Verbal novelty detection within the human hippocampus proper

Abstract

Animal studies and neuropsychological tests of patients with temporal lobe epilepsy have demonstrated the importance of human medial temporal lobes for memory formation. In addition, more recent studies have shown that the human hippocampal region is also involved in novelty detection. However, the exact contribution of the hippocampus proper to these processes is still unknown. To examine further its role we compared event-related potentials recorded within the medial temporal lobes in 29 temporal lobe epilepsy patients with and 21 without hippocampal sclerosis. While in patients with extrahippocampal lesions but without hippocampal sclerosis event-related potentials to first presentations and repetitions of words were reduced on the side of the epileptogenic focus, in patients with hippocampal sclerosis only those to first presentations but not to repetitions were affected. Because sclerosis of the hippocampus proper selectively reduced event-related potentials to new but not old verbal stimuli, it can be concluded that the human hippocampus proper contributes to verbal novelty detection.

Figure 1

Area of electrode locations at which maximal AMTL-N400s to words were recorded and grand averages of AMTL-N400s. (A) Schematics of recording sites of AMTL-N400s to words. Hatched area indicates the area in which maximal AMTL-N400 potentials were recorded in all patients. (B) Grand averages of AMTL-N400s in the nonfocal and focal temporal lobe in patients with extrahippocampal lesions and without hippocampal sclerosis (n = 21) as well as in patients with hippocampal sclerosis (n = 29). HS, hippocampal sclerosis. Solid line, first presentations; dashed line, word repetitions.

Figure 2

Examples of AMTL-N400s to new and old words in six patients with temporal lobe epilepsy. On the nonfocal and focal side the AMTL-N400 from the contact with maximal amplitudes to first presentations was selected. Solid line, first presentations; dashed line, word repetitions. (A) Three patients with hippocampal sclerosis. (B) Three patients with extrahippocampal lesions and without hippocampal sclerosis. Note that in patient S4 there is no new-minus-old repetition effect on the focal side, although both AMTL-N400 to first presentations and repetitions are reduced.