Orientation-specific aftereffects and illusions in the perception of brightness

Lightness depends on immediately prior experience

The lightness hangover illusion is an unusually robust, long-lasting, prior-experience-based lightness effect. The effect occurs in the Mondrian world, a miniature chamber with interior walls covered with dark gray to black patches. The lightest patch in this scene, physically dark gray, looks white. When real whites and light grays are added to the scene, all the patches darken, but at an unusually slow rate. For several seconds, the white patches look self-luminous and the other patches continue to look very light. The luminosity fades and the other patches darken only after 2 min. We tested three possible explanations for this illusion: retinal adaptation, lightness persistence, and anchor persistence. The results clearly support anchor persistence, which is caused by the presence of steady patches, surfaces that retain their luminance values across scenes. The data also show that the size of the illusion varies directly with the number of these steady patches.

Colour at edges and colour spreading in McCollough effects

Broerse and O'Shea [(1995) Vision Research, 35, 207-226] proposed that the subjective colours in McCollough effects (MEs) consist of two components: edge colours appearing along the edges of contours, and spread colours radiating from edge colours into adjacent uncontoured regions of test patterns. This proposal was examined in five experiments. First, we demonstrated that fine coloured lines located immediately adjacent to the edges of otherwise achromatic square-wave gratings (i.e. colour-fringed gratings) are sufficient to induce MEs comparable in strength to MEs induced with desaturated versions of traditional uniformly-coloured gratings (Experiments 1 & 2). We then quantified edge and spread colours while varying light/dark duty cycles (white-bar width) in gratings with colour-fringed edges (Experiment 3), uniformly-coloured gratings (Experiment 4), and in achromatic gratings tinged with ME colours after adaptation to colour-fringed gratings (Experiment 5). Whereas the perceived magnitude of edge colours remained constant in all cases, spread colours remained constant only for uniformly-coloured gratings. For both MEs and gratings with colour-fringed edges, spread colours decreased as a function of increasing duty cycle, confirming that conventional MEs may be simulated by gratings with colour-fringed edges. We propose that edge colours arise as a consequence of neural operations correcting for the eye's chromatic aberration, while spread colours reveal a neural filling-in process operating to achieve colour constancy. In seeking to implement these suggestions, we present a putative framework based on the receptive-field properties of single cells described in contemporary neurophysiological investigations of colour.

Isoluminance and contingent color aftereffects

In the typical induction of the orientation-contingent color aftereffect (CCAE), the stimuli are composed of elements that differ in both color and luminance. Three experiments are reported that show that chromatic contrast between stimulus elements is insufficient for the induction of the orientation-CCAE and that luminance contrast is necessary. These experiments expand on previous research concerned with the role of luminance contrast in the induction of orientation-CCAEs by eliminating alternative explanations.

Visual aftereffect of texture density contigent on color of frame

An aftereffect of perceived texture density contingent on the color of a surrounding region is reported. In a series of experiments, participants were adapted, with fixation, to stimuli in which the relative density of two achromatic texture regions was perfectly correlated with the color presented in a surrounding region. Following adaptation, the perceived relative density of the two regions was contingent on the color of the surrounding region or of the texture elements themselves. For example, if high density on the left was correlated with a blue surround during adaptation (and high density on the right with a yellow surround), then in order for the left and right textures to appear equal in the assessment phase, denser texture was required on the left in the presence of a blue surround (and denser texture on the right in the context of a yellow surround). Contingent aftereffects were found (1) with black-and-white scatter-dot textures, (2) with luminance-balanced textures, and (3) when the texture elements, rather than the surrounds, were colored during assessment. Effect size was decreased when the elements themselves were colored, but also when spatial subportions of the surround were used for the presentation of color. The effect may be mediated by retinal color spreading (Pöppel, 1986) and appears consistent with a local associative account of contingent aftereffects, such as Barlow's (1990) model of modifiable inhibition.

Orientation-specific luminance aftereffects

Prolonged viewing of bright vertical (horizontal) gratings alternating with dim horizontal (vertical) gratings generates negative brightness aftereffects that are contingent on the orientation of orthogonal test gratings. The effect is measured by a brightness cancellation technique, similar to the color cancellation technique used in measuring McCollough effects. Like the latter, brightness aftereffects appear to persist for long periods. The magnitude of these aftereffects is a positive monotonic function of the luminance difference between the inducing gratings, and it depends on the conditions of induction; monocular induction generates larger aftereffects than binocular induction does. The aftereffect transfers interocularly, although its magnitude in the contralateral eye is substantially attenuated; binocular measurement, following monocular induction, results in even smaller aftereffects. An attempt to understand these findings within the computational model of brightness perception developed by Grossberg and Mingolla (1985a, 1985b) is presented.

The effect of the nature of the surround on the perceived lightness of grey bars within square-wave test gratings

The perceived lightness of grey bars within grey-white and grey-black square-wave test gratings was measured in the presence of different surrounding regions (plain black, plain white, or black-white square-wave gratings). The results for the grey-white test gratings are explained in terms of three separate processes: (i) lightness contrast; (ii) lightness assimilation resulting from the limited ability of the visual system to deal with grating contrast at higher spatial frequencies; and (iii) lightness assimilation resulting from lateral inhibition between pattern detecting channels mediating the perception of the gratings in the test regions and the surrounds. The results for the grey-black test gratings were explained without reference to the third process. It is concluded that techniques involving lightness assimilation could provide investigators with a sensitive new method of investigating the specificities of the inhibitory interactions underlying pattern perception.