WHAT ARE THE FACTS OF SEMANTIC CATEGORY-SPECIFIC DEFICITS? A CRITICAL REVIEW OF THE CLINICAL EVIDENCE

Word Finding in the Damaged Brain: Probing Marshall's Caveat

Marshall (1977) constructed a plausible simulation of "anomic" speech out of the 100 most common words in the English language. He suggested that impaired access to lower frequency vocabulary might underlie anomic word finding difficulties. But he also noted that another factor, age of acquisition, may exert an influence, with anomic patients experiencing particular difficulty with later acquired vocabulary. A review of research on word-finding in aphasia and other neuropsychological conditions suggests that Marshall (1977) may have been right on both counts, and that in many patients both frequency of use and age of acquisition influence the likelihood that a given word will be able to be accessed and used. Theoretical accounts of why the age of acquisition of words might affect their retention or loss following brain injury in adulthood are considered.

The role of semantic distance in category-specific impairments for living things: evidence from a case of semantic dementia

In this paper, we describe a patient (LI) suffering from semantic dementia who showed a category-specific naming impairment for living things over and above the effects of several nonsemantic confounding variables. We investigated the characteristics of LI's impairment to address the following three issues raised in three different accounts of category-specific impairments: (i) the role of an imbalance in the loss of sensory compared to nonsensory features (assumed by the Sensory Functional Theory [Warrington, E. K., & Shallice, T. (1984). Category-specific semantic impairments. Brain, 107, 829-859]); (ii) the role of cross domain differences in Feature Correlation (assumed by the Conceptual Structure Account [Moss, H., Tyler, L. K., & Devlin, J. T. (2002). The emergence of category-specific deficits in a distributed semantic system. In: E. M. E. Forde & G. W. Humphreys (Eds.), Category Specificity in Brain and Mind (pp. 115-147). New York: Psychology Press]); (iii) the role of semantic distance (proposed by Cree and McRae [Cree, G. S., & McRae, K. (2003). Analyzing the factors underlying the structure and computation of the meaning of chipmunk, cherry, chisel, cheese, and cello (and many other such concrete nouns). Journal of Experimental Psychology: General, 132, 163-201]). We found that semantic distance was the only factor causally linked to LI's poorer performance on living things. In fact, her naming performance was less accurate on items that had many semantic neighbours, which is typical of living things. On the contrary, a feature listing task revealed that the features available to LI were not predicted by their level of correlation, as expected by the Conceptual Structure Account. Finally, at variance with the Sensory Functional Theory, although LI quoted sensory features less accurately than nonsensory ones, this did not give rise to a disproportionate loss of semantic features in the living domain.

Objective versus estimated age of word acquisition: A study of 202 Italian children

We provide objective data concerning the age of acquisition (AoA) of words from 202 Italian children 34-69 months of age. We investigated picture naming with 80 concrete words belonging to eight semantic categories that are included in a widely used battery for the study of naming and semantic memory. For each word, we calculated three different indices: two directly expressing the age at which a picture was given the correct name by at least 75% of the subjects, and one expressing the overall percentage of our children who were correct in the task. (For the latter index, we provide separate values for boys and girls.). The correlation between objective indices of AoA and adult estimates culled from the literature was not very high. Moreover, objective indices showed low correlations with frequency and familiarity, in contrast to adult ratings. We conclude that adult estimates of AoA present validity problems and should be used with caution. The full set of stimuli is available at www.psychonomic.org/archive.

Knowledge of living, nonliving and "sensory quality" categories in semantic dementia

This article reports the findings from 3 patients with semantic dementia (SD) who were given a novel battery of 33 items from sensory quality categories (SQCs) as previously described by Borgo and Shallice (2001; 2003) and Laiacona, Capitani and Caramazza (2003). Their performance on three tasks (two naming, one word-to-picture matching) was compared with performance on similar tasks using a conventional semantic battery. At the group level, patients performed worse than age-matched controls overall, but neither group showed any differences in performance between domains (i.e., living, nonliving and SQCs). Individual patient analyses, however, showed contrasting profiles in the three patients. The results are discussed in terms of the SFT (Warrington & Shallice, 1984) and individual differences (Lambon-Ralph et al., 2003) accounts of category-specificity in SD.

Evaluating feature-category relations using semantic fluency tasks

The issue of the relationship between semantic features and semantic categories has been raised by Warrington and colleagues, who claimed that sensory and functional-associative features are differentially important in determining the meaning of living and nonliving things (Warrington & McCarthy, 1983, 1987; Warrington & Shallice, 1984). In the present study, the effectiveness of semantic memory search for living and nonliving things with sensory and functional-associative search cues was evaluated through eight different adaptations of the semantic fluency task. More living thing responses and clusters were generated from sensory than from functional-associative search cues, while the reverse pattern holds for nonliving things responses and clusters. The results thus provide consistent empirical support for the assumption that sensory properties are fundamental in the representation of living things, while functional-associative properties are fundamental in the semantic representation of nonliving things.

Category Deficits and Paradoxical Dissociations in Alzheimer's Disease and Herpes Simplex Encephalitis

Most studies examining category specificity are single-case studies of patients with living or nonliving deficits. Nevertheless, no explicit or agreed criteria exist for establishing category-specific deficits in single cases regarding the type of analyses, whether to compare with healthy controls, the number of tasks, or the type of tasks. We examined two groups of patients with neurological pathology frequently accompanied by impaired semantic memory (19 patients with Alzheimer's disease and 15 with Herpes Simplex Encephalitis). Category knowledge was examined using three tasks (picture naming, naming-to-description, and feature verification). Both patient groups were compared with age-and education-matched healthy controls. The profile in each patient was examined for consistency across tasks and across different analyses; however, both proved to be inconsistent. One striking finding was the presence of paradoxical dissociations (i.e., patients who were impaired for living things on one task and nonliving things on another task). The findings have significant implications for how we determine category effects and, more generally, for the methods used to document double dissociations across individual cases in this literature.

Modulation of the semantic system by word imageability

A prevailing neurobiological theory of semantic memory proposes that part of our knowledge about concrete, highly imageable concepts is stored in the form of sensory-motor representations. While this theory predicts differential activation of the semantic system by concrete and abstract words, previous functional imaging studies employing this contrast have provided relatively little supporting evidence. We acquired event-related functional magnetic resonance imaging (fMRI) data while participants performed a semantic similarity judgment task on a large number of concrete and abstract noun triads. Task difficulty was manipulated by varying the degree to which the words in the triad were similar in meaning. Concrete nouns, relative to abstract nouns, produced greater activation in a bilateral network of multimodal and heteromodal association areas, including ventral and medial temporal, posterior-inferior parietal, dorsal prefrontal, and posterior cingulate cortex. In contrast, abstract nouns produced greater activation almost exclusively in the left hemisphere in superior temporal and inferior frontal cortex. Increasing task difficulty modulated activation mainly in attention, working memory, and response monitoring systems, with almost no effect on areas that were modulated by imageability. These data provide critical support for the hypothesis that concrete, imageable concepts activate perceptually based representations not available to abstract concepts. In contrast, processing abstract concepts makes greater demands on left perisylvian phonological and lexical retrieval systems. The findings are compatible with dual coding theory and less consistent with single-code models of conceptual representation. The lack of overlap between imageability and task difficulty effects suggests that once the neural representation of a concept is activated, further maintenance and manipulation of that information in working memory does not further increase neural activation in the conceptual store.

Two Distinct Neural Mechanisms for Category-selective Responses

The cognitive and neural mechanisms mediating category-selective responses in the human brain remain controversial. Using functional magnetic resonance imaging and effective connectivity analyses (Dynamic Causal Modelling), we investigated animal- and tool-selective responses by manipulating stimulus modality (pictures versus words) and task (implicit versus explicit semantic). We dissociated two distinct mechanisms that engender category selectivity: in the ventral occipito-temporal cortex, tool-selective responses were observed irrespective of task, greater for pictures and mediated by bottom-up effects. In a left temporo-parietal action system, tool-selective responses were observed irrespective of modality, greater for explicit semantic tasks and mediated by top-down modulation from the left prefrontal cortex. These distinct activation and connectivity patterns suggest that the two systems support different cognitive operations, with the ventral occipito-temporal regions engaged in structural processing and the dorsal visuo-motor system in strategic semantic processing. Consistent with current semantic theories, explicit semantic processing of tools might thus rely on reactivating their associated action representations via top-down modulation. In terms of neuronal mechanisms, the category selectivity may be mediated by distinct top-down (task-dependent) and bottom-up (stimulus-dependent) mechanisms.

Relative preservation of 'animate' knowledge in an atypical presentation of herpes simplex virus encephalitis

A comprehensive battery of neuropsychological tests designed to assess primary cognitive functions, including language and semantic memory, was given to MG, a patient with confirmed herpes simplex virus encephalitis. MG's initial jargon aphasia resolved over time to leave her with a mild phonological impairment. She had a very mild amnesia that was worse for verbal material and a category-specific impairment of semantic memory. This latter impairment resulted in a significant anomia that was worse for manmade/artefact items than for animate kinds. Her naming difficulties were associated with a mild impairment in comprehension that was not specific to category or feature type. MRI revealed a strongly asymmetric and atypical distribution of pathology in MG with the disease affecting the left medial temporal lobe, temporal pole, left frontotemporal and temporoparietal regions.

Distinctions between manipulation and function knowledge of objects: evidence from functional magnetic resonance imaging

A prominent account of conceptual knowledge proposes that information is distributed over visual, tactile, auditory, motor and verbal-declarative attribute domains to the degree to which these features were activated when the knowledge was acquired [D.A. Allport, Distributed memory, modular subsystems and dysphagia, In: S.K. Newman, R. Epstein (Eds.), Current perspectives in dysphagia, Churchill Livingstone, Edinburgh, 1985, pp. 32-60]. A corollary is that when drawing upon this knowledge (e.g., to answer questions), particular aspects of this distributed information is re-activated as a function of the requirements of the task at hand [L.J. Buxbaum, E.M. Saffran, Knowledge of object manipulation and object function: dissociations in apraxic and non-apraxic subjects. Brain and Language, 82 (2002) 179-199; L.J. Buxbaum, T. Veramonti, M.F. Schwartz, Function and manipulation tool knowledge in apraxia: knowing 'what for' but not 'how', Neurocase, 6 (2000) 83-97; W. Simmons, L. Barsalou, The similarity-in-topography principle: Reconciling theories of conceptual deficits, Cognitive Neuropsychology, 20 (2003) 451-486]. This account predicts that answering questions about object manipulation should activate brain regions previously identified as components of the distributed sensory-motor system involved in object use, whereas answering questions about object function (that is, the purpose that it serves) should activate regions identified as components of the systems supporting verbal-declarative features. These predictions were tested in a functional magnetic resonance imaging (fMRI) study in which 15 participants viewed picture or word pairs denoting manipulable objects and determined whether the objects are manipulated similarly (M condition) or serve the same function (F condition). Significantly greater and more extensive activations in the left inferior parietal lobe bordering the intraparietal sulcus were seen in the M condition with pictures and, to a lesser degree, words. These findings are consistent with the known role of this region in skilled object use [K.M. Heilman, L.J. Gonzalez Rothi, Apraxia, In: K.M. Heilman, E. Valenstein (Eds.), Clinical Neuropsychology, Oxford University Press, New York, 1993, pp. 141-150] as well as previous fMRI results [M. Kellenbach, M. Brett, K. Patterson, Actions speak louder than functions: the importance of manipulability and action in tool representation, Journal of Cognitive Neuroscience, 15 (2003) 30-46] and behavioral findings in brain-lesion patients [L.J. Buxbaum, E.M. Saffran, Knowledge of object manipulation and object function: dissociations in apraxic and non-apraxic subjects, Brain and Language, 82 (2002) 179-199]. No brain regions were significantly more activated in the F than M condition. These data suggest that brain regions specialized for sensory-motor function are a critical component of distributed representations of manipulable objects.

The influence of gender and lesion location on naming disorders for animals, plants and artefacts

The aim of this systematic review of single case studies of patients showing a category-specific disorder was to evaluate the influence of gender and lesion location on category-specific disorders for biological versus artefact categories and, within the former, for animals versus plant life categories. Two complementary studies were made, taking into account all the available single case reports of category-specific disorders found in the literature. The first study consisted of an overall statistical evaluation of the influence that gender and lesion location can have upon naming scores obtained with these different categories in patients selected only because they showed some kind of category-specific disorder. The second study assessed the influence of these variables on more selected groups of patients, contrasting those showing a categorical impairment for living things versus artefacts and, respectively, for animals versus plant life categories. Results of these studies consistently showed that: (a) Lesion location has a strong influence on the distinction between biological and artefacts categories, but not on that between animals and plant life domains. In patients with a prevalent impairment either for animals or for plant life items, lesions usually encroach upon the anterior or the posterior parts of the ventral stream of visual processing, whereas in patients with a prevalent impairment for artefacts they are located elsewhere (usually on more dorsal structures of the brain). (b) Gender, on the contrary, does not influence the distinction between living and non-living things, but, within the living entities, has a strong influence on the distinction between animals and plant life. Consistent with data obtained in normal people, which show that men are more familiar with animals and women with fruit and vegetables, men were, indeed, more impaired with plant life categories, whereas women were more impaired with animals.

Semantic relevance best predicts normal and abnormal name retrieval

The relevance of a semantic feature measures its contribution to the "core" meaning of a concept. In a naming-to-description task, we investigated the predictive power of relevance in comparison with frequency, familiarity, typicality, and Age-of-Acquisition. In a group of Alzheimer patients with semantic disorder, relevance turned out to be the best predictor of name retrieval accuracy in a naming-to-description task. The same pattern of results was observed in normal controls. Relations between semantic relevance and the parameters of the concepts are discussed in order to highlight the mechanism of concept activation in a naming-to-description task.

Semantic relevance and semantic disorders

Semantic features are of different importance in concept representation. The concept elephant may be more easily identified from the feature <trunk> than from the feature <four legs>. We propose a new model of semantic memory to measure the relevance of semantic features for a concept and use this model to investigate the controversial issue of category specificity. Category-specific patients have an impairment in one domain of knowledge (e.g., living), whereas the other domain (e.g., nonliving) is relatively spared. We show that categories differ in the level of relevance and that, when concepts belonging to living and nonliving categories are equated to this parameter, the category-specific disorder disappears. Our findings suggest that category specificity, as well as other semantic-related effects, may be explained by a semantic memory model in which concepts are represented by semantic features with associated relevance values.

Object concepts and action: extracting affordances from objects parts

Two experiments with a part-generation task show that rated salience and production order of parts in artifacts are first predicted by their relevance for canonical actions, but also that they vary, depending on the current situation. In three further experiments participants read sentences describing actions (e.g., 'The woman shares the orange') followed by objects' parts from which it was easy or not to extract affordances (e.g., 'slice' vs. 'pulp'). They had to perform a part verification task or to evaluate whether or not the combination made sense. Parts from which it was easy to derive affordances were processed earlier and the combination was evaluated as the one which made more sense. Overall, results support the view that sensory-motor simulations underlie conceptualization and that concepts are action-based.

Category specificity in normal episodic learning: Applications to object recognition and category-specific agnosia

Studies of patients with category-specific agnosia (CSA) have given rise to multiple theories of object recognition, most of which assume the existence of a stable, abstract semantic memory system. We applied an episodic view of memory to questions raised by CSA in a series of studies examining normal observers' recall of newly learned attributes of familiar objects. Subjects first learned to associate arbitrarily assigned colors or textures to objects in a training phase, and then attempted to report the newly learned attribute of each object in a recall task. Our subjects' pattern of recall errors was similar both quantitatively and qualitatively to the identification deficits among patients with CSA for biological objects. Furthermore, errors tended to reflect conceptually and structurally based confusions. We suggest that object identification involves recruitment and integration of information across distributed episodic memories and that this process is susceptible to interference from objects that are structurally similar and conceptually related.

The organization of conceptual knowledge: the evidence from category-specific semantic deficits

Questions about the organization of conceptual knowledge in the human brain can be addressed by studying patients with category-specific semantic deficits: disproportionate and even selective impairment of conceptual knowledge of one category of objects compared with other categories. Recently, consensus has emerged regarding the basic facts of category-specific semantic deficits: (1) the categories that can be disproportionately impaired or spared are 'animals', 'fruit/vegetables', and 'artifacts'; and (2) category-specific semantic deficits are not associated with disproportionate deficits for a type or modality of knowledge. Together with findings in functional neuroimaging, these data indicate a complex organization of conceptual knowledge characterized by several independent dimensions of organization.