Use of complex tones in the assessment of auditory cortical function in patients undergoing surgery for temporal lobe epilepsy

Previous reports described three types of auditory evoked potentials (AEP) obtained with continuous synthesized musical instrument tones, two generated in the supratemporal cortex, a third one in the lateral temporal cortex, more prominently on the right. The objective of this work was to examine those AEP in a patient population undergoing surgery for temporal lobe epilepsy, which may direct or indirectly affect their generators. A group of 31 normal subjects and 148 epileptic patients were evaluated with AEPs to onset of tones, changes in their spectral and changes in their spectro-temporal patterns. Patients were divided according to their clinical and magnetic resonance imaging (MRI) characteristics: 43 had right hippocampal sclerosis (HS), 55 had left HS, 35 had lesions that were not HS, 15 had complex partial epilepsy with normal MRI. Forty-two patients with HS or other cerebral lesions were selected for surgery and were re-evaluated 3-6 months post surgery. In the pre surgical evaluation, the incidence of AEP abnormalities was 90 to 100%. Post surgically, the incidence of abnormalities did not differ significantly from pre surgical ones. Serial AEP assessment revealed deterioration and improvement in comparable numbers of cases. Correlation tests were performed between AEP surgical evolution and neuropsychological surgical evolution. Some correlations were observed to the right HS subgroup, but the pattern did not clearly suggest a common cause for any deficit. No correlations were seen for the left HS subgroup. No significant correlation was seen between surgical volume of removed tissue and AEP changes for any patient group. In conclusion, no evidence was found for any deleterious effect of surgery on the AEPs. The high incidence of pre surgical abnormalities may point to a generalized mild dysfunction of auditory cortical processes in this group of patients