NONINVASIVE DETECTION OF NEURONAL POPULATION ACTIVITY IN HUMAN HIPPOCAMPUS

Medial temporal brain structures and hippocampus are associated with m emory encoding and activate during registration and transfer of inform ation within cortico-hippocampal circuits. Hippocampal dysfunction may cause learning and memory deficits as well as epileptogenesis. We mon itored neuronal population activity of the human hippocampal formation with a whole-head magnetoencephalographic array. Attention-dependent hippocampal responses were elicited by auditory oddball stimuli. The m ost prominent responses emerged 200-500 ms after attended oddballs in good agreement with earlier intracranial recordings. Some activation o f hippocampal source areas was seen at 60-100 ms. This completely non- invasive method permits access to cortico-hippocampal neural networks on the millisecond time scale during normal and abnormal brain activat ion and thus provides a new tool for evaluation of cognitive processes and pathological conditions involving hippocampal formation.