Magnetic fields in the human hippocampal area evoked by a somatosensory oddball task

We recorded neuromagnetic fields evoked by a somatosensory oddball task requiring subjects to discriminate target stimuli from nontarget stimuli, which are different in stimulation intensity, with a mental count of the target stimuli. A whole head type 80-channel magnetoencephalography (MEG) system with a 50-mm baseline gradiometer array was used. Targets and nontargets were somatosensory stimuli with an electrical current intensity of twice and three times the sensory threshold, respectively. The source current locations of the evoked magnetic fields in three dimensions and the equivalent current dipole (ECD) moments were calculated by using a single dipole model, assuming the brain as a sphere. In 28 of 63 recording sessions for 7 subjects, the loci of neuronal activities were observed in the left and/or right hippocampal areas. The latency of left hippocampal activity (293 +/- 49 ms) was significantly shorter than that of the right (333 +/- 45 ms) (P = 0.027, non-paired t-test). In view of previous studies that have shown the time window of sensory integration as approximately 200-300 ms, sensory information may be transferred to hippocampal areas following sensory integration.