Magnetoencephalographic Approach to the Cognitive Process of Audiovisual Input

Abstract

For the first step toward understanding physiological processes underlying learning in biofeedback, we measured neuromagnetic fields in the cognitive process of sensory inputs using a whole - cortex type magnetoencephalography system. A combination of dichotic bimodal (visual and auditory) inputs was applied to subjects who were instructed to respond to a specified combination of the bimodal input by mentally counting it. The evoked magnetic fields elicited by the target detection were then analyzed by the source modelling method. The equivalent current dipoles detected were located on the magnetic resonance image of subjects' brain. The results showed that (1) at an early stage (about 90-130 ms) their locations were restricted in visual and auditory areas, suggesting that at this stage input information would be processed in a modality-specific manner, (2) in about 130-150 and 220-230 ms they were estimated mainly in subcortical areas including the hippocampus and the basal ganglia, and (3) otherwise, they were diversified into many cortical areas including the parietal, the temporal and the frontal association areas, as well as subcortical areas including the limbic system and the basal ganglia, suggesting activities in the ganglia-thalamocortical loop and the prefrontal-hippocampal system. The neuromagnetic fields also showed P300m- like waveforms in their late component (after about 350 ms) in relation to target discrimination, which would correspond to P3b in electroencephalography. By this time discrimination of the input signal would be made. The results show that the cortical processes in discrimination of the sensory inputs would be made incorporated with the relevant subcortical activities, suggesting a possibility of relationship between cognition and learning processes in biofeedback at the subcortical level.