Monocular and binocular distance cues: insights from visual form agnosia I (of III)

The human nervous system constructs a Euclidean representation of near (personal) space by combining multiple sources of information (cues). We investigated the cues used for the representation of personal space in a patient with visual form agnosia (DF). Our results indicated that DF relies predominantly on binocular vergence information when determining the distance of a target despite the presence of other (retinal) cues. Notably, DF was able to construct an Euclidean representation of personal space from vergence alone. This finding supports previous assertions that vergence provides the nervous system with veridical information for the construction of personal space. The results from the current study, together with those of others, suggest that: (i) the ventral stream is responsible for extracting depth and distance information from "monocular" retinal cues (i.e. from shading, texture, perspective) and (ii) the dorsal stream has access to binocular information (from horizontal image disparities and vergence). These results also indicate that DF was not able to use size information to gauge target distance, suggesting that intact temporal cortex is necessary for "learned size" to influence distance processing. Our findings further suggest that in neurologically intact humans, object information extracted in the ventral pathway is combined with the products of dorsal stream processing for guiding prehension. Finally, we studied the "size-distance paradox" in visual form agnosia in order to explore the cognitive use of size information. The results of this experiment were consistent with a previous suggestion that the paradox is a cognitive phenomenon.

The role of size and binocular information in guiding reaching: insights from virtual reality and visual form agnosia III (of III)

Reaching out to grasp an object requires information about the size of the object and the distance between the object and the body. We used a virtual reality system with a control population and a patient with visual form agnosia (DF) in order to explore the use of binocular information and size cues in prehension. The experiments consisted of a perceptual matching task in addition to a prehension task. In the prehension task, control participants modified their reach distance in response to step changes in vergence in the absence of any clear reference for relative disparity. Their reach distance was unaffected by equivalent step changes in size, even though they used this information to modify grasp and showed a size bias in a distance matching task. Notably, DF showed the same pattern of results as the controls but was far more sensitive to step changes in vergence. This finding complements previous research suggesting that DF relies predominantly on vergence information when gauging target distance. The results from the perceptual matching tasks confirmed previous findings suggesting that DF is unable to make use of size information for perceptual matching, including distance comparisons. These data are discussed with regard to the properties of the pathways subserving the two visual cortical processing streams.

The effect of obstacle position on reach-to-grasp movements

Numerous everyday tasks require the nervous system to program a prehensile movement towards a target object positioned in a cluttered environment. Adult humans are extremely proficient in avoiding contact with any non-target objects (obstacles) whilst carrying out such movements. A number of recent studies have highlighted the importance of considering the control of reach-to-grasp (prehension) movements in the presence of such obstacles. The current study was constructed with the aim of beginning the task of studying the relative impact on prehension as the position of obstacles is varied within the workspace. The experimental design ensured that the obstacles were positioned within the workspace in locations where they did not interfere physically with the path taken by the hand when no obstacle was present. In all positions, the presence of an obstacle caused the hand to slow down and the maximum grip aperture to decrease. Nonetheless, the effect of the obstacle varied according to its position within the workspace. In the situation where an obstacle was located a small distance to the right of a target object, the obstacle showed a large effect on maximum grip aperture but a relatively small effect on movement time. In contrast, an object positioned in front and to the right of a target object had a large effect on movement speed but a relatively small effect on maximum grip aperture. It was found that the presence of two obstacles caused the system to decrease further the movement speed and maximum grip aperture. The position of the two obstacles dictated the extent to which their presence affected the movement parameters. These results show that the anticipated likelihood of a collision with potential obstacles affects the planning of movement duration and maximum grip aperture in prehension.

Vertical gaze angle: absolute height-in-scene information for the programming of prehension

One possible source of information regarding the distance of a fixated target is provided by the height of the object within the visual scene. It is accepted that this cue can provide ordinal information, but generally it has been assumed that the nervous system cannot extract "absolute" information from height-in-scene. In order to use height-in-scene, the nervous system would need to be sensitive to ocular position with respect to the head and to head orientation with respect to the shoulders (i.e. vertical gaze angle or VGA). We used a perturbation technique to establish whether the nervous system uses vertical gaze angle as a distance cue. Vertical gaze angle was perturbed using ophthalmic prisms with the base oriented either up or down. In experiment 1, participants were required to carry out an open-loop pointing task whilst wearing: (1) no prisms; (2) a base-up prism; or (3) a base-down prism. In experiment 2, the participants reached to grasp an object under closed-loop viewing conditions whilst wearing: (1) no prisms; (2) a base-up prism; or (3) a base-down prism. Experiment 1 and 2 provided clear evidence that the human nervous system uses vertical gaze angle as a distance cue. It was found that the weighting attached to VGA decreased with increasing target distance. The weighting attached to VGA was also affected by the discrepancy between the height of the target, as specified by all other distance cues, and the height indicated by the initial estimate of the position of the supporting surface. We conclude by considering the use of height-in-scene information in the perception of surface slant and highlight some of the complexities that must be involved in the computation of environmental layout.

The size of the visual size cue used for programming manipulative forces during precision grip

We used a perturbation technique to quantify the contribution of visual size cues to the programming of target force when lifting an object. Our results indicate that the nervous system attaches a reasonable weight to visual size cues when programming the target grip force for a novel object. In a subsequent lift of the same object, however, the confidence attached to the visual size cue fell dramatically. It is not clear whether the decrease in the use of size information was accelerated by the presence of a cue conflict or whether the fall represents the normal shift towards the use of a memory-based representation for programming grip force. In a second experiment, we used the "size-weight illusion" to explore the relationship between the verbal report of an object's weight and the programming of the grip and load force. We found that erroneous motor programming (as indexed by a number of measures) was neither necessary nor sufficient for the size-weight illusion to occur. These findings call for a re-evaluation of a previous explanation for the size-weight illusion. We suggest that the illusion arises because the cognitive system attempts to rationalise the fact that objects of apparently equal density but different size feel as if they have the same weight.

A test between two hypotheses and a possible third way for the control of prehension

We used an obstacle avoidance task to test two opposing accounts of how the nervous system controls prehension. The visuomotor account supposes that the system independently controls the grip formation and transport phase of prehensile movements. In contrast, the digit channel hypothesis suggests that the system controls the thumb and finger more or less independently. Our data strongly favoured the traditional visuomotor channel hypothesis and demonstrated that the time taken to grasp an object in the presence of obstacles was well predicted by a Fitts' law relationship. We suggest a "thirdway" hypothesis in order to retain the advantages of the digit channel hypothesis within the visuomotor framework. The third-way hypothesis suggests that the nervous system selects a single digit to transport to the object. We speculate that the actual digit selected might depend upon attention and the nature of the prehension task. This hypothesis is able to account for most of the empirical findings unearthed by researchers investigating the control of prehension.

Vergence provides veridical depth perception from horizontal retinal image disparities

One useful source of depth information available to the human nervous system is present in the horizontal disparities that exist between the two retinal images (stereoscopic depth). The relationship between horizontal disparity and depth varies with viewing distance so that an interpreting signal is required if disparities are to yield useful information. One potentially useful interpreting signal is available from ocular vergence. A number of studies have concluded, however, that a vergence signal does not provide veridical stereoscopic depth. All of these studies required observers to make a range of judgements under conditions of uncertainty (often using random dot stimuli) and we suggest that the lack of veridicality arose because of a contraction bias: a general tendency to bias judgements towards the centre of the range of possible responses. We re-examined the role of ocular vergence in the maintenance of stereoscopic depth constancy for real three-dimensional objects. Our results question the conclusions reached by previous studies and suggest that vergence can provide a veridical interpretation of stereoscopic depth. Our results indicate that horizontal retinal image disparities are not interpreted by a 'higher order' signal (i.e. the 'perceived distance' of the fixation point). The results of the experiment have significant implications for models of depth processing from disparity.

Kinaesthetic judgements and refinement of striking action

Little is known about the developmental course of striking action. This cross-sectional study explored the refinement of striking in 28 children aged between 4 and 12 years and investigated how well they could use kinaesthesis to gauge the length of an unseen bat. The kinematic data (including smoothness of movement) showed quantitative differences between the age groups. In contrast, no differences were found in the children's ability to judge the length of the unseen bat: within three strikes all of the children had made a clean hit, indicating that they had successfully judged bat length. The children then appeared to memorize the bat with which they had accurately hit the target and made: (1) minimal errors when using this bat in later trials and (2) predictable errors when using two other bats of different sizes. The results show that the striking action becomes optimized over childhood, with smoothness of movement providing an index of this refinement. The findings also suggest that young children have a higher level of kinaesthetic sensitivity than has been assumed previously on the basis of static limb positioning tasks. The results suggest that the striking task used in this study might be a useful tool for investigating the development of movement skills in children with developmental disorders.

Getting the measure of vergence weight in nearness perception

Combining multiple sources of information allows the human nervous system to construct an approximately Euclidean representation of near (personal) space. Within this space, binocular vergence is an important source of egocentric distance information. We investigated how the nervous system determines the significance (weight) accorded to vergence information when other (retinal) distance cues are present. We found that weight decreases with (1) increasing discrepancy between vergence information and other cues and (2) reduced vergence demand. The results also provided evidence that the nervous system represents vergence related distance information in units of nearness (the reciprocal of distance).

Modelling adaptation effects in vergence and accommodation after exposure to a simulated virtual reality stimulus

Natural viewing conditions place equal demands on the vergence and accommodation systems. The two responses are co-ordinated via the interactive components which couple the two systems: accommodative vergence and vergence accommodation. The crosslink components are usually quantified by the AC/A and CA/C ratios. Whether these ratios are stable entities, genetically determined, or modifiable by experience has been the subject of some debate. A 'Virtual Reality' stimulus was used to place unequal demands on the vergence and accommodation systems. Pre-task and post-task measures of AC/A and CA/C were objectively determined. The changes in the post-task measures are discussed with reference to a dual interactive model of vergence and accommodation. Model simulations suggest that adaptation of the vergence and accommodative controllers (tonic adaptation) may also play a part by altering the open loop bias. The results therefore support the idea that vergence accommodation and accommodative vergence are capable of adaptation.

Ordinal depth information from accommodation?

The ability to judge egocentric distance was assessed in two groups of six observers using a manual pointing task. The purpose of the study was to determine the extent to which blur-driven accommodation can provide information on target distance in the absence of any retinal cues to distance. Observers were extremely accurate when carrying out the pointing task in a 'full-cue' condition. In contrast, observers were extremely poor at carrying out the task when accommodation was the only distance cue available. Responses on individual trials bore little relationship to the actual target distance in any of the observers. On the other hand, accommodation weakly biased the mean responses in some observers. This bias appears to be due to the observers' effective use of accommodation to determine whether the target presented in one trial was nearer or further away than the target presented in the previous trial. Accommodation therefore appears to provide ordinal information, although the distance signal may actually arise from accommodation-driven vergence. The poverty of accommodation as a source of metric information was highlighted in a second group of observers who all demonstrated a strong bias when perceiving distance in the presence of an initially ambiguous retinal cue. It is concluded that accommodation can act as a source of ordinal distance information in the absence of other cues to distance but the contribution of accommodation to normal distance perception in full-cue conditions is questioned.

Some recent studies on the extraretinal contribution to distance perception

Some recent studies on the extraretinal contribution to distance perception are reviewed. These experiments demonstrate that vergence can provide reliable information for judgments on the distance of proximal targets in the absence of all other cues. We argue that, although vergence is an unreliable cue at large fixation distances and is subject to a strong contraction bias when studied in isolation, these facts do not imply a minor role for vergence in near-space perception. When additional depth and distance cues are added, the contribution of vergence information becomes more complicated. We present results which indicate that the different cues to depth and distance are combined in a manner that can result in unexpected distortions of visual space. A simple heuristic model which can produce the observed distortions is outlined.

The Judd illusion: evidence for two visual streams or two experimental conditions?

In the Judd illusion, observers inaccurately bisect the shaft located between two arrowheads pointing in the same direction. The magnitude of error is greater when verbal judgements are compared to action based responses (reaching out and grasping the centre of the bar). This difference has been attributed to the presence of two visual streams within cortical processing. In contrast, we provide evidence that the improved accuracy in the reaching condition may be due to occlusion of the illusory background during the transport phase of the movement. We suggest that caution is required when interpreting performance differences between two conditions that are not strictly equivalent.

Visual display height

We examined the influence of backrest inclination and vergence demand on the posture and gaze angle that workers adopt to view visual targets placed in different vertical locations. In the study, 12 participants viewed a small video monitor placed in 7 locations around a 0.65-m radius arc (from 65 degrees below to 30 degrees above horizontal eye height). Trunk posture was manipulated by changing the backrest inclination of an adjustable chair. Vergence demand was manipulated by using ophthalmic lenses and prisms to mimic the visual consequences of varying target distance. Changes in vertical target location caused large changes in atlanto-occipital posture and gaze angle. Cervical posture was altered to a lesser extent by changes in vertical target location. Participants compensated for changes in backrest inclination by changing cervical posture, though they did not significantly alter atlanto-occipital posture and gaze angle. The posture adopted to view any target represents a compromise between visual and musculoskeletal demands. These results provide support for the argument that the optimal location of visual targets is at least 15 degrees below horizontal eye level. Actual or potential applications of this work include the layout of computer workstations and the viewing of displays from a seated posture.

The use of vergence information in the programming of prehension

Human prehension requires accurate information on the properties of an object and on the position of the object relative to the body. In principle, prehension might be more accurate with binocular rather than monocular vision. Previous studies have shown that the kinematics of prehension are altered when one eye is covered. Unfortunately, the source of the useful binocular information cannot be established using this approach. In the current study, we used a perturbation technique to explore whether the human nervous system uses a signal from vergence in prehension. Perturbing vergence caused predictable changes in the kinematics of prehension. Our results thus provide clear evidence that the nervous system uses vergence information in the programming of prehensile movement.

The size-distance paradox is a cognitive phenomenon

The perceived size of a fixated object is known to be a function of the perceived fixation distance. The size-distance paradox has been posited as evidence that the perceived distance of a fixated object is, in turn, influenced by the object's perceived size. If this is correct then it challenges a widely accepted account (modified weak fusion) of how the nervous system combines multiple sources of information. We hypothesised that the influence of perceived size on the perception of distance is likely to be restricted to conscious perceptual judgements. If our hypothesis is correct then the size-distance paradox should not be observed when observers make action-based distance judgements. In line with this expectation we observed the size-distance paradox when participants made verbal reports on target distance but found no paradoxical judgements in a group who were asked to point at the target. We therefore suggest that the size-distance paradox should not be taken as evidence that perceived size feeds back into distance perception.

Increasing confidence in vergence as a cue to distance

Multiple cues contribute to the visual perception of an object's distance from the observer. The manner in which the nervous system combines these various cues is of considerable interest. Although it is accepted that image cues play a significant role in distance perception, controversy exists regarding the use of kinaesthetic information about the eyes' state of convergence. We used a perturbation technique to explore the contribution of vergence to visually based distance estimates as a function of both fixation distance and the availability of retinal information. Our results show that the nervous system increases the weighting given to vergence as (i) fixation distance becomes closer; and (ii) the available retinal image cues decrease. We also identified the presence of a strong contraction bias when distance cues were studied in isolation, but we argue that such biases do not suggest that vergence provides an ineffectual signal for near-space perception.

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.

Eye movements, prematurity and developmental co-ordination disorder

Horizontal pursuit eye movements were investigated in two separate groups of children: One group exhibited developmental co-ordination disorder (n = 8) whilst another group of children were born prematurely (n = 8). Both studies found a reduced gain in pursuit eye movements when the respective populations were compared with control groups (n = 32). A difference was also found in the ability of some children to temporally synchronize their tracking response to the stimulus, which was indicative of poor predictive control rather than lags in the control system. We suggest that horizontal eye movements may be a sensitive indicator of more general motor deficits during childhood development.

Gaze angle: a possible mechanism of visual stress in virtual reality headsets

It is known that some Virtual Reality (VR) head-mounted displays (HMDs) can cause temporary deficits in binocular vision. On the other hand, the precise mechanism by which visual stress occurs is unclear. This paper is concerned with a potential source of visual stress that has not been previously considered with regard to VR systems: inappropriate vertical gaze angle. As vertical gaze angle is raised or lowered the 'effort' required of the binocular system also changes. The extent to which changes in vertical gaze angle alter the demands placed upon the vergence eye movement system was explored. The results suggested that visual stress may depend, in part, on vertical gaze angle. The proximity of the display screens within an HMD means that a VR headset should be in the correct vertical location for any individual user. This factor may explain some previous empirical results and has important implications for headset design. Fortuitously, a reasonably simple solution exists.

The contribution of vision and proprioception to judgements of finger proximity

We sought to determine whether an increase in judged egocentric distance created by increasing vergence-specified distance would be negated when participants pointed at their own finger. It was found that ocular position dominates limb proprioception in the judgement of finger distance in the sagittal plane when vision is available. In contrast, an increase in perceived egocentric distance was largely attenuated by the presence of limb proprioception in reduced visual cue conditions. We conclude that the relative contribution of vergence to perceived distance depends upon the strength of the vergence effort signal when there are other cues present. Furthermore, if the distance percept includes a major contribution from retinal cues, then the visual component will dominate the limb proprioception component. If the visual component is largely determined by vergence information, limb proprioception will make a significant contribution and actually dominate when the vergence effort signal is weak. The results extend previous studies that have found a similar relationship between ocular position and limb proprioception in the perception of a finger's location in the coronal plane.

Improving vision: neural compensation for optical defocus

Anecdotal reports abound of vision improving in myopia after a period of time without refractive correction. We explored whether this effect is due to an increased tolerance of blur or whether it reflects a genuine improvement in vision. Our results clearly demonstrated a marked improvement in the ability to detect and recognize letters following prolonged exposure to optical defocus. We ensured that ophthalmic change did not occur, and thus the phenomenon must be due to a neural compensation for the defocus condition. A second set of experiments measured contrast sensitivity and found a decrease in sensitivity to mid-range (5-25 cycles deg-1) spatial frequencies following exposure to optical defocus. The results of the two experiments may be explained by the unmasking of low contrast, high spatial frequency information via a two-stage process: (1) the pattern of relative channel outputs is maintained during optical defocus by the depression of mid-range spatial frequency channels; (2) channel outputs are pooled prior to the production of the final percept. The second set of experiments also provided some evidence of inter-ocular transfer, indicating that the adaptation process is occurring at binocular sites in the cortex.

Aftereffects and sense of presence in virtual environments: formulation of a research and development agenda

This report represents a committee summary of the current state of knowledge regarding aftereffects and sense of presence in virtual environments (VEs). The work presented in this article, and the proposed research agenda, are the result of a special session that was set up in the framework of the Seventh International Conference on Human Computer Interaction. Recommendations were made by the committee regarding research needs in aftereffects and sense of presence, and, where possible, priorities were suggested. The research needs were structured in terms of the short, medium, and long term and, if followed, should lead toward the effective use of VE technology. The 2 most critical research issues identified were (a) standardization and use of measurement approaches for aftereffects and (b) identification and prioritization of sensorimotor discordances that drive aftereffects. Identification of aftereffects countermeasures (i.e., techniques to assist users in readily transitioning between the real and virtual worlds), reduction of system response latencies, and improvements in tracking technology were also thought to be of critical importance.

Looking at the task in hand: vergence eye movements and perceived size

A retinal afterimage of the hand changes size when the same unseen hand is moved backwards and forwards in darkness. We demonstrate that arm movements per se are not sufficient to cause a size change and that vergence eye movements are a necessary and sufficient condition for the presence of the illusory size change. We review previous literature to illustrate that changing limb position in the dark alters vergence angle and we explain the illusion via this mechanism. A discussion is provided on why altering limb position causes a change in vergence and we speculate on the underlying mechanisms.

Synaesthesia in the normal limb

We explored the degree to which vision may alter kinaesthetic perception by asking participants to view their hand through a prism, introducing different horizontal deviations, while trying to align their fingers above and below a thin table. When the visual image of one hand was displaced this overwhelmed kinaesthetic judgements and participants reliably reported that they felt their limbs were aligned, even when they were laterally mis-aligned by as much as 10 cm. This effect, however, was mediated by 'visual capture' and when the task was attempted in a darkened room with limb position indicated by an LED taped to the finger, kinaesthesis dominated and participants reported that the LED seemed to become detached from their finger tip. In both light and dark conditions the finger was clearly visible and only the background detail was extinguished. Hence, in perceiving limb position, it appears that we believe in what we see, rather than in what we feel, when the visual background is rich, and in what we feel when the visual background is sparse.

Health issues with virtual reality displays

During the late 1980s and early 1990s virtual reality (VR) technology enjoyed a prolonged honeymoon with the international media who presented a glossy futuristic image of the technology. It was inevitable that the media would eventually tire of this image and look for new journalistic angles, such as the negative effects of VR. Some speculation then ensued about the negative social consequences of VR in the public domain ("Social autism" and "The end of civilisation as we know it…" --- Stone, 1992, BBC Horizon), although these speculations now appear unfounded.The first direct assertion, in the international media, that there might be visual safety issues with VR technology, came from the reporting of findings from Mon-Williams, Wann and Rushton in 1993 [5]. Since then a steady trickle of media features have strongly hinted at virtual problems that may arise (e.g. Business Week, July 10, 1995; New Scientist, Jan 27, 1996), each being followed by accusations of "scare-mongering" from some sectors of the VR community. Our research findings have fueled some of the negative reports, and our position has angered some VR protagonists. Also, like other research groups in this field, we are often approached by journalists in pursuit of a sensationalist story.Hence we think it is timely to examine what we know about the effects of virtual reality displays; what we don't know about virtual reality displays; and what research should be undertaken to resolve the unknown issues.

Natural problems for stereoscopic depth perception in virtual environments

The use of virtual reality (VR) display systems has escalated over the last 5 yr and may have consequences for those working within vision research. This paper provides a brief review of the literature pertaining to the representation of depth in stereoscopic VR displays. Specific attention is paid to the response of the accommodation system with its cross-links to vergence eye movements, and to the spatial errors that arise when portraying three-dimensional space on a two-dimensional window. It is suggested that these factors prevent large depth intervals of three-dimensional visual space being rendered with integrity through dual two-dimensional arrays.

Binocular vision in a virtual world: visual deficits following the wearing of a head-mounted display

The short-term effects on binocular stability of wearing a conventional head-mounted display (HMD) to explore a virtual reality environment were examined. Twenty adult subjects (aged 19-29 years) wore a commercially available HMD for 10 min while cycling around a computer generated 3-D world. The twin screen presentations were set to suit the average interpupillary distance of our subject population, to mimic the conditions of public access virtual reality systems. Subjects were examined before and after exposure to the HMD and there were clear signs of induced binocular stress for a number of the subjects. The implications of introducing such HMDs into the workplace and entertainment environments are discussed.