscious contents appear as if from a first-person perspective (Gibson Reference Gibson1979; Merker Reference Merker2013c; J. Prinz Reference Prinz, Velmans and Schneider2007). The conscious field is imbued with this first-person perspective during waking, in dreaming, and for illusions in which, through clever experimental manipulations and the presentation of certain stimuli, the perspective is momentarily perceived as if from outside of the body (Ehrsson Reference Ehrsson2007).Footnote ¹⁴ From this standpoint, the demands of adaptive action selection require the creation of a first-person perspective, which is a primitive form of “self.”

4.3. Tenet: The conscious field serves as a frame that represents encapsulated contents for collective influence over, not itself, but the skeletal muscle output system

It seems that the conscious field is like a mosaic of discrete, heterogeneous contents, rendering the field to be combinatorial. Each content is well-crafted and unambiguous (Dietrich & Markman Reference Dietrich and Markman2003; Freeman Reference Freeman2004; Köhler Reference Köhler1947; Merker Reference Merker, Shimon, Tomer and Zach2012; Scholl Reference Scholl2001). The contents cannot directly influence each other,Footnote ¹⁵ and the content generators cannot duplicate the generative abilities of each other. Thus, the resultant contents from these modules are encapsulated from each other. These mosaic-like Gestalts (i.e., the conscious field) arise in consciousness in a discontinuous manner, with each conscious moment, lasting for fractions of a second, having an updated version of all of the contents in the conscious field. For action to be adaptive, the refresh rate of the entire field must be faster than the quickest rate at which voluntary actions can be produced. Hence, the refresh must occur more rapidly than the rate at which the fastest actions (e.g., saccades) can be emitted (Merker Reference Merker2013c). (For theorizing regarding the temporal properties of such an updating process, see Libet [Reference Libet2004] and Merker [Reference Merker, Shimon, Tomer and Zach2012, p. 56; 2013c, p. 12].)

Importantly, the collective influence of the combination of contents in the conscious field is not toward the conscious field itself; instead, according to PRISM, the conscious field is apprehended by the (unconscious) mechanisms composing the skeletomotor output system. Thus, the conscious contents of blue, red, a smell, or the urge to blink are the tokens of a mysterious language understood, not by consciousness itself (nor by the physical world), but by the unconscious action mechanisms of the skeletomotor output system. Why do things appear the way they do in the field? Because, in order to benefit action selection, they must differentiate themselves from all other tokens in the field – across various modalities/systems but within the same decision space.

Although possessing elements of a “Cartesian Theater” (Dennett Reference Dennett1991), this arrangement does not introduce the “homunculus fallacy,” because, in the skeletomotor output system, there are many motor-homunculi, each incapable of duplicating the functions of the system as a whole (Dennett Reference Dennett1991). Unlike the “workspace” models associated with the integration consensus (e.g., Baars Reference Baars1988; Dehaene Reference Dehaene2014), which propose that conscious representations are “broadcast” to modules engaged in both stimulus interpretation and content generation, in our framework (as in Merker Reference Merker2007), the contents of the conscious field are directed only at response modules in the skeletomotor output system. In short, conscious contents are “sampled” only by unconscious action systems that are charged with (specifically) skeletal muscle control.