Visual backward masking: Deficits in locating targets are specific to schizophrenia and not related to intellectual decline

Visual backward masking deficits have been postulated as potential vulnerability markers for schizophrenia. This study investigated the diagnostic specificity of a location and an identification variant of the backward masking task for schizophrenia and analyzed masking performance during the course of the tasks. The influence of schizophrenia patients' intellectual decline on masking performance was also examined. Twenty-eight schizophrenia patients were compared to 28 patients with unipolar depression and 28 healthy controls on a letter location task and a letter identification task applying a low spatial frequency mask. Schizophrenia patients made significantly more detection errors on the location task than depressives at an interstimulus interval (ISI) of 50 ms and healthy controls at ISIs of 16.7, 33.3, 50, and 66.7 ms. Thus, the location masking dysfunction of schizophrenia patients was distinctive at a rather long interstimulus interval (50 ms). On the identification task the performance of schizophrenia patients did not differ from that of the two control groups. Identification but not location masking performance improved during the course of the task for all groups. Intellectual deterioration of schizophrenia patients was not correlated with location or identification masking performance. Schizophrenia patients are characterized by specific impairments in spatial visual processing which appear to be independent of intellectual decline. Potential explanations of the location masking deficit found in schizophrenia are discussed.