Abstracts

In human, the functional connectivity of the hippocampus has been well established within the temporo-limbic structures, using intra-cerebral stimulation coupled with evoked potentials (EP). Conversely, this approach has not yet been used to study the hippocampal connectivity with the majority of other brain regions. Taking advantage of stereotactic EEG procedures (SEEG), we have investigated this issue in 9 epileptic patients. These nine patients proved to suffer from refractory partial epilepsy of temporal lobe origin in seven (including one bi-temporal), and frontal in two. Intracranial electrodes were placed in a single hemisphere in five patients, and bilaterally in four, with at least one electrode targeted to the hippocampus ipsilateral to the seizure onset zone in all. A total of 107 intra-cerebral electrodes were implanted, including 55 in the temporal lobe, 41 in the frontal lobe, and 7 in the parietal lobe. Nine of these electrodes also sampled the insula. Electrical bipolar stimulations, produced by a current-regulated neurostimulator designed for a safe diagnostic stimulation of the human brain, were delivered at 45 hippocampal sites. The stimulations parameters were chosen to avoid tissue damage and consisted in two series of 25 pulses of 1 msec duration, 0.2 Hz frequency, and 3 mA intensity. EP were averaged over the 25 stimuli of each series, over all recorded sites, and considered significant when reproducible on the two consecutive series, with an amplitude at least twice that of the background level. Highly reproducible EP were elicited in many brain regions in all patients. However, the same anatomofunctional structure did not always demonstrate an EP across all patients. The structures which most often displayed an EP were the following : cingulate gyrus (85%), amygdala (83%), entorhinal cortex (80%), temporal pole (80%), orbito-frontal cortex (75%), insula (67%), mesial prefrontal (50%), lateral frontal (33%), SMA (28%), pre-central operculum (28%). No response were recorded in the post-central operculum nor in the contralateral hippocampus. Our results show that the coupling of intra-cerebral low frequency stimulation and EP recordings is an effective method to study cortico-cortical connectivity, and could help to more precisely define the potential propagation pathways of epileptic discharges in human. Data from hippocampal stimulations appear generally consistent with our knowledge on hippocampal functionnal anatomy. However, to which extend the recorded EP reflect physiological responses or abnormal cortical activation related to the epilepsy remains to be determined. (Supported by Claude Bernard University)