Spatial and temporal influences on the contrast gauge

AbstractA contrast gauge consists of a narrow bar shaded from dark on bottom to light on top [Shapiro, A. G., Charles, J. P., & Shear-Heyman, M. (2005). Visual illusions based on single-field contrast asynchronies. Journal of Vision, 5(10), 764–782]. The perceptual division between dark and light on the bar depends on the luminance level of the surround: when the surround has a high luminance level, the perceptual divider moves up the bar; when the surround has a low luminance level, the perceptual divider moves down the bar. This paper examines the extent to which the perceptual division between light and dark can be used as an indicator to mark the zero contrast level between the bar and the surround. In the experiments, the bar was surrounded by a field whose luminance modulated in time. Three observers marked the maximum and minimum levels of the perceptual divider as a function of modulation amplitude, chromaticity (R, G, B, W), temporal frequency, and width of the surround. Linear changes in the modulation amplitude of the surround produced linear changes in the observers’ settings of the indicator. Observer settings matched zero luminance contrast when the surround was wide (12.5deg), was modulating at less than or equal to 1Hz, and had W or G chromaticity, but not when the surround was narrow, or was modulating faster than 1Hz, or had R or B chromaticity. The effects of surround size suggest that the perceived minimum contrast results from processes that operate over multiple spatial scales. To test this hypothesis, the paper presents a new configuration in which near and far contrast information create different perceptual signatures. Under normal viewing conditions, the motion of the indicator follows the contrast information from the nearest edge, but when high spatial frequency information is removed (through image blur), the motion follows the contrast from the far spatial edge. It is therefore likely that the setting for the indicator for the contrast gauge depends on multiple processes and is not a simple indicator of luminance contrast. The perceptual response to low spatial frequency contrast appears to be given less perceptual weight when high spatial frequencies are present in the image