The role of vergence in the perception of distance: a fair test of Bishop Berkeley's claim

Binocular eye movements were measured while subjects perceived the wallpaper illusion in order to test the claim made by Bishop Berkeley in 1709 that we perceive the distance of nearby objects by evaluating the vergence angles of our eyes. Four subjects looked through a nearby fronto-parallel array of vertical rods (28-35 cm away) as they binocularly fixated a point about 1 meter away. The wallpaper illusion was perceived under these conditions, i.e. the rods appeared farther away than their physical location. We found that although binocular fixation at an appropriate distance was needed to begin perceiving the wallpaper illusion (at least for naive observers), once established, the illusion was quite robust in the sense that it was not affected by changing vergence. No connection between the apparent localization of the rods and vergence was observed. We conclude that it is unlikely that vergence, itself, is responsible for the perceived distance shift in the wallpaper illusion, making it unlikely that vergence contributes to the perception of distance as Bishop Berkeley suggested. We found this to be true even when vergence angles were relatively large (more than 2 deg), the region in which the control of vergence eye movements has been shown to be both fast and effective.     

Dyslexic children are confronted with unstable binocular fixation while reading

Reading requires three-dimensional motor control: saccades bring the eyes from left to right, fixating word after word; and oblique saccades bring the eyes to the next line of the text. The angle of vergence of the two optic axes should be adjusted to the depth of the book or screen and--most importantly--should be maintained in a sustained manner during saccades and fixations. Maintenance of vergence is important as it is a prerequisite for a single clear image of each word to be projected onto the fovea of the eyes. Deficits in the binocular control of saccades and of vergence in dyslexics have been reported previously but only for tasks using single targets. This study examines saccades and vergence control during real text reading. Thirteen dyslexic and seven non-dyslexic children read the French text "L'Allouette" in two viewing distances (40 cm vs. 100 cm), while binocular eye movements were measured with the Chronos Eye-tracking system. We found that the binocular yoking of reading saccades was poor in dyslexic children (relative to non-dyslexics) resulting in vergence errors; their disconjugate drift during fixations was not correlated with the disconjugacy during their saccades, causing considerable variability of vergence angle from fixation to fixation. Due to such poor oculomotor adjustments during reading, the overall fixation disparity was larger for dyslexic children, putting larger demand on their sensory fusion processes. Moreover, for dyslexics the standard deviation of fixation disparity was larger particularly when reading at near distance. We conclude that besides documented phoneme processing disorders, visual/ocular motor imperfections may exist in dyslexics that lead to fixation instability and thus, to instability of the letters or words during reading; such instability may perturb fusional processes and might--in part--complicate letter/word identification.     

A framework for considering the role of afference and efference in the control and perception of ocular position

It has been well established that extra-retinal information is used in the perception of visual direction and distance. Furthermore, a number of studies have established that both efference copy and afferent discharge contribute to the extra-retinal signal. Despite this, no model currently exists to explain how the signals which arise through oculomotor control contribute to perception. This paper attempts to provide such a framework. The first part of the paper outlines the framework [the cyclopean equilibrium point (EP) model] and considers the binoculus or cyclopean eye from the perspective of a current account of motor control (the EP hypothesis). An existing model is used to describe how the nervous system could utilise available efference copy and afferent extra retinal signals when determining the direction and distance of cyclopean fixation. Although the cyclopean EP model is speculative, it allows for a parsimonious framework when considering the oculomotor contribution to perception. The model has the additional advantage of being consistent with current theories regarding the control and perception of limb movement. The second part of the paper shows that the model is biologically plausible, demonstrates the use of the proposed model in describing the central control of eye movements with regard to non-conjugate peripheral adaptation and reconciles seemingly disparate empirical findings.     

Frogs use retinal elevation as a cue to distance

Summary We have investigated the role that retinal elevation plays in a frog's (Rana pipiens) estimate of prey distance. We dissociated retinal elevation from other depth cues by artificially increasing the height of the frogs' eyes above the ground. Frogs then snapped short of their prey in their ventral visual field as if their estimate of distance were determined primarily by the retinal elevation of the image of the prey. The data suggest that the frog assumes its eyes to be about 3 cm above the ground. Other cues modify depth judgements when targets are close to this assumed groundplane.     

Vergence eye movement control and multivalent perception of autostereograms

We introduce a dynamical model for automatic vergence eye movement control. In connection with our dynamical system of binocular model neurons that solves the correspondence problem of stereo-vision, we present a complete model for stereo-vision. Our automatic vergence eye movement control adjusts an image segment, which is of momentary interest to the observer. The adjustment is done in such a way that we only need to define a disparity search range of minimal extension. ecently, a new method of encoding (3D) three-dimenional information in 2D pictures was designed in the form of computer-generated patterns of colored dots. At first glimpse, these so-called autostereograms appear as structured but meaningless patterns. After a certain period of observation, a 3D pattern emerges suddenly in an impressive way. Applying our algorithm to autostereograms, we find a fully satisfactory agreement with the multivalent perception experienced by humans. As in nature, in our model the phase transition between the initial state and the 3D perception state takes place in a very short time. Our algorithm is very robust against noise, and there is no need to interpolate a sparse depth map.     

Self versus Environment Motion in Postural Control

To stabilize our position in space we use visual information as well as non-visual physical motion cues. However, visual cues can be ambiguous: visually perceived motion may be caused by self-movement, movement of the environment, or both. The nervous system must combine the ambiguous visual cues with noisy physical motion cues to resolve this ambiguity and control our body posture. Here we have developed a Bayesian model that formalizes how the nervous system could solve this problem. In this model, the nervous system combines the sensory cues to estimate the movement of the body. We analytically demonstrate that, as long as visual stimulation is fast in comparison to the uncertainty in our perception of body movement, the optimal strategy is to weight visually perceived movement velocities proportional to a power law. We find that this model accounts for the nonlinear influence of experimentally induced visual motion on human postural behavior both in our data and in previously published results.     

How haptic size sensations improve distance perception

Determining distances to objects is one of the most ubiquitous perceptual tasks in everyday life. Nevertheless, it is challenging because the information from a single image confounds object size and distance. Though our brains frequently judge distances accurately, the underlying computations employed by the brain are not well understood. Our work illuminates these computions by formulating a family of probabilistic models that encompass a variety of distinct hypotheses about distance and size perception. We compare these models' predictions to a set of human distance judgments in an interception experiment and use Bayesian analysis tools to quantitatively select the best hypothesis on the basis of its explanatory power and robustness over experimental data. The central question is: whether, and how, human distance perception incorporates size cues to improve accuracy. Our conclusions are: 1) humans incorporate haptic object size sensations for distance perception, 2) the incorporation of haptic sensations is suboptimal given their reliability, 3) humans use environmentally accurate size and distance priors, 4) distance judgments are produced by perceptual "posterior sampling". In addition, we compared our model's estimated sensory and motor noise parameters with previously reported measurements in the perceptual literature and found good correspondence between them. Taken together, these results represent a major step forward in establishing the computational underpinnings of human distance perception and the role of size information.     

Vestibular facilitation of optic flow parsing

Simultaneous object motion and self-motion give rise to complex patterns of retinal image motion. In order to estimate object motion accurately, the brain must parse this complex retinal motion into self-motion and object motion components. Although this computational problem can be solved, in principle, through purely visual mechanisms, extra-retinal information that arises from the vestibular system during self-motion may also play an important role. Here we investigate whether combining vestibular and visual self-motion information improves the precision of object motion estimates. Subjects were asked to discriminate the direction of object motion in the presence of simultaneous self-motion, depicted either by visual cues alone (i.e. optic flow) or by combined visual/vestibular stimuli. We report a small but significant improvement in object motion discrimination thresholds with the addition of vestibular cues. This improvement was greatest for eccentric heading directions and negligible for forward movement, a finding that could reflect increased relative reliability of vestibular versus visual cues for eccentric heading directions. Overall, these results are consistent with the hypothesis that vestibular inputs can help parse retinal image motion into self-motion and object motion components.     

Isotropic integration of binocular disparity and relative motion in the perception of three-dimensional shape

Richards (1985) showed that veridical three-dimensional shape may be recovered from the integration of binocular disparity and retinal motion information, but proposed that this integration may only occur for horizontal retinal motion. Psychophysical evidence supporting the combination of stereo and motion information is limited to the case of horizontal motion (Johnston et al., 1994), and has been criticised on the grounds of potential object boundary cues to shape present in the stimuli. We investigated whether veridical shape can be recovered under more general conditions. Observers viewed cylinders that were defined by binocular disparity, two-frame motion or a combination of disparity and motion, presented at simulated distances of 30 cm, 90 cm or 150 cm. Horizontally and vertically oriented cylinders were rotated about vertical and horizontal axes. When rotation was about the cylinder's own axis, no boundary cues to shape were introduced. Settings were biased for the disparity and two-frame motion stimuli, while more veridical shape judgements were made under all conditions for combined cue stimuli. These results demonstrate that the improved perception of three-dimensional shape in these stimuli is not a consequence of the presence of object boundary cues, and that the combination of disparity and motion is not restricted to horizontal image motion.     

Virtual-reality techniques resolve the visual cues used by fruit flies to evaluate object distances

Insects can estimate distance or time-to-contact of surrounding objects from locomotion-induced changes in their retinal position and/or size. Freely walking fruit flies (Drosophila melanogaster) use the received mixture of different distance cues to select the nearest objects for subsequent visits. Conventional methods of behavioral analysis fail to elucidate the underlying data extraction. Here we demonstrate first comprehensive solutions of this problem by substituting virtual for real objects; a tracker-controlled 360 degrees panorama converts a fruit fly's changing coordinates into object illusions that require the perception of specific cues to appear at preselected distances up to infinity. An application reveals the following: (1) en-route sampling of retinal-image changes accounts for distance discrimination within a surprising range of at least 8-80 body lengths (20-200 mm). Stereopsis and peering are not involved. (2) Distance from image translation in the expected direction (motion parallax) outweighs distance from image expansion, which accounts for impact-avoiding flight reactions to looming objects. (3) The ability to discriminate distances is robust to artificially delayed updating of image translation. Fruit flies appear to interrelate self-motion and its visual feedback within a surprisingly long time window of about 2 s. The comparative distance inspection practiced in the small fruit fly deserves utilization in self-moving robots.     

Evidence for implication of primate area V1 in neural 3-D spatial localization processing.

We investigated the neural mechanisms underlying visual localization in 3-D space in area V1 of behaving monkeys. Three different sources of information, retinal disparity, viewing distance and gaze direction, that participate in these neural mechanisms are being reviewed. The way they interact with each other is studied by combining retinal and extraretinal signals. Interactions between retinal disparity and viewing distance have been shown in foveal V1; we have observed a strong modulation of the spontaneous activity and of the visual response of most V1 cells that was highly correlated with the vergence angle. As a consequence of these gain effects, neural horizontal disparity coding is favoured or refined for particular distances of fixation. Changing the gaze direction in the fronto-parallel plane also produces strong gains in the visual response of half of the cells in foveal V1. Cells tested for horizontal disparity and orientation selectivities show gain effects that occur coherently for the same spatial coordinates of the eyes. Shifts in preferred disparity also occurred in several neurons. Cells tested in calcarine V1 at retinal eccentricities larger than 10 degrees , show that horizontal disparity is encoded at least up to 20 degrees around both the horizontal and vertical meridians. At these large retinal eccentricities we found that vertical disparity is also encoded with tuning profiles similar to those of horizontal disparity coding. Combinations of horizontal and vertical disparity signals show that most cells encode both properties. In fact the expression of horizontal disparity coding depends on the vertical disparity signals that produce strong gain effects and frequent changes in peak selectivities. We conclude that the vertical disparity signal and the eye position signal serve to disambiguate the horizontal disparity signal to provide information on 3-D spatial coordinates in terms of distance, gaze direction and retinal eccentricity. We suggest that the relative weight among these different signals is the determining factor involved in the neural processing that gives information on 3-D spatial localization.     

Position,guidance, and mapping in the developing visual system

Positional identity in the visual system affects the topographic projection of the retina onto its central targets. In this review we discuss gradients and positional information in the retina, when and how they arise, and their functional significance in development. When the axons of retinal ganglion cells leave the eye, they navigate through territory in the central nervous system that is rich in positional information. We review studies that explore the navigational cues that the growth cones of retinal axons use to orient towards their target and organize themselves as they make this journey. Finally, these axons arrive at their central targets and make a precise topographic map of visual space that is crucial for adaptive visual behavior. In the last section of this review, we examine the topographic cues in the tectum, what they are, when, and how they arise, and how retinal axons respond to them. We also touch on the role of neural activity in the refinement of this topography. © 1993 John Wiley & Sons, Inc.     

Ocular tracking as a measure of auditory motion perception.

Motion is a potent sub-modality of vision. Motion cues alone can be used to segment images into figure and ground and break camouflage. Specific patterns of motion support vivid percepts of form, guide locomotion by specifying directional heading and the passage of objects, and in case of an impending collision, the time to impact. Visual motion also drives smooth pursuit eye movements (SPEMs) that serve to stabilize the retinal image of objects in motion. In contrast, the auditory system does not appear to be particularly sensitive to motion. We review the ambiguous status of auditory motion processing from the psychophysical and electrophysiological perspectives. We then report the results of two experiments that use ocular tracking performance as an objective measure of the perception of auditory motion in humans. We examine ocular tracking of auditory motion, visual motion, combined auditory + visual motion and imagined motion in both the frontal plane and in depth. The results demonstrate that ocular tracking of auditory motion is no better than ocular tracking of imagined motion. These results are consistent with the suggestion that, unlike the visual system, the human auditory system is not endowed with low-level motion sensitive elements. We hypothesize however, that auditory information may gain access to a recently described high-level motion processing system that is heavily dependent on 'top-down' influences, including attention.     

An investigation of the mechanisms of eye movement control

Summary Feedback mechanisms exist in all the periferal sense organs including the eye, which acts as a highly efficient position control servo system. Histological studies so far have not revealed the precise circuitry of the eye movement control system but some information about it can be obtained by a study of the sources of feedback. Existing theories have considered three types of feedback originating in the oculomotor tract, in the proprioceptive fibres of the extrinsic eye muscles and from retinal image displacement. In the present experiments an optical arrangement has been used to vary or eliminate the amount of information available from retinal image motion, and the response of the eye to simple harmonic displacement of a target has been recorded. The response curves of gain (eyeball movement divided by target motion) against frequency indicate that the system is lion linear when the image falls in the retinal region which is insensitive to position. Outside this area, retinal image position is used as negative feedback but the information from the oculomotor tract must be regenerative. There is also evidence for feedback proportional to the first derivative of eyeball position and this function is ascribed to the proprioceptive signals; this form of feedback appears to saturate for large amplitude movements, thus avoiding heavy damping of the flick movements.A schematic eye movement control system having the same characteristics as the eye is proposed. The transfer function of this system indicates that it should be unstable if the sign of the retinal image feedback loop is reversed. Experiments with this form of feedback show that steady fixation is impossible and the eye performs a pendular nystagmus.     

Binocular coordination: reading stereoscopic sentences in depth

The present study employs a stereoscopic manipulation to present sentences in three dimensions to subjects as they read for comprehension. Subjects read sentences with (a) no depth cues, (b) a monocular depth cue that implied the sentence loomed out of the screen (i.e., increasing retinal size), (c) congruent monocular and binocular (retinal disparity) depth cues (i.e., both implied the sentence loomed out of the screen) and (d) incongruent monocular and binocular depth cues (i.e., the monocular cue implied the sentence loomed out of the screen and the binocular cue implied it receded behind the screen). Reading efficiency was mostly unaffected, suggesting that reading in three dimensions is similar to reading in two dimensions. Importantly, fixation disparity was driven by retinal disparity; fixations were significantly more crossed as readers progressed through the sentence in the congruent condition and significantly more uncrossed in the incongruent condition. We conclude that disparity depth cues are used on-line to drive binocular coordination during reading.     

Absence of cue-recruitment for extrinsic signals: sounds, spots, and swirling dots fail to influence perceived 3D rotation direction after training

The visual system can learn to use information in new ways to construct appearance. Thus, signals such as the location or translation direction of an ambiguously rotating wire frame cube, which are normally uninformative, can be learned as cues to determine the rotation direction. This perceptual learning occurs when the formerly uninformative signal is statistically associated with long-trusted visual cues (such as binocular disparity) that disambiguate appearance during training. In previous demonstrations, the newly learned cue was intrinsic to the perceived object, in that the signal was conveyed by the same image elements as the object itself. Here we used extrinsic new signals and observed no learning. We correlated three new signals with long-trusted cues in the rotating cube paradigm: one crossmodal (an auditory signal) and two within modality (visual). Cue recruitment did not occur in any of these conditions, either in single sessions or in ten sessions across as many days. These results suggest that the intrinsic/extrinsic distinction is important for the perceptual system in determining whether it can learn and use new information from the environment to construct appearance. Extrinsic cues do have perceptual effects (e.g. the "bounce-pass" illusion and McGurk effect), so we speculate that extrinsic signals must be recruited for perception, but only if certain conditions are met. These conditions might specify the age of the observer, the strength of the long-trusted cues, or the amount of exposure to the correlation.     

A precisely timed asynchronous pattern of ON and OFF retinal ganglion cell activity during propagation of retinal waves

Patterns of coordinated spontaneous activity have been proposed to guide circuit refinement in many parts of the developing nervous system. It is unclear, however, how such patterns, which are thought to indiscriminately synchronize nearby cells, could provide the cues necessary to segregate functionally distinct circuits within overlapping cell populations. Here, we report that glutamatergic retinal waves possess a substructure in the bursting of neighboring retinal ganglion cells with opposite light responses (ON or OFF). Within a wave, cells fire repetitive nonoverlapping bursts in a fixed order: ON before OFF. This pattern is absent from cholinergic waves, which precede glutamate-dependent activity, providing a developmental sequence of distinct activity-encoded cues. Asynchronous bursting of ON and OFF retinal ganglion cells depends on inhibition between these parallel pathways. Similar asynchronous activity patterns could arise throughout the nervous system, as inhibition matures and might help to separate connections of functionally distinct subnetworks.     

Does vertical disparity scale the perception of stereoscopic depth?

It has been suggested that a measure of the gradients of vertical disparity over a surface may scale the mapping between horizontal disparity and perceived depth. We have investigated this possibility by obtaining estimates of the depth within stereograms that simulated two apposed fronto-parallel planes placed at different distances from an observer. The gradients of vertical disparity in a stereogram were set to simulate those appropriate to a viewing distance of 12.5 cm, 25 cm, 50 cm or 100 cm, whereas the distance specified by vergence and accommodative cues was always fixed at 50 cm. Judgements of the perceived depth between the two planes were uninfluenced by changes in the gradients of vertical disparity. It thus seems that the human visual system does not employ vertical disparity as a scaling parameter in stereoscopic depth judgements.