Spatial and temporal influences on the contrast gauge

Helix rotation: luminance contrast controls the shift from two-dimensional to three-dimensional perception

We present the helix rotation phenomenon, an array of moving dots that creates a conflict between two potential perceptions: a 3D Pulfrich-like horizontal rotation and a low-spatial-frequency up-down motion. We show that observers perceive up-down motion when the dots are equiluminant with the background and when the display is blurred; that the addition of sparse luminance information to equiluminant and blurred displays produces 3D perception; and that the balance between the perception of 3D rotation and up-down motion depends on the magnitude of the luminance contrast. The results are discussed in terms of the luminance capture of equiluminant information.

The Perpetual Diamond: Contrast Reversals Along Thin Edges Create the Appearance of Motion in Objects

The Perpetual Diamond produces motion continuously and unambiguously in one direction despite never physically changing location. The phenomenon consists of a steady, mid-luminance diamond bordered by four thin edge strips and a surrounding background field. The direction of motion is determined by the relative phases of the luminance modulation between the edge strips and the background. Because the motion is generated entirely by changing contrast signals between the edge strips and background, the stimulus is a valuable tool for tests of spatial contrast, temporal contrast, contrast gain, and color contrast. We demonstrate that observers see motion even when the edge strips subtend only seconds of arc on the retina (which is less than the frequently reported 10 minutes of arc) and that perceived motion is due entirely to changes in the difference in contrast phase modulation, independent from the luminance phase.

Perceptual Experience of Visual Motion Activates hMT+ Independently From the Physical Reality: fMRI Insights From the Looming Pinna Figure

The human motion processing area, hMT+, has been labeled the critical neural area for processing of real and illusory visual motion in radial 2D patterns. However, the activation in hMT+ during perception of illusory rotation in the looming double-circular Pinna Figure (PF) generated in 3D space has not been observed yet. To do so, an optic-flow like motion of rings (looming) in PF was generated on a computer screen. A psychophysically precise nulling procedure allowed quantifying the individual amount of the perceived illusory rotation in PF (PI) for each participant. The interpolation of the individual illusory motion parameters created a subjectively non-rotating PF and a physically rotating control stimulus of identical rotary strength as the PI. The physically rotating control was a double-circular figure which diverged from PF only in its arrangement of luminance gradients. In a 3-Tesla scanner, participants were presented with a random order of rotating and non-rotating figures (illusory, real, no rotation, and nulled PI). Both types, illusory and real rotation, when equal in perceptual strength for the observer, were found to be processed by hMT+.

Contrast-contrast asynchronies

We introduce the "contrast-contrast asynchrony," a dynamic stimulus configuration that combines elements of the Shapiro contrast asynchrony with elements of the Chubb contrast-contrast illusion. In the contrast-contrast asynchrony, static textured fields surround two textured fields; one surround has high-contrast texture, and the other has low-contrast texture. The contrasts of the center fields modulate in phase with each other at 1 Hz, and as a consequence, the difference between the contrast of the centers and the contrast of the respective surround modulates in antiphase. Most observers report an antiphase appearance for high-contrast, fine-grained centers. These observers therefore respond to the difference between the center contrast and surround contrast. We also document three observers who do not see the asynchrony for high-contrast modulations of the center, suggesting possibly interesting individual differences.

The separation of monocular and binocular contrast

The contrast asynchrony is a stimulus configuration that illustrates the visual system's separable responses to luminance and luminance contrast information (Shapiro, 2008; Shapiro et al., 2004). When two disks, whose luminances modulate in phase with each other, are each surrounded by a disk, one light and one dark, observers can see both the in-phase brightness signals and the antiphase contrast signals and can separate the two. Here we present the results of experiments in which observers viewed a similar stimulus dichoptically. We report that no asynchrony is perceived when one eye is presented with modulating disks and the other eye is presented with the black and white surround rings, nor is an asynchrony perceived in gradient versions of the contrast asynchrony. We also explore the "window shade illusion" (Shapiro, Charles, & Shear-Heyman, 2005) dichoptically and find that when a modulating disk is presented to one eye and a horizontally split black/white annulus is presented to the other, observers perceive a "shading" motion up and down the disk. This shading can be seen in either direction in the binocular condition, but it is almost always seen as moving towards low contrast in the monocular condition. These findings indicate the presence of separable retinal and cortical networks for contrast processing at different temporal and spatial scales.

Effect of stimulus width on simultaneous contrast

Perceived brightness of a stimulus depends on the background against which the stimulus is set, a phenomenon known as simultaneous contrast. For instance, the same gray stimulus can look light against a black background or dark against a white background. Here we quantified the perceptual strength of simultaneous contrast as a function of stimulus width. Previous studies have reported that wider stimuli result in weaker simultaneous contrast, whereas narrower stimuli result in stronger simultaneous contrast. However, no previous research has quantified this relationship. Our results show a logarithmic relationship between stimulus width and perceived brightness. This relationship is well matched by the normalized output of a Difference-of-Gaussians (DOG) filter applied to stimuli of varied widths.

Relative Brightness in Natural Images Can Be Accounted for by Removing Blurry Content

One critical question regarding visual cognition concerns how the physical properties of the visual world are represented in early vision and then relayed to high-level vision. Here, we posit a simple theory: Processes that encode object appearance reduce their response to spatial content that is coarser than the size of the attended object. We show that a filtering procedure based on this theory can account for the relative brightness levels of test patches placed in images of natural scenes and for many hard-to-explain brightness illusions. The implication is that the perception of brightness differences in most brightness illusions actually corresponds to physical differences present in the images. Portions of the visual system may encode these physical differences by means of neural processes that adaptively reduce their response to low-spatial-frequency content.

Edges can eliminate the appearance of the contrast asynchrony

Recent work in the Shapiro laboratory has suggested that the visual response to changes in chromaticity/luminance can be separated from the visual response to changes in spatial contrast. Here, we examine how spatial edges affect the relative perceptual weighting of these two types of responses. In the experiments, we separate color from color contrast with a 'contrast asynchrony' stimulus in which the luminance of two identical rectangles varies sinusoidally over time. We use two different stimulus configurations: in one configuration, one rectangle is placed on a black background, and the other is placed on a white background; in the other configuration, the two rectangles are placed on a striped background (similar to Munker-White's background), with one rectangle set against a white stripe and the other against a black stripe. Experiment 1 documents that the rectangle placed on the solid white background appears to modulate out of phase with the rectangle placed on the solid black background, and that the two rectangles placed on the striped background appear to modulate in phase with each other. Experiment 2 measured the length the background stripes must be to shift from the perception of in-phase modulation to antiphase modulation (and vice versa). In the solid background configuration, the perceived shift from in-phase to antiphase occurred when edges above and below the rectangles were about 0.5°; and in the striped background configuration, the perceived shift from antiphase to in-phase occurred when the edges were < 10 min of arc. Experiment 3 showed that edges that could engender the perception of the contrast asynchrony in the striped background configuration had no effect on the perceived brightness of the bars. The results indicate that edges placed on opposite sides of the modulating field can inhibit the contrast response but do not necessarily affect the perceived brightness.

Spillmann's weaves are more resilient than Hermann's grid

In a classic Hermann grid display, faint and transient (illusory) spots are produced at the intersections of a white grid superimposed on a black background (or vice versa). In a variant of the Hermann grid developed by Spillmann and Levine (Spillmann, L., & Levine, J. (1971). Contrast enhancement in a Hermann grid with variable figure-ground ratio. Experimental Brain Research, 13, 547-559), the vertical and horizontal bars have different reflectance levels. In previous studies, the illusory spots in the Hermann and Spillmann and Levine grids have been treated analogously. Here, we focus on differences by introducing two types of 'weaves': one type consists of intertwined vertical and horizontal bars with the same luminance levels (hereinafter referred to as 'equiluminant weaves'); the vertical bars in the other type of weave differ in luminance level from the horizontal bars (hereinafter referred to as 'luminance-mismatched weaves'). The Hermann grid is a type of equiluminant weave, and the portion of the Spillmann and Levine grid in which the bars have different reflectance levels is similar to the luminance-mismatched weave. We demonstrate differences between illusory spots produced by luminance-mismatched weaves (and therefore Spillmann and Levine displays) and spots produced by equiluminant weaves (and therefore the Hermann grid): (1) low-pass equiluminant weaves create scintillating patterns, whereas low-pass luminance-mismatched weaves do not; (2) unlike spots for equiluminant weaves, the spots for the luminance-mismatched weaves are not abolished by jagged bars, wavy bars, thick bars, or orientation changes; (3) unlike the spots for equiluminant weaves, the spots for luminance-mismatched weaves occur foveally; and (4) unlike the spots for equiluminant weaves, luminance-mismatched weaves can be created with contrast variation (contrast-contrast, or 2nd-order weaves). We suggest three possible explanations for these results: (1) equiluminant weaves are just a liminal case among luminance-mismatched weaves; (2) the spots arise out of the co-activation of cortical simple cells and color-selective cells, where color-selective cells represent both hue and achromatic sensations; and (3) the spots for both equiluminant and luminance-mismatched weaves are present in high spatial frequency content, but the appearance or disappearance of the spots indicates the interplay between luminance and contrast responses at multiple spatial scales.