Evoked potential study of hippocampal efferent projections in the human brain

ObjectiveTo explore in human potential hippocampal projections within and outside the temporal lobe.MethodsWe performed intra-cerebral electrical stimulations in seven patients investigated by depth electrodes for refractory epilepsy and analyzed the presence of evoked potentials (EPs) in all brain regions explored. Bipolar electrical stimulations, consisting of two series of 25 pulses of 1 ms duration, 0.2 Hz frequency, and 3 mA intensity, were delivered in a total of 36 hippocampal stimulations sites.ResultsReproducible EPs were recorded in several brain regions with variable latencies, amplitudes and morphologies. Within the temporal lobe, EPs were present in the amygdala, entorhinal cortex, temporal pole and temporal neocortex. EPs were also observed in the frontal lobe, anterior cingulate gyrus and orbito-frontal cortex, midcingulate and posterior cingulate gyrus, insula and thalamic pulvinar nucleus.ConclusionOur results demonstrate a large distribution of direct or indirect hippocampal projections.SignificanceThis widespread connectivity supports the previous definition of different networks involved mainly in memory and behavioral processes, implicating the temporal lobe, the cingulate gyrus or the prefrontal region. Our data provide some clues to further evaluate potential pathways of propagation of mesial temporal lobe seizure, via the insula or the pulvinar nucleus.

► Using electrical hippocampal stimulations in humans, intra-cranial evoked potentials were recorded in the temporal lobe, orbito-frontal region, cingulate gyrus, insula and thalamic pulvinar nucleus.
► This widespread distribution of direct or indirect hippocampal projections specifies the previously described networks involved mainly in memory and behavioral processes.
► These data provide some clues to further evaluate potential pathways involving the insula or the pulvinar nucleus, which may participate in the propagation of mesial temporal lobe seizures.