The effect of stimulus intensity on visual evoked potential estimates of interhemispheric transmission time.

Visual evoked potentials (VEPs) to bright or dim lateralised light flashes were recorded from homologous occipital and central sites. In a GO/NOGO reaction time task (Experiment 1) the latency of the N160 component of the VEP was found to be shorter from the contralateral hemisphere by approximately 16 ms at occipital sites, but only 3 ms centrally. In addition, there was a trend for the occipital contralateral latency advantage to increase with decreasing stimulus brightness. In Experiment 2 a wider intensity range and a simple visual reaction time task were employed. Contralateral N160 latency advantages were again found to be larger occipitally (approx 13 ms) than centrally (3 ms). Furthermore the occipital contralateral latency advantage was significantly increased at the lower stimulus intensity, while that from central sites remained constant. These data suggest that two types of interhemispheric relay can be distinguished-a sensory one recorded occipitally and a non-sensory one recorded from central sites.