Peter Bright

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.

The National Adult Reading Test as a measure of premorbid intelligence: A comparison with estimates derived from demographic variables

Since its publication in 1982, the National Adult Reading Test (NART; Revised Version, NART-R) has become a widely accepted method for estimating premorbid levels of intelligence in neuropsychological research. However, the assumption that NART/NART-R performance is relatively independent of brain damage has been increasingly challenged in recent years. In a number of conditions, including Alzheimer dementia and Korsakoffs syndrome, studies have indicated a deterioration in reading ability, leading to an underestimated premorbid IQ. In a reaction to these studies, some researchers have advocated the use of demographic variables as a more suitable foundation for accurately predicting premorbid intelligence. We addressed this issue by calculating IQ estimates on the basis of NART/NART-R, demographic variables, and a combination of the two approaches and by comparing these with current WAIS/WAIS-R IQ in patients with Korsakoffs syndrome, Alzheimer dementia, frontal or temporal lobe lesions, and in healthy controls. Estimated premorbid IQs did not differ across groups, whether derived from NART/NART-R or demographic variables. Those based on NART/NART-R demonstrated higher correlations with current WAIS/WAIS-R IQ in controls and patients than those derived from demographic variables. An equation combining NART scores with demographic variables did not significantly increase the amount of variance in IQ explained by NART only, either in patients or controls. The data offer reassurance regarding the continued use of NART as a valid estimate of premorbid intelligence in a number of conditions.

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.

Neural processing of nouns and verbs: the role of inflectional morphology.

Dissociations of nouns and verbs following brain damage have been interpreted as evidence for distinct neural substrates underlying different aspects of the language system. Some neuroimaging studies have supported this claim by finding neural differentiation for nouns and verbs [Brain 122 (1999) 2337] while others have argued against neural specialisation [Brain 119 (1996) 159; Brain 124 (2001) 1619]. We suggest that one reason why these inconsistencies may have arisen is because the morphological structure of nouns and verbs has been ignored. In an event-related functional magnetic resonance imaging (fMRI) study we test the hypothesis that the neural processing of nouns and verbs differs when they are inflected. We contrasted the processing of regularly inflected nouns (dogs) with regularly inflected verbs (hitting), and found that the LIFG was more strongly activated in processing regularly inflected verbs compared to regularly inflected nouns. Moreover, regions of LIFG that were more active in the fMRI study for inflected verbs partially overlapped with the lesions in patients who have particular problems with verb morphology. Taken together with previous studies, these results suggest that noun and verb stems do not differ in terms of their representation, but when verbs are morphologically complex they differentially engage those neural systems which are involved in processes of morpho-phonology and syntax.

Grammatical SLI: A distinct subtype of developmental language impairment?

Grammatical specific language impairment (G-SLI) has been proposed as a distinct subtype of language impairment. We assessed a large sample of twins between the ages of 7 and 13 years on language comprehension tests sensitive to G-SLI. The sample included 37 same-sex twin pairs selected for the presence of language impairment (LI) in one or both twins and 104 twin pairs from the general population. The number and patterns of errors in those with LI replicated findings from previous studies of G-SLI. Qualitative markers of G-SLI were derived from the tests. Out of 144 children for whom complete data were available, 2 scored positive on all five markers and 9 scored positive on four of the five markers. Most children who made grammatical errors characteristic of G-SLI had co-occurring deficits in other areas. These results raise questions for theories that treat this disorder as an all-or-none modular deficit.

Do semantic categories activate distinct cortical regions? Evidence for a distributed neural semantic system

A key issue in cognitive neuroscience concerns the neural representation of conceptual knowledge. Currently, debate focuses around the issue of whether there are neural regions specialised for the processing of specific semantic attributes or categories, or whether concepts are represented in an undifferentiated neural system. Neuropsychological studies of patients with selective semantic deficits and previous neuroimaging studies do not unequivocally support either account. We carried out a PET study to determine whether there is any regional specialisation for the processing of concepts from different semantic categories using picture stimuli and a semantic categorisation task. We found robust activation of a large semantic network extending from left inferior frontal cortex into the inferior temporal lobe and including occipital cortex and the fusiform gyrus. The only category effect that we found was additional activation for animals in the right occipital cortex, which we interpret as being due to the extra visual processing demands required in order to differentiate one animal from another. We also carried out analyses in specific cortical regions that have been claimed to be preferentially activated for various categories, but found no evidence of any differential activation as a function of category. We interpret these data within the framework of cognitive accounts in which conceptual knowledge is represented within a nondifferentiated distributed system.

The anatomy of object processing : The role of anteromedial temporal cortex

How objects are represented and processed in the brain remains a key issue in cognitive neuroscience. We have developed a conceptual structure account in which category-specific semantic deficits emerge due to differences in the structure and content of concepts rather than from explicit divisions of conceptual knowledge in separate stores. The primary claim is that concepts associated with particular categories (e.g., animals, tools) differ in the number and type of properties and the extent to which these properties are correlated with each other. In this review, we describe recent neuropsychological and neuroimaging studies in which we have extended our theoretical account by incorporating recent claims about the neuroanatomical basis of feature integration and differentiation that arise from research into hierarchical object processing streams in nonhuman primates and humans. A clear picture has emerged in which the human perirhinal cortex and neighbouring anteromedial temporal structures appear to provide the neural infrastructure for making fine-grained discriminations among objects, suggesting that damage within the perirhinal cortex may underlie the emergence of category-specific semantic deficits in brain-damaged patients.

Longitudinal studies of semantic dementia: The relationship between structural and functional changes over time

The pattern of brain atrophy in semantic dementia and its associated cognitive effects have attracted a considerable body of research, but the nature of core impairments remains disputed. A key issue is whether the disease encompasses one neurocognitive network (semantics) or two (language and semantics). In order to address these conflicting perspectives, we conducted a longitudinal investigation of two semantic dementia patients, in which behavioural performance across a range of measures of language and semantic performance was assessed and interpreted in the context of annually acquired MRI scans. Our results indicated a core semantic impairment in early stages of the disease, associated with atrophy of the inferior, anterior temporal cortex. Linguistic impairments emerged later, and were contingent on atrophy having spread into areas widely believed to subserve core language processes (left posterior perisylvian, inferior frontal and insular cortex). We claim, therefore, that phonological, syntactic and morphological processing deficits in semantic dementia reflect damage to core language areas. Further, we propose that much of the current controversy over the nature of deficits in semantic dementia reflect a tendency in the literature to adopt a static perspective on what is a progressive disease. An approach in which the relationship between progressive neural changes and behavioural change over time is carefully mapped, offers a more constraining data-set from which to draw inferences about the relationship between language, semantics and the brain.

Deficits for Semantics and the Irregular Past Tense: A Causal Relationship?

The regular and irregular past tense has become a focus for recent debates about the structure of the language processing system, asking whether language functions are subserved by different neural and functional mechanisms or whether all processes can be accommodated within a single unified system. A critical claim of leading single mechanism accounts is that the relationship between an irregular stem and its past tense form is primarily semantic and not morphological in nature. This predicts an obligatory relationship between semantic performance and access to the irregular past tense, such that a semantic deficit necessarily leads to impairments on the irregulars. We tested this claim in a series of studies probing the comprehension and production of regular and irregular past tense forms in four semantic dementia patients, all of whom had profound semantic deficits. In two elicitation tasks and one auditory priming study, we found that three out of the four patients did not have a deficit for the irregular past tense, in spite of their semantic deficits. This argues against the view that the relationship between irregular past tense forms and their stems is primarily semantic, and more generally against the single system claim that morphological structure can be captured solely based on phonological and semantic relationships.

Correlations of regional cerebral metabolism with memory performance and executive function in patients with herpes encephalitis or frontal lobe lesions.

Cerebral [18F]fluorodeoxyglucose positron emission tomography (18FDG-PET) data from patients suffering amnesia following herpes encephalitis (n=7) or frontal lobe pathology (n=14) were compared with data from age-matched nonamnesic subjects (n=10). All subjects received structural MRI, resting 18FDG-PET scans, and neuropsychological evaluation. PET data were analyzed using complementary statistical parametric mapping and region-of-interest methods. Differential patterns of hypometabolism were found in patients relative to healthy controls. Factor analysis of the neuropsychological data revealed that memory performance was associated with retrosplenial and medial temporal metabolism, and executive function was associated with dorsolateral frontal metabolism. The association between memory performance and retrosplenial metabolism remained statistically significant after accounting for measures of cerebral atrophy using MRI. The significance of the retrosplenium as a major relay station between the thalamus and the medial temporal and frontal lobes--sensitive to changes in either--is discussed in the light of the findings.