Object recognition under semantic impairment: The effects of conceptual regularities on perceptual decisions

Although patients with semantic deficits can sometimes show good performance on tests of object decision, we present evidence that this pattern applies when nonsense-objects do not respect the regularities of the domain. In a newly designed test of object-decision, 20 patients with semantic dementia viewed line drawings of a real and chimeric animal side-by-side, and were asked to decide which was real. The real animal was either more typical (real > nonreal) or less typical (nonreal > real) than the chimera. Performance was significantly better in the real > nonreal condition, and success in both conditions was modulated by patients' degree of semantic impairment. A similar effect of item typicality was revealed in a subset of items selected from a standard test battery. Object-decision scores were highly correlated with other pictorial and verbal assessments of conceptual knowledge, suggesting that impaired performance on all tasks resulted from the degradation of a unitary underlying system.