When Leopards Lose Their Spots: Knowledge of Visual Properties in Category-specific Deficits for Living Things

Several cases of category-specific semantic deficits for living things have now been reported, most of whom have a greater difficulty with knowledge of visual properties of living things than with non-visual properties. This has motivated two different kinds of account of category-specific deficits for living things. One account is that the impairment for living things arises as a result of general damage to visual properties, and so it predicts that impairment for visual properties of living things will be accompanied by some degree of impairment for visual properties of other categories. The second account posits explicit category-based organisation of semantic memory, such that visual properties of living and non-living things are stored separately, and so can be independently impaired. We investigated the semantic impairment of a patient SE, who had received a diagnosis of herpes simplex encephalitis several years previously, resulting in a mild category-specific semantic deficit for living things. Both off-line and on-line tasks revealed that SE had a highly selective impairment for the visual properties of living things, with no sign of problems for the visual properties of non-living things. This raises a challenge for models in which category-specific deficits emerge as a result of a general deficit determined by some other factor. We suggest one way in which these models may account for the data by encoding inter-relations between the function and form of objects in semantic memory.