Minding the facts: a comment on Thompson-Schill et al.’s “A neural basis for category and modality specificity of semantic knowledge”
Introduction
In a recent issue of Neuropsychologia, Thompson-Schill et al. [22] (1999) report having carried out a test of a “… model of how damage to interactive, modality-specific neural regions might give rise to [semantic] categorical impairments”. These are impairments in which certain semantic categories are disproportionately impaired relative to other categories. The most frequently investigated categorical distinction is that between living and non-living things, but other category-specific deficits have been reported (see [2], [8] for brief reviews). Thompson-Schill et al. used functional MRI to determine whether the left fusiform gyrus responds differentially during retrieval of visual and non-visual knowledge about living and non-living things. They found increased fusiform activity, relative to a baseline, for visual and non-visual knowledge of living things; they also found increased fusiform activity for visual knowledge of non-living things but not for non-visual properties. These results were interpreted as providing support for the interactive, modality-specific account of semantic category-specific deficits proposed by Farah and McClelland [7]. However, I will argue that there is a substantial body of data that is highly problematic for Thompson-Schill et al.’s modality-specific account of category-specific deficits. I will further argue that it is not at all obvious that the predictions they derive from Farah and McClelland’s computational version of the modality-specific hypothesis follow from that model. Finally, I will argue that the fMRI results reported could as easily be interpreted to reflect imagery processes involved in solving difficult semantic judgment tasks of the sort used in their study, and they may have nothing to say about the causes of category-specific semantic deficits or the organization of the semantic system.
Section snippets
The modality-specific hypothesis and category-specific deficits: what are the ‘facts’
The interactive, modality-specific hypothesis is a variant of the sensory/functional theory of semantic organization, which was originally proposed by Warrington and Shallice [25] to account for the existence of semantic category-specific deficits.1
The modality-specific hypothesis: fMRI predictions
Thompson-Schill et al. derive the following two fMRI predictions from the IMSH:
There are two critical predictions of the interactive modality-specific hypothesis which can be tested … activity should be present in the left fusiform gyrus during retrieval of non-visual information about living things. This is a straightforward prediction of the claim that visual knowledge is obligatory during the retrieval of any information about living things. Second, activity in the left fusiform gyrus should
Conclusion
In this commentary on a recent paper by Thompson-Schill et al. [22] I have argued that the authors defend a theory of the causes of semantic category-specific deficits that is at variance with well-established empirical facts. No attempt is made by the authors to deal with these recalcitrant facts. I have also argued (1) that the predictions they make about fusiform gyrus activation on the basis of the IMSH do not obviously follow from this model or are wrong, (2) that simulations are needed to
Acknowledgements
The writing of this comment was supported in part by NIH grant NS22201. I am grateful to two anonymous referees for their helpful comments.
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