Magnetoencephalographic study of speed-dependent responses in apparent motion

There have been only few studies of visually-evoked cortical responses to apparent motion as a function of stimulus speed. Most earlier findings on evoked peak magnitudes and latencies, utilizing various types of smooth and apparent motion stimuli, have demonstrated that greater spatial separation/speed resulted in enhanced peak magnitudes, decreasing onset latencies in individual extrastriate neurons and in shorter motor reaction times in subjects. However, some reports using partial-coverage magnetoencephalography stated that increasing the stimulus displacement actually triggered a substantial reduction of the evoked main peak latency while the magnitude showed no clear change.

To resolve the issue of the dependency of evoked responses on stimulus speed in apparent motion, we presented moving bar stimuli to 6 subjects at velocities within a 100 fold range and investigated the ensuing evoked visual cortical activity using a whole-cortex magnetoencephalograph. The magnitude and the latency of the first major evoked peak M1 was measured and compared for 6 discrete bar-stimuli displacements in all subjects.

Our results showed clearly that the M1 peak response magnitudes increased in a nonlinear way with higher apparent speeds (larger displacements), in compliance with the logarithmic Fechner law. We observed also that the fluctuations of the mean evoked M1 peak latency (140+/-10.6 ms) did not reach significance over the tested range of stimulus velocities.

These findings probably reflect global motion processing mechanisms which rely on nonlinear speed-dependent feedback connectivity between striate and extrastriate visual cortex areas.