Loss of stored knowledge of object structure: Implications for "category-specific" deficits

Following a right-hemisphere lesion, the patient SM had impaired object rec ognition, with good elementary visual abilities, and could derive informati on about object structure. He was also impaired on all tasks tapping stored structural knowledge, even when tested in the verbal modality. This sugges ts that SM has a disorder affecting stored knowledge of object structure, t hough he remains able to assemble novel structural descriptions. I-Iis obje ct recognition ability also appeared significantly worse for nonliving thin gs. By contrast, existing models relating to stored knowledge would predict that SM would show greater impairment with living things. We argue that SM 's deficit reflects the loss of a type of structural knowledge that relates to the "within-item structural diversity" of items. It is argued that livi ng things show less structural variation than objects in the natural world, and might arguably be easier to recognise, because the image of the to-be- recognised object would be similar to the stored representation. Hence, a d eficit affecting this aspect of stored knowledge would differentially impac t upon nonliving things. This argument receives confirming independent supp ort from the finding that normal subjects ratings for the within-item struc tural diversity of visual stimuli are (unlike other "critical" variables) s ignificant predictors of SM's naming performance.