Hippocampal event-related potentials (ERPs) in patients with unilateral Ammon's horn sclerosis

Introduction: In the hippocampus, visual presentation of words elicits a positive deflection (P600), followed by a long lasting late negative component (LNC). These ERP components are enhanced by word repetition, presumably reflecting hippocampal memory processes. Previous studies on patients with unilateral Ammon's horn sclerosis (AHS) have indicated that the LNC response to repetitions of words, as well as the P300 in oddball paradigms, are strongly reduced in the sclerotic hippocampus (Grunwald et al., 1995). The present study extends previous findings. Methods: The study sample consisted of 19 patients with temporal lobe epilepsy (10 left, 9 right unilateral AHS). Multicontact depth electrodes were implanted along the longitudinal axis of the hippocampus as part of the presurgical evaluation process. In a word recognition test, subjects had to indicate whether each word was new or already presented. In an oddball test, patients had to respond to rare target stimuli. The most anterior and the most posterior hippocampal electrodes were selected for measuring the peak amplitude of the P300 and P600. The less sharply peaking LNC was quantified by taking the mean amplitude between 600 and 900 ms. Results: In the word recognition paradigm, the P600 to repeated words was higher in amplitude than the response to newly presented words on focal and non-focal sides. The P600 was significantly reduced on the sclerotic side. The LNC was reduced on the focal side as compared to the non-focal side, but only in response to repeated words and only at posterior electrodes. Additionally, in the oddball paradigm, the P300 was significantly reduced on the sclerotic side. All components were more pronounced at posterior electrodes as compared to the anterior electrodes. Conclusions: As reported in earlier studies, P600 and LNC responses to correctly identified repeated words were higher in amplitude than responses to new words. Hippocampal ERPs were significantly reduced on the sclerotic side in both paradigms. This suggests that hippocampal processing is disrupted by the sclerosis and that measuring ERP parameters can be helpful in the presurgical evaluation process. Lesion effects are most pronounced in the P600 time range and at posterior electrodes. References: Grunwald et al. (1995), Electroencephalogr Clin Neurophysiol, 95(1), 53–62