Prism adaptation with opposed base orientation: The weighting of direction information from the two eyes

The aftereffect of wearing 20-diopter vertically displacing prisms with bases oriented in opposite directions on the two eyes was measured in each eye. Five groups of 20 Ss differed in the density of a neutral filter worn over one eye. With no filter, the base-down prism dominated the directional judgments of both eyes, although there was a difference between the eyes consistent in direction with their differential treatment. The characteristics of the function relating aftereffect measures to filter density were found to depend upon the base orientation of the prism associated with the filter.     

Prism distortion and accommodative change

The literature concerning oculomotor changes during adaptation to prism distortion has dealt mainly with the question of eye movements. The present study examined accommodation during exposure to distortion by ophthalmic prisms. Ss were asked to fixate four targets of equal visual angle at four distances. Accommodation was measured at each distance by means of a Laser-Badal optometer. Repeated measures for the same target distances were obtained prior to during and after exposure to binocular prisms. The results indicated that prisms induce underaccommodation at each of the target distances, although the total range of predistortion and distortion accommodation was restricted. Significant recovery from the effects of induced underaccommodation was observed at near target distances. No significant aftereffects of wearing the prisms was observed. In a second experiment, where target background contrast was increased, consistent underaccommodation was again observed and recover was observed at the near target distance. The range of accommodative change was also considerably improved. It was concluded that the angular magnification of ophthalmic prisms induced underaccommodation. It was suggested that partial recovery from the prism effect may be related to alterations in vergence.     

Visuomotor coordination and intermanual transfer for a proprioceptive reaching task

Two experiments were conducted to determine the role of head constraint, whether present or absent and arm exposure type (terminal or continuous) on the production of intermanual transfer to two types of visual distortion. Experiment 1 investigated intermanual transfer to binocular, lateral prism displacement where the prism base orientation for both eyes was in the same direction. Experiment 2 determined whether intermanual transfer could be produced to squint prism viewing where the prism base orientation for each eye was in an opposite direction (base-out prisms). In both experiments transfer was produced when either head movement during prism exposure was unconstrained or when a terminal arm exposure was employed. Maximal transfer was produced when both of these conditions were employed.     

Additivity of aftereffects of maintained head and eye rotations: An alternative to recalibration

Seven groups of 10 subjects each were exposed to various combinations of left and right head and eye rotations for a period of 10 min. Both head and eye produced significant aftereffects of prior position as measured by pointing at a visible target with the unseen hand, but there was no significant interaction. Thus, aftereffects of sustained head and eye rotation were shown to be additive and to account fully for the results of Craske and Crawshaw (1975). Eye muscle potentiation rather than recalibration may be assumed to be the cause of the altered direction of gaze resulting from exposure to displacing prisms.     

Recalibration of the convergence system.

Adaptation to convergence-altering prism spectacles was studied. The subjects were trained for 8 min with either convergence-increasing or convergence-decreasing prisms. Before and after training they estimated the distance to a binocular dot varying in the stimulus it provided to convergence. Both the preadaptation and postadaptation estimates showed that the reciprocal of estimated target distance is linearly related to convergence. Further there was a consistent change in the direction of adaptation expected from the preadaptation to the postadaptation test. This change was accounted for in terms of a recalibration of the effective absolute convergence level.     

Eye-position aftereffects of backward head tilt manifested by illusory visual direction

Three experiments showed posttest-minus-pretest shifts in subjective straight-ahead eye position when subjects read for 3, 6, or 9 min with their heads tilted back 20° from upright. These shifts were significant relative to control conditions in which subjects read with their heads upright. All subjects read with the same straight-ahead eye-in-head position. Variability-reducing procedures were developed to provide better measures over Experiments 1, 2. and 3. Explanations in terms of deliberate compensation, head-position asymmetries, eye-position asymmetries, and progressive error were ruled out. It was hypothesized that the shifts were caused by negative aftereffects of compensation for the doll reflex. The doll reflex rotates the eyes down without central registration. causing an upward illusory shift of visual direction similar to what is caused by wedge prisms. Perceptual-motor adaptation to this shift, i.e., doll adaptation, causes an illusory shift in the opposite direction when the head is returned to upright.     

Left to right: representational biases for numbers and the effect of visuomotor adaptation

Adaptation to right-shifting prisms improves left neglect for mental number line bisection. This study examined whether adaptation affects the mental number line in normal participants. Thirty-six participants completed a mental number line task before and after adaptation to either: left-shifting prisms, right-shifting prisms or control spectacles that did not shift the visual scene. Participants viewed number triplets (e.g. 16, 36, 55) and determined whether the numerical distance was greater on the left or right side of the inner number. Participants demonstrated a leftward bias (i.e. overestimated the length occupied by numbers located on the left side of the number line) that was consistent with the effect of pseudoneglect. The leftward bias was corrected by a short period of visuomotor adaptation to left-shifting prisms, but remained unaffected by adaptation to right-shifting prisms and control spectacles. The findings demonstrate that a simple visuomotor task alters the representation of space on the mental number line in normal participants.     

Serial adaptation to conflicting prismatic rearrangement effects in monkey and man.

Seven adult male squirrel monkeys adapted successfully to prisms displacing in equal and opposite directions. After protracted training under certain conditions, they succeeded in conserving adaptation to both prisms without benefit of readaptation trials. Results obtained with thirty-six undergraduates under similar conditions of test (although for a much shorter period) are also reported.     

Determinants of the rod and frame effect: The role of retinal size

In Experiment 1, base-out prisms were used to alter perceived size and distance to a luminous rod and frame while the retinal size remained unchanged. The rod-and-frame effect (RFE) was the same, whether the display was viewed directly or through the prisms. In Experiment 2, one large and one small rod-and-frame display were placed at distances such that they produced identical retinal angles. This was replicated at three different sets of distances. Perceived size and distance of the large and small frame of identical retinal angle interacted with the observation distance, such that at near distances the large frame was perceived as larger and farther than the small frame while, at far distances, both types of estimates converged to a constant value. In contrast, the RFE was identical for the large and small frames matched in retinal angle, but diminished with distance. In both experiments, the RFE varied precisely with variation in retinal angle. Implications of the role of retinal angle in the RFE and for the interpretation of individual differences were discussed.     

Adaptation of the two arms to opposite prism displacements

In normal subjects, the two arms were exposed separately to prismatic displacements of opposite sign, using the eye ipsilateral to the exposed arm. Opposite adaptive shifts were induced on each arm whether the eye ipsilateral to the arm (i.e. exposed to a displacement of the same sign as the arm) or the eye controlateral to the arm (i.e. exposed to a displacement of opposite sign) was used during testing. This result precludes the possible role of oculomotor signals in this type of prism adaptation.     

Prism Adaptation and Aftereffect: Specifying the Properties of a Procedural Memory System

Prism adaptation, a form of procedural learning, is a phenomenon in which the motor system adapts to new visuospatial coordinates imposed by prisms that displace the visual field. Once the prisms are withdrawn, the degree and strength of the adaptation can be measured by the spatial deviation of the motor actions in the direction opposite to the visual displacement imposed by the prisms, a phenomenon known as aftereffect. This study was designed to define the variables that affect the acquisition and retention of the aftereffect. Subjects were required to throw balls to a target in front of them before, during, and after lateral displacement of the visual field with prismatic spectacles. The diopters of the prisms and the number of throws were varied among different groups of subjects. The results show that the adaptation process is dependent on the number of interactions between the visual and motor system, and not on the time spent wearing the prisms. The results also show that the magnitude of the aftereffect is highly correlated with the magnitude of the adaptation, regardless of the diopters of the prisms or the number of throws. Finally, the results suggest that persistence of the aftereffect depends on the number of throws after the adaptation is complete. On the basis of these results, we propose that the system underlying this kind of learning stores at least two different parameters, the contents (measured as the magnitude of displacement) and the persistence (measured as the number of throws to return to the baseline) of the learned information.     

The interaction of perceived distance with the perceived direction of visual motion during movements of the eyes and the head.

A horizontally moving target was followed by rotation of the eyes alone or by a lateral movement of the head. These movements resulted in the retinal displacement of a vertically moving target from its perceived path, the amplitude of which was determined by the phase and amplitude of the object motion and of the eye or head movements. In two experiments, we tested the prediction from our model of spatial motion (Swanston, Wade, & Day, 1987) that perceived distance interacts with compensation for head movements, but not with compensation for eye movements with respect to a stationary head. In both experiments, when the vertically moving target was seen at a distance different from its physical distance, its perceived path was displaced relative to that seen when there was no error in perceived distance, or when it was pursued by eye movements alone. In a third experiment, simultaneous measurements of eye and head position during lateral head movements showed that errors in fixation were not sufficient to require modification of the retinal paths determined by the geometry of the observation conditions in Experiments 1 and 2.     

The interaction of perceived distance with the perceived direction of visual motion during movements of the eyes and the head

A horizontally moving target was followed by rotation of the eyes alone or by a lateral movement of the head. These movements resulted in the retinal displacement of a vertically moving target from its perceived path, the amplitude of which was determined by the phase and amplitude of the object motion and of the eye or head movements. In two experiments, we tested the prediction from our model of spatial motion (Swanston, Wade, & Day, 1987) that perceived distance interacts with compensation for head movements, but not with compensation-for eye movements with respect to a stationary head. In both experiments, when the vertically moving target was seen at a distance different from its physical distance, its perceived path was displaced relative to that seen when there was no error in pereived distance, or when it was pursued by eye movements alone. In a third experiment, simultaneous measurements of eye and head position during lateral head movements showed that errors in fixation were not sufficient to require modification of the retinal paths determined by the geometry of the observation conditions in Experiments 1 and 2.     

Oculomotor adaptation to prisms is not simply a muscle potentiation effect

An experiment is reported in which subjects pointed to a visual target before and after exposure to prisms. The exposure condition required the subject to look at his feet through leftward deviating prisms while holding his eyes to the right. Aftereffects on pointing were significantly to the right. This result is opposite to that predicted by the muscle potentiation hypothesis put forward by Ebenholtz and Wolfson (1975), but consistent with recalibration of the visual direction system caused by spatial discordance.     

Contralateral transfer of inhibitory and excitatory eyelid conditioning in the rabbit

In Experiment I, rabbits received training to establish a clicker as a conditioned inhibitor. In a subsequent test phase this stimulus was used as a signal for shock either to the eye reinforced during initial training or to the opposite eye. Learning to the clicker was slower in both conditions than in the appropriate control groups. The second experiment replicated the results of those subjects trained and tested with opposite eyes and ruled out the possibility that the slower learning was due to the effects of latent inhibition. Experiment III demonstrated that excitatory conditioning to a clicker to one eye facilitated future excitatory conditioning to that stimulus to the opposite eye. These results are consistent with the view that inhibitory and excitatory conditioning both involve the acquisition of a general, motivational conditioned response which is capable of mediating the transfer of conditioning across different response systems.     

Alternating Adaptation of Eye and Hand Movements to Opposite Directed Double Steps

Eye and hand movements can adapt to a variety of sensorimotor discordances. Studies on adaptation of movement directions suggest that the oculomotor and the hand motor system access the same adaptive mechanism related to the polarity of a discordance, because concurrent adaptations to opposite directed discordances strongly interfere. The authors scrutinized whether participants adapt their hand and eye movements to opposite directions (clockwise/counterclockwise) when both motor systems are alternatingly exposed to opposite directed double steps, and whether such adaptation is influenced by the allocation of effector to adaptation direction. The results showed that hand and eye movements adapted to opposite directions, but adaptation was biased to the counterclockwise direction. Aftereffects emerged nearly unbiased and independently for both motor systems. The authors conclude that the oculomotor and the hand motor system use independent mechanisms when they adapt to opposite polarities, although they interact during adaptation or concurrent performance.     

No retinal component in prism adaptation

Cohen's (1966) report of a retinal component in adaptation to prisms was re-examined and some possible artifacts identified. An experiment is briefly reported that overcame these problems. Although adaptation of registered eye position was found, there was no evidence for any retinal involvement.     

In sight, out of mind: the role of eye movements in the rapid resumption of visual search

Three experiments investigated the role of eye movements in the rapid resumption of an interrupted search. Passive monitoring of eye position in Experiment 1 showed that rapid resumption was associated with a short distance between the eye and the target on the next-to-last look before target detection. Experiments 2 and 3 used two different methods for presenting the target to the point of eye fixation on some trials. If eye position alone is predictive, rapid resumption should increase when the target is near fixation. The results showed that gaze-contingent targets increased overall search success, but that the proportion of rapid responses decreased dramatically. We conclude that rather than depending on a high-quality single look at a search target, rapid resumption of search depends on two glances; a first glance in which a hypothesis is formed, and a second glance in which the hypothesis is confirmed.     

Visually directed pointing as a function of target distance,direction, and available cues

In pointing at visual targets without sight of the hand, large errors occur. There is a tendency to overreach targets, and this tendency is much greater (about 25 em) when convergence is the only cue to distance than when there are many cues (2–11 cm). Angular errors of up to 10 deg also occur. These tend to be to the side opposite the sighting eye, when the favored hand is used. The variance of the pointing response with convergence alone is reduced by approximately half with the introduction of several spatial cues. These results are interpreted as indicating that, for a target within the reach of the arm and with convergence alone as a cue, the depth signal produced by the visual system corresponds to a greater distance than that produced when many cues are available. The results are also consistent with the hypothesis that perceived direction tends to approximate direction from the sighting eye.     

Gaze direction and the extraction of egocentric distance

The angular declination of a target with respect to eye level is known to be an important cue to egocentric distance when objects are viewed or can be assumed to be resting on the ground. When targets are fixated, angular declination and the direction of the gaze with respect to eye level have the same objective value. However, any situation that limits the time available to shift gaze could leave to-be-localized objects outside the fovea, and, in these cases, the objective values would differ. Nevertheless, angular declination and gaze declination are often conflated, and the role for retinal eccentricity in egocentric distance judgments is unknown. We report two experiments demonstrating that gaze declination is sufficient to support judgments of distance, even when extraretinal signals are all that are provided by the stimulus and task environment. Additional experiments showed no accuracy costs for extrafoveally viewed targets and no systematic impact of foveal or peripheral biases, although a drop in precision was observed for the most retinally eccentric targets. The results demonstrate the remarkable utility of target direction, relative to eye level, for judging distance (signaled by angular declination and/or gaze declination) and are consonant with the idea that detection of the target is sufficient to capitalize on the angular declination of floor-level targets (regardless of the direction of gaze).