The principle of "common fate" tells us that objects that move together, group together. But what happens when physical paths are shared and illusory paths differ? Here, horizontally aligned targets move up and down over columns containing static, oblique gratings. If all columns share the same orientation, the target group appears to drift in that direction, as per the furrow illusion. However, when the two outer "bookend" targets have a different background orientation from the inner "book" targets, group motion is entirely captured by the bookends. This occurs despite the inner books having discriminable features and recoverable relative motion. What is missing is any sense of their opposing illusory path. We suggest that spatial proximity and common vertical motion help create a single grouped object representation. Since rigid objects tend not to move in two directions at once, the salient bookends dominate our perception of where the group is going.

At sunset, your shadow can be more than 10 times longer than your own height. This long shadow may appear to have a disproportionally small head and long legs, but the disproportion is not in the physical shape of the shadow. The head looks tapered because it is far away and is viewed from a fixed position. But unlike any other long object, our own shadows seem to fascinate us. We discuss the reasons behind this unique phenomenon.

In 1992, Randolph Blake, in collaboration with Robert Cormack and Eric Hiris, reported a strong deviation in perceived direction for a target moving over an oblique, static grating. Here we follow up on this effect, subsequently called the furrow illusion, to determine its origin. We find, unlike Cormack et al., that it is influenced by the luminance of the target and that it does not survive smooth pursuit of a moving fixation that stabilizes the target on the retina. We also introduce an inverted version of the furrow stimulus with the static grating visible only within the moving target rather than only around it. This "peep-hole" furrow stimulus shows a similar deviation in its direction and is quite similar to the well-known double-drift stimulus (Lisi & Cavanagh, 2015). Like the double-drift but unlike the furrow stimulus, its illusory direction persists when tracking a fixation that moves in tandem with the target. The main source for the illusion in both cases appears to be the terminators where the grating's bars meet the target contour. These terminators move laterally along the target's contour as the target moves vertically and the combination of these two directions creates the illusory oblique motion. However, the loss of the illusion for the tracked furrow stimulus suggests either a contribution from negative afterimages within the target or from induced motion in this case.

Two spots moving simultaneously along the same path in opposite directions can appear either to bounce or stream. Bouncing is promoted by spots that turn back and retrace their path. Streaming is promoted by same-size spots, moving along a straight path and viewed peripherally. Both percepts are driven by Gestalt grouping.

A large and a small disk with radii R and r, side by side, exchange positions repetitively at 1.33 Hz. The motion is ambiguous. If r/ R < 0.8 then observers perceive a single large disk jumping back and forth across a small static disk. But if r/ R > 0.8, observers perceive two static disks that expand and contract in counter phase, with no motion across the gap between the disks. Observers respond to the movement of the centroid, which is greatest when r/ R < 0.8.

Moving frames produce large displacements in the perceived location of flashed and continuously moving probes. In a series of experiments, we test the contributions of the probe's displacement and the frame's displacement on the strength of the frame's effect. In the first experiment, we find a dramatic position shift of flashed probes whereas the effect on a continuously moving probe is only one-third as strong. In Experiment 2, we show that the absence of an effect for the static probe is a consequence of its perceptual grouping with the static background. As long as the continuously present probe has some motion, it appears to group to some extent with the frame and show an illusory shift of intermediate magnitude. Finally, we informally explored the illusory shifts seen for a continuously moving probe when the frame itself has a more complex path. In this case, the probe appears to group more strongly with the frame. Overall, the effects of the frame on the probe demonstrate the outcome of a competition between the frame and the static background in determining the frame of reference for the probe's perceived position.

Backward masking is a powerful phenomenon that can reduce, often to zero, the visibility of targets. Here, we show that when the masking is less than completely effective so that the target remains visible, the masking has other effects, specifically reducing the perceived size of the target.

Motion can produce large changes in the apparent locations of briefly flashed tests presented on or near the motion. These motion-induced position shifts may have a variety of sources. They may be due to a frame effect where the moving pattern provides a frame of reference for the locations of events within it. The motion of the background may act through high-level mechanisms that track its explicit contours or the motion may act on position through the signals from low-level motion detectors. Here we isolate the contribution of low-level motion by eliminating explicit contours and trackable features. In this case, motion still supports a robust shift in probe locations with the shift being in the direction of the motion that follows the probe. Although robust, the magnitude of the shift in our first experiment is about 20% of the shift seen in a previous study with explicit frames and, in the second, about 45% of that found with explicit frames. Clearly, low-level motion alone can produce position shifts although the magnitude is much reduced compared to that seen when high-level mechanisms can contribute.

A rotating stimulus of alternating red and white sectors generates a faint pink fill throughout the image. The trailing cyan after images of the red sectors quickly become the brightest regions in the image, providing an index of the overall illumination that triggers a shift of the white point. Actual white areas then shift in the opposite direction and appear pink.

Targets flashed within a frame that moves back and forth are perceptually offset by as much as the frame’s displacement (Özkan et al, 2021; Cavanagh et al, 2022), a far larger offset than that seen on stationary or continuously moving targets (induced motion, Duncker, 1929; Wallach et al, 1978). To understand the difference between continuous and flashed targets we varied the frequency from a single flash per transit of the frame up to continuously present (30 Hz). We find a monotonic decrease in the illusory offset as the flash frequency increases. However, the setting for a stationary target, where there is little or no effect, does not lie on this function, suggesting that the stationary target may group with the steady background instead of the frame. To test this, we presented continuously visible targets that were steady or jiggled vertically while the frame moved horizontally. When the continuous target had any movement, the illusory horizontal movement was visible (induced motion). It did not matter if the frame itself also jiggled vertically, synchronously or asynchronously with the probe. In our original frame effect where the target flashes once per transit, the apparent separation between the two flashes is maximum and there is no sense of motion between the two flashes. When there are multiple flashes or continuous movement, there is illusory movement but the apparent distance travelled is reduced compared to the separation seen when there is just the one flash at each end of the frame’s travel. When the target is stationary, no illusory movement is seen at the frame speeds used here, most likely because it groups with the stationary background.

Ambiguous patterns have a tendency to appear to point up. This bias makes sense as most objects are on the ground, pointing up. However, we discover that the source of the up bias is the preference for seeing depth receding from the lower to the upper visual field.

A white test disk is embedded in a surround that alternates, in either space or time, between red and white. Simultaneous contrast should make the disk look green, but it does not. It looks pink.

Probes flashed within a moving frame are dramatically displaced (Özkan et al, 2021; Wong &Mack, 1981). The effect is much larger than that seen on static or moving probes (induced motion, Duncker, 1929; Wallach et al, 1978). These flashed probes are often perceived with the separation they have in frame coordinates — a 100% effect. Here we explore this frame effect on flashed tests with several versions of the standard stimulus. We find that the frame effect holds for smoothly or abruptly displacing frames, even when the frame changed shape or orientation between the endpoints of its travel. The path could be non-linear, even circular. The effect was driven by perceived not physical motion. When there were competing overlapping frames, the effect was determined by which frame was attended. There were a number of constraints that limited the effect. A static anchor near the flashes suppressed the effect but an extended static texture did not. If the probes were continuous rather than flashed, the effect was abolished. The observational reports of 30 online participants suggest that the frame effect is robust to many variations in its shape and path and leads to a perception of flashed tests in their locations relative to the frame as if the frame were stationary. Our results highlight the role of frame continuity and of the grouping of the flashes with the frame in generating the frame effect.

OPINION article Front. Psychol., 07 February 2022Sec. Perception Science Volume 12 - 2021 | https://doi.org/10.3389/fpsyg.2021.817745

Two versions of the flash grab illusion were used to examine the relative contributions of motion before and motion after the test flash to the illusory position shift. The stimulus in the first two experiments was a square pattern that expanded and contracted with an outline square flashed each time the motion reversed producing a dramatic difference in perceived size between the two reversals. Experiment 1 showed a strong illusion when motion was present before and after the flashed tests or just after the flashes, but no significant effect when only the pre-flash motion was present. In Experiment 2, motion always followed the flash, and the duration of the pre-flash motion was varied. The results showed a significant increase in illusion strength with the duration of pre-flash motion and the effect of the pre-flash motion was almost 50% that of the post-flash motion. Finally, Experiment 3 tested the position shifts when the linear motion of a disk before the flash was orthogonal to its motion after the flash. Here, the results again showed that the pre-flash motion made a significant contribution, about 32% that of the post-flash motion. Several models are considered and even though all fail to some degree, they do offer insights into the nature of the illusion. Finally, we show that the empirical measure of the relative contribution of motion before and after the flash can be used to distinguish the mechanisms underlying different illusions.

Knowing where things are is important. Here we examine the effect of motion context, using a frame that moves back and forth, on the perceived position of probes (Cavanagh, Anstis, & Wexler, VSS 2019). When two probe dots are flashed inside the frame at the same physical location, each at one extreme of the frame’s movement, a very large illusory offset is seen between the probes, roughly equal to the frame’s travel. Here we examine the effects of the distance, duration, and speed of the frame’s travel on the perceived spatial offset. A total of 274 York University undergraduates completed an online task (PsychoJS, hosted on Pavlovia). After screening participants for appropriate devices and self-reported understanding of the task (60), and response outliers (73), 141 remained. Reliable monitor calibration was available for about 40% of participants. The size of the stimuli in degrees of visual angle did not affect illusion strength so we combined all data. The perceived spacing approximately matched the distance the frame moved, both when varying the speed (r²=0.97, p=.001) and the duration (r²=0.92, p=.006) of the frame’s motion. Conclusion: stimuli flashed before and after a frame’s motion are seen in their coordinates relative to the frame as if the frame were stationary.

To capture where things are and what they are doing, the visual system may extract the position and motion of each object relative to its surrounding frame of reference [K. Duncker, Routledge and Kegan Paul, London 161-172 (1929) and G. Johansson, Acta Psychol (Amst.) 7, 25-79 (1950)]. Here we report a particularly powerful example where a paradoxical stabilization is produced by a moving frame. We first take a frame that moves left and right and we flash its right edge before, and its left edge after, the frame's motion. For all frame displacements tested, the two edges are perceived as stabilized, with the left edge on the left and right edge on the right, separated by the frame's width as if the frame were not moving. This stabilization is paradoxical because the motion of the frame itself remains visible, albeit much reduced. A second experiment demonstrated that unlike other motion-induced position shifts (e.g., flash lag, flash grab, flash drag, or Fröhlich), the illusory shift here is independent of speed and is set instead by the distance of the frame's travel. In this experiment, two probes are flashed inside the frame at the same physical location before and after the frame moves. Despite being physically superimposed, the probes are perceived widely separated, again as if they were seen in the frame's coordinates and the frame were stationary. This paradoxical stabilization suggests a link to visual stability across eye movements where the displacement of the entire visual scene may act as a frame to stabilize the perception of relative locations.

We present a novel ‘dazzle’ illusion, in which a black/white negative bar embedded in a grating and viewed in near peripheral vision can look doubled, as if there were two bars lying side by side touching each other.