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James Macdonald, Rufin VanRullen; Perceptual Echoes At 10Hz In Human EEG. Journal of Vision 2010;10(7):924. doi: 10.1167/10.7.924.
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© ARVO (1962-2015); The Authors (2016-present)
Physiology studies have revealed an oscillatory ‘afterdischarge’ or ‘augmenting response’ at 10 Hz that may be attributed to cortico-thalamic reverberation. Such reverberation should be visible in the visual impulse response function (VIRF - the electrophysiological response of the brain to a brief luminance increment); however, VIRF estimates in human EEG via the classic ‘visually evoked potential’ method do not reveal any reverberation. Here, we estimated the VIRF by reverse-correlating the EEG response to a prolonged (6.25 s), dynamic stimulus that was luminance-modulated by a broadband temporal function (0-80 Hz white noise generated anew for each trial). The reverse correlation revealed a pure oscillation at ∼10 Hz that persisted for between 5 and 10 cycles, located over occipito-parietal cortex. The specific frequency and amplitude of this oscillation was closely related to each participant’s resting state alpha frequency and amplitude, although it was not a mere reflection of the presence of alpha, since it disappeared when luminance sequences and EEG epochs were shuffled before the analysis. In a second experiment, we presented two independent random luminance sequences, one on each side of fixation, and participants attended to one or the other during each trial. The oscillation in the VIRF in response to the attended stimulus was 20% greater than for the unattended; this contrasts with the typical decrease of ongoing alpha with attention. Our findings demonstrate that the brain resonates with the random fluctuations in our the noise stimulus at its natural frequency (∼10 Hz), and disregards other frequencies; in accordance with this interpretation, our participants tended to perceive the stimulus as periodic flicker rather than as a broadband white noise signal. Therefore each luminance modulation in our stimulus, i.e., each ‘event’, generated not just one potential, but a periodic series of potentials - ‘echoes’ of the first, akin to cortico-thalamic reverberation.
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