Visual evoked potentials to double-pulse pattern presentation.

The temporal resolution and summation characteristics of human cortical processes were investigated by recording an individual VEP component, C1, to the double-pulse presentation of pattern pairs of both the same (+ve/+ve; -ve/-ve) and of opposite (+ve/-ve; -ve/+ve) contrast polarity at varying onset-to-onset intervals (SOA). The results show that comparable limiting SOA values of 40-50 msec are needed for the C1 components to the two patterns to separate out to form a double-peaked response and for the stimulus to be seen as two distinct events. Also, the amplitude variations of the single-peaked response obtained at shorter SOAs show evidence of complete precortical response integration for SOA values below 5-10 msec and partial integration for values up to 30-40 msec, which again correspond to the results of related psychophysical studies. These VEPs show no reflection, however, of the inhibition/summation effects reported for subjective responses to pattern pairs of the same/opposite contrast polarity at SOA values between 40 and 70 msec. The implications of these findings are discussed.