11/7/2023 0 Comments Occipital contour definition![]() The duration of this type of contour persistence is typically 1–3 s, and contour visibility ends instantaneously when contour elements are physically removed (Ferber, Humphrey, & Vilis, 2003, 2005 Large, Aldcroft, & Vilis, 2005 Strother et al., 2011 Strother, Lavell, & Vilis, 2012), consistent with offset-initiated reset and the prevention of contour “hallucination” in the absence of supporting visual input (Li, 1998). In the demonstration, a contour-defined bird becomes visible by virtue of structure from motion, and critically, it persists perceptually even after it stops moving (left demo), but only when the contour segments remain embedded in the camouflaging background (right demo). Perceptual effects consistent with these predictions are evident in a demonstration ( ) adapted from Regan ( 1986). Positive-feedback models of contour processing in visual cortex predict that persistent contour binding should occur in the absence of an inhibitory reset signal, such as stimulus offset (Francis, 1999). If so, this would imply that persistent contour binding plays a foundational role in stable object perception, a prediction we test here with respect to smoothness sensitivity and the association-field mechanism of contour integration. In the context of contour perception, some have proposed that excitatory feedback circuits bind contour features and exhibit persistence (Francis, 1996 Francis, Grossberg, & Mingolla, 1994). The existence of this type of persistent neural activity in visual cortex may be indicative of positive feedback loops within visual cortex (Grossberg, 2015). For example, O'Herron and von der Heydt ( 2009, 2011) reported persistent figure–ground border-ownership signals in single neurons in visual cortex, which they interpreted as the basis for stable figure–ground segmentation under fluctuating stimulus conditions. The neural basis of this ability is not fully understood, but it may be partly due to intrinsic memory properties of early visual cortical mechanisms. ![]() Stable perception relies on the capacity of neural circuits to temporarily maintain perceptual representations under constantly changing sensory input. Here we are interested in the possibility that an association-field or similar neural mechanism supports the persistence of contour integration, and thus promotes perceptual continuity in both space and time. The existence of an association-field and other qualitatively similar contour-integration mechanisms is consistent with patterns of long-range horizontal connections in visual cortex (Sigman, Cecchi, Gilbert, & Magnasco, 2001), as well as an abundance of corroborative psychophysical data. For example, the association-field model of contour integration binds discrete edge information into perceptually continuous contours through mutual facilitation between visual cortical neurons with similarly oriented classical receptive fields (Field, Hayes, & Hess, 1993). Visual sensitivity to the physical properties of contours is consistent with the statistical properties of contours in natural images (Geisler & Perry, 2009 Geisler, Perry, Super, & Gallogly, 2001), and this is often reflected in neurally plausible models of contour integration. We conclude that the strong modulatory effects of contour smoothness on persistence are due to the sustained reverberation of local and global contour-binding mechanisms in visual cortex, which form an important basis of perceptual continuity and stable object perception.Ĭontours are powerful indicators of object shape and surface boundaries. We distinguish the effect of contour smoothness on contour persistence from observer bias, which also contributes to the surprisingly long duration of contour persistence. Here we show that the duration of contour persistence reflects the persistent operation of visual mechanisms sensitive to contour smoothness, which also influences contour visibility more generally under highly camouflaging stimulus conditions. Some neurophysiological models of feature binding in early visual cortex predict persistent contour perception under certain stimulus conditions. ![]() The mechanisms by which such representations are formed and maintained are not fully understood but presumably involve interplay between early and higher tier visual cortical mechanisms. Stable object perception relies on persistent yet temporary neural representations under constantly fluctuating stimulus conditions.
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