Differential Latencies Sculpt the Time Course of Contextual Effects on Spatial Perception.

The ability to judge an object's orientation with respect to gravitational vertical relies on an egocentric reference frame that is maintained using not only vestibular cues but also contextual cues provided in the visual scene. Although much is known about how static contextual cues are incorporated into the egocentric reference frame, it is also important to understand how changes in these cues affect perception, since we move about in a world that is itself dynamic. To explore these temporal factors, we used a variant of the rod-and-frame illusion, in which participants indicated the perceived orientation of a briefly flashed rod (5-msec duration) presented before or after the onset of a tilted frame. The frame was found to bias the perceived orientation of rods presented as much as 185 msec before frame onset. To explain this postdictive effect, we propose a differential latency model, where the latency of the orientation judgment is greater than the latency of the contextual cues' initial impact on the egocentric reference frame. In a subsequent test of this model, we decreased the luminance of the rod, which is known to increase visual afferent delays and slow decision processes. This further slowing of the orientation judgment caused the frame-induced bias to affect the perceived orientation of rods presented even further in advance of the frame. These findings indicate that the brain fails to compensate for a mismatch between the timing of orientation judgments and the incorporation of visual cues into the egocentric reference frame.

Differential visual and auditory effects in a crossmodal induced Roelofs illusion.

For vision and audition to accurately inform judgments about an object's location, the brain must reconcile the variable anatomical correspondence of the eyes and ears, and the different frames of reference in which stimuli are initially encoded. To do so, it has been suggested that multisensory cues are eventually represented within a common frame of reference. If this is the case, then they should be similarly susceptible to distortion of this reference frame. Following this reasoning, we asked participants to locate visual and auditory probes in a crossmodal variant of the induced Roelofs effect, a visual illusion in which a large, off-center visual frame biases the observer's perceived straight-ahead. Auditory probes were mislocalized in the same direction and with a similar magnitude as visual probes due to the off-center visual frame. However, an off-center auditory frame did not elicit a significant mislocalization of visual probes, indicating that auditory context does not elicit an induced Roelofs effect. These results suggest that the locations of auditory and visual stimuli are represented within a common frame of reference, but that the brain does not rely on stationary auditory context, as it does visual, to maintain this reference frame. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

The Roelofs and induced Roelofs effects.

The visual image provides important cues for an observer's sense of location and orientation within the world. Occasionally, though, these cues can be misleading, resulting in illusions. In the Roelofs and induced Roelofs effects, for example, a large illuminated frame, offset from the observer's midline in otherwise complete darkness, tends to bias the observer's judgment of straight ahead, causing the position of the frame, and anything contained within it, to be misperceived. Studies of these illusions have provided much insight into the processes that establish an observer's egocentric reference frame, and the manner in which object locations are encoded relative to this frame for perception and action.

The Two-Wrongs model explains perception-action dissociations for illusions driven by distortions of the egocentric reference frame.

Several studies have demonstrated a dissociation of the effects of illusion on perception and action, with perception generally reported to be susceptible to illusions, while actions are seemingly immune. These findings have been interpreted to support Milner and Goodale's Two Visual Systems model, which proposes the existence of separate visual processing streams for perception and action. However, an alternative interpretation suggests that this type of behavioral dissociation will occur for any illusion that is caused by a distortion of the observer's egocentric reference frame, without requiring the existence of separate perception and action systems that are differently affected by the illusion. In this scenario, movements aimed at illusory targets will be accurate if they are guided within the same distorted reference frame used for target encoding, since the error of motor guidance will cancel with the error of encoding (hence, for actions, two wrongs do make a right). We further test this Two-Wrongs model by examining two illusions for which the hypothesis makes very different predictions: the rod-and-frame illusion (which affects perception but not actions) and the simultaneous-tilt illusion (which affects perception and actions equally). We demonstrate that the rod-and-frame illusion is caused by a distortion of the observer's egocentric reference frame suitable for the cancellation of errors predicted by the Two-Wrongs model. In contrast, the simultaneous-tilt illusion is caused by local interactions between stimulus elements within an undistorted reference frame, precluding the cancellation of errors associated with the Two-Wrongs model such that the illusion is reflected in both perception and actions. These results provide evidence for a class of illusions that lead to dissociations of perception and action through distortions of the observer's spatial reference frame, rather than through the actions of functionally separate visual processing streams.

The role of the right superior parietal lobule in processing visual context for the establishment of the egocentric reference frame.

Visual cues contribute to the creation of an observer's egocentric reference frame, within which the locations and orientations of objects can be judged. However, these cues can also be misleading. In the rod-and-frame illusion, for example, a large tilted frame distorts the observer's sense of vertical, causing an enclosed rod to appear tilted in the opposite direction. To determine the brain region responsible for processing these spatial cues, we used TMS to suppress neural activity in the superior parietal lobule of healthy observers. Stimulation of the right hemisphere, but not the left, caused a significant reduction in rod-and-frame susceptibility. In contrast, a tilt illusion caused by a mechanism that does not involve a distortion of the observer's egocentric reference frame was unaffected. These results demonstrate that the right superior parietal lobule is actively involved in processing the contextual cues that contribute to our perception of egocentric space.

Adaptation to leftward-shifting prisms enhances local processing in healthy individuals.

In healthy individuals, adaptation to left-shifting prisms has been shown to simulate the symptoms of hemispatial neglect, including a reduction in global processing that approximates the local bias observed in neglect patients. The current study tested whether leftward prism adaptation can more specifically enhance local processing abilities. In three experiments, the impact of local and global processing was assessed through tasks that measure susceptibility to illusions that are known to be driven by local or global contextual effects. Susceptibility to the rod-and-frame illusion - an illusion disproportionately driven by both local and global effects depending on frame size - was measured before and after adaptation to left- and right-shifting prisms. A significant increase in rod-and-frame susceptibility was found for the left-shifting prism group, suggesting that adaptation caused an increase in local processing effects. The results of a second experiment confirmed that leftward prism adaptation enhances local processing, as assessed with susceptibility to the simultaneous-tilt illusion. A final experiment employed a more specific measure of the global effect typically associated with the rod-and-frame illusion, and found that although the global effect was somewhat diminished after leftward prism adaptation, the trend failed to reach significance (p=.078). Rightward prism adaptation had no significant effects on performance in any of the experiments. Combined, these findings indicate that leftward prism adaptation in healthy individuals can simulate the local processing bias of neglect patients primarily through an increased sensitivity to local visual cues, and confirm that prism adaptation not only modulates lateral shifts of attention, but also prompts shifts from one level of processing to another.

Shifts of visuospatial attention do not cause the spatial distortions of the Roelofs effect.

When a visible frame is offset left or right of an observer's objective midline, subjective midline is pulled toward the frame's center, resulting in an illusion of perceived space known as the Roelofs effect. However, a large frame is not necessary to generate the effect-even a small peripheral stimulus is sufficient, raising the possibility that the effect would be brought about by any stimulus that draws attention away from the midline. To assess the relationship between attention and distortions of perceived space, we adopted a paradigm that included a spatial cue that attracted the participant's attention, and an occasional probe whose location was to be reported. If shifts of attention cause the Roelofs effect, the probe's perceived location should vary with the locus of attention. Exogenous attentional cues caused a Roelofs-like effect, but these cues created an asymmetry in the visual display that may have driven the effect directly. In contrast, there was no mislocation after endogenous cues that contained no asymmetry in the visual display. A final experiment used color-contingent attentional cues to eliminate the confound between cue location and asymmetry in the visual display, and provided a clear demonstration that the Roelofs effect is caused by an asymmetric visual display, independent of any shift of attention.

Activation in a frontoparietal cortical network underlies individual differences in the performance of an embedded figures task.

The Embedded Figures Test (EFT) requires observers to search for a simple geometric shape hidden inside a more complex figure. Surprisingly, performance in the EFT is negatively correlated with susceptibility to illusions of spatial orientation, such as the Roelofs effect. Using fMRI, we previously demonstrated that regions in parietal cortex are involved in the contextual processing associated with the Roelofs task. In the present study, we found that similar parietal regions (superior parietal cortex and precuneus) were more active during the EFT than during a simple matching task. Importantly, these parietal activations overlapped with regions found to be involved during contextual processing in the Roelofs illusion. Additional parietal and frontal areas, in the right hemisphere, showed strong correlations between brain activity and behavioral performance during the search task. We propose that the posterior parietal regions are necessary for processing contextual information across many different, but related visuospatial tasks, with additional parietal and frontal regions serving to coordinate this processing in participants proficient in the task.

Attentional control settings modulate susceptibility to the induced Roelofs effect.

When a visible frame is offset laterally from an observer's objective midline, the subjective midline is pulled toward the frame's center, causing the frame and any enclosed targets to be misperceived as being shifted somewhat in the opposite direction. This illusion, the Roelofs effect, is driven by environmental (bottom-up) visual cues, but whether it can be modulated by top-down (e.g., task-relevant) information is unknown. Here, we used an attentional manipulation (i.e., the color-contingency effect) to test whether attentional filtering can modulate the magnitude of the illusion. When observers were required to report the location of a colored target, presented within an array of differently colored distractors, there was a greater effect of the illusion when the Roelofs-inducing frame was the same color as the target. These results indicate that feature-based attentional processes can modulate the impact of contextual information on an observer's perception of space.

Dissociations of perception and action do not automatically imply independent visual processing streams.

Abstract Since the publication of Milner and Goodale's perception-action model of visual processing, there has been a general tendency to attribute any dissociation in the performance of perceptual and action tasks to a difference in the abilities and limitations of the ventral and dorsal visual streams. However, behavioral dissociations do not necessarily imply different underlying neural systems. In particular, there is a class of illusions, brought about by distortions of the observer's egocentric reference frame, that can cause perception-action dissociations without requiring or implying the existence of separate visual processing streams.

A specific autistic trait that modulates visuospatial illusion susceptibility.

Although several accounts of autism have predicted that the disorder should be associated with a decreased susceptibility to visual illusions, previous experimental results have been mixed. This study examined whether a link between autism and illusion susceptibility can be more convincingly demonstrated by assessing the relationships between susceptibility and the extent to which several individual autistic traits are exhibited as a continuum in a population of college students. A significant relationship was observed between the systemizing trait and susceptibility to a subset of the tested illusions (the rod-and-frame, Roelofs, Ponzo and Poggendorff illusions). These results provide support for the idea that autism involves an imbalance between the processing of local and global cues, more heavily weighted toward local features than in the typically developed individual.

Visuospatial contextual processing in the parietal cortex: an fMRI investigation of the induced Roelofs effect.

Neighboring contextual elements can dramatically affect the manner in which the brain processes the perceptual characteristics of an object. Indeed, many well-known visual illusions rely on misleading contextual cues to create misperceptions of size, length or orientation (e.g., in the Ebbinghaus, Muller-Lyer or rod-and-frame illusions, respectively). However, little is known about the brain regions underlying these integrative computations. The current study used fMRI to delineate the brain areas responsible for processing visuospatial contextual information. Participants were asked to determine whether a small target was positioned left or right of midline in the presence of an offset rectangle designed to induce a shift in the participant's perception of straight-ahead (the induced Roelofs effect). We found localized, bilateral regions in superior parietal cortex and precuneus that were specifically active when participants judged the target location in the presence of this shifted context; significantly less activation was present when a color judgment was made with identical stimuli, or when the location judgment was made without a Roelofs-inducing frame. We propose that this portion of parietal cortex is selectively involved in processing visuospatial contextual information. Additional findings support the notion that perceptual judgments of target location based on an egocentric frame of reference fall within the purview of the dorsal stream of visual processing, rather than the ventral stream.

Transcranial magnetic stimulation over MT/MST fails to impair judgments of implied motion.

The medial temporal and medial superior temporal cortex (MT/MST) is involved in the processing of visual motion, and fMRI experiments indicate that there is greater activation when subjects view static images that imply motion than when they view images that do not imply motion at all. We applied transcranial magnetic stimulation (TMS) to MT/MST in order to assess the functional necessity of this region for the processing of implied motion represented in static images. Area MT/MST was localized by the use of a TMS-induced misperception of visual motion, and its location was verified through the monitored completion of a motion discrimination task. We controlled for possible impairments in general visual processing by having subjects perform an object categorization task with and without TMS. Although MT/MST stimulation impaired performance in a motion discrimination task (and vertex stimulation did not), there was no difference in performance between the two forms of stimulation in the implied motion discrimination task. MT/MST stimulation did, however, improve subjects' performance in the object categorization task. These results indicate that, within 150 msec of stimulus presentation, MT/MST is not directly involved in the visual processing of static images in which motion is implied. The results do, however, confirm previous findings that disruption of MT/MST may improve efficiency in more ventral visual processing streams.

Fragments of the Roelofs effect: a bottom-up effect equal to the sum of its parts.

The Roelofs effect is a distortion of perceived space that occurs when a large frame whose center is offset left or right from the objective midline is presented visually to an observer and causes a bias in the observer's subjective judgment of midline. Experiments were designed to test whether an isolated fragment (left or right end) of a Roelofs-inducing frame was capable of generating the Roelofs effect and to determine whether prior experience with intact frames would provide a top-down influence that would bias the Roelofs effect resulting from fragment presentation. Although the fragments did induce an effect, top-down information did not play a significant role even after a 5-day training paradigm. Instead, we found that the effect generated by an intact frame was equal in magnitude to the sum of the effects generated by the individual fragments. In addition, perception was found to be differentially affected by the two ends of the frame, with fragments falling in the right visual field causing a larger effect than those falling in the left.

Perception, action, and Roelofs effect: a mere illusion of dissociation.

A prominent and influential hypothesis of vision suggests the existence of two separate visual systems within the brain, one creating our perception of the world and another guiding our actions within it. The induced Roelofs effect has been described as providing strong evidence for this perception/action dissociation: When a small visual target is surrounded by a large frame positioned so that the frame's center is offset from the observer's midline, the perceived location of the target is shifted in the direction opposite the frame's offset. In spite of this perceptual mislocalization, however, the observer can accurately guide movements to the target location. Thus, perception is prone to the illusion while actions seem immune. Here we demonstrate that the Roelofs illusion is caused by a frame-induced transient distortion of the observer's apparent midline. We further demonstrate that actions guided to targets within this same distorted egocentric reference frame are fully expected to be accurate, since the errors of target localization will exactly cancel the errors of motor guidance. These findings provide a mechanistic explanation for the various perceptual and motor effects of the induced Roelofs illusion without requiring the existence of separate neural systems for perception and action. Given this, the behavioral dissociation that accompanies the Roelofs effect cannot be considered evidence of a dissociation of perception and action. This indicates a general need to re-evaluate the broad class of evidence purported to support this hypothesized dissociation.

The induced Roelofs effect: two visual systems or the shift of a single reference frame?

Cognitive judgments about an object's location are distorted by the presence of a large frame offset left or right of an observer's midline. Sensorimotor responses, however, seem immune to this induced Roelofs illusion, with observers able to accurately point to the target's location. These findings have traditionally been used as evidence for a dissociation of the visual processing required for cognitive judgments and sensorimotor responses. However, a recent alternative hypothesis suggests that the behavioral dissociation is expected if the visual system uses a single frame of reference whose origin (the apparent midline) is biased toward the offset frame. The two theories make qualitatively distinct predictions in a paradigm in which observers are asked to indicate the direction symmetrically opposite the target's position. The collaborative findings of two laboratories clearly support the biased-midline hypothesis.

Evidence against a central bottleneck during the attentional blink: multiple channels for configural and featural processing.

When a visual target is identified, there is a period of several hundred milliseconds when the processing of subsequent targets is impaired, a phenomenon labeled the attentional blink (AB). The emerging consensus is that the identification of a visual target temporarily occupies a limited attentional resource that is essential for all visual perception. The present results challenge this view. With the same digit discrimination task that impaired subsequent letter discrimination for several hundred milliseconds, we found no disruption of subsequent face discrimination. These results suggest that all stimuli do not compete for access to a single resource for visual perception. We propose a multi-channel account of interference in the AB paradigm.