One of the continuing debates in the field of face recognition is whether the defect in prosopagnosia is truly selective for faces, or if faces are only the most obvious and dramatic stimulus type affected by a problem in making fine discriminations between highly similar exemplars within any object category (Damasio et al.
; Farah et al.
). This debate is also mirrored in the contrasting views of the fusiform face area as a face-specific cortical module or a region involved in making expertise judgements for many object categories (Gauthier et al.
; Kanwisher, 2000
; Yovel and Kanwisher, 2004
While there are cases of prosopagnosic patients who have also lost the ability to recognize types of other objects such as cars, flowers, animals, clothing or buildings (Bornstein et al.
; Newcombe, 1979
; Assal et al.
; Gomori and Hawryluk, 1984
; Damasio et al.
; de Renzi, 1986
; Henke et al.
), there are also reports of other prosopagnosic patients who have retained the ability to discriminate between such objects (Cole and Perez-Cruet, 1964
; de Renzi, 1986
; McNeil and Warrington, 1993
; Henke et al.
; Riddoch et al.
). While variations in lesion anatomy can always be invoked to account for such discrepancies, a critical methodological issue also confounds attempts to address object specificity: how well should any given subject be able to identify other types of objects? Because recognition performance varies with perceptual expertise, it may be difficult to deduce from a certain level of performance whether the processing of other object categories is intact in a particular individual. For example, correct naming of a car as a Volkswagen may be sufficient evidence of intact car recognition in a novice observer, but insufficient in an automobile enthusiast, from whom a more detailed answer might be expected. This problem of accounting for pre-morbid expertise reflects the fact that, while it is reasonable to assume that most humans have a relatively uniform and significant expertise with face recognition, the same cannot be said for most other objects, for which we vary in our interest and experience.
To provide more definitive evidence regarding recognition in other object domains besides faces in prosopagnosia, it would be helpful to have a method of adjusting perceptual scores for pre-morbid expertise. One possibility we considered was whether one could index visual recognition for some object category by some measure of the subject's non-visual semantic knowledge about that category. This novel strategy, however, requires a strong correlation between visual semantic knowledge about objects—which would be involved in visual object recognition—and semantic knowledge about them in a non-visual modality. We first review the grounds for believing that such a correlation exists.
Semantic knowledge or memory, also known as conceptual knowledge, is a ‘general knowledge of objects, word meanings, facts and people, without connection to any particular time or place’ (Patterson et al.
). There are many varieties of semantic knowledge. In neuropsychology, distinctions have been made between information categories (e.g. living versus non-living things, actions versus objects) and modalities (e.g. visual semantic knowledge versus verbal knowledge), based on the dissociations between different forms of semantic knowledge in patients (McCarthy and Warrington, 1994
; Gainotti, 2006
Neurological disorders are not the only source of inter-subject variability in semantic knowledge. Semantic information is acquired through experience with the world: thus ‘meaning is updated or transformed by a dynamic memory system that learns continuously from personal experience’ (Funnell, 2001
), and others define semantic memory as ‘knowledge of the world acquired during experience, which contributes to the formation and long-term representation of concepts, categories, facts, word meanings and so on’ (Moscovitch et al.
). ‘Multiple trace theory’ holds that the creation of multiple traces from specific episodes results in the extraction of common data and the formation of semantic information (Moscovitch et al.
). Thus in healthy subjects, semantic knowledge about a specific category will reflect the degree and frequency of past experiences with that category: this is the basis of expertise effects.
Some propose that category and modality are orthogonal variables in semantic knowledge (McCarthy and Warrington, 1994
). Nevertheless, it is likely that for most humans the acquisition of semantic information about a particular category is frequently multimodal, either within or across the episodes from which semantic memory is created, and that, as a result, expertise in one modality is usually paralleled by expertise in another. (This may not always be the case: one can envision anomalous exceptions, such as that of a painter of birds who develops detailed visual semantic knowledge about birds but little familiarity with their habits, their calls or even their names.) If so, then it should be possible to predict the degree of knowledge in one modalilty from the degree measured in another, as we propose.
This assumption underlies recent examinations of subjects with Alzheimer's disease (Hodges et al.
; Lambon Ralph et al.
) and semantic dementia (Lambon Ralph et al.
), which tested these subjects for their ability to provide information in response to either the names or the pictures of objects. When asked to give verbal definitions to the spoken names of the objects, Alzheimer's patients had more difficulty with objects whose pictures they also could not name. Furthermore, for the objects that they could not name by sight, their semantic difficulty was greater for structural (i.e. perceptual) properties than for other types of information, such as the function of the object (Hodges et al.
). In a clearer contrast, in patients with semantic dementia the ability to provide core definitions for pictures of objects paralleled their ability to provide similar definitions for names of objects (Lambon Ralph et al.
While these data have been used to support an amodal concept of semantic knowledge (Patterson et al.
), it is not clear whether the intuitive assumption that semantic knowledge correlates across different modalities holds in healthy subjects. In a small group of 10 control subjects, the ability to provide correct semantic information did not differ between objects they could or could not name by sight, although there may have been a difference in the ability to provide core definitions (Hodges et al.
). However, the number of items not named by sight was too small to permit definitive conclusions.
This review thus suggests that, while there are grounds to believe that semantic knowledge may be correlated across different modalities, further study is required to establish the relationship between visual and verbal semantic knowledge before we can confidently use this strategy to index visual object recognition in prosopagnosia. We decided to examine knowledge about a specific object category for which the normal population shows a wide range of knowledge due to expertise effects: cars. This object category has been frequently used in studies of perceptual expertise in healthy humans (Gauthier et al.
; Rossion et al.
; Tanaka and Corneille, 2007
) and prosopagnosic subjects (Gomori and Hawryluk, 1984
; Damasio et al.
; Sergent and Signoret, 1992
; Henke et al.
; Rossion et al.
; Duchaine and Nakayama, 2005
). While almost all humans in our culture have daily experience with cars, car knowledge is highly variable, reflecting factors such as occupation and interest.
Our first goal was to establish, in healthy subjects, whether car knowledge accessed through the verbal domain (names of car models) is related to car knowledge accessed through the visual domain (pictures of car models). If so, this relationship could be used to create a test that adjusts visual car recognition scores for each individual's level of expertise, as gauged by performance on a test of verbal semantic car knowledge. Our second goal was to use this test to determine, in a group of prosopagnosic subjects, if the relationship between visual recognition and verbal semantic knowledge differed from that of healthy subjects.