The role of the hippocampus in auditory processing studied by event-related electric potentials and magnetic fields in epilepsy patients before and after temporal lobectomy.

Abstract

To clarify the relationship between the hippocampus and the event-related responses in auditory information processing, we recorded event-related potentials (ERPs) and event-related magnetic fields (ERFs) associated with the auditory oddball paradigm in 12 patients with temporal lobe epilepsy before and after surgical treatment, and in eight age-matched healthy volunteers. Lesions in the patients were hippocampal sclerosis (8), cyst (2), cavernoma (1) and calcified arteriovenous malformation (1), all in the unilateral temporal lobe. Standard temporal lobectomy (8), selective amygdalohippocampectomy (2), selective hippocampectomy (1) and inferior lateral temporal resection (1) were carried out. ERPs were recorded in nine patients before surgery, in all 12 patients after surgery, and in all normal subjects. P300 was maximal at Pz in the patients both before and after surgery, and in normal subjects. The peak latency and amplitude of P300 measured at Pz in the patients either before or after surgery did not differ significantly from those in normal subjects. After surgery, only the amplitude of P300 over the anterior and mid-temporal area on the resected side was attenuated, while it was symmetric before surgery regardless of the side of epileptogenic focus. ERFs were recorded in three patients before surgery and in six normal subjects by using a whole-head neuromagnetometer. ERFs in response to the target stimuli at a latency of approximately 400 ms were recognized at the anterior, middle and posterior lateral channels on each hemisphere (M400). The latency and dipole moments for M400 did not differ significantly between the patients before surgery and the normal subjects. As a result of analysis using the time-varying multidipole model, three dipoles for M400 were estimated in two patients in whom ERFs were available before surgery for the analysis, and in normal subjects: mesial temporal area, superior temporal area and inferior parietal area on each hemisphere. After surgery, in four out of six patients in whom ERFs were recordable, M400 at the anterior temporal channels on the resected side disappeared, and the activity in the affected mesial temporal area was lost. In one patient who underwent inferior lateral temporal resection, M400 waveforms and its sources were preserved in all regions. There were no significant differences in the latency and dipole moments of the unaffected source of M400 before versus after surgery. These results suggest that the hippocampus contributes to the scalp-recorded P300 only at the corresponding anterior temporal region, and does not influence its general waveform and predominant distribution over the scalp.