Effects of adaptation on visual sensitivity and MEG responses to the envelope of AM flicker

Abstract

For a sinusoidal flickering light amplitude- modulated (AM) sinusoidally, flicker with a periodicity of a temporal envelope (modulation-frequency) of the light is perceived even though there is no such frequency component. However, the mechanisms underlying the perception are still unclear. In human visual system, it is known that there are two temporal channels with a lower and a higher peak frequency. Therefore, we investigated whether the two temporal channels affected the perception of the envelope of the AM flickering light. Visual sensitivity and magnetoencephalographic (MEG) responses to the envelope periodicity were measured when the sensitivity of each of the temporal channel was reduced by adaptation. Sinusoidal flickering lights with a low (2 Hz) and a high (16 Hz) temporal frequency were used as adapting stimuli. The sinusoidal flickering lights of 2 and 16 Hz and an AM flickering light with a modulation-frequency of 2 Hz and a carrier-frequency of 16 Hz were presented as test stimuli. The results showed that the sensitivity and MEG responses to the sinusoidal flickering light decreased more when the sensitivity of the temporal channel tuned for the test stimulus frequency was reduced than when that of the other temporal channel was reduced. On the other hand, the sensitivity and MEG responses to the envelope periodicity decreased more when the sensitivity to the frequency corresponding to the envelope periodicity was reduced than when that to the carrier frequency was reduced, even though a frequency component corresponding to the envelope periodicity did not exist in the stimulus. The results indicated that the envelope component of AM flickering light was affected by the low-frequency channel and suggest the possibility that the envelope component arises before or while the AM flickering signal passes the low-frequency channel. © Springer-Verlag 2010.