Abstracts

Rationale: Human limbic system is not described in detail. Recent diffusion tract imaging on MRI technique enables to visualize the human fornical circuit or cingulum. However this technique demonstrates partially anatomical connectivity not functional connectivity, therefore so far nobody could elucidate limbic functional connectivity. Cortico-Cortical Evoked Potential (CCEP) method was first introduced by Matsumoto and Nair et al in 2004 (Matsumoto et al.,2004). They clearly showed the functional connectivity in human language between Broca and Wernicke area. After this report, using this technique connectivity of human cortical motor (Matsumoto et al., 2007), bilateral cortical motor (Terada et al., 2008), bitemporal connectivity (Umeoka et al., 2009) were also reported. Today this seems to be one of the established methods to ensure functional connectivity between two interest areas. The purpose of this study is to neurophysiologically clarify the functional connectivity of human limbic system with CCEP method. Especially in this study, we focused on the connectivity between hippocampus and cingulated gyrus. Methods: We currently recruited five intractable focal epileptic patients who implanted stereoelectroencephalogam electrodes (SEEG) in brain area for exploring ictal onset area. After finishing recording interictal and ictal events, antiepileptic medications were restarted and then CCEP was performed. Electric pulse stimuli were alternatively delivered in a bipolar fashion to two adjacent SEEG contacts on hippocampus. Stimulus parameter was described in elsewhere (Matsumoto et al., 2004). Cortico-cortical evoked potentials were recorded by averaging electroencephalogram from other contacts. We did twice of each session for confirming reproducibility. This study was approved by Cleveland Clinic IRB. We obtained the informed consent from all patients in the research use. Each patient s locations of electrodes were different in each patient, therefore we selected five patients who had contacts both hippocampus and cingulate gyrus to examine the limbic connectivity. Results: All patients showed prominent CCEP responses in cingulated gyrus compared with another surrounding cortex such as medial occipital cortex or medial part of superior parietal lobule. Especially responses in posterior part of cingulate gyrus were more obvious than other cingulate gyrus. We could preliminary found the functional connectivity between hippocampus and posterior cingulate gyrus. For detail analysis, we should recruit more cases. Conclusions: We could preliminary identify the neurophysiologial connectivity between hippocampus and cingulate gyrus. And the connectivity of other brain areas such as insular and orbitofrontal cortex will be investigated. This finding also leads to the understanding propagation pattern of the medial temporal lobe epilepsy, and might be clarifying of the Alzheimer s disease and psychiatric disorder such as depression or schizophrenia.