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.

Remote controls illusion: strange interactions across space cannot be explained by simple contrast filters.

The visual system has separable visual encoding for luminance and for contrast modulation [J. Vis.8(1), B152 (2008)1534-736210.1167/8.6.1]; the two dimensions can be represented with a luminance contrast versus luminance plane. Here we use a contrast asynchrony paradigm to explore contextual effects on luminance contrast modulation: two identical rectangular bars (0.5°×2.5°) have luminance levels that modulate at 2 Hz; when one bar is placed on a bright field and the other bar on a dark field, observers perceive the bars modulating in antiphase with each other and yet becoming light and dark at the same time. The antiphase perception corresponds to the change in contrast between the bars and their surrounds (a change along the contrast axis of the plane); the in-phase perception corresponds to the luminance modulation (a change along the luminance axis of the plane). We examine spatial interaction by adding bright rectangular (0.5°×2.5°) flankers on both sides of the dark-field bar and dark flankers on both sides of the bright-field bar. Remarkably, flankers produce an in-phase appearance when separated from the bars by between 2' and 4' of visual angle, and produce antiphase appearance when they directly adjoin the bars or are separated by more than 8'. To estimate the dimensions of the spatial interaction, we parametrically adjust the size of the gap between bars and flankers and the length of the flankers. We attempt to account for the results with models based on rectified difference of Gaussian filters and with rectified oriented difference of Gaussian filters. The models can account for the results when the flankers are the same height as bars, but are unable to account for the effects of increasing the flanker length. The models therefore suggest that the spatial interaction across distances requires more complex interactions of contrast filters.

Spatial filtering, color constancy, and the color-changing dress.

The color-changing dress is a 2015 Internet phenomenon in which the colors in a picture of a dress are reported as blue-black by some observers and white-gold by others. The standard explanation is that observers make different inferences about the lighting (is the dress in shadow or bright yellow light?); based on these inferences, observers make a best guess about the reflectance of the dress. The assumption underlying this explanation is that reflectance is the key to color constancy because reflectance alone remains invariant under changes in lighting conditions. Here, we demonstrate an alternative type of invariance across illumination conditions: An object that appears to vary in color under blue, white, or yellow illumination does not change color in the high spatial frequency region. A first approximation to color constancy can therefore be accomplished by a high-pass filter that retains enough low spatial frequency content so as to not to completely desaturate the object. We demonstrate the implications of this idea on the Rubik's cube illusion; on a shirt placed under white, yellow, and blue illuminants; and on spatially filtered images of the dress. We hypothesize that observer perceptions of the dress's color vary because of individual differences in how the visual system extracts high and low spatial frequency color content from the environment, and we demonstrate cross-group differences in average sensitivity to low spatial frequency patterns.

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.

Paradoxical effect of spatially homogenous transparent fields on simultaneous contrast illusions.

In simultaneous brightness contrast (SBC) demonstrations, identical mid-luminance disks appear different from each other when one is placed on a black background while the other is placed on a white background. The strength of SBC effects can be enhanced by placing a semi-transparent layer on top of the display (Meyer's effect). Here, we try to separate the causes of Meyer's effect by placing a spatially homogenous transparent layer over a standard SBC display, and systematically varying the transmission level (alpha=0, clear; alpha=1, opaque) and color (black, gray, white) of the semi-transparent layer. Spatially homogenous transparent layers, which lack spatial cues, cannot be unambiguously interpreted as transparent fields. We measure SBC strength with both matching and ranking procedures. Paradoxically, with black layers, increasing alpha level weakens SBC when measured with a ranking procedure (no Meyer's effect) and strengthens SBC when measured with a matching procedure (Meyer's effect). With white and gray layers, neither procedure produces Meyer's effect. We account for the differences between white and black layers by positing that the visual system separates luminance from contrast. The results suggest that observers attend to different information in the matching and ranking procedures.

The maintenance and disambiguation of object representations depend upon feature contrast within and between objects.

The brain processes many aspects of the visual world separately and in parallel, yet we perceive a unified world populated by objects. In order to create such a "bound" percept, the visual system must construct object-centered representations out of separate features and then maintain the representations across changes in space and time. Here, we examine the role of features themselves in maintaining and disambiguating the representations of the objects to which they belong. In three experiments, we measure how the perceived motion of two objects traversing ambiguous trajectories is affected by the contrast between the features and surrounding fields, by the contrast between features, and by changes to orientation of texture within objects. We report that the maintenance and disambiguation of object representations depend on the contrast of the features relative to their surrounds and on the extent of feature differences between the two objects. These feature dependencies indicate that object representation relies on relative response to many stimulus dimensions.

Using a variant of the optomotor response as a visual defect detection assay in zebrafish.

We describe a visual stimulus that can be used with both larval and adult zebrafish (Danio rerio). This protocol is a modification of a standard visual behavior analysis, the optomotor response (OMR). The OMR is often used to determine the spatial response or to detect directional visuomotor deficiencies. An OMR can be generated using a high contrast grated pattern, typically vertical bars. The spatial sensitivity is measured by detection and response to a change in grating bar width and is reported in cycles per degree (CPD). This test has been used extensively with zebrafish larvae and adults to identify visual- and/or motor-based mutations. Historically, when tested in adults, the grated pattern was presented from a vertical perspective, using a rotating cylinder around a holding tank, allowing the grating to be seen solely from the sides and front of the organism. In contrast, OMRs in zebrafish larvae are elicited using a stimulus projected below the fish. This difference in methodology means that two different experimental set-ups are required: one for adults and one for larvae. Our visual stimulus modifies the stimulation format so that a single OMR stimulus, suitable for use with both adults and larvae, is being presented underneath the fish. Analysis of visuomotor responses using this method does not require costly behavioral tracking software and, using a single behavioral paradigm, allows the observer to rapidly determine visual spatial response in both zebrafish larvae and adults.

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.

Separating color from color contrast.

Visual objects can be described by their color and by their color contrast. For example, a red disk in front of a white background appears "red with high color contrast," whereas a red disk in front of a slightly less-saturated red background will appear "red with low color contrast." This paper examines the visual response to color contrast in a cone-based color space. The stimulus consists of two disks whose chromaticity and/or luminance modulate in time along a line in a DKL color space; the chromaticity and luminance levels of the two disks are always identical. One disk is surrounded by a static ring whose color is at one end of the color line, and the other disk is surrounded by a static ring whose color is at the opposite end of the color line. The disks appear to modulate in antiphase (following the contrast information), yet they can also appear to be approximately the same color (following the chromatic/luminance information). The observers' task was to adjust the color angle of modulating disks until the antiphase appearance was eliminated-creating a contrast null. Observers set contrast nulls at a color angle approximately 90 deg away from the line connecting the colors of the surround rings; this result occurred in both chromoluminant and equiluminant color planes, although two observers showed a flattening near equiluminance in the chromoluminance planes. To account for the data, I present a model that contains one pathway for color and another pathway for color contrast. I show that (1) the model correctly predicts orthogonal directions in color space for the contrast nulling task; (2) the response of the contrast pathway appears to be faster than the response of the color pathway; (3) the response of the contrast pathway may mediate detection thresholds under some conditions (a finding that can account for some of the effects of surround luminance on temporal sensitivity); (4) the asynchronous modulation can be seen even when the stimulus is blurred; and (5) the asynchrony does not require a disk-ring configuration.

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.

Visual illusions based on single-field contrast asynchronies.

A single-field contrast asynchrony refers to a stimulus configuration in which there is a single temporally modulated field and multiple sources of contrast information; the sources of contrast information modulate at different temporal phases or at different temporal frequencies. In this paper we show how single-field contrast asynchronies can lead to a wide variety of visual illusions. We investigate, in depth, the window shade/rocking disk configuration, in which a temporally modulated disk is surrounded by a split annulus (i.e., the top half is dark, and the bottom half is light). When the annulus is thick, the disk appears spatially inhomogeneous (shading); when the annulus is thin, the disk appears to rock back and forth (shifting). We measure the proportion of trials that a disk appears to shade or, on separate trials, appears to shift as a function of modulation amplitude, surround thickness, temporal frequency, and disk size. We account for the shading effects by postulating a combination of separate first- and second-order responses and/or a multi-scale spatial filtering process. We account for the shifting effects by examining four elemental motion conditions. For luminance modulation, the direction of the shift follows the same pattern as that produced by the rectified output of an array of spatial center-surround filters applied to the X, t plot. For equiluminant modulation, the direction of the shifts is consistent with a sequence-tracking (or third-order) motion response.

The Star Wars Scroll Illusion.

The Star Wars Scroll Illusion is a dynamic version of the Leaning Tower Illusion. When two copies of a Star-Wars-like scrolling text are placed side by side (with separate vanishing points), the two scrolls appear to head in different directions even though they are physically parallel in the picture plane. Variations of the illusion are shown with one vanishing point, as well as from an inverted perspective where the scrolls appear to originate in the distance. The demos highlight the conflict between the physical lines in the picture plane and perspective interpretation: With two perspective points, the scrolling texts are parallel to each other in the picture plane but not in perspective interpretation; with one perspective point, the texts are not parallel to each other in the picture plane but are parallel to each other in perspective interpretation. The size of the effect is linearly related to the angle of rotation of the scrolls into the third dimension; the Scroll Illusion is stronger than the Leaning Tower Illusion for rotation angles between 35° and 90°. There is no effect of motion per se on the strength of the illusion.

Cone-specific mediation of rod sensitivity in trichromatic observers.

PURPOSE: The slope of the rod threshold versus the illuminance (TVI) function changes with the wavelength of the background light. This study was conducted to determine whether the changes in slope are due to the stimulation of specific cone classes. METHODS: An eight-channel optical system was used to generate lights that differed in cone and rod photoreceptor illuminance. Rod flicker TVI functions were measured in normal trichromatic observers at mesopic light levels. The independent variables were (1) the relative contribution of the short (S)- and long (L)- wavelength cones to the background light (i.e., the background lights varied along S-only and L-only lines), and (2) the temporal frequency of the flickering lights (4, 7.5, and 15 Hz). RESULTS: The 4-Hz rod flicker TVI function had a slope of 0.87 when measured near W (MacLeod-Boynton chromaticity of 0.66, 1.0). At 4 and 7.5 Hz, an increase in the relative L-cone illuminance steepened the slope of the rod-only TVI curve, but an increase in the relative S-cone illuminance had no effect. The slope of the 7.5-Hz TVI function decreased at higher illuminance levels. At 15 Hz, the thresholds could be measured over only a limited range. CONCLUSIONS: The L-cone system contributes to the desensitization of the rod system at mesopic light levels, whereas, in the range of lights used in these experiments, the S-cone system apparently does not. The possibility that S-cone stimulation desensitizes the response to rod signals at higher levels of S-cone illumination cannot be eliminated.

Time-course of S-cone system adaptation to simple and complex fields.

We examine the temporal nature of adaptation at different stages of the S-cone color system. All lights were restricted to the S-cone-only (a constant L and M) cardinal axis in color space passing through mid-white (W). The observer initially adapted to a steady uniform field with a chromaticity on the -S end of the axis or on the +S end of the axis or a complex field composed of chromaticy -S and +S (+/-S adaptation). The observer then readapted to a steady uniform field of chromaticity W for a variable length of time (i.e., 0, 0.1, 0.25, 0.5, 1.0, or 2.0 s). A probe-flash technique was used to measure S-cone discrimination at various points along the S-cone-only cardinal axis. This allowed estimation of the response of the S-cone system over an extended response range. Following exposure to the -S and +S uniform fields, sensitivity was maximal at or near the chromaticity of the initial adaptation field and decreased linearly away from the adapting point. The shift from +S to W occurred more rapidly than the shift from -S to W; both of these shifts can be described by a multiplicative scaling of the S-cone signal. Following +/-S adaptation the threshold curve initially had a shape similar to that measured following -S adaptation, but returned rapidly to the W adaptation state. The shift following +/-S adaptation cannot be described by the multiplicative model, but can be explained by a change in the shape of the non-linearity. The results suggest the existence of fast post-receptoral processes.

Induced contrast asynchronies.

We document a new type of perceptual effect in which asynchronous contrast signals are presented simultaneously with synchronous luminance signals. The template for the basic effect consists of two physically identical disks (.75-deg diameter, 40 cd/m2), one surrounded by a dark annulus (1.5 deg, 20 cd/m2) and the other by a light annulus (1.5 deg, 60 cd/m2). The center disks are modulated in time, with a maximum luminance of 55 cd/m2 and a minimum luminance of 25 cd/m2. With this stimulus configuration, the luminance signals of the disks modulate in phase with each other while the contrast signals relative to the surrounds modulate in anti-phase. Observers can track the contrast and luminance signals when the luminance is modulated at 1 Hz but perceive primarily the contrast signal at 2-6 Hz. We show that the asynchrony can be perceived with a thin annular surround, that the appearance of the asynchrony is dependent on the modulation amplitude, and that a decrease in the relative strength of the asynchrony at 1 Hz corresponds to the band-pass shape of the temporal contrast sensitivity function in the presence of light and dark edges. We also introduce variations of the induced contrast asynchrony principle in which a single modulated disk is surrounded by a half-light and half-dark split annulus; we refer to these configurations as the window-shade and rocking-disk illusions.

Feature- and Face-Exchange illusions: new insights and applications for the study of the binding problem.

THE BINDING PROBLEM IS A LONGSTANDING ISSUE IN VISION SCIENCE: i.e., how are humans able to maintain a relatively stable representation of objects and features even though the visual system processes many aspects of the world separately and in parallel? We previously investigated this issue with a variant of the bounce-pass paradigm, which consists of two rectangular bars moving in opposite directions; if the bars are identical and never overlap, the motion could equally be interpreted as bouncing or passing. Although bars of different colors should be seen as passing each other (since the colors provide more information about the bars' paths), we found "Feature Exchange": observers reported the paradoxical perception that the bars appear to bounce off of each other and exchange colors. Here we extend our previous findings with three demonstrations. "Peripheral Feature-Exchange" consists of two colored bars that physically bounce (they continually meet in the middle of the monitor and return to the sides). When viewed in the periphery, the bars appear to stream past each other even though this percept relies on the exchange of features and contradicts the information provided by the color of the bars. In "Face-Exchange" two different faces physically pass each other. When fixating centrally, observers typically report the perception of bouncing faces that swap features, indicating that the Feature Exchange effect can occur even with complex objects. In "Face-Go-Round," one face repeatedly moves from left to right on the top of the monitor, and the other from right to left at the bottom of the monitor. Observers typically perceive the faces moving in a circle-a percept that contradicts information provided by the identity of the faces. We suggest that Feature Exchange and the paradigms used to elicit it can be useful for the investigation of the binding problem as well as other contemporary issues of interest to vision science.

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.

A first- and second-order motion energy analysis of peripheral motion illusions leads to further evidence of "feature blur" in peripheral vision.

BACKGROUND: Anatomical and physiological differences between the central and peripheral visual systems are well documented. Recent findings have suggested that vision in the periphery is not just a scaled version of foveal vision, but rather is relatively poor at representing spatial and temporal phase and other visual features. Shapiro, Lu, Huang, Knight, and Ennis (2010) have recently examined a motion stimulus (the "curveball illusion") in which the shift from foveal to peripheral viewing results in a dramatic spatial/temporal discontinuity. Here, we apply a similar analysis to a range of other spatial/temporal configurations that create perceptual conflict between foveal and peripheral vision. METHODOLOGY/PRINCIPAL FINDINGS: To elucidate how the differences between foveal and peripheral vision affect super-threshold vision, we created a series of complex visual displays that contain opposing sources of motion information. The displays (referred to as the peripheral escalator illusion, peripheral acceleration and deceleration illusions, rotating reversals illusion, and disappearing squares illusion) create dramatically different perceptions when viewed foveally versus peripherally. We compute the first-order and second-order directional motion energy available in the displays using a three-dimensional Fourier analysis in the (x, y, t) space. The peripheral escalator, acceleration and deceleration illusions and rotating reversals illusion all show a similar trend: in the fovea, the first-order motion energy and second-order motion energy can be perceptually separated from each other; in the periphery, the perception seems to correspond to a combination of the multiple sources of motion information. The disappearing squares illusion shows that the ability to assemble the features of Kanisza squares becomes slower in the periphery. CONCLUSIONS/SIGNIFICANCE: The results lead us to hypothesize "feature blur" in the periphery (i.e., the peripheral visual system combines features that the foveal visual system can separate). Feature blur is of general importance because humans are frequently bringing the information in the periphery to the fovea and vice versa.

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.

Last but not least.

A central tenet of Gestalt psychology is that the visual scene can be separated into figure and ground. The two illusions we present demonstrate that Gestalt processes can group spatial contrast information that cuts across the figure/ground separation. This finding suggests that visual processes that organise the visual scene do not necessarily require structural segmentation as their primary input.