Neuroimaging

Functional magnetic resonance imaging (fMRI) or positron emission tomography (PET) allow for the examination of the human brain with a high spatial resolution. These methods register brain activity by measuring cerebral blood flow or metabolism, and due to the rather slow changes of these indices their temporal resolution is low compared to EEG and MEG.

Numerous studies have been undertaken to explore possible neurophysiological correlates of the

A.1.2 : Introduction - Neuroanatomy of object processing and categorization: empirical findings processing of different categories of objects and a synoptical table can be found in Appendix A.

A.1.2.3.1 Studies focusing on face processing

One subgroup of imaging studies focused on the neuroanatomical substrates of face recognition, which represents a particular class of objects, because a) single cells in animals have been found probably tuned to the processing of faces (Gross et al., 1972; Perrett et al., 1992), b) face processing was suggested to be innate (Goren et al., 1975; Valenza et al., 1996) and c) prosopagnostic patients have been described in the literature (Barton, 2003; Sergent & Signoret, 1992). A region in the (predominantly right) fusiform gyrus, the FFA (fusiform face area) is consistently activated by the processing of individual faces (e.g. Chao et al., 1999a; Gorno-Tempini & Price, 2001; Grill-Spector et al., 2004). While this was interpreted as clear and compelling evidence for a separable and dis-tinct 'face module' in the brain (Kanwisher et al., 1999; Kanwisher, 2001), the face-specific re-sponding of this area was criticized by Gauthier et al. and others (e.g. Gauthier et al., 1999; Tarr &

Gauthier, 2000). Subjects were trained to categorize novel stimuli (3-dimensional figures, so-called 'greebles') and functional brain imaging showed that the FFA becomes more active with increasing expertise in the categorization task (Gauthier et al., 1999). Expertise-related modulation of the FFA and an additional face-selective region in the right occipital lobe ('OFA') was further demonstrated in another study by Gauthier et al. (2000a). Bird and car experts performed a one-back-repetition judgment task including pictures of faces, birds, cars and common objects. The level of expertise was manifested in an interaction between FFA activity and the level of expertise. Bird experts showed increased activity in the FFA when they processed bird stimuli, but not car stimuli, while car experts showed the reversed pattern of results. This indicates that activation of the FFA is not limited to the processing of facial stimuli.

Furthermore, not only the level of expertise, but also the level of categorization modulated activity in the FFA (Gauthier et al., 1997, 2000b). Subjects categorized stimuli at the subordinate level (e.g.

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A.1.2 : Introduction - Neuroanatomy of object processing and categorization: empirical findings sparrow) as well as at the basic level (e.g. bird). Activity in the FFA was higher for categorization at the subordinate level.

A.1.2.3.2 Studies focusing on the processing of different object categories

While the aforementioned studies examined if the processing of faces is localized in a specialized area, other neuroimaging studies attempted to find category-specific brain regions for different ob-ject categories. Using PET, Martin et al. (1996) presented pictures of animals and tools and revealed category-specific activation. Naming of animals led to activation of the left medial occipital lobe, which was not activated by tools. On the other hand, naming of tools elicited significantly greater activity in the left premotor cortex as well as a region in the left middle temporal gyrus. In a similar study, Moore & Price (1999) used two categories of man-made objects (vehicles, tools) as well as two categories of natural objects (animals, fruits). Compared to natural objects, man-made objects evoked increased activity in the left medial extrastriate cortex. Natural objects elicited activation in the anterior temporal cortex (bilateral) and the right posterior middle temporal cortex. These results were found across two different tasks (naming; word-picture matching).

In another fMRI study, Chao et al. (1999b) presented pictures of animals and tools and got compa-rable results across several tasks (naming, delayed match-to-sample, passive viewing). Animals (relative to tools) activated the lateral region of the fusiform gyrus (including the occipitotemporal sulcus, whereas tools (relative to animals) led to bilateral activation in the medial fusiform gyrus (including the collateral sulcus). In addition, pictures of faces and houses were also included in the study. Facial stimuli clustered in the same region as animals and houses elicited a response in the 'tools region'.

A study by Aguirre et al. (1998) focused on 'buildings' and found an area sensitive for this category close to the 'houses area' of the study by Chao et al. (1999b). This specific activity was limited to the right hemisphere in the lingual sulcus. A similar region for famous and non-famous buildings

A.1.2 : Introduction - Neuroanatomy of object processing and categorization: empirical findings (relative to faces) was also reported by Gorno-Tempini & Price (2001) in the parahippocampal/lin-gual cortex.

Ishai et al. (1999) found evidence for regions along the ventral visual pathway responding to differ-ent stimulus categories (faces: lateral fusiform gyrus, houses: medial fusiform gyrus, chairs: inferior temporal sulcus). However, these areas were not activated exclusively by the categories and there was considerable overlapping of areas activated by each category. In a subsequent paper (Ishai et al., 2000), the same distributed differential activation pattern was also found in the ventral occipital cortex, whereas no such consistent topological arrangement could be discerned in the dorsal part of the occipital cortex. Category-specific response patterns have also been examined in a recent study by Haxby et al. (2001). Pictures of faces, cats, and five different man-made objects were presented within a one-back repetition paradigm. Each object-category was found to be represented by a high-ly distinct activation pattern in the ventral temporal lobe, indexed by within-category correlation be-tween the response patterns of odd and even runs of each object. This within-category similarity could even still be demonstrated after removal of those regions that responded maximally to a given object.

Given that there is considerable variance between the studies with regard to methods (PET, fMRI), tasks (passive viewing, naming, n-back, etc), stimulus materials (line drawings, black and white photographs, color photographs, words), aim of the study (differentiate between man-made objects and artifacts; find representations of single object categories) and differences in statistical analysis (Huettel et al., 2004) it is not surprising that results are rather inconsistent across studies. Although each study succeeded in identifying category-related areas, many of these ‘category-specific’ re-gions are not activated consistently across studies for a given object category.

However, some tendencies that a region is more likely activated by a specific category are evident across studies. Kanwisher and colleagues (Downing et al., 2001; Epstein & Kanwisher, 1998;

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A.1.2 : Introduction - Neuroanatomy of object processing and categorization: empirical findings Kanwisher, 2000) have proposed a limited number of specialized processing modules for the recognition of faces (fusiform face area), places (parahippocampal place area) and body parts (extrastriate body area) - other object categories are proposed to be processed by the remaining ventral temporal cortex. Rather than being localized in category-specific areas or modules, it was suggested by Haxby et al. (2001) that object categories are organized in distinct response patterns all across the occipito-temporal cortex (‘ object form topography’). Each object category has its unique distributed “signature” across the whole occipito-temporal cortex, which is replicable if the specific exemplar or the view-point are changed (Spiridon & Kanwisher, 2002).

In sum, neuroimaging studies revealed that the categorical structure of the semantic system is related to the ventral occipito-temporal cortex. Objects might be represented by a discrete functional topography of cortical regions along the ventro-temporal cortex. The variance between studies that attempted to localize category-specific areas is considerable.