Abstracts

Rationale: The brain s default network is an anatomically defined system that is active during undisturbed thinking. Two of its subsystems, the medial temporal lobe (MTL), involved in memory retrieval, and the medial prefrontal subsystem, involved in construction of mental simulations, converge on the posterior cingulate (PC) cortex. While neuroimaging has demonstrated functional connectivity among these structures, our aim was to demonstrate connectivity between the PC and MTL by neurophysiological methods. Methods: Four women (ages 23 to 58 years) with intractable nonlesional temporal lobe epilepsy underwent implantation of depth electrodes for seizure-focus localization. Implanted areas included anterior, middle, and posterior hippocampi; amygdalae; and PC. Single pulse stimulation (1 Hz, pulse width 0.3 msec, at 2-10 mA) was applied to implanted structures. Results: Cerebro-cerebral evoked potentials (CCEPs) with early and late components were recorded bidirectionally between PC and ipsilateral hippocampus (n=3). Stimulation of PC elicited responses throughout the hippocampus, with maximal amplitude posteriorly (n=2). Stimulation of the anterior, middle, and posterior hippocampal electrodes elicited CCEPs in the PC region, with maximum amplitude upon stimulation of the posterior hippocampus (n=2), and anterior hippocampus (n=1). The location of the PC electrodes in the patient with no MTL CCEPs was different from PC electrode locations in the others. Conclusions: Cerebro-cerebral evoked potentials (CCEPs) with early and late components were recorded bidirectionally between PC and ipsilateral hippocampus (n=3). Stimulation of PC elicited responses throughout the hippocampus, with maximal amplitude posteriorly (n=2). Stimulation of the anterior, middle, and posterior hippocampal electrodes elicited CCEPs in the PC region, with maximum amplitude upon stimulation of the posterior hippocampus (n=2), and anterior hippocampus (n=1). The location of the PC electrodes in the patient with no MTL CCEPs was different from PC electrode locations in the others.