The effects of chiasmal compression on the pattern visual evoked potential.

The visual evoked potentials (VEPs) to pattern reversal stimulation (26' individual check subtense, 11 degrees total field) have been examined in 10 patients with proven chiasmal compression. Bipolar occipital-sylvian and occipital-parietal recordings were taken with electrodes placed according to the Modified Maudsley system. In all patients abnormalities, taking the form of latency delays or amplitude reductions, were found in response to monocular stimulation which, without exception, were maximal in the traces corresponding to the hemisphere contralateral to the field defect. In seven patients responses to a 13' pattern were also examined. In six of these the lateralisation of abnormality was enhanced. The discrepancy between these and previous findings is discussed, the importance of stimulus and recording parameters being stressed.