DISTRIBUTED CORTICAL NETWORK FOR VISUAL-ATTENTION

The contribution of prefrontal and posterior association cortex to vol untary and involuntary visual attention was assessed using electrophys iological techniques in patients with focal lesions in prefrontal (n = 11), temporal-parietal (n = 10), or lateral parietal cortex (n = 7). Subjects participated in a task requiring detection of designated targ et stimuli embedded in trains of repetitive stimuli. Infrequent and ir relevant novel visual stimuli were randomly interspersed with the targ et and background stimuli. Controls generated attention dependent N1 ( 170 msec) and N2 (243 msec) potentials maximal over extrastriate corte x. Anterior and posterior association cortex lesions reduced the ampli tude of both the N1 and N2 potentials recorded over extrastriate corte x of the lesioned hemisphere. The pattern of results obtained reveals that an intrahemispheric network involving prefrontal and posterior as sociation cortex modulates early visual processing in extrastriate reg ions. Voluntary target detection generated a parietal maximal P300 res ponse (P3b) and irrelevant novel stimuli generated a more fronto-centr ally distributed P300 (P3a). Cortical lesions had differential effects on P3a and P3b potentials. The P3b was not significantly reduced in a ny cortical lesioned group. Conversely, the P3a was reduced by both pr efrontal and posterior lesions with decrements most severe throughout the lesioned hemisphere. These data provide evidence that an associati on cortex network involving prefrontal and posterior regions is activa ted during orientation to novel events. The lack of a significant effe ct on the visual target P3b in patients with novelty P3a reductions su pports the notion that different neural systems are engaged during vol untary vs involuntary attention to visual stimuli.