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Farshad Moradi, Shinsuke Shimojo; Adaptation to invisible gratings in Troxler filling-in. Journal of Vision 2005;5(8):962. doi: 10.1167/5.8.962.
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Under strict fixation, a stationary or slowly changing peripheral stimulus gradually disappears from awareness (Troxler 1804). This phenomenon is often attributed to early sensory adaptation at the level of retinal ganglion cells or LGN. Yet, evidence from binocular rivalry, motion-induced blindness, and induced disappearance indicates a cortical origin for disappearance. Here, we examined whether Troxler fading occurs before processing orientation information in V1. Six participants viewed two low-contrast peripheral (11.5 deg eccentricity) Gabor patterns for 15 s. Gratings drifted slowly (0.1 Hz) to reduce receptor adaptation. Observers monitored the visibility of one of the gratings by holding a key while it disappeared. On the average, the grating was invisible for 2.2 s. The second grating served as a control and was erased from the screen whenever the first was reported as invisible. After adaptation, a test Gabor pattern (with either the same or orthogonal orientation) appeared at one of the two locations, and observers were asked to report its location and alignment. Results: Orientation-selective adaptation was stronger when the adapting stimulus was physically present than when it was erased (48% vs. 64% correct detection for same orientation, p < 0.01). Subjects had little difficulty detecting an orthogonal grating in both conditions (89% vs. 90%, n.s.). Notably, the aftereffect was stronger in trials that observer reported fading during adaptation (42% vs. 56% for the same orientation, 94% vs. 82% for orthogonal). Thus, fading from awareness did not result in reduction of orientation-selective aftereffect compared to the control. We conclude that consistent with other disappearance phenomena, Troxler fading occurs at least in part after the site of orientation-selective processing. Results may be accounted for by attentional modulation of the visibility of peripheral targets.
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