Academic journal article Perception and Psychophysics

Knowledge Alters Visual Contrast Sensitivity

Academic journal article Perception and Psychophysics

Knowledge Alters Visual Contrast Sensitivity

Article excerpt

Research has shown that the visual system's sensitivity to variations in luminance (visual contrast) within a particular area of the retina is affected in a bottom-up fashion by the ambient contrast levels in nearby regions. Specifically, changes in the ambient contrast in areas surrounding the target area alter the sensitivity to visual contrast within the target area. More recent research has shown that paying attention to the target or target area modulates contrast sensitivity, suggesting a top-down influence over contrast sensitivity that is mediated by attention. Here we report another form of top-down influence over contrast sensitivity that is unlikely to be mediated by attention. In particular, we show that knowledge and/or expectations about the levels of visual contrast that may appear in upcoming targets also affect how sensitive the observer is to the contrast in the target. This sort of knowledge-driven, top-down contrast sensitivity control could be used to preset the visual system's contrast sensitivity to maximize discriminability and to protect contrast-sensitive processes from a contrast overload. Overall, our results suggest that existing models of contrast sensitivity might benefit from the inclusion of top-down control mechanisms.

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Abrupt changes of luminance within a static visual display almost always occur at borders between an object and its background, and the change of pattern of luminance across the surface of an object is often a cue as to the texture of the surface and to the object's identity. Hence, the ability of the visual system to detect local changes in luminance (visual contrast) is important for object perception. The visual system, however, faces a particular challenge in the processing of luminance contrasts: The range of ambient contrast levels is larger than the range of contrasts that a cortical contrast-sensitive neuron is able to represent at one time. A contrast-sensitive unit responds to changes in luminance across its receptive field (the area of the retina that provides input to these units). Low levels of contrast do not activate the unit until its threshold value is reached. As contrast is raised above the threshold value, the unit's rate of firing increases until it saturates, with the range between threshold and saturation (the unit's dynamic range) typically being 1/10 to 1/5 as large as the range of contrasts typically found in visual scenes (Albrecht & Hamilton, 1982; Frazor & Geisler, 2006). Therefore, if the dynamic range of a contrast-sensitive neuron were to be fixed, it would be unable to process most of the contrasts that would fall into its receptive field when the observer was scanning the visual scene.

How then does the visual system operate over the entire range of contrasts present in the environment when it is composed of neurons whose dynamic range is much smaller? At least part of the answer can be found in the remarkable ability of the visual system to adjust the sensitivity of contrast-sensitive neurons to the local ambient contrast level close to the unit's receptive field. For example, if the local ambient contrast level changes from high to low, the visual system increases the sensitivity of the contrast-sensitive neurons. In other words, the unit's contrast sensitivity is adjusted to permit it to function effectively over a large range of ambient contrasts. A number of studies, both behavioral (e.g., Boynton & Foley, 1999; Foley, 1994; Ross & Speed, 1991; Wilson & Humanski, 1993) and neural (e.g., Albrecht & Geisler, 1991; Albrecht & Hamilton, 1982; Gardner et al, 2005; Ohzawa, Sclar, & Freeman, 1982), have found evidence for the existence of a contrast sensitivity control mechanism. They have demonstrated that contrast sensitivity in a confined region of the visual field is modulated by the spatial and temporal frequency composition of nearby stimuli (see, e. …

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