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Katie L.H. Gray, Greta Vilidaitė, Rebecca E. Kitching, Kirstie H. Wailes-Newson, Daniel H. Baker; Dynamic interocular suppression is uncorrelated with perception in early visual areas. Journal of Vision 2014;14(10):961. doi: https://doi.org/10.1167/14.10.961.
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Steady-state visual evoked potentials (SSVEPs) are contrast-dependent oscillations in the EEG signal induced by flickering visual stimuli. SSVEPs have been used to explore the mechanisms underlying binocular rivalry (Sutoyo & Srinivasan, 2009, Brain Res, 1251: 245-255), but to date they have not been used to compare activity between conscious and nonconscious vision during continuous flash suppression (CFS; Tsuchiya & Koch, Nat Neurosci, 8: 1096-1101). In CFS, high-contrast broadband masks are presented to one eye, causing stimulI in the other eye to be excluded from awareness. We investigated the effect of CFS on the SSVEP response to 1c/deg gratings (contrasts of 4-64%) or face stimuli. Targets were presented at 9Hz (sinusoidal on/off flicker) to one eye, with a Mondrian mask refreshing at 10Hz in the other eye, for trials of 11 seconds. We recorded EEG signals at 64 electrode sites, and Fourier transformed the waveforms to estimate the amplitude of the neural response at each stimulus frequency. The CFS mask reduced SSVEP responses to the targets (both gratings and faces) across the majority of the scalp. However, targets were not typically suppressed from awareness for the entire trial. When categorised by the subjective reports of the participant, no differences in SSVEP amplitude were found between conscious and nonconscious viewing at occipital electrodes. This implies a fixed level of interocular suppression in early visual areas. We also observed no amplitude modulation at more frontal electrodes, but this is likely due to insufficient activity at these sites owing to our necessarily small (~4 deg) target stimuli. We conclude that since CFS produces high levels of non-dynamic suppression in early visual areas, brain regions that correlate strongly with perception must lie further up the cortical hierarchy. Locating these with SSVEPs is challenging given the requirement to use small target stimuli to ensure complete suppression.
Meeting abstract presented at VSS 2014
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