Preferred gaze direction and the vergence system at rest]

In two experiments the effects of binocular vergence, head tilt, and prolonged gaze inclination on the preferred vertical gaze direction were studied. Concurrently, dark vergence was measured to test a hypothesis on the effect that binocular vergence has on the preferred vertical direction of gaze. The previously reported influence of observation distance on the preferred gaze inclination was replicated. When the head was tilted backward, the preferred gaze inclination was lower and the effect of observation distance was reduced; when the head was tilted forward, the preferred gaze inclination was higher and the effect of observation distance was increased. After prolonged upward or downward gaze inclination the preferred vertical gaze direction was shifted up or down, respectively, while the effect of observation distance remained constant. Overall the change of the preferred vertical direction of gaze upon binocular fixation of near stimuli was accompanied by a reduced discrepancy between convergence and the resting state of the vergence system; additional implications of the hypothesis on the relation between individual resting vergence and the effect of observation distance on the preferred inclination of gaze could not be substantiated.     

Vertical gaze direction and the resting posture of the eyes

With horizontal gaze, the resting posture of binocular vergence typically corresponds to a distance of about 1 m. The effect of vertical direction of gaze on this basic resting posture was investigated. The dark vergence of twenty-four subjects was measured while they fixated a dim monocular light-point at vertical directions ranging from -45 degree (lowered) to +30 degrees (elevated). In one condition, gaze was varied by changes in eye position with the head held upright; in a second condition, gaze was varied by changes in head inclination with the eyes held in constant (horizontal) position with respect to the head. In both conditions, dark vergence shifted in the convergent (nearer) direction with lowered gaze and in the divergent (farther) direction with elevated gaze. The effect of varied eye inclination was larger, more variable across subjects, and more stable over time than that of varied head inclination. These findings indicate that multiple mechanisms contribute to gaze-related variations of the resting posture of the eyes. They may help to explain the variations of space perception and visual fatigue that are observed with different gaze inclinations.     

Oculomotor tonus and visual adaptation

The contribution of oculomotor efference to visual perception and performance can be clarified by considering the functions of tonus. Accommodation and vergence typically ‘relax’ at an intermediate distance, reflecting tonic innervation of the ciliary and extraocular muscles, which varies widely among individuals who have normal vision. In many situations, especially when stimulation is degraded, fixation and focusing responses are biased toward the individual's resting state. Moreover, unusual circumstances, such as alteration of the relation of eye position and distance or prolonged exposure to near work, induce adaptive modification of the resting posture. These normal variations of oculomotor toms affect the accuracy and the effort required to fixate objects, and they may help explain problems of space perception and visual fatigue.     

Distance perception within near visual space

We investigated the perception of distance of visual targets with constant size and luminance presented between 20 and 120 cm from subjects' eyes. When retinal disparity cues were present, the subjects could reproduce very accurately the distance of a seen reference in this area. When only extraretinal information was available, distance perception was still correct for distances of 40 cm or less. However, distances beyond 60 cm were underestimated. When forced to evaluate the distance between a reference and themselves, e.g. when evaluating the absolute distance or half the distance or twice the distance of a reference, subjects used an egocentric plane of reference located on average 10.4 cm in front of their eyes. Measurements of binocular eye movements indicated a clear relationship between vergence angle and target distance. The egocentric plane of reference at 10.4 cm also corresponds to the maximum achievable vergence. These results suggest that ocular convergence can be used as a reliable cue for distance within the arm's reaching space.     

Oculomotor adjustments in darkness and the specific distance tendency

Two experiments were performed to investigate the relationship between the oculomotor adjustments assumed in total darkness and perceived distance under reduced visual conditions. Experiment I compared the dark focus of accommodation with the perceived distance of a monocular light point presented in a dark environment. Experiment II compared the convergence angle assumed in darkness (dark convergence) with the perceived distance of the light point. Both accommodation and convergence were found to assume intermediate values in darkness. Perceived distance of the monocular light point was significantly correlated with dark convergence and unrelated to the dark focus of accommodation. It was suggested that ocular vergence is a major determinant of perceived distance under reduced visual conditions, and thus provides a possible mechanism for the specific distance tendency.     

Apparent size as a function of vertical gaze direction: new tests of an old hypothesis

The hypothesis was tested that the decline of apparent size with elevated gaze results from a latent tendency of the eyes to diverge and thus increased vergence effort. Through the use of a method of category estimation, the decline of apparent size on elevation or depression of gaze was found not to be different between subjects with larger or smaller changes of dark vergence and thus vergence effort. In a 2nd experiment, vergence effort was varied by varying gaze elevation and the angle of convergence. With vergence effort constant, apparent size was dependent on the angle of convergence rather than being constant. It is concluded that apparent size does not depend on vergence effort and that the effect of gaze elevation on apparent size cannot be attributed to its concomitant effect on dark vergence.     

Vergence eye movements during figure-ground perception

Figure-ground, that is the segmentation of visual information into objects and their surrounding backgrounds, provides structure for visual attention. Recent evidence shows a novel role of vergence eye movements in visual attention. In the present work, vergence responses during figure-ground segregation tasks are psychophysically investigated. We show that during a figure-ground detection task, subjects convergence their eyes. Vergence eye movements are larger in figure trials than in ground trials. In detected figures trials, vergence are stronger than in trials where the figure went unnoticed. Moreover in figure trials, vergence responses are stronger to low-contrast figures than to high-contrast figures. We argue that these discriminative vergence responses have a role in figure-ground.     

Localization of targets by strabismic subjects: Contrasting patterns in constant and alternating suppressors

Strabismic subjects in whom vision in one eye is constantly suppressed localized targets at different points in space when using each eye. Whether they were esotropes or exotropes determined the relation between the two locations and the angle of their vergence error correlated with the magnitude of the interocular discrepancy. These relationships suggest that constant suppressors utilize information about posture of the dominant eye in reaching toward targets presented to the normally suppressed eye. Neither orthotropes nor strabismics in whom vision in each eye is alternately suppressed showed this anomaly. These results were attributed to differences in the way in which visual space is represented, an outcome of the pattern of use of the eyes during early development.     

Perceived size and perceived distance in stereoscopic vision and an analysis of their causal relations

Effects of visual angle and convergence upon the perceived sizes and perceived distances of a familiar object (playing card) and a nonrepresentational object (blank white card) were investigated by means of a projector stereoscope with polarizing filters. The results obtained with six Ss indicated that size estimates increased nearly proportionally as the visual angle increased and decreased nearly linearly as the convergence increased. Distance estimates decreased nearly linearly as either the visual angle or the convergence increased. The ratio of the size estimate to the distance estimate for a given visual angle was almost constant irrespective of convergence. In this sense, the size-distance invariance hypothesis held. No clear effect of familiarity was found. Partial correlations were used to discriminate direct and indirect causal relationships between the stimulus variables and perceptual estimates. Both perceived size and perceived distance were found to be determined directly by the two stimulus variables, but to be mutually related only indirectly.     

True three-dimensional displays that allow viewers to dynamically shift accommodation, bringing objects displayed at different viewing distances into and out of focus.

Under natural viewing conditions, viewers do not just passively perceive. Instead, they dynamically scan the visual scene by shifting their eye fixation and focus between objects at different viewing distances. In doing so, the oculomotor processes of accommodation (eye focus) and vergence (angle between lines of sight of two eyes) must be shifted synchronously to place new objects in sharp focus in the center of each retina. Accordingly, nature has reflexively linked accommodation and vergence, such that a change in one process automatically drives a matching change in the other. Conventional stereoscopic displays force viewers to try to decouple these processes, because while they must dynamically vary vergence angle to view objects at different stereoscopic distances, they must keep accommodation at a fixed distance--or else the entire display will slip out of focus. This decoupling generates eye fatigue and compromises image quality when viewing such displays. In an effort to solve this accommodation/vergence mismatch problem, we have built various prototype displays that can vary the focus of objects at different distances in a displayed scene to match vergence and stereoscopic retinal disparity demands and better simulate natural viewing conditions. By adjusting the focus of individual objects in a scene to match their stereoscopic retinal disparity, the cues to ocular accommodation and vergence are brought into agreement. As in natural vision, the viewer brings different objects into focus by shifting accommodation. As the mismatch between accommodation and vergence is decreased, natural viewing conditions are better simulated and eye fatigue should decrease.     

The perception of size and distance under monocular observation

A relative-perceived-size hypothesis is proposed to account for the perception of size and distance under monocular observation in reduced-cue settings. This hypothesis is based on two assumptions. In primary processing, perceived size is determined by both proximal stimulation on the retina and distance information from primary cues such as oculomotor cues. In secondary processing, the relation of two primary perceived sizes determines another relation of secondary perceived distances, so that an object of smaller primary perceived size is judged to be further away. An experiment was designed to test this hypothesis, especially the assumption of secondary processing, by making ratio judgments of perceived size and perceived distance for two successively presented targets. The Standard square was presented at a constant distance and varied in visual angle; the variable square was presented with a constant visual angle in distance. The results showed that an inverse relation between size and distance estimates held regardless of whether the visual angles of the targets were the same or different.     

Distance adaptation depends upon plasticity in the oculomotor control system

Maintaining binocular fixation on a target at 20 cm in the absence of secondary cues to distance produced changes in apparent distance and lateral phoria. Positive lenses of 0, .5, 2.0, 3.5, and 5.0 spherical diopters (SD) were used to manipulate the level of accommodative convergence in force during the period of maintained fixation. An inverse relationship was found between the stimulus to accommodation and the magnitude of the induced esophoria, the phoria being linearly related to an increase in apparent distance. The distance aftereffect obtained in the condition with the lowest net accommodative stimulus li.e., 0 D) equaled that typically produced by base-out prism adaptation with full secondary cues to distance available. In a second experiment, subjects walked through a well-lit hallway while viewing through a pair of 5h base-out prisms. It was shown that increasing the stimulus to accommodation by adding negative lenses of 0, 1.5, 3.5, and 5.5 SD reduced the adaptive change in apparent distance, as well as the change in phoria produced by the conventional base-out prism adaptation paradigm. It was concluded that a change in the resting tonus of the disparity vergence system underlies such adaptation, rather than recalibration of the oculomotor cues to distance. Monocular exposure data indicated that a small change in the tonus control for the accommodative system may be present as well.     

Oculomotor Effects in the Size-Distance Paradox and the Moon Illusion

Drawing on 2 concepts—the resting position of the eyes and a binocular geometry for perceived size, the moon illusion is explained as the consequence of different oculomotor adjustments caused by change in the direction of gaze contingent upon the viewing conditions of the moon. Hence, each particular moon will be viewed with a different vergence state which, in turn, yields a different amount of binocular disparity. The vergence state will determine the perceived size of an object whereas disparity will determine its perceived distance. It is further contended that the perceived size of the moon is based on a new binocular information source for size perception enabling the size of an object to be perceived even in the absence of egocentric distance information. Discussion focuses on the paradoxical aspect of the moon illusion and how the size-distance invariance hypothesis may have contributed to its effect.     

Perceptual aftereffects of sustained fixation and oculomotor changes

Several conditions induce correlated perceptual and oculomotor changes. Often the perceptual aftereffects are treated as a result of the oculomotor ones. Some results are reported that cast doubt on this interpretation of the aftereffects of sustained convergence. First, the inspection distance that induces no aftereffect is nearer than the vergence resting position. Second, there are at least two situations in which the expected correlation between perceptual and oculomotor changes breaks down: (1) inspection of a near stimulus below eye level tends to induce a subsequent overestimation of the distance of a test stimulus that is accompanied by an exophoric shift instead of the expected exophoric one, and (2) closing one's eyes for some minutes induces an exophoric shift without a perceptual aftereffect. These results suggest that at least in some situations perceptual and oculomotor changes may be functionally independent.     

Variation of stereoscopic acuity with observation distance and eye movement characteristics

It has puzzled research workers in perception that stereoscopic acuity decreases with decreasing observation distance, or, equivalently, increasing angle of convergence. Experimental results presented in this paper, as well as evidence published previously, show that the reduction in acuity might be due to rotation of the eyes around their visual axes during convergence which would introduce a disparity for the test lines. Stereoscopic acuity is known to be worse for disparate images. It was proved experimentally that cyclodisparity did lead to a decrease of stereoscopic acuity, supporting this explanation.     

Effect of disparity and viewing distance on perceived depth

It is shown that veridical depth perception presupposes the processing of both the magnitude of retinal disparity and observation distance according to a square-law function specified by the underlying geometrical stimulus relations. In the present study, after testing its existence, this constancy of depth perception was investigated by measuring perceived depth as a function of retinal disparity and observation distance. In addition, the relative effectiveness of convergence and accommodation as possible indicators of distance was examined through a conflicting-cues paradigm. It was shown that in the perception of depth the visual system computes distance by taking into account the convergence parameter only, rather than that of accommodation or of both.     

Dark vergence in human infants: implications for the development of binocular vision

The resting position of binocular alignment in 1- to 4-month-olds, 6- to 18-month-olds, and adults was estimated by obtaining photographic measures of interpupillary distance in total darkness. The mean dark vergence position in the adult group corresponded to a distance of approximately 100 cm, similar to a previous estimate of 120 cm (Owens and Leibowitz 1976). The mean dark vergence position in the group of 1- to 4-month-olds was approximately 25 cm and in the group of 6- to 18-month-olds was approximately 50 cm. These near dark vergence positions in infants provide strong evidence that inaccuracies in binocular fixation during early infancy are not the result of a divergence bias. The developmental implications of dark vergence, accommodative vergence, and fusional vergence for the control of binocular alignment and distance perception during infancy are discussed.     

Depth,size and distance in stereoscopic vision

Evidence is presented to show that in stereoscopic vision a constant ratio of perceived size to perceived distance corresponds to a constant visual angle (the size-distance invariance hypothesis). The functions relating the size/ distance ratio to visual angle and the depth/distance ratio to disparity are determined for three as using the methods of magnitude estimation and magnitude production. The results for each a may be represented by power functions, the depth/ distance function having the higher exponent. These scales are used to predict the outcome of an experiment in which depth is matched to size. The agreement of predictions with results is good for the combined data of the group, but significant deviations occur from curves predicted for individual as. An experiment in which an oblique line is matched to a frontal extent yields data consistent with Luneburg’s hypothesis that the intrinsic geometry of visual space is non-Euclidean. The indicated curvature is negative for two as and varying from positive to negative for the third.     

Relations among age, visual memory, and resting cerebral metabolism in 40 healthy men

Visual memory, as measured by the Benton Visual Retention Test (BVRT), and resting regional cerebral metabolic rates for glucose (rCMRglc), as measured by positron emission tomography (PET) and [18-F]fluorodeoxyglucose, were examined in 40 very healthy men, aged 21 to 83 years. Age-related differences in visual memory were found but were significantly smaller than differences reported in the general population. This discrepancy is attributable to our rigorous health screening. The age-related differences found in this sample are estimates of the lower limit of population differences unconfounded by disease. Age-related differences on undistracted delayed visual memory were greater than differences on immediate visual memory, suggesting age-related differences in spontaneous elaborative visual information processing. No relation between visual memory and resting rCMRglc was found, supporting the hypothesis that mental abilities are unrelated to resting brain metabolism unless both functions are influenced by disease.     

A comparison of accommodative and fusional convergence as cues to distance

The effectiveness of fusional as compared with accommodative convergence (with accommodation present in both cases) in determining perceived distance was investigated in this study. Luminous frames of two different visual angles at a nearly constant distance were viewed binocularly to provide fusional convergence and monocularly to provide accommodative convergence. Although some differences in reported size and distance of the frames occurred on the first presentations for binocular as compared to monocular observation, the most systematic differences between these two types of observation were present for the second (successive) presentations of the two frame sizes to the same Os. This is attributed to the relative size cue to distance occurring as a function of the different retinal sizes on the successive presentations. It was found that this relative size cue was more effective in modifying the perceived size and distance of the second presentations for monocular than for binocular observation. It is suggested that this reflects the greater effectiveness as a cue to distance of fusional as compared with accommodative convergence. This conclusion is of importance for studies concerned with the evaluation of convergence as a determiner of perceived distance.