When fixating at the center of the left display, the top circles look closer than the bottom circles; however, the top and bottom circles are aligned. (The display on the right is exactly the same except the direction of motion has been reversed.) This demonstrates that the direction of motion affects the perceived position of a moving stimulus.

2007

Position shifts following crowded second order motion adaptation reveal processing of local and global motion without awareness.

Adaptation to first-order (luminance defined) motion produces not only a motion aftereffect but also a position aftereffect, in which a target pattern's perceived location is shifted opposite the direction of adaptation. These aftereffects can occur passively (when the direction of motion adaptation cannot be detected) and remotely (when the target is not at the site of adaptation). Although second-order (contrast defined) motion produces these aftereffects, it is unclear whether they can occur passively or remotely. To address these questions, we conducted two experiments. In the first, we used crowding to remove a local adapter's second-order motion from awareness and still found a significant position aftereffect. In the second experiment, we found that the direction of motion in one region of a crowded array could produce a position aftereffect in an unadapted, spatially separated region of the crowded array. The results suggest that second-order motion influences perceived position over a large spatial range even without awareness.

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2006

Contribution of bottom-up and top-down motion processes to perceived position.

Perceived position depends on many factors, including motion present in a visual scene. Convincing evidence shows that high-level motion perception--which is driven by top-down processes such as attentional tracking or inferred motion--can influence the perceived position of an object. Is high-level motion sufficient to influence perceived position, and is attention to or awareness of motion direction necessary to displace objects' perceived positions? Consistent with previous reports, the first experiment revealed that the perception of motion, even when no physical motion was present, was sufficient to shift perceived position. A second experiment showed that when subjects were unable to identify the direction of a physically present motion stimulus, the apparent locations of other objects were still influenced. Thus, motion influences perceived position by at least two distinct processes. The first involves a passive, preattentive mechanism that does not depend on perceptual awareness; the second, a top-down process that depends on the perceptual awareness of motion direction. Each contributes to perceived position, but independently of the other.

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Second-order motion shifts perceived position.

Many studies have documented that first-order motion influences perceived position. Here, we show that second-order (contrast defined) motion influences the perceived positions of stationary objects as well. We used a Gabor pattern as our second-order stimulus, which consisted of a drifting sinusoidal contrast modulation of a dynamic random-dot background; this second-order carrier was enveloped by a static Gaussian contrast modulation. Two vertically aligned Gabors had carrier motion in opposite directions. Subjects judged the relative positions of the Gabors' static envelopes. The positions of the Gabors appeared shifted in the direction of the carrier motion, but the effect was narrowly tuned to low temporal frequencies across all tested spatial frequencies. In contrast, first-order (luminance defined) motion shifted perceived positions across a wide range of temporal frequencies, and this differential tuning could not be explained by differences in the visibility of the patterns. The results show that second-order motion detection mechanisms contribute to perceived position. Further, the differential spatial and temporal tuning of the illusion supports the idea that there are distinct position assignment mechanisms for first and second-order motion.

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2005

Motion distorts perceived position without awareness of motion.

To view a Quicktime demo of the stimuli, click on the image above. Fixate on the bull's-eye on the left of the screen and notice the array of drifting patches in your periphery. While fixating, if you try to judge the direction of motion in the patch that is three from the left and two from the top of the array, you'll find it is extremely difficult. Continue fixating on the bull's-eye and notice that the two single patches, presented during the test period, look misaligned (and may appear to move). Those two test patches are physically static and physically aligned. The illusory misalignment is caused by motion adaptation that was crowded out of your awareness. Observers are not able to distinguish the direction of motion in the central patches, but the exposure to that motion influences subsequent judgments of object location. This demonstrates that the visual system's passive motion detection mechanisms influence the coding of object location.

A number of striking illusions show that visual motion influences perceived position [1];in all of these, the perceived shift is accompanied or preceded by a visible and salient motion signal. Observers can easily scrutinize the motion: they can attentively track, or at least perceive via inference, the moving features [2, 3 and 4]. With position shifts that accompany the motion aftereffect (MAE) [5, 6, 7, 8, 9 and 10], for example, observers can attentively track the moving adaptation stimulus [11 and 12]. Even if the shifted test pattern does not display any perceived motion [6 and 10], the moving adaptation stimulus is clearly visible, and it could be the visibility of the adaptation stimulus that causes the perceived shift in the test stimulus position. If awareness of motion, mediated by high-level or top-down mechanisms, explains all motion-induced position shifts, then there should be no shift in perceived position without the perception of directional motion. Here, we show that perceived position can be shifted even without awareness of motion.

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2003

Motion adaptation shifts apparent position without the motion aftereffect.

Adaptation to motion can produce effects on both the perceived motion (the motion aftereffect) and the position (McGraw, Whitaker, Skillen, & Chung, 2002; Nishida & Johnston, 1999; Snowden, 1998; Whitaker, McGraw, & Pearson, 1999) of a subsequently viewed test stimulus. The position shift can be interpreted as a consequence of the motion aftereffect. For example, as the motion within a stationary aperture creates the impression that the aperture is shifted in position (De Valois & De Valois, 1991; Hayes, 2000; Ramachandran & Anstis, 1990), the motion aftereffect may generate a shift in perceived position of the test pattern simply because of the illusory motion it generates on the pattern. However, here we show a different aftereffect of motion adaptation that causes a shift in the apparent position of an object even when the object appears stationary and is located several degrees from the adapted region. This position aftereffect of motion reveals a new form of motion adaptation--one that does not result in a motion aftereffect--and suggests that motion and position signals are processed independently but then interact at a higher stage of processing.

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2002

The influence of visual motion on perceived position.

The ability of the visual system to localize objects is one of its most important functions and yet remains one of the least understood, especially when either the object or the surrounding scene is in motion. The specific process that assigns positions under these circumstances is unknown, but two major classes of mechanism have emerged: spatial mechanisms that directly influence the coded locations of objects, and temporal mechanisms that influence the speed of perception. Disentangling these mechanisms is one of the first steps towards understanding how the visual system assigns locations to objects when there are motion signals present in the scene.

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Surrounding motion affects the perceived locations of moving stimuli.

The perceived position of an object is determined not only by the retinal location of the object but also by gaze direction, eye movements, and the motion of the object itself. Recent evidence further suggests that the motion of one object can alter the perceived positions of stationary objects in remote regions of visual space (Whitney & Cavanagh, 2000). This indicates that there is an influence of motion on perceived position, and that this influence can extend over large areas of the visual field. Yet, it remains unclear whether the motion of one object shifts the perceived positions of other moving stimuli. To test this we measured two well-known visual illusions, the Fröhlich effect and representational momentum, in the presence of extraneous surrounding motion. We found that the magnitude of these mislocalizations was altered depending on the direction and speed of the surrounding motion. The results indicate that the positions assigned to stationary and moving objects are affected by motion signals over large areas of space and that both types of stimuli may be assigned positions by a common mechanism.

2000

Motion distorts visual space: shifting the perceived position of remote stationary objects.

Try fixating on the center fixation point (center of the image, though any of the three fixation points will work). Notice that there are two briefly presented flashed lines near the top of the image that straddle the gratings. Most people (>90%, hundreds have observed at conferences) perceive the flashes misaligned: the right flash appears below the left flash. Also notice the two flashed lines toward the bottom of the image. Most people report that the flash on the right is above the flash on the left. This misalignment is always shifted in a direction consistent with the nearest direction of motion. Both pairs of flashes are actually physically aligned. You might also try fixating on the upper or lower fixation points, which are in fact precisely aligned with the two pairs of flashes, respectively.

To perceive the relative positions of objects in the visual field, the visual system must assign locations to each stimulus. This assignment is determined by the object's retinal position, the direction of gaze, eye movements, and the motion of the object itself. Here we show that perceived location is also influenced by motion signals that originate in distant regions of the visual field. When a pair of stationary lines are flashed, straddling but not overlapping a rotating radial grating, the lines appear displaced in a direction consistent with that of the grating's motion, even when the lines are a substantial distance from the grating. The results indicate that motion's influence on position is not restricted to the moving object itself, and that even the positions of stationary objects are coded by mechanisms that receive input from motion-sensitive neurons.

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