Figure

The architecture of a connectionist model proposed by Farah and McClelland (1991).

eaten; chairs are for sitting on). Why are there three times as many visual units as functional units within the semantic system? Human participants were provided with dictionary definitions of living and non-living objects, and asked to classify the descriptors as visual or functional. Three times more of the descriptors were classified as visual than as functional. Of particular importance, the ratio of visual to functional descriptors was 7.7:1 for non-living objects, but only 1.4:1 for non-living objects. This difference between living and non-living objects was built into the semantic system of the model.

The computational model was tested by training it on object recognition (linking the visual and verbal representations) of 10 living and 10 non-living objects. Its performance was perfect after only 40 training trials. After that, Farah and McClelland (1991) simulated the effects of visual associative agnosia by means of "lesions" to the semantic system. This involved deactivating some of the semantic units. Damage to the visual units in the semantic system had much more severe consequences for object recognition of living than of non-living objects. Damage to the functional units had much less effect. It produced only a small reduction in object recognition, and that was limited to non-living objects.

Evaluation

The computational model of Farah and McClelland (1991) has various strengths. First, it provides a simple account of key processes involved in object recognition. Second, the model explains the double dissociation that has been found, with some patients having greater object recognition with living than with non-living objects, whereas some have the opposite pattern. Third, it also helps to explain why there are many more patients who have impaired ability to recognise living objects than those who have problems in recognising non-living objects.

On the negative side, the processes involved in object recognition are more complex than is suggested by the model. In addition, it is not clear that the semantic system is organised neatly into visual and functional sub-systems. It is possible that it is organised in part on a categorical basis, with different categories (e.g., animals; fruits) being stored in different regions of the brain. This possibility was explored by Damasio et

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