Prolonged interhemispheric neural conduction time evaluated by auditory-evoked magnetic signal and cognitive deterioration in elderly subjects with unstable gait and dizzy sensation

Magnetoencephalography (MEG) studies have showed that the latency of auditory-evoked neuronal action peak (N100m peak) detected at the temporal cortex ipsilateral to the auditory stimulation is delayed as compared with that detected at the contralateral side. Our recent auditory evoked magnetic fields (AEFs) study has indicated that auditory impulses, originated from the unilateral ear, first arrive at the contralateral temporal cortex and later reach the ipsilateral temporal cortex through interhemispheric neural connections, thus leading to the delay of ipsilateral N100m peak latency. Such a conduction pathway of auditory impulses makes it possible to measure interhemispheric neural conduction time (INCT). We measured INCT in 33 elderly patients (72±10 years of age) complaining of unstable gait and dizzy sensation to study its relationship with cognitive function. Cognitive function was estimated with mini-mental state examination (MMSE) scores. The patients were classified into two groups, such as Group A with normal cognitive function (MMSE score⩾24, n=23) and Group B with cognitive dysfunction (MMSE score⩽23, n=10). INCT was significantly longer in Group B (50.5±14.7 ms) than in Group (15.6±13.9 ms, p<0.05). In the entire patient group, INCT was prolonged negatively correlating with MMSE scores (r=−0.84, p<0.001). The results of the present study suggest that the impairment of cognitive function may be closely related with the prolongation of INCT. The measurement of INCT with AEFs may be useful for early detection of cognitive impairment in elderly patients with dizziness who may later develop dementia.