Meridian interference reveals neural locus of motion-induced position shifts

Flash grab effect within the regions of modal and amodal completions

When a rotating grating reverses its direction and is accompanied by a briefly flashed stimulus on top, the flash's apparent position shifts in direction after the reversal. This phenomenon, termed the flash-grab effect, can induce an illusory position shift of several degrees of visual angle, prompting investigation into scenarios in which the expected position coincides with another visual event. We investigated two such situations: perceptual filling-in at the blind spot and amodal completion behind a visible occluder. By inducing a position shift in the flash presented just outside such completed patterns, we measured the perceived angular position of the flash in the perceptual matching paradigm. We found subjective localization within the completed region of the moving inducer. Consistent results were found even when the flash was presented at a less optimal time for the flash-grab effect. Illusion size had a certain dependency on stimulus configuration, suggesting that various sources of spatial referencing are involved in the position processing around the blind-spot/occluder region. These findings imply that the visual system does not necessarily avoid a region that is devoid of physical motion stimuli when determining perceived flash position, reaching a consistent perceptual solution that integrates the motion-induced position shift and perceptual completion.

Effects of internal and external velocity on the perceived direction of the double-drift illusion

In the double-drift illusion, the combination of the internal and external motion vectors produces large misperceptions of both position and direction of motion. Here, we investigate the role that speed plays in determining how these two sources of motion are combined to produce the double-drift illusion. To address this question, we measure the size of the illusion at seven internal speeds combined with six external speeds. We find that the illusion increases with increasing internal speed and decreases with increasing external speed. We model this by combining the external and internal vectors to produce the resulting, illusory direction (Tse & Hsieh, 2006). The relative effect of the two vectors is specified by a constant K in this model and the data reveal that K decreases linearly as external speed increases. This critical role of external speed in modulating the vector combination uncovers new details about how the visual system combines different sources of motion information to produce a global motion percept.

A position anchor sinks the double-drift illusion

When the internal texture of a Gabor patch moves orthogonally to its envelope's motion, the perceived path, viewed in the periphery, shifts dramatically in position, and direction relative to the true path (the double-drift illusion). Here, we examine positional uncertainty as a critical factor underlying this illusory shift. We presented participants with an anchoring line at different distances from the drifting Gabor's physical path. Our results indicate that placing an anchor (a fixed line) close to the Gabor's path halved the magnitude of the illusion. This suppression was symmetrical for anchors placed on either side of the Gabor. In a second experiment, we used crowding to degrade the anchoring line's position information by embedding it in a set of parallel lines. In this case, despite the presence of the same lines that reduced the illusion when presented in isolation, the illusory shift was now largely restored. We suggest that the adjacent lines crowded each other, reducing their positional certainty, and thus their ability to anchor the location of the moving Gabor. These findings indicate that the positional uncertainty of the equiluminant Gabor patch is critical for the illusory position offset.

Attentional tracking takes place over perceived rather than veridical positions

Illusions can induce striking differences between perception and retinal input. For instance, a static Gabor with a moving internal texture appears to be shifted in the direction of its internal motion, a shift that increases dramatically when the Gabor itself is also in motion. Here, we ask whether attention operates on the perceptual or physical location of this stimulus. To do so, we generated an attentional tracking task where participants (N = 15) had to keep track of a single target among three Gabors that rotated around a common center in the periphery. During tracking, the illusion was used to make three Gabors appear either shifted away from or toward one another while maintaining the same physical separation. Because tracking performance depends in part on target to distractor spacing, if attention selects targets from perceived positions, performance should be better when the Gabors appear further apart and worse when they appear closer together. We find that tracking performance is superior with greater perceived separation, implying that attentional tracking operates over perceived rather than physical positions.

Pop-out for illusory rather than veridical trajectories with double-drift stimuli

If a patch of texture drifts in one direction while its internal texture drifts in the orthogonal direction, the perceived direction of this double-drift stimulus (also known as the infinite regress and curveball illusions) deviates strongly from its physical direction. Here, we use double-drift stimuli to construct two types of search arrays: The first had an oddball target in terms of the physical trajectories, but no oddball for the perceived trajectory, whereas the second had a perceptual oddball, but no physical oddball. We used these two arrays to determine whether pop-out operates over physical or perceived trajectories. Participants reported the location of the odd double-drift stimulus that had either a unique physical or perceived trajectory in a set of four or eight items. When the distractors all shared one perceived trajectory, but the target had an odd perceived trajectory, it popped out even though the physical trajectories of the stimuli were mixed: Accuracy rates were at ceiling, and response times decreased with increasing set size. In contrast, participants were significantly less accurate and slower at finding the physical oddball when all the paths had a common perceived trajectory. Moreover, responses became less accurate and slower with increasing set size. Our findings suggest that, at least for this type of stimulus, perceptual features can be processed rapidly, whereas the search for physical features is very inefficient.

Neural Correlates of the Conscious Perception of Visual Location Lie Outside Visual Cortex

When perception differs from the physical stimulus, as it does for visual illusions and binocular rivalry, the opportunity arises to localize where perception emerges in the visual processing hierarchy. Representations prior to that stage differ from the eventual conscious percept even though they provide input to it. Here, we investigate where and how a remarkable misperception of position emerges in the brain. This "double-drift" illusion causes a dramatic mismatch between retinal and perceived location, producing a perceived motion path that can differ from its physical path by 45° or more. The deviations in the perceived trajectory can accumulate over at least a second, whereas other motion-induced position shifts accumulate over 80-100 ms before saturating. Using fMRI and multivariate pattern analysis, we find that the illusory path does not share activity patterns with a matched physical path in any early visual areas. In contrast, a whole-brain searchlight analysis reveals a shared representation in anterior regions of the brain. These higher-order areas would have the longer time constants required to accumulate the small moment-to-moment position offsets that presumably originate in early visual cortical areas and then transform these sensory inputs into a final conscious percept. The dissociation between perception and the activity in early sensory cortex suggests that consciously perceived position does not emerge in what is traditionally regarded as the visual system but instead emerges at a higher level.

Vertical and horizontal meridian modulations suggest areas with quadrantic representations as neural locus of the attentional repulsion effect

The attentional repulsion effect (ARE) is a perceptual bias attributed to a covert shift of attention toward a peripheral cue, which, in turn, repulses the perceived position of a subsequently presented probe (Suzuki & Cavanagh, 1997). So far, probes were mainly presented around the vertical meridian. Other studies of perceptual biases reported disruptions when stimuli were presented across the vertical meridian. These disruptions were explained by separate representations of the left and right visual hemifields, projecting to opposite anatomical hemispheres. As the ARE is typically examined through two-alternative, forced-choice tasks in which the estimation of the probe's position is based on the cue's effectiveness to repulse the probe across the vertical meridian, no such asymmetry has been reported. To test for similar meridian disruptions in the ARE, we collected absolute estimations (computer mouse responses) of the perceived probe positions (Experiment 1a). As absolute estimations of memorized positions are associated with overestimated distances in reproduction, results had to be compared to a no-cue baseline condition (Experiment 1b). Through this new methodological approach, we found the ARE to be strongest when the attentional capturing cue and the subsequently presented probe were displayed in the same hemifield (Experiment 2a). In a further experiment (Experiment 2b), we observed that the ARE is not only disrupted at the vertical, but also at the horizontal meridian. These disruptions at both meridians suggest the involvement of visual neural areas with quadrantic representations, such as V2 and/or V3 in the generation of the ARE.