Helen E. Moss

Towards a distributed account of conceptual knowledge

How is conceptual knowledge organized and represented? Are domains (such as living things) and categories (such as tools, fruit) represented explicitly or can domain and category structure emerge out of a distributed system? Taken at face value, evidence from brain-damaged patients and neuroimaging studies suggests that conceptual knowledge is explicitly structured in independent content-based stores. However, recent analyses of the fine-grained details of semantic impairments, combined with research using connectionist modelling, suggest a different picture - one in which concepts are represented as patterns of activation over multiple semantic properties within a unitary distributed system. Within this context, category-specific deficits emerge as a result of differences in the structure and content of concepts rather than from explicit divisions of conceptual knowledge in separate stores.

Susceptibility-Induced Loss of Signal: Comparing PET and fMRI on a Semantic Task

Functional magnetic resonance imaging (fMRI) has become a popular tool for investigations into the neural correlates of cognitive activity. One limitation of fMRI, however, is that it has difficulty imaging regions near tissue interfaces due to distortions from macroscopic susceptibility effects which become more severe at higher magnetic field strengths. This difficulty can be particularly problematic for language tasks that engage regions of the temporal lobes near the air-filled sinuses. This paper investigates susceptibility-induced signal loss in the temporal lobes and proposes that by defining a priori regions of interest and using the small-volume statistical correction of K. J. Worsley, S. Marrett, P. Neelin, A. C. Vandal, K. J. Friston, and A. C. Evans (1996, Hum. Brain Mapp. 4: 58-83), activations in these areas can sometimes be detected by increasing the statistical power of the analysis. We conducted two experiments, one with PET and the other with fMRI, using almost identical semantic categorization paradigms and comparable methods of analysis. There were areas of overlap as well as differences between the PET and fMRI results. One anticipated difference was a lack of activation in two regions in the temporal lobe on initial analyses in the fMRI data set. With a specific region of interest, however, activation in one of the regions was detected. These experiments demonstrate three points: first, even for almost identical cognitive tasks such as those in this study, PET and fMRI may not produce identical results; second, differences between the two methods due to macroscopic susceptibility artifacts in fMRI can be overcome with appropriate statistical corrections, but only partially; and third, new data acquisition paradigms are necessary to fully deal with susceptibility-induced signal loss if the sensitivity of the fMRI experiment to temporal lobe activations is to be enhanced.

Accessing Different Types of Lexical Semantic Information: Evidence From Priming

The types of semantic information that are automatically retrieved from the mental lexicon on hearing a word were investigated in 3 semantic priming experiments. The authors probed for activation of information about a words category membership by using prime-target pairs that were members of a common semantic category (e.g., pig-horse) and 2 types of functional semantic properties: instrument relations (e.g., broom-floor) and script relations (e.g., restaurant-wine). The authors crossed type of semantic relation between prime and target with degree of normative association strength. In a paired and a single-word presentation version of an auditory lexicaldecision priming task, the authors found significant priming for category and functionally related targets, both with and without an additional associative relation. In all cases there was a significant associative boost. However, in a visual version of the single-word lexical-decision paradigm, a different pattern of results was found for each type of semantic relation. Category coordinates primed only when they were normatively associated, instrument relations primed both with and without association, and script relations primed in neither condition.

Conceptual Structure and the Structure of Concepts: A Distributed Account of Category-Specific Deficits ☆

We present a new account of the fine-grained structure of semantic categories derived from neuropsychological, behavioral, and developmental data. The account places theoretical emphasis on the functions of the referents of concepts. We claim (i) that the distinctiveness of functional features correlated with perceptual features varies across semantic domains; and (ii) that category structure emerges from the complex interaction of these variables. The representational assumptions that follow from these claims make strong predictions about what types of semantic information are preserved in patients showing category-specific deficits following brain damage. These claims are illustrated with a connectionist simulation which, when damaged, shows patterns of preservation of distinctive and shared functional and perceptual information which varies across semantic domains. The data model both dissociations between knowledge for artifacts and for living things and recent neuropsychological evidence concerning the robustness of functional information in the representation of concepts.

Modulation of motor and premotor cortices by actions, action words and action sentences.

Recent research has indicated that processing different kinds of action verbs, such as those related to arm or leg movements (e.g. grab, kick), engages regions along the motor strip responsible for the execution of the corresponding actions. It has been proposed that this activation reflects action-related meaning and that these regions are automatically triggered whenever action words are encountered. However, this view is not universally shared by cognitive studies that have shown that the representation of verbs is highly dependent on the interactions with the semantic context. We investigated these views in a set of fMRI studies, in which participants performed a movement localiser task and listened to arm- and leg-related verbs that were presented in isolation (e.g. kick), in literal sentences (as in kick the ball) and idiomatic sentences (as in kick the bucket). We found significant activation in motor regions when action verbs were presented in isolation, and, to a lesser extent, in literal sentential contexts. When the same verbs were presented in idiomatic contexts, activation was found in fronto-temporal regions, associated with language processing, but not in motor and premotor cortices. These results suggest that motor responses were context-dependent, rather than automatic and invariable. These findings lend support to cognitive theories of semantic flexibility, by showing that the nature of the semantic context determines the degree to which alternative senses and particularly relevant features are processed when a word is heard.

Processing Objects at Different Levels of Specificity

How objects are represented and processed in the brain is a central topic in cognitive neuroscience. Previous studies have shown that knowledge of objects is represented in a featurebased distributed neural system primarily involving occipital and temporal cortical regions. Research with nonhuman primates suggest that these features are structured in a hierarchical system with posterior neurons in the inferior temporal cortex representing simple features and anterior neurons in the perirhinal cortex representing complex conjunctions of features (Bussey & Saksida, 2002; Murray & Bussey, 1999). On this account, the perirhinal cortex plays a crucial role in object identification by integrating information from different sensory systems into more complex polymodal feature conjunctions. We tested the implications of these claims for human object processing in an event-related fMRI study in which we presented colored pictures of common objects for 19 subjects to name at two levels of specificity-basic and domain. We reasoned that domain-level naming requires access to a coarsergrained representation of objects, thus involving only posterior regions of the inferior temporal cortex. In contrast, basic-level naming requires finer-grained discrimination to differentiate between similar objects, and thus should involve anterior temporal regions, including the perirhinal cortex. We found that object processing always activated the fusiform gyrus bilaterally, irrespective of the task, whereas the perirhinal cortex was only activated when the task required finer-grained discriminations. These results suggest that the same kind of hierarchical structure, which has been proposed for object processing in the monkey temporal cortex, functions in the human.

Is there an anatomical basis for category-specificity? Semantic memory studies in PET and fMRI

Patients with semantic impairments sometimes demonstrate category-specific deficits suggesting that the anatomical substrates of semantic memory may reflect categorical organisation, however, neuroimaging studies have failed to provide consistent data in support of a category-based account. We conducted three functional neuroimaging experiments to investigate the neural correlates of semantic processing, two with positron emission tomography (PET) and a third with functional magnetic resonance imaging (fMRI). The first experiment used a lexical decision task to search for brain regions selectively activated by concepts from four different categories--animals, fruit, tools, and vehicles. The second experiment used a semantic categorisation task to increase the demands on the semantic system and to look for evidence of consistent activations for the domains of natural kinds or man-made items. The final experiment was a replication of the semantic categorisation task using fMRI to increase the spatial resolution and statistical sensitivity of the experiment. The results of these experiments reliably identified a distributed neural system common to both natural kinds and artifacts but failed to find robust evidence of functional segregation by domain or categories. Category effects were neither reliable nor consistently present across experiments although some were consistent with previous studies. We discuss the implications of these findings, arguing that they are most consistent with a semantic system undifferentiated by category at the neural level.

PERCEPTUAL DISTANCE AND COMPETITION IN LEXICAL ACCESS

Two experiments use rhyme priming techniques to explore the decision space for lexical access. The 1st experiment, using intramodal (auditory-auditory) priming, covaried the phonological distance of a spoken rhyme prime (e.g., pomato) from its source word (e.g., tomato) with the presence or absence of close lexical competitors. The results showed strong effects of phonological distance and no significant effects of competitor environment. The 2nd experiment, using ambiguous rhyme primes in a cross-modal (auditory-visual) priming task, showed that phonetically ambiguous primes could fully match their source words, but only in the appropriate lexical environment. The results support a view of lexical access in which the listeners perceptual experience is based on strict requirements for a bottom-up match with the speech input, and in which competitor environment does not directly modulate the on-line goodness-of-fit computation.

Unitary vs multiple semantics: PET studies of word and picture processing

In this paper we examine a central issue in cognitive neuroscience: are there separate conceptual representations associated with different input modalities (e.g., Paivio, 1971, 1986; Warrington & Shallice, 1984) or do inputs from different modalities converge on to the same set of representations (e.g., Caramazza, Hillis, Rapp, & Romani, 1990; Lambon Ralph, Graham, Patterson, & Hodges, 1999; Rapp, Hillis, & Caramazza, 1993)? We present an analysis of four PET studies (three semantic categorisation tasks and one lexical decision task), two of which employ words as stimuli and two of which employ pictures. Using conjunction analyses, we found robust semantic activation, common to both input modalities in anterior and medial aspects of the left fusiform gyrus, left parahippocampal and perirhinal cortices, and left inferior frontal gyrus (BA 47). There were modality-specific activations in both temporal poles (words) and occipitotemporal cortices (pictures). We propose that the temporal poles are involved in processing both words and pictures, but their engagement might be primarily determined by the level of specificity at which an object is processed. Activation in posterior temporal regions associated with picture processing most likely reflects intermediate, pre-semantic stages of visual processing. Our data are most consistent with a hierarchically structured, unitary system of semantic representations for both verbal and visual modalities, subserved by anterior regions of the inferior temporal cortex.

‘Two Eyes of a See-through’: Impaired and Intact Semantic Knowledge in a Case of Selective Deficit for Living Things

Abstract We report a patient, RC, with a category-specific semantic deficit for living things, who shows the following pattern of behaviour: (1) he has difficulty in distinguishing among individual exemplars for living thing categories, (2) he has no impairment in grouping living things according to their shared properties and (3) in property verification and definition tasks, RC shows good knowledge of shared but not distinctive properties of living things, (especially biological functional properties) and the reverse pattern for non-living things. These findings cannot be accounted for by selective damage to a particular type or category of information. We argue that RCs deficit is predicted by a unitary model of semantic memory in which the patterns of correlation among distinctive and shared perceptual and functional properties are different for concepts in the domains of living and non-living things. These differences in conceptual structure give rise to category-specific impairments when the semant...