Dynamics of cognitive processing in the human hippocampus by neuromagnetic and neurochemical assessments

Various features of cognitive processing have been studied using event-related electric potentials and magnetoencephalography (MEG), as well as neuroimaging. Magnetic resonance spectroscopy (MRS) is a unique, noninvasive approach of measuring cellular metabolism that reflects the static metabolic state. The present study is the first to show noninvasively the dynamic neurophysiological and metabolic changes that occur during cognitive processing in vivo in the human hippocampus, as measured by MEG and spin-echo dynamic (1)H-MRS time-locked to the onset of the stimulus. The stimuli consisted of unpleasant and pleasant pictures of faces of human or primate babies. The event-related synchronization of theta activity and levels of creatine/phosphocreatine and choline-containing compounds relative to the respective level in the resting condition increased significantly, more in the right hippocampus than in the left, during the target discrimination task and also more in the right hippocampus in response to the unpleasant target picture than the pleasant one. These results suggest that excitatory postsynaptic metabolism in the hippocampus, especially in the right hippocampus, is involved in discriminative and cognitive processing of emotional information. This newly devised method combining event-related MEG with MRS can be used to noninvasively elucidate the dynamic features of neurophysiology and neurochemical metabolism and represents a promising approach toward improving our understanding of brain pathophysiology.