Alterations of intrahippocampal cognitive potentials in temporal lobe epilepsy

During presurgical evaluation event-related potentials were recorded with depth electrodes located longitudinally within the hippocampas in 25 patients suffering from unilateral temporal lobe epilepsy. Rare stimuli in a visual oddball paradigm elicited a pronounced negativity (“No”) in the hippocampal body. Amplitudes were significantly reduced on the side of the primary epileptogenic area. Visual presentations of words in a recognition paradigm evoked an earlier negativity (“ENw”) in anterior and a later negativity (“LNw”) in posterior hippocampal structures. Both were sensitive for recognition effects and showed reduced amplitudes on the side of the primary epileptogenic area. Relating the differences of left and right hippocampal “No” and “ENw”, amplitudes proved to be a sensitive method for topological diagnosis and allowed a correct lateralization of the primary epileptogenic area in all patients. Amplitudes of the hippocampal ENw evoked in the dominant hemisphere by first presentations, strongly correlated with the recognition rate, when the primary epileptogenic area was situated in the contralateral temporal lobe. This correlation was reduced by the presence of ipsilateral epileptogenic foci.