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Paul M. Corballis, Nathan A. Parks, Mary K. Holder, Arthur G. Shapiro; Identification of luminance and contrast modulation signatures in the steady-state visual evoked potential. Journal of Vision 2005;5(8):488. doi: 10.1167/5.8.488.
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© ARVO (1962-2015); The Authors (2016-present)
Shapiro and colleagues (2004; http://www.journalofvision.org/4/6/5/) recently reported a class of visual effects that demonstrate a perceptual separation between a luminance response (1st order) and a contrast response (2nd order). The basic version of the effect consists of two identical patches whose luminances are modulated sinusoidally in time. One patch is surrounded by a dark annulus, and the other by a light annulus. Thus, the 1st-order luminance information in the two patches modulates in phase while the 2nd-order contrast information modulates in antiphase. At 1 Hz, observers track both the 1st- and 2nd-order information, creating the paradoxical impression that the disks modulate in antiphase but become light and dark at the same time. At 3 Hz, the 2nd-order information dominates, so that the patches appear to modulate in antiphase. In addition, a disk surrounded by mid-gray appears to modulate faster than a disk surrounded by black or white, indicating the presence of a rectified 2nd-order visual response. In the present study, we used steady-state visual evoked potentials (SSVEPs) to explore the electrophysiological correlates of an analogous effect. We presented observers with a circular patch (6 degrees in diameter, centered at the point of fixation) that modulated in luminance at 3.2 Hz. The patch was surrounded by an annulus (12 degrees in diameter) that did not modulate. When the annulus was light or dark, the 1st- and 2nd-order information modulated at 3.2 Hz, but when the patch was mid-luminance (gray), the rectified 2nd-order information modulated at 6.4 Hz. SSVEPs recorded from occipital electrodes revealed a strong signal at 3.2 Hz — corresponding to the 1st-order modulation signal — for all three conditions. The SSVEP power at the second harmonic (6.4 Hz) was higher for the mid-luminance annulus than for the light or dark annulus. These results indicate that visual response to 1st- and 2nd-order information can be identified in the VEP.
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