Phase-locked oscillatory approximately 15- to 30-Hz response to transient visual contrast stimulation: neuromagnetic evidence for cortical origin in humans.

We present neuromagnetic evidence that the human oscillatory (-15-30 Hz; "gamma band") mass response to transient visual (contrast) stimulation originates from cortical areas also generating the conventional pattern-evoked response (VERs). The oscillatory response has shorter latency from stimulus and earlier temporal evolution than the VERs, with different orientation of the source currents. These results suggest the activation of (partly) distinct generating neuronal assemblies with contributions to the development of the VER response. A functional role in stimulus-related cortical synchronization during early visual processing is further suggested and appears consistent with the results of single-unit/multiunit animal research.