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.     

Oculomotor correlates of context-guided learning in visual search

Previous studies have shown that context-facilitated visual search can occur through implicit learning. In the present study, we have explored its oculomotor correlates as a step toward unraveling the mechanisms that underlie such learning. Specifically, we examined a number of oculomotor parameters that might accompany the learning of context-guided search. The results showed that a decrease in the number of saccades occurred along with a fall in search time. Furthermore, we identified an effective search period in which each saccade monotonically brought the fixation closer to the target. Most important, the speed with which eye fixation approached the target did not change as a result of learning. We discuss the general implications of these results for visual search.     

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.     

Oculomotor adaptation to wedge prisms with no part of the body seen

When S looks at a visual target through prisms, adaptive shifts in reaching behavior occur even though he sees no part of his body through the prisms. These shifts are caused by a change in the judgment of the direction of gaze (oculomotor change), which in turn is caused by two secondary prismatic effects: (a) asymmetry of the visual display and (b) apparent rotation about a vertical axis of a panel or wall facing S. The “asymmetry” factor contributes 22% of the total oculomotor change, and the “rotation” effect contributes the remaining 78%. Oculomotor change is not facilitated by eye-movement activity. The adaptive oculomotor change induces a non-adaptive proprioception change about one-tenth as large as the oculomotor change. These findings are capable of accounting for the previously unexplained results reported by Wooster in 1923, and also for the current controversy about the role of reafferent stimulation in sensorymotor adaptation.     

Oculomotor adaptation to wedge prisms with no part of the body seen

When S looks at a visual target through prisms, adaptive shifts in reaching behavior occur even though he sees no part of his body through the prisms. These shifts are caused by a change in the judgment of the direction of gaze (oculomotor change), which in turn is caused by two secondary prismatic effects: (a) asymmetry of the visual display and (b) apparent rotation about a vertical axis of a panel or wall facing S. The “asymmetry” factor contributes 22% of the total oculomotor change, and the “rotation” effect contributes the remaining 78%. Oculomotor change is not facilitated by eye-movzment activity. The adaptive oculomotor change induces a non-adaptive proprioception change about one-tenth as large as the oculomotor change. These findings are capable of accounting for the previously unexplained results reported by Wooster in 1923, and also for the current controversy about the role of reafferent stimulation in sensorymotor adaptation.     

Model of visual adaptation and contrast detection

A three-component model of spatial vision is proposed, consisting of (1) a feedback stage, (2) a feedforward stage, (3) a threshold detector. The components correspond to physiological processes; in particular, the feedforward control signal corresponds to the “surround’s” signal in the receptive fields of retinal ganglion cells. The model makes appropriate qualitative predictions of: (l)a square-root law (Δl ∞ l^1/2) for detection at low luminances, (2) a Weber law (Δl ∞ l) at high luminances, (3) additivity of threshold masking effects at high background luminances, (4) receptive fields that, in the dark, consist only of an excitatory center and that, in the light, also contain inhibitory surrounds, (5) the variation of spatial characteristics of receptive fields depending on the temporal characteristic of the test stimulus used to measure them, (6) the subjective appearance of Mach bands, (7) sine-wave contrast-threshold transfer functions, (8) the frequent failure of disk-detection experiments to demonstrate inhibitory surrounds, and (9) various second-order threshold effects, such as reduced spatial integration for long-duration stimuli, reduced temporal integration for large-area stimuli, and the increased effect of background luminance on the detection of large-area stimuli. Predictions are improved by assuming there exist various sizes of receptive fields that determine thresholds jointly.     

Perception and tonus

Abstract.— A review is given of the achievements of more than one hundred years of research concerning the influence of muscular tonus on the perceptual phenomena related to human spatial orientation. The tonal conditions of both the extrinsic eye muscles and the complex system of skeletal muscles have been shown to have decisive influence on "egocentric localization". The factual median is perceived as being displaced in the direction of the side of the body with greater tonus. This is the case whether the tonic asymmetry is experimentally induced or natural. Corresponding to this fact, a spot of light in the dark situated at the factual eye level will be perceived as being situated above the experienced eye level. The influence on egocentric localization exercised by prolonged fixation of greater or smaller parts of the muscular system (especially of the eye muscles) is pointed out. This "fixation effect" seems to be in accordance with conceiving the postural system of the body by analogy with a servo-mechanical system, the feed-back part of which is restrained.     

Decay of visual adaptation to tilt and displacement

Persistence in the dark following 48 min of visual adaptation to tilt and displacement was compared in two experiments to determine if same or different processes are involved in the two kinds of adaptation. Decay of tilt adaptation occurred rapidly, all within about 16 min. However, it was not complete and some residual tilt adaptation persisted for at least as long as 56 min. Decay of displacement adaptation occurred more slowly but was clearly complete after at most 56 min in the dark. Displacement adaptation appears to be entirely subject to decay, while tilt adaptation involves an additional, more long-term component. Results are interpreted in terms of independent systems for the perception of location and orientation.     

Effects of movement duration and visual feedback on visual and proprioceptive components of prism adaptation

While looking through laterally displacing prisms, subjects pointed sagittally 80 times at an objectively straight-ahead target, completing a reciprocal out-and-back pointing movement ever 1, 3, or 6 s. Visual feedback was available early in the pointing movement or only late at the end of the movement. Aftereffect measures of adaptive shift (obtained after every 10 pointing trials) showed adaptive change only in limb position sense (i.e., proprioceptive adaptation) when movement duration was 1 s, regardless of visual feedback condition; but as movement duration increased, adaptive change in the eye position sense (i.e., visual adaptation) increased while proprioceptive adaptation decreased, especially for the late visual feedback condition. Regardless of visual feedback condition, proprioceptive adaptation showed the maximal rate of growth with the 1-s movement duration, whereas visual adaptation showed maximal growth with the 6-s movement duration. These results provide additional support for a model of adaptive spatial mapping in which the direction of strategically flexible coordination (guidance) between eye and limb (and consequently the locus of adaptive spatial mapping) is jointly determined by movement duration and timing of visual feedback. An additional effect of movement duration is to determine the rate of discordant inputs. Maximal growth of adaptation occurs when the input rate matches the response time of the spatial mapping function.     

Modulation of gait during visual adaptation to dark

The authors investigated the modulation of gait during dark adaptation. Twenty-five women (mean age = 72 years, SD = 5 years) walked back and forth on an arbitrarily uneven walkway during normal lighting at speeds ranging from slow to fast. Participants then performed 20 trials at preferred speed after sudden reduction of lighting; the authors compared those trials with point estimates at equivalent speeds representing normal lighting. The authors estimated speed, cadence, mediolateral trunk acceleration, and mediolateral interstep trunk-acceleration variability for each trial. Participants compensated for sudden reduction of lighting by reducing their walking speed. Compared with performance at equivalent speeds during normal lighting, cadence, trunk acceleration, and interstep trunk-acceleration variability initially increased. All variables showed an asymptotic approximation toward normal values during 60-90 s of walking in subdued lighting. The authors suggest that the sudden transition from normal to marginal lighting, rather than marginal lighting itself, may challenge locomotor control.     

Reduced felt arm sensation effects on visual adaptation

Proprioception is often considered to be critically involved in producing adaptation to a prism-induced visual displacement. The present study focused on reduction of proprioceptive feedback during prism exposure by means of hypnotically induced anesthesia in the adapting arm. In addition, intermanual transfer was considered. Results showed adaptation occurring in situations where S could feel arm sensations while viewing arm movement during a prism exposure. However, if the adapting arm was hypnotically anesthetized while still remaining mobile, adaptation did not occur. No intermanual transfer was found between the adapted arm and the unadapted arm.     

Concurrent adaptation to opposite visual distortions: impairment and cue

The present study compared single and dual adaptation to visuomotor rotations in different cueing conditions. Participants adapted either to a constant rotation or to opposing rotations (dual adaptation) applied in an alternating order. In Experiment 1, visual and corresponding postural cues were provided to indicate different rotation directions. In Experiment 2, either a visual or a postural cue was available. In all cueing conditions, substantial dual adaptation was observed, although it was attenuated in comparison to single adaptation. Analysis of switching costs determined as the performance difference between the last trial before and the first trial after the change of rotation direction suggested substantial advantage of the visual cue compared to the postural cue, which was in line with previous findings demonstrating the dominance of visual sense in movement representation and control.     

Accuracy and adaptation of reaching and pointing in pitched visual environments

Visually perceived eye level (VPEL) and the ability of subjects to reach with an unseen limb to targets placed at VPEL were measured in a statically pitched visual surround (pitchroom). VPEL was shifted upward and downward by upward and downward room pitch, respectively. Accuracy in reaching to VPEL represented a compromise between VPEL and actual eye level. This indicates that VPEL shifts reflect in part a change in perceived location of objects. When subjects were provided with terminal visual feedback about their reaching, accuracy improved rapidly. Subsequent reaching, with the room vertical, revealed a negative aftereffect (i.e., reaching errors that were opposite those made initially in the pitched room). In a second study, pointing accuracy was assessed for targets located both at VPEL and at other positions. Errors were similar for targets whether located at VPEL or elsewhere. Additionally, pointing responses were restricted to a narrower range than that of the actual target locations. The small size of reaching and pointing errors in both studies suggests that factors other than a change in perceived location are also involved in VPEL shifts.     

Oculomotor adjustments and size constancy

The effect on matched size of the oculomotor adjustments was determined by stimulation and relaxation of accommodation and convergence by means of spherical lenses. The normal coupling between accommodation and convergence was maintained by introducing the amount of convergence appropriate to the lens power and each S’s interpupillary distance. Data indicate that the oculomotor adjustments are adequate to account for size constancy up to approximately 1 m, beyond which their effect progressively decreases. The actual accommodation in force was assessed by means of the laser scintillation technique. It was determined that the magnitude of accommodation responds accurately to the spherical lens introduced up to about 1 m observation distance, beyond which underaccommodation was noted. Examination of the matched size as a function of the actual accommodation distance reveals a very close correspondence to the size constancy prediction up to about 1 m.     

Frequency specific chromatic adaptation in the human visual system

Subjects were exposed to a vertical chromatic grating alternating with horizontal chromatic grating of the identical frequency. They were then tested with a series of test gratings of varied spatial frequencies to examine whether the responses were effected by the spatial frequency of the adaptation pattern in relation to the test pattern. It was found that maximum response occurred when adaptation and test gratings had the same spatial frequency, the effect was asymmetric and finally that enhancements were found at octaves. Thus the experiment further demonstrated that neural elements specific to spatial frequency exist in the human visual system.     

Oculomotor adjustments and size-distance perception

The relationship between perceived size and distance and oculomotor adjustments were assessed in two experiments. In both experiments, Ss were required to make scalar linear size, angular size, and distance judgments of stimuli subtending a constant retinal image size at different levels of convergence. The results of the first experiment indicate that the perceived linear size, angular size, and distance of the stimulus decreased with increased convergence, the decrease in perceived linear size being greater than that of perceived angular size. While again showing a decrease in perceived linear and angular size, the results of the second experiment also show that there was a smaller decrease in perceived distance with increased convergence when Ss continued to view the stimulus as convergence was changed than when they did not view the stimulus as convergence was changed. The implications these results have for size and distance perception are discussed.     

Individual differences in the visual component of prism adaptation

The centrality of individual differences in the visual component of perceptual adaptation was examined in a massed-practice-terminal-exposure, prism-viewing paradigm. With positive (adaptive) adjustments in the judgment of the visual straight-ahead, target-pointing aftereffects were found to be equivalent to the sum of the visual and proprioceptive (head-arm) aftereffects. For subjects showing negative visual adjustments to prism exposure, the target-pointing aftereffect was not significantly different from the change in proprioception alone. Implications of these findings for hypotheses concerning the process of perceptual adaptation are discussed.