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J. Jason McAnany, Michael W. Levine; Magnocellular- and parvocellular-pathway processing in a novel visual illusion. Journal of Vision 2005;5(8):56. doi: 10.1167/5.8.56.
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© ARVO (1962-2015); The Authors (2016-present)
If viewed peripherally, a white disk presented in an intersection of gray alleys in a grid of black squares is not detected. Previous work showed that both retinal and cortical mechanisms contribute to this “blanking phenomenon” (McAnany & Levine, 2005). Here, we examine magnocellular (MC) and parvocellular (PC) pathway contributions to this novel form of visual disappearance. In these experiments, grids of black squares were continuously presented 15° above and below fixation on a 46 cd/m2 gray background (background and alley luminance was always equal); a white disk appeared in a randomly chosen intersection. Subjects were asked to identify which intersection (left, middle, right) contained the disk.
In one condition, the disk was presented as a 36 ms pulse, a presentation duration that favors the MC-pathway. Subjects identified the correct intersection with near-perfect accuracy in this condition. In a second condition, the disk was ramped on, held for 36 ms, and then ramped off (minimizing temporal transients, thus biasing processing toward the PC-pathway). Subjects' responses to the ramped stimulus were essentially random. A third condition was intended to saturate the MC-pathway by introducing a large luminance transient concurrent with the disk presentation (Leonova, Pokorny, Smith, 2003). To induce the luminance transient, the background and alley gray luminance before and after the pulsed disk presentation was 0.3 log above or below the 46 cd/m2 gray. Contrast threshold under this condition was significantly higher than when the disk was pulsed without a luminance change.
There are two possible explanations for these results. First, the MC-pathway may be solely responsible for detecting the disk; the disk cannot be detected when the MC-pathway is saturated or ineffective. Alternatively, either pathway can detect the disk, but the PC-pathway includes processing that causes blanking in the presence of the grid.
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