Spatio-Temporal Analysis of Mixed AEF and SEF using Dynamic Singular Value Decomposition

Measurements of somatosensory evoked field (SEF) by median nerve stimulation and auditory evoked field (AEF) by tone burst to right ear were performed. MEG measurements of multiple sources, AEF and SEF overlapped in time, were carried out. In the mixed AEF and SEF, stimulation to the median nerve was delivered 80 and 200 ms later than to the right ear, respectively. Dynamic singular value decomposition (DSVD) method, having a time window and shift time, was applied to each spatio-temporal magnetic data. It showed that the time course of the singular value corresponded to the change of magnetic field waveforms. Furthermore, in order to discuss hemispheric characteristics, the DSVD was also applied to the spatio-temporal magnetic data of bilateral hemisphere (contra and ipsilateral to the stimulation). It showed that the time course of the singular value of all magnetic data well matched to the results of the contralateral hemisphere. Correlation coefficient was calculated between the results of the DSVD in each hemisphere. The correlation coefficient of the contralateral (left) hemisphere showed a higher value than that of the ipsilateral (right, over 0.7). Finally, Time-frequency analysis was applied to the magnetic data of the contralateral hemisphere. We conclude that the DSVD, having time window and shift time, is useful for analyzing spatio-temporal activity of the human cortex.