Magnetoencephalographic study of speed-dependent responses in apparent motion

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