Mechanisms of backward masking in schizophrenia

Backward masking is a technique that assesses early visual information processing. Schizophrenic patients consistently require longer interstimulus intervals to identify target stimuli than normal controls. However, the interpretation of this masking deficit has been difficult, mainly because the masking methods used with schizophrenic patients typically combined two masking mechanisms (integration and interruption) and two types of visual channels (transient and sustained). The present project altered the masking procedures to allow better specification of the nature of the deficit. First, we increased the target:mask energy ratio to obtain a non-monotonic masking function. Such a function would indicate that masking occurred primarily through interruption. Second, we lowered the spatial frequency of the target to reduce reliance on sustained visual channels during target identification. Manic patients were included to examine the specificity of the masking deficit to schizophrenia. Schizophrenic patients differed significantly from normal controls on the non-monotonic function, with targets of both high and low spatial frequency. Manic patients performed comparably to the schizophrenic patients. The results suggest that the masking deficits in schizophrenia are: (1) not entirely due to increased susceptibility to masking by integration; (2) not explained solely by deficits in sustained visual channels; and (3) present in manic patients to a comparable extent.