Hippocampal function and visual object processing in temporal lobe epilepsy M. Vannucci, 1,2,CA Th. Dietl, 1 N. Pezer, 1 M. P. Viggiano, 2 C. Helmstaedter, 1 C. Schaller, 3 C. E. Elger 1 and Th. Grunwald 1,4 1 Departments of Epileptology and 3 Neurosurgery, University of Bonn, Germany; 2 Department of Psychology, University of Florence, via S Niccolo' 93, 50125 Firenze, Italy; 4 Swiss Epilepsy Center, Zurich, Switzerland CA,2 Corresponding Author: manila.vannucci@psico.uni¢.it Received 29 December 2002; accepted14 January 2003 DOI: 10.1097/01.wnr.0000079895.11980.23 Limbic event-related potentials related to verbal but not to visual recognition memory have been found to be attenuated within the epileptic hippocampus of patients with temporal lobe epilepsy (TLE). To identify hippocampal contributions to visual processing and memory we recorded intracranial ERPs directly from within the epileptic and the non-epileptic hippocampus in 12 patients with unilateral TLE during a visual object decision and naming task. While the non-epileptic hippocampus di¡erentiated reliably between real and nonsense objects, this e¡ect was completely eliminated within the epileptic mesial temporal lobe. This ¢nding suggests that the hippocampus proper contributes to the semantic processing of visual objects and may help to explain visual memory de¢cits in TLE. NeuroReport 14 :1489^1492  c 2003 Lippincott Williams & Wilkins. Key words: Epilepsy; Event-related potentials; Hippocampus; Temporal lobe epilepsy; Visual object processing INTRODUCTION The occasional need to implant depth electrodes into the medial temporal lobes of patients with temporal lobe epilepsy (TLE) undergoing evaluation for possible epilepsy surgery makes it possible to record event-related potentials (ERPs) directly from hippocampal and parahippocampal limbic structures. It has been known for a long time that these medial temporal brain regions play a crucial role in declarative memory processes [1–3]. However, the possi- bility of correlating depth ERPs with the graded memory deficits of TLE patients has contributed to the under- standing of the participation of different medial temporal lobe subregions in memory processes [4–7]. In particular, a negative field potential peaking around 400 ms has been recorded within the anterior medial temporal lobe (AMTL- N400) during language and memory tasks with both words and pictures [8–11]. This potential is generated by a parahippocampal (entorhinal) generator [12,13] and it is reduced in amplitude by stimulus repetition [14]. AMTL-N400 elicited by words and by pictures have been found to be differentially affected by the epileptogenic process in TLE patients: AMTL-N400s to first presentations of words (new) in a continuous verbal recognition paradigm were found to be attenuated within the epileptic temporal lobe [4,15] so that the associated new-minus-old repetition effect is also diminished [16,17]. These unilateral attenu- ations of limbic ERPs proved to be very helpful in lateralizing the epileptogenic focus predicting surgical outcome [18]. By contrast, up to now no study could demonstrate any effects of the epileptogenic process on the AMTL-N400 elicited by both abstract pictures [19] and pictures of common objects [9]. This discrepancy is difficult to under- stand since seizures originating within the non-dominant temporal lobe are associated with non-verbal memory deficits [20,21]. In fact, if the hippocampal memory system contributes to the memory related processing of both verbal and non-verbal stimuli, epileptogenicity should somehow affect limbic ERPs elicited by figurative stimuli. Recently, functional brain imaging and electrophysiologi- cal studies have shown that the MTL does participate in visual object processing tasks like object decision [22], visual object naming [23] and categorization [24] and the specific amount and location of the activation of MTL structures in these tasks was affected by the meaning of the pictures: within the left MTL, meaningful pictures of real objects elicited a significantly higher activity than meaningless pictures of nonsense objects [22,23]. These recent findings, together with the earlier conflicting results in studies using non-verbal vs verbal recognition paradigms, suggest that the hippocampal memory system could make different contributions to the encoding of verbal and non-verbal stimuli. To further delineate the hippocampal participation in the processing of non-verbal material, we recorded limbic ERPs invasively from within the human hippocampus proper during a visual object decision and naming task. In particular, we aimed to identify possible effects of the 0959- 4965  c Lippincott Williams & Wilkins Vol 14 No 11 6 August 2003 1489 COGNITIVE NEUROSCIENCE AND NEUROPSYCHOLOGY NEUROREPORT Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.