Perceptual symbol systems provide a welcome alternative to amodal encapsulated means of cognitive processing. However, the relations between perceived reality and internal mentation require a more differentiated approach, reflecting both developmental differences between infant and adult experience and qualitative differences between consciously perceived and mentally represented contents. Neurological evidence suggests a developmental trajectory from initial perceptual states in infancy to a more differentiated consciousness from two years of age on. Children's processing of and verbal expressions regarding motion events provides an example of the changing capacity for mental experience.

Barsalou's interesting model might benefit from defining simulation and clarifying the implications of prior critiques for simulations (and not just for perceptual symbols). Contrary to claims, simulators (or frames) appear, in the limit, to be amodal. In addition, the account of abstract terms seems extremely limited.

The notion of introspection is unparsimonious and unnecessary to explain the experiential grounding of our mentalistic concepts. Instead, we can look at subtle proprioceptive experiences, such as the experience of agency in planning motor acts, which may be explained in part by the phenomenon of collateral discharge or efference copy. Proprioceptive sensations experienced during perceptual and motor activity may account for everything that has traditionally been attributed to a special mental activity called “introspection.”

Barsalou's theory of perceptual symbol systems is considered from a metacognitive perspective. Two examples are discussed in terms of the proposed perceptual symbol theory. First, recent results in research on feeling-of-knowing judgement are used to argue for a representation of familiarity with input cues. This representation should support implicit memory. Second, the ability of maintaining a theory of other people's beliefs (theory of mind) is considered and it is suggested that a purely simulation-based view is insufficient to explain the available evidence. Both examples characterize areas where Barsalou's theory would benefit from additional detail.

Barsalou's hypothesis that mental representations are constructed by selecting parts of percepts encounters the same difficulties as other empiricist theories: They cannot explain concepts for which instances do not share perceptible features (e.g., furniture) or for which there are no relevant percepts (e.g., the end of time). Barsalou's attempt to reduce falsity to failed pattern matching is an elementary error, and the generativity of his simulators cannot be attained without nonterminal symbols. There is not now, and there never was, any reason to be interested in empiricist theories of knowledge. Abstraction is a fundamental aspect of human cognition.

We propose a way in which Barsalou could strengthen his position and at the same time make a considerable dent in the category/abstraction problem (that he suggests remains unsolved). There exists a class of connectionist models that solves this problem parsimoniously and provides a mechanistic underpinning for the promising high-level architecture he proposes.

I criticize three claims concerning simulators: (1) That a simulator provides the best-fitting simulation of the perceptual impression one has of an object does not guarantee, pace Barsalou, that the object belongs to the simulator's category. (2) The people described by Barsalou do not acquire a concept of truth because they are not sensitive about the potential inadequacy of their sense impressions. (3) Simulator update prevents Barsalou's way of individuating concepts (i.e., identifying them with simulators) from solving the problem of intra-personal concept stability because to update a simulator is to change its content, and concepts with different contents are distinct.

We attempt to resolve some details of Barsalou's theory. (1) The mechanism that guides selection of perceptual information may be the efferent control of activity. (2) Information about a world that is not accessible to the senses can be constructed in the process of semiotic mediation. (3) Introspection may not be a kind of perception; rather, semiotically mediated information processing might be necessary for the emergence of introspection.

Barsalou's theory rightly emphasizes the perceptual basis of cognition. However, the perceptual symbols that he proposes seem ill suited to carry the representational burden entailed by the architecture in which they function, given that Barsalou accepts the requirement for productivity. A more radical proposal is needed in which symbols are largely external to the cognizer and linked to internal states via perception.

The meanings of abstract concepts depend on context. Perceptual symbol systems (PSS) provide a powerful framework for representing such context. Whereas a few expected difficulties for simulations are consistent with empirical findings, the theory does not clearly predict simulations of specific abstract concepts in a testable way and does not appear to distinguish abstract noun concepts (like truth) from their stem concepts (such as true).

Perceptual symbol systems form a theoretically plausible alternative to amodal symbol systems. At this point it is unclear whether there is any truly diagnostic empirical evidence to decide between these systems. We outline some possible avenues of research in the domain of language comprehension that might yield such evidence. Language comprehension will be an important arena for tests of the two types of symbol systems.

Prior to the twentieth century, theories of knowledge were inherently perceptual. Since then, developments in logic, statistics, and programming languages have inspired amodal theories that rest on principles fundamentally different from those underlying perception. In addition, perceptual approaches have become widely viewed as untenable because they are assumed to implement recording systems, not conceptual systems. A perceptual theory of knowledge is developed here in the context of current cognitive science and neuroscience. During perceptual experience, association areas in the brain capture bottom-up patterns of activation in sensory-motor areas. Later, in a top-down manner, association areas partially reactivate sensory-motor areas to implement perceptual symbols. The storage and reactivation of perceptual symbols operates at the level of perceptual components – not at the level of holistic perceptual experiences. Through the use of selective attention, schematic representations of perceptual components are extracted from experience and stored in memory (e.g., individual memories of green, purr, hot). As memories of the same component become organized around a common frame, they implement a simulator that produces limitless simulations of the component (e.g., simulations of purr). Not only do such simulators develop for aspects of sensory experience, they also develop for aspects of proprioception (e.g., lift,run) and introspection (e.g., compare,memory,happy, hungry). Once established, these simulators implement a basic conceptual system that represents types, supports categorization, and produces categorical inferences. These simulators further support productivity, propositions, and abstract concepts, thereby implementing a fully functional conceptual system. Productivity results from integrating simulators combinatorially and recursively to produce complex simulations. Propositions result from binding simulators to perceived individuals to represent type-token relations. Abstract concepts are grounded in complex simulations of combined physical and introspective events. Thus, a perceptual theory of knowledge can implement a fully functional conceptual system while avoiding problems associated with amodal symbol systems. Implications for cognition, neuroscience, evolution, development, and artificial intelligence are explored.

Various defenses of amodal symbol systems are addressed, including amodal symbols in sensory-motor areas, the causal theory of concepts, supramodal concepts, latent semantic analysis, and abstracted amodal symbols. Various aspects of perceptual symbol systems are clarified and developed, including perception, features, simulators, category structure, frames, analogy, introspection, situated action, and development. Particular attention is given to abstract concepts, language, and computational mechanisms.

During active vision, the eyes continually scan the visual environment using saccadic scanning movements. This target article presents an information processing model for the control of these movements, with some close parallels to established physiological processes in the oculomotor system. Two separate pathways are concerned with the spatial and the temporal programming of the movement. In the temporal pathway there is spatially distributed coding and the saccade target is selected from a “salience map.” Both pathways descend through a hierarchy of levels, the lower ones operating automatically. Visual onsets have automatic access to the eye control system via the lower levels. Various centres in each pathway are interconnected via reciprocal inhibition. The model accounts for a number of well-established phenomena in target-elicited saccades: the gap effect, express saccades, the remote distractor effect, and the global effect. High-level control of the pathways in tasks such as visual search and reading is discussed; it operates through spatial selection and search selection, which generally combine in an automated way. The model is examined in relation to data from patients with unilateral neglect.

New experimental evidence throws doubt on postulating no relationship between saccade control and visual object recognition. The control of saccades during reading depends on the perceptual system mediating object recognition.

We have shown that FEF lesion-induced extinction could be compensated for by changing the relative temporal onsets of two targets presented on either side of the midline. Monkeys were trained to make saccades to either of two identical visual stimuli presented with various stimulus onset asynchronies (SOA). In intact animals the targets were chosen with equal probability when they appeared simultaneously. After unilateral FEF lesions an SOA of 67–116 msec had to be introduced, with the contralesional target appearing first, to obtain equal probability choice. With a smaller target separation, averaging saccades occurred with highest frequency at similar SOAs. Our findings suggest that neglect may be attributable to more time being required in the damaged hemisphere for converting sensory information into motor responses.

Findlay & Walker's target article questions whether covert attention plays any role in normal visual scanning (overt attention). My commentary suggests that there is indeed a very close link between the processes that govern covert and overt attention.

The Findlay-Walker model does not consider saccades generated by auditory targets or nontargets (distractors), or by bimodal stimulation. Empirical results suggest that the effects of auditory stimulation cannot easily be incorporated into the model, neither in the WHEN nor in the WHERE system. A two-stage model by Colonius and Arndt gives a quantitative account of the facilitative effects of auditory distractors on saccadic latencies toward visual targets.

Patients with schizophrenia have an impairment in the inhibition of reflexive saccades, as a consequence of a functional impairment of the prefrontal cortex, which has not yet been encapsulated in terms of a formal model. A number of novel and testable hypotheses can be generated from the framework proposed by Findlay & Walker that will stimulate further research. Their framework therefore marks an important step in the development of a comprehensive functional model of saccadic eye movements. Further advances will be assisted by (1) a recognition of important distinctions in the executive control of volitional saccades and (2) addressing the capacity for cross-model integration of spatial information in the generation of the spatial properties of saccadic eye movements.

Removing attention from the saccade generation system should also remove the paradoxical loops that can occur with attention terminology. At least one such loop is still apparent in the current model, however. The benefits of an attention-free approach are assessed through comparison with a recent theory of attention (Logan 1996).