Middle latency auditory evoked potentials in cortical lesions. Critical of interhemispheric asymmetry

Middle latency auditory evoked potentials after monaural stimulation have been recorded in 16 normal subjects and in 21 patients with unilateral cortical or subcortical lesions determined by computed tomographic scan examination. An interhemispheric index was studied to quantify the asymmetry of Na and Pa middle latency auditory evoked potential components recorded over each hemisphere. This index has been calculated for each ear separately and after adding the responses of both ears. Its reliability is shown to be better in the latter situation. In all of the patients the interhemispheric asymmetry index was in the normal range for the Na component. For the Pa component this index was abnormal in 11 patients with cortical temporal lobe or subcortical lesions interrupting acoustic radiations. These results confirm that the Pa is dependent on the integrity of acoustic radiations and auditory cortex in the supratemporal plane, whereas the Na component would be generated at a subcortical level. Unilateral extinction of the dichotic listening test was found to correlate with abnormal Pa component asymmetry in the case of lesions involving the auditory structures.