The form of reference frames in vision: The case of intermediate shape-centered representations.

Although a great deal is known about the early sensory and the later, perceptual, stages of visual processing, far less is known about the nature of intermediate representational units and reference frames. Progress in understanding intermediate levels of representations in vision is hindered by the complexity of interactions among multiple levels of representation in the visual system, making it difficult to isolate and study the nature of each particular level. Nature occasionally provides the opportunity to peer inside complex systems by isolating components of a system through accidental damage or genetic modification of neural components. We have recently reported the case of a young woman who perceives 2D bounded regions of space as if they were plane-rotated by 90, 180 or 270° around their center, mirrored across their own axes, or both. This suggested that an intermediate stage of processing consists in representing mutually exclusive 2D bounded regions extracted from the retinal image in their own "shape-centered" perceptual frame. We proposed to refer to this level of representation as "intermediate shape-centered representation" (ISCR). Here, we used Davida's pattern of errors across 9 experiments as a tool for specifying in greater detail the geometrical properties of the reference frame in which elongated and/or symmetrical shapes are represented at the level of the ISCR. The nature of Davida's errors in these experiments suggests that ISCRs are represented in reference frames composed of orthogonal axes aligned with and centered on the most elongated segment of elongated shapes and, for symmetrical shapes deprived of a straight segment, aligned with their axis of symmetry, and centered on their centroid.

Typical neural representations of action verbs develop without vision.

Many empiricist theories hold that concepts are composed of sensory-motor primitives. For example, the meaning of the word "run" is in part a visual image of running. If action concepts are partly visual, then the concepts of congenitally blind individuals should be altered in that they lack these visual features. We compared semantic judgments and neural activity during action verb comprehension in congenitally blind and sighted individuals. Participants made similarity judgments about pairs of nouns and verbs that varied in the visual motion they conveyed. Blind adults showed the same pattern of similarity judgments as sighted adults. We identified the left middle temporal gyrus (lMTG) brain region that putatively stores visual-motion features relevant to action verbs. The functional profile and location of this region was identical in sighted and congenitally blind individuals. Furthermore, the lMTG was more active for all verbs than nouns, irrespective of visual-motion features. We conclude that the lMTG contains abstract representations of verb meanings rather than visual-motion images. Our data suggest that conceptual brain regions are not altered by the sensory modality of learning.

Judging semantic similarity: an event-related fMRI study with auditory word stimuli.

Much of mental life consists in thinking about object concepts that are not currently within the scope of perception. The general system that enables multiple representations to be maintained and compared is referred to as "working memory" [Repovs G, Baddeley A (2006) Neuroscience 139:5-21], and involves regions in medial and lateral parietal and frontal cortex [e.g., Smith EE, Jonides J (1999) Science 283:1657-1661]. It has been assumed that the contents of working memory index information in regions of the brain that are critical for processing and storing object knowledge. To study the processes involved in thinking about common object concepts, we used event related fMRI to study BOLD activity while participants made judgments of conceptual similarity over pairs of sequentially presented auditory words. Through a combination of conventional fMRI analysis approaches and multi-voxel pattern analysis (MVPA), we show that the brain responses associated with the second word in a pair carry information about the conceptual similarity between the two members of the pair. This was the case in frontal and parietal regions involved in the working memory and decision components of the task for both analysis approaches. However, in other regions of the brain, including early visual regions, MVPA permitted classification of semantic distance relationships where conventional averaging approaches failed to show a difference. These findings suggest that diffuse and statistically sub-threshold "scattering" of BOLD activity in some regions may carry substantial information about the contents of mental representations.

When nominal features are marked on verbs: a transcranial magnetic stimulation study.

It has been claimed that verb processing (as opposed to noun processing) is subserved by specific neural circuits in the left prefrontal cortex. In this study, we took advantage of the unusual grammatical characteristics of clitic pronouns in Italian (e.g., lo and la in portalo and portala 'bring it [masculine]/[feminine]', respectively)-the fact that clitics have both nominal and verbal characteristics, to explore the neural correlates of verb and clitic processing. We used repetitive transcranial magnetic stimulation (rTMS) to suppress the excitability of the left prefrontal cortex and to assess its role in producing verb+det+noun and verb+clitic phrases. Results showed an interference effect for both kinds of phrases when stimulation was applied to the left but not to the right prefrontal cortex. However, the interference effect was significantly greater for the verb+clitic than for the verb+det+noun phrases. These findings support the view that clitics increase the morphosyntactic complexity of verbs.

What are the facts of semantic category-specific deficits? A critical review of the clinical evidence.

In this study we provide a critical review of the clinical evidence available to date in the field of semantic category-specific deficits. The motivation for undertaking this review is that not all the data reported in the literature are useful for adjudicating among extant theories. This project is an attempt to answer two basic questions: (1) what are the categories of category-specific deficits, and (2) is there an interaction between impairment for a type of knowledge (e.g., visual, functional, etc.) and impairment for a given category of objects (e.g., biological, artefacts, etc.). Of the 79 case studies in which the reported data are sufficiently informative with respect to the aims of our study, 61 presented a disproportionate impairment for biological categories and 18 presented a disproportionate impairment for artefacts. Less than half of the reported cases provide statistically and theoretically interpretable data. Each case is commented upon individually. The facts that emerge from our critical review are that (1) the categories of category-specific semantic deficits are animate objects, inanimate biological objects, and artefacts (the domain of biological objects fractionates into two independent semantic categories: animals, and fruit/vegetables); (2) the types of category-specific deficits are not associated with specific types of conceptual knowledge deficits. Other conclusions that emerge from our review are that the evidence in favour of the existence of cases of reliable category-specific agnosia or anomia is not very strong, and that the visual structural description system functions relatively autonomously from conceptual knowledge about object form.

The specific-word frequency effect: implications for the representation of homophones in speech production.

In a series of experiments, the authors investigated whether naming latencies for homophones (e.g., /nlambdan/) are a function of specific-word frequency (i.e., the frequency of nun) or a function of cumulative-homophone frequency (i.e., the sum of the frequencies of nun and none). Specific-word but not cumulative-homophone frequency affected picture-naming latencies. This result was obtained in 2 languages (English and Chinese). An analogous finding was obtained in a translation task, where bilingual speakers produced the English names of visually presented Spanish words. Control experiments ruled out that these results are an artifact of orthographic or articulatory factors, or of visual recognition. The results argue against the hypothesis that homophones share a common word-form representation, and support instead a model in which homophones have fully independent representations.

Grammatical distinctions in the left frontal cortex.

Selective deficits in producing verbs relative to nouns in speech are well documented in neuropsychology and have been associated with left hemisphere frontal cortical lesions resulting from stroke and other neurological disorders. The basis for these impairments is unresolved: Do they arise because of differences in the way grammatical categories of words are organized in the brain, or because of differences in the neural representation of actions and objects? We used repetitive transcranial magnetic stimulation (rTMS) to suppress the excitability of a portion of left prefrontal cortex and to assess its role in producing nouns and verbs. In one experiment subjects generated real words; in a second, they produced pseudowords as nouns or verbs. In both experiments, response latencies increased for verbs but were unaffected for nouns following rTMS. These results demonstrate that grammatical categories have a neuroanatomical basis and that the left prefrontal cortex is selectively engaged in processing verbs as grammatical objects.

The dissociation of color from form and function knowledge.

We report on two brain-damaged subjects who exhibit the uncommon pattern of loss of object color knowledge, but spared color perception and naming. The subject P.C.O., as in previously reported patients, is also impaired in processing other perceptual and functional properties of objects. I.O.C., in contrast, is the first subject on record to have impaired object color knowledge, but spared knowledge of object form, size and function. This pattern of performance is consistent with the view that semantic information about color and other perceptual properties of objects is grounded in modality-specific systems. Lesion analysis suggests that such grounding requires the integrity of the mesial temporal regions of the left hemisphere.

Agrammatic Broca's aphasia is not associated with a single pattern of comprehension performance.

One influential hypothesis posits that the brain regions implicated in Broca's aphasia are responsible for specific syntactic operations that are necessary for the comprehension and production of sentences (Grodzinsky, 1986, 1990, in press). The empirical basis of this hypothesis is the claim that Broca's aphasics have no difficulty understanding sentences in the active voice (and other "canonical" sentence types, such as subject relatives and clefts with negative predicates), but perform at chance level with passive voice constructions (and other "noncanonical" sentences such as object-gap relatives and object clefts). In the face of well-established results indicating that Broca's aphasics can exhibit several different performance patterns on these sentence types, Grodzinsky, Piñango, Zurif, and Drai (1999) argued that these conflicting results do not challenge the theory when the data are analyzed appropriately. They carried out a creative statistical analysis of the comprehension performance of published cases of Broca's aphasia and concluded that all of these cases are in agreement with the predicted pattern: chance on passives and 100% correct on actives. Here we show that the statistical reasoning adopted by Grodzinsky et al. (1999) is flawed. We also show that the comprehension performance of a substantial number of the Broca's aphasics in their own sample does not conform to the pattern required. Rather, contrary to these authors' claim, Broca's aphasia is not associated with a consistent pattern of sentence comprehension performance, but allows for a number of distinct patterns in different patients.

Sometimes a noun is just a noun: comments on Bird, Howard, and Franklin (2000).

Bird, Howard, and Franklin (2000) have proposed a semantic-conceptual explanation of grammatical category-specific deficits that attributes impairments in noun and verb processing to two distinct mechanisms. According to their account, apparent deficits in verb production are not category specific, but rather result from the lower imageability of verbs compared to concrete nouns. Noun deficits are said to result from differences in the distribution of semantic feature types such that damage to sensory features results in disproportionate impairments in naming nouns, especially animate nouns, compared to verbs. However, this hypothesis, which we call the "extended sensory/functional theory" (ESFT), fails on several counts. First, the assumption that representations of living things are more heavily freighted with sensory features than are those of nonliving objects does not have any reliable empirical basis. Second, the ESFT incorrectly predicts associations between deficits in processing sensory features and living things or functional features and nonliving things. Finally, there are numerous cases of patients with grammatical category-specific deficits that do not seem to be consistent with damage at the semantic level. All of this suggests that the ESFT is not a useful model for considering grammatical (or semantic) category-specific deficits.

Serial order effects in spelling errors: evidence from two dysgraphic patients.

This study reports data from two dysgraphic patients, TH and PB, whose errors in spelling most often occurred in the final part of words. The probability of making an error increased monotonically towards the end of words. Long words were affected more than short words, and performance was similar across different output modalities (writing, typing and oral spelling). This error performance was found despite the fact that both patients showed normal ability to repeat the same words orally and to access their full spelling in tasks that minimized the involvement of working memory. This pattern of performance locates their deficit to the mechanism that keeps graphemic representations active for further processing, and shows that the functioning of this mechanism is not controlled or "refreshed" by phonological (or articulatory) processes. Although the overall performance pattern is most consistent with a deficit to the graphemic buffer, the strong tendency for errors to occur at the ends of words is unlike many classic "graphemic buffer patients" whose errors predominantly occur at word-medial positions. The contrasting patterns are discussed in terms of different types of impairment to the graphemic buffer.

Set size and repetition in the picture--word interference paradigm: implications for models of naming.

Caramazza and Costa (Cognition 75 (2000) B51) reported results which demonstrate that a semantically related word distractor interferes in picture naming even when it is not in the response set and there is no possibility for mediated interference. They interpreted the results to be problematic for the model of lexical access proposed by Levelt, Roelofs, and Meyer (Behavioral and Brain Sciences 22 (1999) 1). Roelofs (Cognition 80 (2001, this issue 283--90)) argues that those results are not inconsistent with Levelt et al.'s model when certain new assumptions about the mechanism of lexical selection are considered. Here we show that even with these assumptions the model still makes the wrong predictions. We report new results which demonstrate that the semantic interference and facilitation effects that are obtained respectively in the basic-level and category-level naming variants of the picture-word interference paradigm are not the result of response set size and response repetitions.

Dissociation of semantic and phonological errors in naming.

We report the naming performance of a fluent aphasic, DP, who shows a striking dissociation between semantic and phonological (nonword) errors: he produced numerous semantic errors but virtually no phonological errors. DP's pattern of performance is the reverse of that reported for patient DM (Caramazza, Papagno, & Ruml, 2000), who only made phonological errors in a naming task. These patterns of performance are inconsistent with the proposal by Dell, Schwartz, Martin, Saffran, and Gagnon (1997) that the naming deficit in fluent aphasia is the result of global damage to all levels of the lexical access system and support instead the hypothesis that brain damage can selectively disrupt distinct subcomponents of the lexical processing system.

The selective impairment of phonological processing in speech production.

We report the naming performance of a patient (DM) with a fluent progressive aphasia who made phonological errors in all language production tasks. The pattern of errors in naming was strikingly clear: DM made very many phonological errors that resulted almost always in nonword responses. The complete absence of semantic errors and the very low ratio of formal errors relative to nonword errors (1.6:30.3) in DM's performance are discussed in the context of recent claims about the nature of naming deficits in fluent aphasics. We argue that DM's performance makes highly improbable the claim that fluent aphasia results from global lesions affecting all levels of the lexical access system equally.

The cognate facilitation effect: implications for models of lexical access.

Do nonselected lexical nodes activate their phonological information? Catalan-Spanish bilinguals were asked to name (a) pictures whose names are cognates in the 2 languages (words that are phonologically similar in the 2 languages) and (b) pictures whose names are noncognates in the 2 languages. If nonselected lexical nodes are phonologically encoded, naming latencies should be shorter for cognate words, and because the cognate status of words is only meaningful for bilingual speakers, this difference should disappear when testing monolingual speakers. The results of Experiment 1 fully supported these predictions. In Experiment 2, the difference between cognate and noncognate words was larger when naming in the nondominant language than when naming in the dominant language. The results of the 2 experiments are interpreted as providing support to cascaded activation models of lexical access.

An evaluation of a computational model of lexical access: comment on Dell et al. (1997).

The computational model of lexical access proposed by G. S. Dell, M. F. Schwartz, N. Martin, E. M. Saffran, and D. A. Gagnon (1997) is evaluated. They argued that fits of their model to naming data obtained from normal and brain-damaged patients support assumptions regarding interactivity in the lexicon, global damage in aphasia, and continuity between normal and aphasic naming behavior. Additional analysis reveals that the model fits the empirical data poorly and that the claims Dell et al. made on the basis of the model's performance would not follow even if the model were accurate. Although use of a novel automatic regression procedure improved the model's fit, it cannot account for 5 of Dell et al.'s 21 patients (24%), and its limitations were found to be inherent in its design. It is argued that claims such as those made by Dell et al. can only be addressed by considering evidence from multiple related tasks and by comparing multiple computational models.

Minding the facts: a comment on Thompson-Schill et al.'s "A neural basis for category and modality specificity of semantic knowledge".

In this comment on a recent paper by Thompson-Schill et al. (1999) I argue that the authors failed to consider important empirical facts that are at variance with their favored theory of the causes of semantic category-specific deficits. I also argue that the predictions they make about fusiform gyrus activation on the basis of the interactive modality-specific hypothesis of semantic organization do not obviously follow from that model. I point out that simulations are needed in order to derive predictions from the model. Finally, I argue that the fMRI results they obtained are not obviously relevant to our understanding of the causes of semantic category-specific deficits.

The semantic interference effect in the picture-word interference paradigm: does the response set matter?

In three picture-word interference experiments we explore some properties of the semantic interference (SI) effect in the picture-word interference paradigm. In Experiments 1 and 2, we test whether SI may be obtained when the distractor words are not part of the response set and when only one picture per semantic category is included in the experiment. In Experiment 3, we explore if the magnitude of the SI effect depends on whether or not the distractor words are part of the response set. Reliable SI effects were obtained in all three experiments and the magnitude of the effect did not vary as a function of whether or not distractor words are part of the response set. These results are problematic for the selection mechanism in the WEAVER++ lexical access model (Levelt, W. J. M., Roelofs, A., & Meyer, A. S. (1999). A theory of lexical access in speech production. Behavioral and Brain Sciences. 22, 1-75; Roelofs, A. (1992). A spreading-activation theory of lemma retrieval in speaking. Cognition, 42, 107-142).

Separable processing of consonants and vowels.

There are two views about the nature of consonants and vowels. One view holds that they are categorically distinct objects that play a fundamental role in the construction of syllables in speech production. The other view is that they are convenient labels for distinguishing between peak (vowel) and non-peak (consonant) parts of a continuous stream of sound that varies in sonority (roughly the degree of openness of the vocal apparatus during speech), or that they are summary labels for bundles of feature segments. Taking the latter view, consonants and vowels do not have an independent status in language processing. Here we provide evidence for the possible categorical distinction between consonants and vowels in the brain. We report the performance of two Italian-speaking aphasics who show contrasting, selective difficulties in producing vowels and consonants. Their performance in producing individual consonants is independent of the sonority value and feature properties of the consonants. This pattern of results suggests that consonants and vowels are processed by distinct neural mechanisms, thereby providing evidence for their independent status in language production.