Academic journal article Perception and Psychophysics

Interocular Suppression Differentially Affects Achromatic and Chromatic Mechanisms

Academic journal article Perception and Psychophysics

Interocular Suppression Differentially Affects Achromatic and Chromatic Mechanisms

Article excerpt

Results from a series of psychophysical experiments show that interocular suppression produced by continuous flash suppression (CFS) differentially affects visual features of a target viewed by the other eye. When CFS stimuli are defined by luminance contrast, target color can be reliably identified but percent-correct discrimination of target orientation is near chance. When the colored target is moving, color identification deteriorates with motion speed but direction of motion discrimination improves with target speed. Color's immunity to suppression is also weakened when interocular suppression is induced by equiluminant CFS stimuli that presumably stimulate the chromatic pathway. These results are discussed in terms of functional segregation of achromatic and chromatic processing in the visual system.

Color has a beguiling, promiscuous quality, in that it tends to show up in places where it does not belong. For example, color can spread beyond its figural boundaries into neighboring regions of the visual field, imparting a vivid sense of color where none actually exists. Compelling examples of color's propensity to spread include neon color spreading (Bressan, Spillmann, Mingolla, & Watanabe, 1989) and the watercolor effect (Pinna, Brelstaff, & Spillmann, 2001). The color of a given object can also be erroneously assigned to other objects in the visual field, causing those objects to appear a color they are not. Examples include illusory binding in visual search (Treisman & Gelade, 1980), illusory binding during binocular rivalry (Creed, 1935; Holmes, Hancock, & Andrews, 2006), and the (mis)assignment of the color of foveally viewed moving objects to other objects moving in the peripheral visual field (Wu, Kanai, & Shimojo, 2004). These and other, related visual phenomena often are taken as evidence for distributed processing of form, motion, and color within the brain, with the neural signals within those distributed brain areas being imperfectly combined so that color can become unbound from the objects whose surfaces are generating those color signals.

We have discovered another set of conditions under which color becomes conspicuously unbound from form and motion, this one associated with a particularly potent form of interocular suppression called continuous flash suppression (CFS). First described by Tsuchiya and Koch (2005), CFS is produced by exposing one eye to a picture of an object, such as a face, while at the same time exposing the other eye to a rapidly, repetitively changing series of images, such as a montage of rectangles resembling a Mondrian pattern. Under these viewing conditions, the ordinarily visible, static picture can disappear from visual awareness for many seconds at a time. This loss of perceptual awareness is accompanied by a pronounced loss of visual sensitivity in the eye viewing the picture (Tsuchiya, Koch, Gilroy, & Blake, 2006). While constructing CFS displays for use in experiments on color-graphemic synesthesia, we were startled to discover that the color of an object, suppressed from awareness during CFS, could nonetheless be experienced as a diffuse, somewhat faint cloud appearing transparently on the grayscale rectangles forming the Mondrian patterns. This impression of color did not seem to be a surface property of the Mondrian itself but, instead, to be a transparent overlay with no defined shape. To reiterate, the shape of the colored object was itself completely suppressed from awareness. We describe this unbound color phenomenon in detail here and discuss its relevance for isolating and measuring properties of neural mechanisms involved in chromatic and achromatic vision.

METHOD

Participants

Five observers participated in the first two experiments and 4 observers participated in the motion-detection experiment. Observers (M.K., S.H., K.M., and D.B.) other than the authors (S.W.H. and R.B.) were naive about the purpose of the experiments. …

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