Do overinclusion and distorted semantic category boundaries in schizophrenia arise from executive dysfunction?

Semantic memory impairments have been reported extensively in people with schizophrenia. Inefficient search and retrieval strategies, due to an executive dysfunction, rather than a primary loss of semantic knowledge are a primary candidate for such impairments. In order to test this hypothesis we compared the performance of 20 patients meeting DSM-IV-TR criteria for schizophrenia with that of 20 healthy controls and 10 patients with acquired brain injury (ABI) with a dysexecutive syndrome. Seventy percent of the people with schizophrenia and 100% of the ABI patients in this study met criteria for executive impairment. However, the two groups performed significantly differently on a range of semantic memory tests. Whereas 45% of the patients with schizophrenia met criteria for distorted semantic category boundaries (n.b. overinclusion), this was true for only 10% of the ABI patients. In addition, no correlation was found between severity of executive dysfunction and tendency to overinclude in the schizophrenia group. This pattern of neuropsychological findings suggests that overinclusion, or disorganized semantic categorization procedures, in schizophrenia does not result from a classical executive dysfunction. Alternative explanations are discussed.