Prefrontal Cortex Involvement in Low-Level Visual Processing

Abstract

Visual tasks that involve low-level visual features such as luminance contrast are processed in early visual areas such as retina, LGN, or V1. Schizophrenia patients with hypothesized prefrontal dopamine deficits, however, have been shown to perform poorly on two interval forced-choice contrast detection tasks (Sheremata and Chen in press). These deficits appear to be related to the delay component as the same group of subjects performed at normal levels on another contrast modulated task without a delay component. Here we tested the hypothesis that deficits in performance were indeed related to prefrontal dysfunction using transcranial magnetic stimulation (TMS) in normal subjects. Subjects were asked to determine which of two sinusoid gratings presented peripherally had higher luminance contrast while fixating a small square. The gratings were presented for 100 msec either successively (ISI =800 msec) or simultaneously in order to determine whether the effect of stimulation was dependent on the delay component. Performance after TMS to the right dorsolateral prefrontal cortex was significantly impaired as compared with testing before stimulation for the delay condition but not for the non-delay condition. Performance improved to pre-stimulation levels when subjects were again tested after resting for fifteen minutes. These data show that the prefrontal cortex is involved in two-interval forced choice luminance contrast discrimination. We suggest that the prefrontal cortex is involved in low-level visual tasks requiring retention of visual signals over intervals shorter than those used for working memory studies.

Clara Mayo Graduate Fellowship, JSPS