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Daniel Baker, Greta Vilidaitė, Alex Wade; Comparison of four types of suppression using steady-state EEG. Journal of Vision 2015;15(12):1002. doi: 10.1167/15.12.1002.
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© 2017 Association for Research in Vision and Ophthalmology.
It has been proposed that distinct forms of neural suppression occur at different stages of visual processing. At a single neuron level, monocular and dichoptic cross-channel masking have very different properties consistent with pre-cortical and cortical neural substrates respectively (Li, Peterson, Thompson, Duong & Freeman, 2005, J Neurophysiol., 94: 1645-1650). Monocular cross-channel masking produces a lateral shift of the contrast response function (contrast gain) whereas dichoptic masking produces a multiplicative shift (response gain). We investigated how contrast response functions in human visual cortex are affected by four mask types: monocular and dichoptic cross-oriented (overlaid) masks, and aligned and orthogonal surround masks. We measured steady-state EEG responses at the occipital pole to 1c/deg target stimuli (contrasts 6-96%) flickering at 5Hz with or without drifting masks of various contrasts. We interpret the results in the context of a Naka-Rushton model (resp=rCn/(Sn + Cn), where C is contrast) for which the response gain parameter (r), the saturation constant (S), or both parameters could be altered by the mask. All masks reduced target signal-to-noise ratios (SNRs) at high mask contrasts, with monocular overlay and aligned surround masks showing evidence of contrast gain shifts, and dichoptic and orthogonal surround masks consistent with changes in response gain. We also plotted target responses as a function of mask contrast, revealing a novel facilitatory effect of low contrast surround masks that can be explained within the normalization framework. At high mask contrasts there was clear saturation for surround and dichoptic masks above the noise floor, which was not apparent for monocular orthogonal masks. Saturating suppression is consistent with the mask signals having passed through a stage of nonlinear transduction before impacting the target signals. These results bridge single cell studies in animals and psychophysical work in humans in delineating distinct suppressive pathways in the early visual system.
Meeting abstract presented at VSS 2015
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