More thoughts on perceiving and grasping the Müller–Lyer illusion

It has been suggested that the insensitivity of the visuomotor system to various visual illusions is based on mechanisms in the so-called ‘dorsal stream’ of primate extrastriate cortex, which may depend upon binocular cues for their functions. The present study investigated the effects of binocular and monocular viewing on perception of and action to Müller–Lyer figures. Fourteen participants were required to match and grasp the shaft of a Müller–Lyer display under both viewing conditions. In the matching condition, participants were required to show the perceived extent of the central shaft of one of the two Müller–Lyer figures, using the extent of the gap between their finger and thumb. In the grasping task, participants were required to quickly and accurately reach out and grasp the central shaft of the specified Müller–Lyer figure. First, there was a striking effect of the illusion on the matching performance under both viewing conditions. However, the maximum grip aperture remained unaffected by the illusion figures. These results add to the theory of distinct modes of visual processing for perception and action. However, we did not find that grasping performance was affected by the illusion under monocular conditions. It is plausible that monocular depth cues distinct from those responsible for the illusion can successfully drive accurate grasping. Additional concerns regarding claims of action system resistance to the perceptual distortions of various illusions are discussed.     

More thoughts on perceiving and grasping the Müller-Lyer illusion

It has been suggested that the insensitivity of the visuomotor system to various visual illusions is based on mechanisms in the so-called 'dorsal stream' of primate extrastriate cortex, which may depend upon binocular cues for their functions. The present study investigated the effects of binocular and monocular viewing on perception of and action to Müller-Lyer figures. Fourteen participants were required to match and grasp the shaft of a Müller-Lyer display under both viewing conditions. In the matching condition, participants were required to show the perceived extent of the central shaft of one of the two Müller-Lyer figures, using the extent of the gap between their finger and thumb. In the grasping task, participants were required to quickly and accurately reach out and grasp the central shaft of the specified Müller-Lyer figure. First, there was a striking effect of the illusion on the matching performance under both viewing conditions. However, the maximum grip aperture remained unaffected by the illusion figures. These results add to the theory of distinct modes of visual processing for perception and action. However, we did not find that grasping performance was affected by the illusion under monocular conditions. It is plausible that monocular depth cues distinct from those responsible for the illusion can successfully drive accurate grasping. Additional concerns regarding claims of action system resistance to the perceptual distortions of various illusions are discussed.     

Perceiving and acting in depth in Williams syndrome and typical development

Individuals with the neurodevelopmental disorder Williams syndrome (WS) often report difficulty processing and acting in depth, such as crossing roads or reaching for objects; however little research attention has been directed at understanding depth perception and action in depth in WS and whether deficits in depth perception have an ocular or perceptual root in this group. This study assessed the extent and relationship of deficits in stereopsis (binocular, three dimensional vision) and actions performed in depth in WS, as well as in typically developing participants (TD) matched for non-verbal ability. Stereoacuity was age-appropriate in the TD group but at the level of a TD three year old in WS; one third of the WS group did not show evidence of stereopsis. When monocularly acting in depth there was no difference between the WS and TD groups. When binocularly acting in depth the WS group that did not exhibit stereopsis were significantly poorer than the TD group and the WS group that exhibited stereopsis. When assessing the relationship between stereoacuity and action in depth, stereoacuity negatively correlated with binocular action in depth for the WS group with stereopsis, but not the TD group. Therefore, no deficits in monocular depth perception in WS were evidenced, yet significant deficits are exhibited in binocular depth perception and action. Importantly action in depth under binocular viewing may be a useful gross screening measure for stereodeficits in WS. Remediation of depth perception deficits in WS could train further understanding of monocular cues to compensate for poor stereopsis.     

Motion parallax enables depth processing for action in a visual form agnosic when binocular vision is unavailable

Visual-form agnosic patient DF, who has severe difficulties in using visual information about size, shape and orientation for perceptual report, can nevertheless—under normal viewing conditions—use the same information to accurately guide her hand movements. However, her performance of prehension tasks requiring the analysis of visual depth is severely disrupted when binocular vision is prevented. We have suggested that this deterioration in visuomotor control is due to an inability to use pictorial depth cues to compensate for the removal of binocular vision. In the current study we investigated whether DF was able to use motion parallax as an alternative to binocular cues. We asked her to grasp a square plaque slanted at different orientations in depth, under two monocular testing conditions. In one condition her head remained stationary on a chin rest, and in the other condition she made large lateral head movements just prior to each prehension movement. The results confirmed that DF is impaired in adjusting her hand orientation to the orientation of the target object when reaching monocularly with her head stationary. In contrast, when she made head movements, her manual performance was restored to almost normal levels. Our results are consistent with the idea that the processing of pictorial depth cues depends on the cortical ventral stream, which is known to be disrupted by DF’s lesion. They further indicate that orientation in depth can be computed from motion parallax just as well as from binocular cues in the absence of a normally functioning ventral stream.     

Depth perception in cats after cerebral hemispherectomy: comparisons between neonatal- and adult-lesioned animals

Depth perception was studied in adult cats following removal of the left cerebral hemisphere as a neonate or as an adult. Both monocular and binocular thresholds were determined using a visual cliff. Although both age-at-lesion groups showed depth perception deficits, the neonatal-lesioned animals performed much worse under binocular conditions on the visual cliff than either adult-lesioned or intact animals. This was primarily due to the lack of a binocular advantage in the neonatal-lesioned cats since their monocular thresholds were similar to that of adult-lesioned animals. Both lesioned groups showed higher monocular thresholds compared to intact animals but this effect reached significance only for the right eye. In addition, the neonatal-lesioned cats showed ocular misalignment which may have contributed to their lack of binocular depth perception. Regardless of these deficits neonatal-lesioned cats were more like intact controls regarding the types of errors made on the visual cliff. Neonatal-lesioned animals and intact controls made random errors, whereas adult-lesioned animals made most of their errors when the shallow shelf was presented on the animals' right side. This may indicate that the adult-lesioned animals have greater motor and/or visual field biases than do neonatal-lesioned cats.     

Binocular eye movements evoked by self-induced motion parallax

Perception often triggers actions, but actions may sometimes be necessary to evoke percepts. This is most evident in the recovery of depth by self-induced motion parallax. Here we show that depth information derived from one's movement through a stationary environment evokes binocular eye movements consistent with the perception of three-dimensional shape. Human subjects stood in front of a display and viewed a simulated random-dot sphere presented monocularly or binocularly. Eye movements were recorded by a head-mounted eye tracker, while head movements were monitored by a motion capture system. The display was continuously updated to simulate the perspective projection of a stationary, transparent random dot sphere viewed from the subject's vantage point. Observers were asked to keep their gaze on a red target dot on the surface of the sphere as they moved relative to the display. The movement of the target dot simulated jumps in depth between the front and back surfaces of the sphere along the line of sight. We found the subjects' eyes converged and diverged concomitantly with changes in the perceived depth of the target. Surprisingly, even under binocular viewing conditions, when binocular disparity signals conflict with depth information from motion parallax, transient vergence responses were observed. These results provide the first demonstration that self-induced motion parallax is sufficient to drive vergence eye movements under both monocular and binocular viewing conditions.     

Delayed pointing movements to masked Müller-Lyer figures are affected by target size but not the illusion

There is ongoing debate with respect to interpretation of the finding that, in contrast to perceptual size judgments, actions are relatively unaffected by the Müller-Lyer illusion. In normal unrestricted viewing situations observers cannot perform an action directed at an object without simultaneously perceiving the object - this makes it difficult to unequivocally establish whether observed effects are a function of vision for perception, vision for action, a combination of both, or of a single all-purpose visual system. However, there is evidence that observers are capable of performing actions towards objects of which they are not consciously aware, implying that two distinct visual thresholds may exist; one accompanying vision for action and one accompanying vision for perception. To investigate this possibility we created a situation in which visual information was presented below the perception threshold, but above the purported action threshold, allowing examination of action responses independent of contributions from vision for perception. Following a perceptual categorization task, participants performed delayed pointing movements towards briefly exposed masked Müller-Lyer targets of different sizes. When the targets were presented below the perception threshold, participants were unable to discriminate between them, yet their delayed pointing movements were affected by target size (but not the illusion). The results imply that vision for action is functional even after a delay and/or that the pickup of egocentric information is associated with a lower visual threshold than the pickup of allocentric information.     

Dissociations in rod bisection: the effect of viewing conditions on perception and action

Research providing evidence from patients and neurologically healthy participants has demonstrated that visual perception can dissociate from visually guided actions, and that this dissociation can be removed by reducing visual feedback to a monocular view, or by completely occluding vision. Previously we have demonstrated a similar dissociation between perception and action on a rod-bisection task. The current paper examines whether manipulating the viewing conditions can also affect this dissociation. Forty-eight right-handed participants bisected five rods of different lengths, by pointing to the centre and by picking each up by the centre, under three viewing conditions: binocular viewing, monocular viewing and occluded viewing. Binocular viewing resulted in the expected perception-action dissociation: pointing bisection errors were to the right of centre and grasping bisection errors showed no bias. However, this pattern was also evident for the monocular-viewing condition, demonstrating that monocular viewing had no significant effect on bisection. In contrast, complete visual occlusion led to the elimination of the perception-action dissociation, and, in addition, the direction of the pointing errors reversed: both pointing and grasping errors were to the left of centre. These results are compared with a line-bisection task performed under similar conditions. This task resulted in consistent biases for which reducing visual feedback only influenced the extent of the error. The direction of the error was influenced by line position. These results demonstrate that theories for differential processing in the ventral and dorsal streams, used to elucidate perception-action dissociations, may not be compatible with the rod-bisection task and that online visuomotor feedback may better explain the dissociation.     

Impaired binocular depth inversion in patients with alcohol withdrawal

Binocular depth inversion represents an illusion of visual perception. Such inversion does not occur in all cases, especially when objects with a higher degree of familiarity (e.g. photographs of faces) are displayed. Cognitive factors are assumed to override the binocular disparity cues of stereopsis.

We tested the hypothesis that during alcohol withdrawal the human CNS is unable to correct the implausible perceptual hypothesis. Measurements of binocular depth inversion in perception of 3-D objects were performed in 10 patients with mild alcohol withdrawal and in 11 healthy volunteers. The binocular depth inversion scores were highly elevated in the patients group in comparison to the healthy volunteers. The data demonstrates a strong impairment of binocular depth inversion in alcohol withdrawal and support the view that alcohol withdrawal may be accompanied by a disorganization of the interaction between sensory input and generation of perceptual hypotheses.     

Depth Perception in Monocularly Deprived Cats Following Part-time Reverse Occlusion

The behavioural effects of an early period of monocular deprivation can be extremely profound. However, it is possible to achieve a high degree of recovery, even to normal levels of visual acuity, by prompt imposition of certain regimes of part-time reverse occlusion where the initially non-deprived eye is occluded for only part of each day in order to allow a daily period of binocular visual exposure. In this paper we report on the depth perception of five monocularly deprived cats that had recovered normal visual acuity in both eyes following imposition of certain of the above occlusion regimes. Although three of the animals exhibited five-to sevenfold superiority of binocular over monocular depth thresholds, subsequent tests made on two of the animals revealed that they were unable to make stereoscopic discriminations with random-dot stereograms. Despite the recovery of normal visual acuity in both eyes, we conclude that these animals recover at best only local stereopsis.     

Effects of complete monocular deprivation in visuo-spatial memory

Monocular deprivation has been associated with both specific deficits and enhancements in visual perception and processing. In this study, performance on a visuo-spatial memory task was compared in congenitally monocular individuals and sighted control individuals viewing monocularly (i.e., patched) and binocularly. The task required the individuals to view and memorize a series of target locations on two-dimensional matrices. Overall, congenitally monocular individuals performed worse than sighted individuals (with a specific deficit in simultaneously maintaining distinct spatial representations in memory), indicating that the lack of binocular visual experience affects the way visual information is represented in visuo-spatial memory. No difference was observed between the monocular and binocular viewing control groups, suggesting that early monocular deprivation affects the development of cortical mechanisms mediating visuo-spatial cognition.     

Depth-cue integration in grasp programming: no evidence for a binocular specialism

When we grasp with one eye covered, the finger and thumb are typically opened wider than for binocularly guided grasps, as if to build a margin-for-error into the movement. Also, patients with visual form agnosia can have profound deficits in their (otherwise relatively normal) grasping when binocular information is removed. One interpretation of these findings is that there is a functional specialism for binocular vision in the control of grasping. Alternatively, cue-integration theory suggests that binocular and monocular depth cues are combined in the control of grasping, and so impaired performance reflects not the loss of ‘critical’ binocular cues, but increased uncertainty per se. Unfortunately, removing binocular information confounds removing particular (binocular) depth cues with an overall reduction in the available information, and so such experiments cannot distinguish between these alternatives. We measured the effects on visually open-loop grasping of selectively removing monocular (texture) or binocular depth cues. To allow meaningful comparisons, we made psychophysical measurements of the uncertainty in size estimates in each case, so that the informativeness of binocular and monocular cues was known in each condition. Consistent with cue-integration theory, removing either binocular or monocular cues resulted in similar increases in grip apertures. In a separate experiment, we also confirmed that changes in uncertainty per se (keeping the same depth cues available) resulted in larger grip apertures. Overall, changes in the margin-for-error in grasping movements were determined by the uncertainty in size estimates and not by the presence or absence of particular depth cues. Our data therefore argue against a binocular specialism for grasp programming. Instead, grip apertures were smaller when binocular and monocular cues were available than with either cue alone, providing strong evidence that the visuo-motor system exploits the redundancy available in multiple sources of information, and integrates binocular and monocular cues to improve grasping performance.     

The processing of stereoscopic information in human visual cortex: psychophysical and electrophysiological evidence.

Three-dimensional depth perception relies in part on the binocular fusion of horizontally disparate stimuli presented to the left and right eye. The mammalian visual system offers a unique possibility to study electrophysiologically cortical neuronal mechanisms: since the input of the two eyes remains separated up to the level of the visual cortex, evoked potential components that are generated exclusively by cortical structures may be explored when dynamic random-dot stereograms (dRDS) are presented. In a series of independent studies, we determined the scalp topography of dRDS evoked brain activity in different groups of healthy subjects, and we found consistent results. Major differences between stereoscopic and contrast evoked brain activity are seen in the strength of the potential fields as well as in their topography. Our findings suggest that there are fewer neurons in the human visual cortex that are responsive to horizontal disparity, and that higher visual areas like V2 are more engaged with stereoscopic processing than the primary visual cortex. On the other hand, component latencies of evoked brain activity show no effect signifying that the binocular information flow to the visual cortex has a similar time course for both the processing of contrast information and of dRDS stimuli. We could also verify that healthy subjects can learn to perceive 3D structure contained in dRDS. Changes in perceptual ability as measured with psychophysical tests are paralleled by systematic alterations in the topography of stereoscopically evoked potential fields. Stereoscopic VEP recordings may also be of clinical use: in patients with selectively disturbed depth perception but normal visual acuity there is a high correlation between clinical symptoms, perceptual deficiency, and altered VEP amplitudes and latencies.     

Visual 3D illusion: A systems-theoretical approach to psychosis

Binocular depth inversion represents an illusion of visual perception. Such an inversion does not occur in all cases, especially when objects with a higher degree of familiarity (e.g. photographs of faces) are displayed. Cognitive factors are assumed to override the binocular disparity cues of stereopsis. We tested the hypothesis that during psychotic and similar states the human CNS is unable to correct the implausible perceptual hypothesis. Measurements of binocular depth inversion in perception of 3D objects were performed in schizophrenic patients (n=13), in patients with alcohol withdrawal (n=10), in sleep-deprived medical staff (n=10) and in healthy volunteers (n=41). The binocular depth inversion scores were highly elevated in the patient group and in the sleep-deprivated medical staff in comparison with healthy volunteers. The data demonstrate that impairment of binocular depth inversion reflects a common final pathway, characterized by an impairment of adaptive systems regulating perception.     

Matching boxes: familiar size influences action programming

The perception/action model is the dominant account of the primary division of labour in the human visual pathway. Integral to this model is the idea that goal-directed actions are guided spatially by bottom-up vision, independent of perceptual recognition and top-down object knowledge. We question this idea by showing that the expected size of familiar objects (matchboxes) affects the amplitude of reaches made to grasp them, and the pre-shaping of the hand, even when binocular cues are available. This suggests that perceptual recognition routinely influences action programming.     

A new window into the interactions between perception and action

Traditionally, viewing window paradigms have been used to evaluate perceptual features useful in object identification. Participants are presented with a degraded picture of an object on a computer monitor and are asked to identify the object as quickly as possible. A small, user controlled area (the "window") displays the underlying image with normal clarity. Despite their traditional role, viewing window tasks require visuomotor processing, which can be manipulated to illuminate the interactions between the "perception" and "action" based cortical visual streams. As participants performed the present experiment, response times and movement of the window were recorded. The participant's movement of the window was analyzed (the visuomotor scanning pattern), separating the image into four equal sized quadrants and then examining the percentage of time spent in each. A main-effect of quadrant was found, demonstrating the ability of this procedure to identify vertical and horizontal asymmetries in visuomotor scan patterns elicited by the presented objects. This research demonstrates the feasibility of the viewing window as a method of examining the interactions between perceptual and motor information. Importantly, results indicate that the viewing window procedure has the ability discriminate any gross asymmetries in a participant's visuomotor scanpath used to identify these common objects.     

Binocularly deprived cats: binocular tests of cortical cells show regular patterns of interaction

If an animal is prevented from receiving visual experience during an early developmental phase, pronounced dysfunctions are observed. Physiological tests reveal gross abnormalities in the striate cortex. Cells in visual cortex are either unresponsive of their response characteristics are erratic. Although fewer than normal numbers of binocular cells are found in cats reared with binocular lid suture, a population remains that can be activated by stimulation through either eye. We have studied cortical cells in binocularly deprived cats in order to specify monocular and binocular response characteristics. The primary hypothesis we have examined is that the abnormal response properties of these cells are a result of an irregular structure or substructure of the receptive fields. Kittens were binocularly lid-sutured soon after birth, and were studied physiologically at ages between 7 and 11 months. Standard techniques were used to record from single cells in striate cortex. Drifting gratings were presented to either eye or to both eyes together. In the latter case, the relative interocular phase was varied between the gratings so that the retinal disparity of the stimuli was changed. We confirmed the expected finding that most cells were either unresponsive or erratic in their response. Of the cells that responded, monocular tuning functions for orientation and spatial frequency of the stimulus were often irregular. However, even in these cases, binocular interaction patterns of cortical responses were nearly always highly regular and displayed phase-specific profiles. A model is presented that explains this finding and suggests how binocular deprivation may result in disorganized receptive-field structure.     

Difference in binocular rivalry rate between patients with bipolar I and bipolar II disorders

Objective:  When dissimilar figures are presented to each eye individually, perception alternates spontaneously between each monocular view. This phenomenon, known as binocular rivalry, has been used as a powerful tool to investigate conscious visual awareness. Of clinical relevance, Pettigrew and Miller ( Proc Biol Sci 1998; 265: 2141-2148 ) found slow perceptual alternation rates in patients with bipolar I disorder (BD-I). For a better understanding of differences between BD-I and bipolar II disorder (BD-II), we examined whether perceptual alternation rates of binocular rivalry differ between the two subtypes of bipolar disorder.
Methods:  The subjects comprised 25 healthy controls, 11 patients with BD-I, and 17 patients with BD-II. They underwent binocular rivalry examination. One-way analysis of variance was conducted to determine differences in the phase duration of binocular rivalry between the control, BD-I, and BD-II groups.
Results:  Significant differences were observed in the mean phase duration of binocular rivalry between the groups. Although the medication administered did not differ significantly between the BD-I and BD-II patients, the phase duration was significantly longer among the BD-I patients than the BD-II patients and controls, whereas no significant difference was observed in the phase duration between the BD-II patients and controls.
Conclusion:  The present results reveal a significant difference in the mean phase duration of binocular rivalry between subjects with BD-I and those with BD-II, suggesting the presence of some neurobiological difference between these two subtypes of bipolar disorder.     

Trial‐by‐trial reliability of responses in the primary visual cortex on binocular disparity depends on stimulus order

An association of the detrimental effect of monocular deprivation on binocular vision with reduced reliability of neuronal responses in the primary visual cortex has been shown on randomly presented binocular stimuli [V. Vorobyov et al. (2007) Eur J Neurosci. 26(12), 3553–3563]. To examine this effect on biologically relevant signals, binocular gratings of varying relative phase disparity were presented in sequential order, simulating motion, to 55 cats with various types of daily visual experience. During sequential stimulation, the proportions of ‘unstable’ cells (with phase differences exceeding 22.5 ° between peak binocular responses in two consecutive trials) were similar in cats with exclusively binocular experience and with short periods of daily monocular vision (≤ 3.25 h), in mixed binocular–monocular conditions. In contrast, random stimulation was characterized by a significantly enlarged population of ‘unstable’ cells in the latter. After a longer period of monocular vision (6.5 h) or exclusively discordant binocular experience (strabismus), sequential stimulation was accompanied by a significant increase of this population, whereas during randomized stimulation it was very similar to that in cats with short periods of daily monocular vision. Finally, there were no differences in populations of ‘unstable’ cells in cats with long monocular or strabismic vision and those with exclusive monocular experience during sequential stimulation, in contrast with a significant increase in the latter during randomized stimulation. I propose that the detrimental effect of abnormal binocular experience on binocular processing in the primary visual cortex is associated with a disruption of the mechanisms involved in both discrimination of binocular disparity signals and evaluation of their temporal profiles.     

Functional anatomy of macaque striate cortex. I. Ocular dominance, binocular interactions, and baseline conditions

A series of experiments was carried out using 14C-2-deoxy-d-glucose (DG) in order to examine the functional architecture of macaque striate (primary visual) cortex. This paper describes the results of experiments on uptake during various baseline (or reference) conditions of visual stimulation (described below), and on differences in the functional architecture following monocular versus binocular viewing conditions. In binocular "baseline" experiments, monkeys were stimulated either (1) in the dark, (2) with a diffuse gray screen, or (3) with a very general visual stimulus composed of gratings of varied orientation and spatial frequency. In all of these conditions, DG uptake was found to be topographically uniform within all layers of parafoveal striate cortex. In monocular experiments that were otherwise similar, uptake was topographically uniform within the full extent of the eye dominance strip, in all layers. Certain other visual stimuli produce high uptake in the blobs, and still another set of visual stimuli (including high-spatial-frequency gratings) produce highest uptake between the blobs at parafoveal eccentricities, even in an unanesthetized, unparalyzed monkey. Eye movements per se had no obvious effect on striate DG uptake. Endogenous uptake in the blobs (relative to that in the interblobs) appears higher in the squirrel monkey than in the macaque. The pattern of DG uptake produced by binocular viewing was found to deviate in a number of ways from that expected by linearly summing the component monocular DG patterns. One of the most interesting deviations was an enhancement of the representation of visual field borders between stimuli differing from each other in texture, orientation, direction, etc. This "border enhancement" was confined to striate layers 1-3 (not appearing in any of the striate input layers), and it only appeared following binocular, but not monocular, viewing conditions. The border enhancement may be related to a suppression of DG uptake that occurs during binocular viewing conditions in layers 2 + 3 (and perhaps layers 1 and 4B), but not in layers 4Ca, 4Cb, 5 or 6. Another major class of binocular interaction was a spread of neural activity into the "unstimulated" ocular dominance strips following monocular stimulation. Such an effect was prominent in striate layer 4Ca, but it did not occur in layer 4Cb. This "binocular" spread of DG uptake into the inappropriate eye dominance strip in 4Ca may be related to the appearance of orientation tuning and orientation columns in that layer. No DG effects were seen that depended on the absolute disparity of visual stimuli in macaque striate cortex.