Poster : 1 - 4 Basic MEG : Pain and somatosensory function 1 - 4 - 1 : Tracking information flow in the human somatosensory system by means of Partial Directed Coherence

The human visual sensitivity to sinusoidal flickering light is generally band-pass tuned to temporal frequency, probably due to retinal physiological properties consist of low-pass and following high-pass filtering processes. It is also shown that the temporal sensitivity of visual cortex has a band-pass shape. Meanwhile, for the flickering lights with two or more frequency components, flicker at a frequency corresponding to a periodicity of an envelope of luminance oscillation is detected although no flicker-frequency component presents in the stimuli. However, little is known about the detailed characteristics and neurophysiological mechanisms on the detection of flicker at the envelope periodicity. In this study, Exp. 1 investigated detection thresholds for envelope components of AM flickering lights with different-frequency carriers. Exp. 2 using MEG investigated cortical responses to flicker at the envelope periodicity. Exp. 3 investigated the differences between sinusoidal and AM flickering lights in temporal-frequency characteristics of cortical responses. Exp.1 showed that the detection threshold increased steeply for a high-frequency carrier (40 Hz). Exp. 2 showed that the strengths of the MEG responses to flicker at the envelope periodicity increased with increasing modulation depth of AM flickering lights for 10and 20-Hz carriers, whereas the strength for a 40 Hz-carrier was small and did not vary with the modulation depth. These results indicate that flicker at the envelope periodicity is detected for AM flickering light with a low-frequency carrier and the strength of the MEG signal reflects the brightness of detected flicker at the envelope periodicity. Exp. 3 showed that the effects of the high-pass filter were observed in the MEG responses to sinusoidal flickering lights but not to the envelope components of AM flickering lights. This indicates that the envelope component is extracted in the visual pathway after the high-pass filter in the retina.