Bi-hemispheric study of single trial MEG signals of the human auditory cortex

Auditory evoked magnetic fields were recorded from 5 human subjects, simultaneously over each hemisphere, using the BTi twin MAGNES system (2 x 37 channel). Accurate placing of each probe and the use of optimally designed linear sums of signals allow the estimation of the millisecond by millisecond history of activation of the complex of generators responsible for the M100 response in each single trial. Thus it can be shown in much greater detail than any study so far, how the average signal around the M100 period is built up by events in single trials. We found consistent features across subjects, defining a dynamic baseline from which the stimulus related activity can be differentiated. Our analysis indicates that prior to the stimulus onset the activity in each auditory cortex proceeds more or less independently. The onset of the stimulus establishes a binding of activity in the two hemispheres, probably through a common input. The coordinated activity is well described by an exponentially saturating function with a characteristic time constant in the 'magic' 15-30 ms range.