Spatiotemporal separability in the human cortical response to visual motion speed: a magnetoencephalography study.

Humans can estimate the speed of an object's motion independently of other visual information. Although speed-related neural activity is known to exist in the primate brain, there has been no physiological study that investigated where and how the speed of motion is represented in the human brain. Nine different combinations of spatial and temporal frequencies were used to make drifting sinusoidal grating of five different speeds (from 1.5 to 24 deg/s). Using the stimuli, we evaluated whether the magnetoencephalographic response property changes were due to a speed-tuned mechanism or to separable spatial and temporal frequency detection mechanisms. The latency change was caused mainly by an inseparable speed-tuned mechanism. In contrast, the amplitude was inversely related to the spatial frequency and was also affected by the temporal frequency differently depending on the frequency. Our results support the view that the human visual system has three sets of mechanisms tuned to spatial frequency, temporal frequency, and speed.