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John Erik Vanston, Alissa Winkler, Michael Webster, Michael Crognale; Blue-yellow biases in early visual cortex assessed by VEP's. Journal of Vision 2016;16(12):1143. doi: https://doi.org/10.1167/16.12.1143.
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© ARVO (1962-2015); The Authors (2016-present)
Sensitivity in many perceptual tasks tends to be weaker along bluish-yellowish axes than along reddish-greenish axes that are chosen to have equivalent modulations along the LM and S cardinal axes, pointing to a "higher-order" representation of color. We examined whether this bias is manifest in the visual evoked potential (VEP), a measure that can potentially disambiguate early vs. late processing in visual cortex. Subjects fixated the center of the screen while passively viewing fields of gratings that were modulated along different axes in a cone opponent space that was scaled to roughly equate sensitivity to LM and S and was adjusted for equiluminance for individual observers. These axes included nominal bluish-yellowish axes (e.g. 135°-315°), and an orthogonal reddish-green axis (45°-225°). Contrast along each axis was varied over a wide range of levels in a 200ms onset, 800ms offset design, with different axes randomly interleaved. VEP's were recorded at site Oz and referenced to site Pz according to the 10-20 electrode placement system. For each chromatic direction, a function was fit to the subjects' peak-to-trough response amplitudes. Criterion thresholds were taken as the contrast at which the response reached two times the noise level for each axis, and were well above the observer's detection thresholds. The iso-response contours were significantly elevated along the blue-yellow axis relative to the reddish-green axis, t(8) = 5.35, p = 0.001. Notably, we also observed a large asymmetry within the blue-yellow axis, such that yellowish-orange directions had higher thresholds than their bluish counterparts. The weaker sensitivity to blue-yellow is consistent with other recent work (e.g. Goddard et al. 2010) pointing to a non-cardinal bias in early cortical color coding, and may reflect a further signature of cortical adaptation to the stronger blue-yellow variations in natural scenes and lighting.
Meeting abstract presented at VSS 2016
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