Colored aftereffects produced with moving edges

Colored aftereffects that lasted as long as 6 weeks were produced with moving patterns of parallel black and white stripes or with black and white spirals. During adaptation, the patterns moved periodically in opposite directions, each direction paired with one illuminant, red or green. When the moving patterns were later viewed in white light, S saw the red and green colors, but they were related in the opposite way to the direction of motion. The red and green aftereffects were also produced by other pairs of illuminants, red and white, white and green, reddish-yellow and white, and white and greenish-yellow. The aftereffects did not occur unless, during adaptation, the stripes moved in both directions, each direction paired with a different color. The aftereffect was elicited by stripe motion over the retina—it was seen when the eye swept over a pattern of stationary stripes. The aftereffect desaturated when the retinal orientation of the stripes was changed from the adaptation orientation. Saturation was increased by longer exposure and slower speed during adaptation and by faster speed and a more rapid rate of altemation during the test. The luminance of the adaptation light seemed to have little effect. The aftereffect did not transfer from one eye to the other, and it did not change retinal locus, as was shown when clear images of a colored square that lasted several days were produced with a spiral. S ftxated the spiral’s center. The spiral rotated altemately in opposite directions. A red square with a green surround was projected on the center of the spiral when it rotated in one direction; a green square with a red surround was used when it rotated in the other direction. Following 50 min of adaptation, colored images of the squares were seen when the center of the spiral was ftxated and the direction of     

Dynamic form perception in the bending hourglass: effects of contrast reversal and visual persistence

As a white hourglass moves across a black background, the middle appears to lag behind its true position, resulting in the apparent bending of the axis of the hourglass. No such distortion occurs with a black hourglass moving across a white background. The results of two experiments support a visual persistence hypothesis, as opposed to a latency hypothesis.     

Dynamic form perception in the bending

As a white hourglass moves across a black background, the middle appears to lag behind its true position, resulting in the apparent bending of the axis of the hourglass. No such distortion occurs with a black hourglass moving across a white background. The results of two experiments support a visual persistence hypothesis, as opposed to a latency hypothesis.     

White's effect: removing the junctions but preserving the strength of the illusion

White's effect (also known as the Munker White effect) is a lightness illusion in which, contrary to expectations based on simultaneous contrast and Wallach's rule, a gray rectangle predominantly surrounded by white appears lighter than an identical rectangle that is mainly surrounded by black. The illusion is often explained in terms of T-junctions that are formed by the three-way intersection of the gray rectangle, a black stripe, and a white stripe. I present a circular variant of White's effect in which all the junctions have been removed without significantly affecting the strength of the illusion, suggesting that junctions are not an important consideration in all versions of White's effect.     

A static geometrical illusion contributes largely to the footsteps illusion

When a black and a white rectangle drifts across a stationary striped background with constant velocity, the rectangles appear to alternately speed up and slow down. Anstis (2001, Perception 30 785-794; 2004, Vision Research 44 2171-2178) suggested that this 'footsteps' illusion is due to confusion between contrast and velocity signaling in the motion detectors of the human visual system. To test this explanation, three experiments were carried out. In experiment 1, the magnitudes of the footsteps illusion in dynamic and static conditions was compared. If motion detectors play an important role in causing the illusion, it should be reduced in the static condition. Remarkably, however, we found that the illusory misalignment between the black and the white rectangle was even more prominent in the static condition than in the dynamic condition. In experiment 2, we measured the temporal-frequency properties of the footsteps illusion. The results showed that the footsteps illusion was tuned to low temporal frequencies. This suggests that the static illusory misalignment can contribute sufficiently to the dynamic illusory misalignment. In experiment 3, the magnitude of the illusion was measured with the rectangles drifting on a temporally modulated background instead of a spatially modulated background. If contrast affects the apparent velocity of the rectangles, temporal modulation of a uniform background should also cause the footsteps illusion. However, the results showed that the magnitude of the illusion was much reduced in this condition. Taken together, the results indicate that the footsteps illusion can be regarded as a static geometrical illusion induced by the striped background and that motion detectors play a minor role at best.     

Illusory smoke and dazzling fog

It is well known that a flat ellipse rotating in the frontoparallel plane appears, after brief inspection, as a rigid circular disc tilting back and forth in a 3-D space. We here report that rotation of a grey-shaded ellipse on a white or on a black background produces the compelling illusion of a dark smoke or a dazzling fog (depending on the conditions of the background) moving in front of a completely white or completely black tilting disc. The fog effect disappears when there is a luminance contrast all along the perimeter of the ellipse. An experiment is reported showing that the effect can be experienced in static conditions only to a limited extent and mostly in the `dazzling' version, and that relative movement between the contours of the figure and the shaded area is crucial to the occurrence of the effect, while the occurrence of a depth effect is not. Received: 25 February 1999 / Accepted: 11 April 2000     

Spatial offset of test field elements from surround elements affects the strength of motion aftereffects

Static movement aftereffects (MAEs) were measured after adaptation to vertical square-wave luminance gratings drifting horizontally within a central window in a surrounding stationary vertical grating. The relationship between the stationary test grating and the surround was manipulated by varying the alignment of the stationary stripes in the window and those in the surround, and the type of outline separating the window and the surround [no outline, black outline (invisible on black stripes), and red outline (visible throughout its length)]. Offsetting the stripes in the window significantly increased both the duration and ratings of the strength of MAEs. Manipulating the outline had no significant effect on either measure of MAE strength. In a second experiment, in which the stationary test fields alone were presented, participants judged how segregated the test field appeared from its surround. In contrast to the MAE measures, outline as well as offset contributed to judged segregation. In a third experiment, in which test-stripe offset was systematically manipulated, segregation ratings rose with offset. However, MAE strength was greater at medium than at either small or large (180 degrees phase shift) offsets. The effects of these manipulations on the MAE are interpreted in terms of a spatial mechanism which integrates motion signals along collinear contours of the test field and surround, and so causes a reduction of motion contrast at the edges of the test field.     

Visual dissociations of movement, position, and stereo depth: Some phenomenal phenomena

Helmholtz (1867) described as “irradiation” the apparently greater size of a white compared with a dark square, or disc or whatever of the same physical size. The illusory size difference is reversed at low contrasts (Weale, 1974). It is also known that rapid increases in brightness gives apparent movement (gamma movement), though there is no agreed explanation for either phenomenon.

When narrow bordering stripes are added, further systematic phenomena occur. With intensity modulation of an edge-striped grey rectangle, which has a dark stripe on the left side and a light stripe on the right (which is similar to figures used by Stuart Anstis and Brian Rogers), the entire figure shifts, with reversed motion when the background luminance is modulated. By presenting a pair of such figures, mirror reversed one to each eye and fused stereoscopically, the question may be asked: Do these illusory shifts produce stereo depth? The answer is surprising: stereo is produced-but at the cross-over with luminance of the central grey rectangle with the background the depth change is opposite to that given by normal, non-illusory, opposed lateral shifts. We interpret this anomalous stereo depth as a switch of which edges of the stripes are fused, with the change of relative contrast of the edges of the dark and light stripes as the figure-background contrast is changed.

Measures of static shift, lateral movement, and stereo depth, give somewhat different functions. These are considered in terms of different signalled positions, stereo depth, and movement. This study brings out the importance, for explaining such perceptual anomalies, of distinguishing between neural signal channel characteristics and which stimulus features from the display are selected and accepted for perception. Although conceptually clearly distinct these are all too easily confused in psycho-physical experiments.     

Motion fading and the motion aftereffect share a common process of neural adaptation

After prolonged viewing of a slowly drifting or rotating pattern under strict fixation, the pattern appears to slow down and then momentarily stop. Here, we show that this motion fading occurs not only for slowly moving stimuli, but also for stimuli moving at high speed; after prolonged viewing of high-speed stimuli, the stimuli appear to slow down but not to stop. We report psychophysical evidence that the same neural adaptation process likely gives rise to motion fading and to the motion aftereffect.     

A new dynamic visual illusion: The bending hourglass

Attention, Perception, & Psychophysics - A white hourglass moving laterally across a black surface appears to bend in such a way that the narrow portion lags behind its true position, but only...     

Induction of the McCollough effect I: Figural variables

The effect of line orientation and line configuration on the induction of orientation-specific negatively colored aftereffects was investigated in three separate studies. In the first study, subjects viewed magenta-and-black vertical gratings with one eye, alternating with green-and-black vertical gratings to the other. Monocular tests revealed complementary aftereffects in each eye which disappeared when the test patterns were viewed with both eyes together. In Study 2, imposing a single colored bar against a black background induced negatively colored aftereffects in a white bar against a black background and in a black-and-white grating, while imposing a single black bar against a colored background was ineffective. In Study 3, presenting a magenta square outline elicited green aftereffects in vertical and horizontal bars and gratings as well as in outlines of squares and diamonds, while pairing the magenta square with a green cross had no effect. It was concluded that the induction mechanism responsible for the McCollough effect is sensitive to line orientation but not to shape. This specificity appears incompatible with a simple conditioning model.     

The effect of luminance variation on the apparent position of an edge

A gray outline against a white (or black) ground appears to deviate when one of the divided regions turns into black (white). The direction of shift is not predictable on the basis of luminance profile and polarity contrast of this part of contour, called gray edge (to indicate a stepwise gradient from black to gray and from gray to white). Rather, it appears to depend on the luminance profiles of the collinear regions: A gray edge shifts toward the dark side whenever collinear with a gray line traversing a white ground. The same gray edge takes the opposite direction whenever it extends against a black ground. This rule proved to be successful in predicting the illusory convergence of the sides of a square that formed the stimuli of the first experiment, but the magnitude of the phenomenon was affected by luminance ratios and polarity contrasts of the gray edges, in agreement with the findings of the experiments on gray or blurred edge misalignment. A second experiment tested some hypotheses predicting the combined effects of two or more distorting sources. These hypotheses, suggested by the physical theory of vector sum, were partially disproved. A new model is proposed that assumes different ways of integrating local distortions. The third experiment tested predictions of how distorting pulses in opposite directions combine. The illusory misplacement of edge studied in this experiment is proposed as the underlying phenomena of the café wall illusion, the hollow square illusion, and other illusory phenomena observed with blurred areas. A connection with the induction grid phenomena is hypothesized.     

Colour constancy in goldfish and man: influence of surround size and lightness

Colour constancy was investigated by using a series of 10 simultaneously presented surface colours ranging in small steps from green through gray to red-purple. Goldfish were trained to select one medium test field when the entire setup was illuminated with white light. In the tests, either red or green illumination was used. Colour constancy, as inferred from the choice behaviour, was perfect under green illumination when the test fields were presented on a gray or a white background, but imperfect on a black background. Under red illumination and a white background, however, colour constancy was overcompensated. Here, a colour contrast effect was observed. The influence of background lightness was also found when the surround was restricted to a narrow annulus of 4-11 mm width (test field diameter: 14 mm). By applying colour metrics it could be shown that the von Kries coefficient law can describe the overall effect of colour constancy. For an explanation of the effect of surround size and lightness, lateral inhibitory interactions have to be assumed in addition, which are also responsible for simultaneous colour contrast. Very similar results were obtained in experiments with the same colours in human subjects. They had to name the test field appearing 'neutral' under the different illumination and surround conditions, as tested in the goldfish experiment.     

Recovering depth-order from orientation-defined junctions

This study investigated how visual systems recover depth-order from orientation-defined junctions. Stimuli were superimposed stripes defined by Gabor micro-patterns (Gabors). In one stripe (random stripe), Gabor orientation was randomly selected from a given range, while in the other (constant stripe) it was selected so as to be different from the mean orientation of the random stripe by 90 degrees . Observers reported which of the two stripes, the right- or left-tilted one, they perceived as "nearer" than the other. Observers frequently reported that the random stripe was nearer than the constant stripe. The results appeared to stem from detection of discontinuity of texture edges of the constant stripe due to masking by the random stripe at junctions. This idea was confirmed in the following experiments where discontinuity of the texture edges at junctions was introduced by changing the Gabor luminance contrast in one stripe but keeping it intact in the other. The results indicated that processing of texture edges at junctions can contribute to the perception of depth-order.     

Effects of luminance and contrast on direction of ambiguous apparent motion

A study is reported of the role of luminance and contrast in resolving ambiguous apparent motion (AM). Different results were obtained for the short-range (SR) and the long-range (LR) motion-detecting processes. For short-range jumps (7.5 min arc), the direction of ambiguous AM depended on brightness polarity, with AM only from white to white and from black to black. But for larger jumps, or when an interstimulus interval (ISI) was introduced, AM was less dependent on polarity, with white often jumping to black and black jumping to white. Two potential AMs were pitted against each other, one carried by a light stimulus and the other by a dark stimulus. The stimulus whose luminance differed most from the uniform surround captured the AM. Visual response to luminance was linear, not logarithmic. When the stimulus was modified to give continuous AM in one direction it was followed by a negative aftereffect of motion only when the spatial displacement was 1 min arc. A larger displacement (10 min arc) gave good AM but no motion aftereffect. Thus only short-range motion adapts motion-sensitive channels.     

Motion adaptation from surrounding stimuli.

When a narrow uniform gap was surrounded by a moving grating, the gap appeared as a grating in the opposite phase to that of the surround, moving in the same direction with the same speed. Contrast thresholds for moving test-gratings placed in the region of the uniform gap were found to be elevated after prolonged viewing of this pattern, thus demonstrating the existence of motion adaptation in a retinal region surrounded by, but not covered by, a moving pattern. The amplitude of the moving induced-grating was measured by nulling with a real grating moving in the same direction and with the same speed as the surround. When the speed of the inducing grating was varied, the amplitude of the induced effect did not correlate with the magnitude of the threshold elevation. Therefore, it is unlikely that motion adaptation in the uniform gap was due to induced gratings. In some conditions, the adaptation effect of surrounding gratings was no less than the adaptation effect of gratings covering the test region. This result rules out an explantation involving scattered light, and indicates that motion adaptation occurs at a later stage than that consisting of simple motion mechanisms which confound the contrast and velocity of a moving stimulus.     

Circular vection as a function of the relative sizes, distances, and positions of two competing visual displays

In studies where it is reported that illusory self-rotation (circular vection) is induced more by peripheral displays than by central displays, eccentricity may have been confounded with perceived relative distance and area. Experiments are reported in which the direction and magnitude of vection induced by a central display in the presence of a surround display were measured. The displays varied in relative distance and area and were presented in isolation, with one moving and one stationary display, or with both moving in opposite directions. A more distant display had more influence over vection than a near display. A central display induced vection if seen in isolation or through a 'window' in a stationary surrounding display. Motion of a more distant central display weakened vection induced by a nearer surrounding display moving the other way. When the two displays had the same area their effects almost cancelled. A moving central display nearer than a textured stationary surround produced vection in the same direction as the moving stimulus. This phenomenon is termed 'contrast-motion vecton' because it is probably due to illusory motion of the surround induced by motion of the centre. Unequivocal statements about the dominance of an eccentric display over a central display cannot be made without considering the relative distances and sizes of the displays and the motion contrast between them.     

Wakes and spokes: new motion-induced brightness illusions

Under certain conditions, high-contrast moving figures induce adjacent illusory regions, 'wakes' and 'spokes', which have contrast polarity opposite the inducing figures. In this paper we document properties of these novel phenomena. When the illusions are induced by a moving bar, spokes appear on the side of the bar closer to fixation and connect the bar to the fixation point, regardless of the momentary position of the bar or whether it is moving to the left or to the right. Although spokes often extend up to the fixation point, they never extend beyond it. This is not due to blocking of the spoke's spread by the fixation point, because in another experiment spokes extend directly through an intervening figure. Whereas spokes emanate from the end of a horizontally moving bar closest to fixation, wakes emanate from the end farthest from fixation. In contrast to spokes, wakes do not show a towards-fixation bias. Instead, the wake's end trails the position of the bar, like a ship's wake. The higher the bar velocity, the more the end of the wake appears to trail it, suggesting that wakes are caused by a process which spreads from the edge of moving figures. Wakes and spokes, as distinct illusions, should provide significant constraints on theories of human motion and brightness perception processes.     

Differential effect of luminance contrast reduction and noise on motion induction

Motion perception in a region is affected by motion in the surround regions. When a physically static or flickering stimulus surrounded by moving stimuli appears to move in the direction opposite to that of the surround motion, it is referred to as motion contrast. When the centre appears to move in the same direction, it is referred to as motion assimilation. We investigated how noise and luminance contrast affect motion induction by employing static and dynamic counterphase flickering targets. The tendency of motion assimilation was found to be stronger at a high noise level than at a low noise level for both static and dynamic targets. On the other hand, a decrease of luminance contrast tended to strengthen the tendency of motion contrast. However, the addition of noise and the decrease of luminance contrast decreased the visibility of motion comparably. These results suggest that the visual system changes the mode of motion induction according to the noise level, but not the visibility.