Lateralisation of sound in temporal-lobe epilepsy: comparison between pre- and postoperative performances and ERPs

Abstract

Objective: Our aim was to investigate if spatial hearing is impaired in mesial temporal lobe epilepsy and temporal lobectomy has an effect on this function. Methods: Thirteen patients with mesial temporal lobe epilepsy (TLE) due to sclerosis in their left (n = 6) or right (n = 7) hippocampus were studied. Their sound lateralisation performance indexed by d' was tested against that of a group of normal subjects (n = 13). Patients' ERPs to lateralisation shifts induced by interaural disparities of intensity (IID) and time (ITD) were also recorded. Eight of the patients were re-tested after they underwent anterior temporal lobectomy, which involved the resection/removal of medial structures including amygdala, hippocampus and parahippocampal gyrus. Results: The sound-lateralisation performance of the TLE patients was significantly lower than normal subjects, and this disadvantage of the patients was specific to IID-based lateralisation. Amplitudes of their N1 and P2 responses to laterally shifting sounds were much lower than those reported previously for normal subjects. Lobectomy did not have a statistically significant effect on patients' sound-lateralisation performance nor on the amplitude of their auditory directional ERPs. Conclusions: The results show that especially the IID-based sound-lateralisation performance is impaired in TLE patients and that lobectomy should not cause any further deterioration. Significance: This study suggests that a test for assessing the ability of sound lateralisation based on each of the IID and ITD cues should be included in the evaluation of TLE patients.