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

Our aim was to investigate if spatial hearing is impaired in mesial temporal lobe epilepsy and temporal lobectomy has an effect on this function.

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.

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.

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.

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.