Grodzinsky's theory of agrammatic sentence processing fails to account for crucial empirical facts. In contrast to his predictions, the data show that there are (1) degrees of severity and (2) problems with sentences that do not require movement, and that (3) under the right task circumstances, full-fledged syntactic trees are constructed.

It is a major move from the claim that the core linguistic problem in Broca's aphasia is the inability to deal with traces, to the claim that this is the syntactic operation only and that it is exclusively supported by Broca's region. Three arguments plead against this move. First, many Broca patients have no damage to Broca's area. Second, it is not only passive, but also active jabberwocky sentences that activate the frontal operculum in a judgment task. Third, the same area is involved in a phrase-building production task that does not require tense processing.

Grodzinsky's Tree-Pruning Hypothesis can be extended to explain agrammatic comprehension disorders. Although agrammatism is evidence for syntactic modularity, there is no evidence for its anatomical modularity or for its localization in the frontal lobe. Agrammatism results from diffuse left hemisphere damage – allowing the emergence of the limited right hemisphere linguistic competence – rather than from damage to an anatomic module in the left hemisphere.

Grodzinsky presents interesting data on Broca's aphasia, but because of obsolete ideas about neurofunctional organization and an inadequate review of the neuroimaging literature, he fails to put these data into perspective. Rather than supporting a specific linguistic function of Broca's area, the findings should be viewed in terms of working memory functions of the inferior frontal cortex.

Grodzinsky has argued that the traces deleted in Broca's aphasia are “phonetically silent but syntactically active” (sect. 2.). If we assume such traces to be visuospatial in nature, and adopt the term “overwriting” from the author's partial matching theory (1998), we can account for the errors made by Broca's aphasics in comprehending Grodzinsky's Examples (5a), (5b), and (6).

I wholeheartedly endorse Grodzinsky's program of attempting to tie the particular deficits observed in Broca's aphasics' comprehension and production to changes in their mentally represented model of grammar. At the level of detail, however, I see problems with two specific changes that Grodzinsky posits. One is a default Agent-assignment strategy in comprehension. The other is the hypothesis that production involves pruning all functional projections above Agreement Phrase.

The cerebral distinctness of the linguistic and mathematical faculties does not entail their functional independence. Approaches to language that posit a common foundation for the two make claims about design features, not location, and are thus not affected by the finding that one ability can be spared by a neurological accident that compromises the other.

Grodzinsky's general approach to the neuroscience of language is interesting, but the evidence currently available has problems with pragmatic infelicity in experiments involving Japanese scrambling and the interpretation of experimental results on Japanese indirect passives. I will suggest a more direct way of testing the Trace-Deletion Hypothesis (TDH).

Grodzinsky proposes that agrammatism leads to a “pruning” of the syntactic tree in speech production. For German, this assumption predicts that syntactic processes related to functional projections AgrP and CP should be impaired. An analysis of spontaneous-speech data from four Broca's aphasics with respect to subject-verb agreement and verb placement, however, indicates that phrase-structure representations in agrammatism are intact.

Grodzinsky claims that “normal language users demonstrate trace-antecedent relations in real-time tasks.” However, the cited evidence is equally compatible with a traceless account of processing. Moreover, Pickering and Barry (1991) and Traxler and Pickering (1996) have demonstrated that the processor does not wait until the purported trace location before forming the dependency. Grodzinsky's claims about Broca's area should be interpreted in terms of a transformation-free account.

The comprehension problem in Broca's patients does not stem from an inability to represent argument traces. There can be good comprehension in the presence of (object) traces and impaired comprehension can result in constructions where there are no (object) argument traces. This leads to an alternative understanding of Broca's comprehension, one that places the locus of the impairment in an inability to construct syntactic representation on time.

This commentary focuses on the larger implications of Grodzinsky's hypothesis. Although Grodzinsky argues persuasively that the syntactic comprehension deficits in Broca's aphasia involve mainly an inability to comprehend sentences requiring a transformational movement of phrasal constituents, his larger claim for a distinct and dedicated “language organ” in the left hemisphere is much less tenable.

An elaboration on some loose ends in Grodzinsky's analysis shows that data from the field of aphasia contribute to the formulation of theoretical linguistic principles, and provides extra arguments in favor of Grodzinsky's claim that linguistic theory is the best tool for the investigation of aphasia. This illustrates and emphasizes the importance of communication between researchers in the field of (Broca's) aphasia and of theoretical linguistics.

Neuroimaging evidence suggests that the left inferior frontal gyrus (LIFG) supports temporary storage of linguistic material during linguistic tasks rather than computing a syntactic representation. The LIFG is not activated by simple sentences but by complex sentences and maintenance of word lists. Under this hypothesis, agrammatism should only disturb comprehension for constructions in which storage is essential.

Although Grodzinsky's target article has merit, it neglects the importance of neural mechanisms underlying language functions. We present results from our clinical studies on different levels of temporal information processing in aphasic patients and briefly review the existing data on neurobiology of language to cast new light on the main thesis of the target article.

Throughout the history of aphasiology, researchers have identified important premorbid and stroke-related predictors of linguistic performance. Although Grodzinsky discusses some of these variables, exclusion of other variables could lead to unnecessary experimental error and erroneous conclusions. Aspects to consider include sources of experimental bias, premorbid differences, nonlinguistic roles of the frontal regions, and comparison of normal and aphasic performance.

We discuss problematic theoretical and empirical issues and consider alternative explanations for Grodzinsky's hypotheses regarding receptive and expressive syntactic mechanisms in agrammatic aphasia. We also explore his claims pertaining to domain-specificity and neuroanatomical localization.

I begin with a characterization of neurolinguistic theories, trying to pinpoint some general properties that an account of brain/language relations should have. I then address specific criticisms made in the commentaries regarding the syntactic theory assumed in the target article, properties of the Trace Deletion Hypothesis (TDH) and the Tree-Pruning Hyothesis (TPH), other experimental results from aphasia, and findings from functional neuroimaging. Despite the criticism, the picture of the limited role of Broca's area remains unchanged.

In the metaphor of behavioral momentum, the rate of a free operant in the presence of a discriminative stimulus is analogous to the velocity of a moving body, and resistance to change measures an aspect of behavior that is analogous to its inertial mass. An extension of the metaphor suggests that preference measures an analog to the gravitational mass of that body. The independent functions relating resistance to change and preference to the conditions of reinforcement may be construed as convergent measures of a single construct, analogous to physical mass, that represents the effects of a history of exposure to the signaled conditions of reinforcement and that unifies the traditionally separate notions of the strength of learning and the value of incentives. Research guided by the momentum metaphor encompasses the effects of reinforcement on response rate, resistance to change, and preference and has implications for clinical interventions, drug addiction, and self-control. In addition, its principles can be seen as a modern, quantitative version of Thorndike's (1911) Law of Effect, providing a new perspective on some of the challenges to his postulation of strengthening by reinforcement.

This commentary focuses on the stimulus-reinforcer hypothesis of resistance to change. The overall context of reinforcement can account for resistance to extinction. There are ways to systematically test the hypothesis that Pavlovian contingencies account for the behavioral “mass” of discriminated operant behavior.