Preattentive deficit in temporal processing by chronic schizophrenics.

Preattentive visual filtering by two types of neural cells (transient and sustained) constitutes the basic neural correlates underlying visual perception. Transient cells respond abruptly to temporal modulation and to the low spatial frequency components of visual stimuli. Their function is localization of objects and global perception (i.e., detection). Sustained cells respond slowly and selectively to high spatial frequencies and are believed to be responsible for the "oblique effect" (i.e., reduced responsiveness to obliquely oriented high spatial frequencies), which likely occurs at the striate cortex. Their function is to discern the details of an object (i.e., identification). The present study evaluated early visual processing in normal controls and schizophrenics by presenting vertical and oblique (45 degrees) spatial frequency gratings of 0.9 and 12 c/deg that were presented for 50, 100, 175, and 250-msec pulses. Threshold, in milliseconds, to detect an interstimulus interval (ISI) between two grating pulses was the dependent measure. Schizophrenics required longer ISIs for each grating duration, except for 250 msec, and longer ISIs for the 15 c/deg oblique grating. The results indicate that schizophrenics have a preattentive processing deficit that is further elaborated at the cortical level.