Anne-Emmanuelle Priot

Automatic online control of motor adjustments in reaching and grasping

Following the princeps investigations of Marc Jeannerod on action-perception, specifically, goal-directed movement, this review article addresses visual and non-visual processes involved in guiding the hand in reaching or grasping tasks. The contributions of different sources of correction of ongoing movements are considered; these include visual feedback of the hand, as well as the often-neglected but important spatial updating and sharpening of goal localization following gaze-saccade orientation. The existence of an automatic online process guiding limb trajectory toward its goal is highlighted by a series of princeps experiments of goal-directed pointing movements. We then review psychophysical, electrophysiological, neuroimaging and clinical studies that have explored the properties of these automatic corrective mechanisms and their neural bases, and established their generality. Finally, the functional significance of automatic corrective mechanisms-referred to as motor flexibility-and their potential use in rehabilitation are discussed.

Adaptation of egocentric distance perception under telestereoscopic viewing within reaching space

Telestereoscopic viewing provides a method to distort egocentric distance perception by artificially increasing the interpupillary distance. Adaptation to such a visual rearrangement is little understood. Two experiments were performed in order to dissociate the effects of a sustained increased vergence demand, from those of an active calibration of the vergence/distance mapping. Egocentric distances were assessed within reaching space through open-loop pointing to small targets in the dark. During the exposure condition of the first experiment, subjects were instructed to point to the targets without feedback, whereas in the second experiment, hand visual feedback was available, resulting in a modified relationship between vergence-specified distance and reach distance. The visual component of adaptation in the second experiment was assessed on the unexposed hand. In the post-tests of both experiments, subjects exhibited a constant distance overestimation across all targets, with a more than twice larger aftereffect in the second one. These findings suggest two different processes: (1) an alteration in the vergence effort following sustained increased vergence; (2) a calibration of the vergence/distance mapping uncovering the visual component of adaptation.

Visuomotor adaptation needs a validation of prediction error by feedback error

The processes underlying short-term plasticity induced by visuomotor adaptation to a shifted visual field are still debated. Two main sources of error can induce motor adaptation: reaching feedback errors, which correspond to visually perceived discrepancies between hand and target positions, and errors between predicted and actual visual reafferences of the moving hand. These two sources of error are closely intertwined and difficult to disentangle, as both the target and the reaching limb are simultaneously visible. Accordingly, the goal of the present study was to clarify the relative contributions of these two types of errors during a pointing task under prism-displaced vision. In “terminal feedback error” condition, viewing of their hand by subjects was allowed only at movement end, simultaneously with viewing of the target. In “movement prediction error” condition, viewing of the hand was limited to movement duration, in the absence of any visual target, and error signals arose solely from comparisons between predicted and actual reafferences of the hand. In order to prevent intentional corrections of errors, a subthreshold, progressive stepwise increase in prism deviation was used, so that subjects remained unaware of the visual deviation applied in both conditions. An adaptive aftereffect was observed in the “terminal feedback error” condition only. As far as subjects remained unaware of the optical deviation and self-assigned pointing errors, prediction error alone was insufficient to induce adaptation. These results indicate a critical role of hand-to-target feedback error signals in visuomotor adaptation; consistent with recent neurophysiological findings, they suggest that a combination of feedback and prediction error signals is necessary for eliciting aftereffects. They also suggest that feedback error updates the prediction of reafferences when a visual perturbation is introduced gradually and cognitive factors are eliminated or strongly attenuated.

Short exposure to telestereoscope affects the oculomotor system

Under natural viewing conditions, the accommodation and vergence systems adjust the focus and the binocular alignment of the eyes in response to changes in viewing distance. The two responses are linked via cross‐coupling and proceed almost simultaneously. Some optical devices, such as virtual reality or helmet mounted displays, create an oculomotor conflict by modifying demands on both vergence and accommodation. Previous studies extensively investigated the effect of such a conflict on the cross‐coupling between vergence and accommodation, but little is known about the plasticity of the whole oculomotor system. In the present study, an oculomotor conflict was induced by a telestereoscope which magnified the standard inter‐pupillary separation threefold and thus increased the convergence demand while accommodation remained almost unchanged. The effect of a 10 min exposure was assessed via a series of optometric parameters selected on the basis of existing oculomotor models. Associated with subject’s visual complaints, most of the oculomotor parameters tested were modified: there was (1) deterioration of stereoscopic threshold; (2) increase in AC/A ratio; (3) increase in near and far phorias; and (4) shift of the zone of clear and single binocular vision towards convergence. These results showed a change in gain of accommodative vergence and a shift of vergence reserves towards convergence in response to telestereoscopic viewing. The subject’s binocular behaviour tended towards esophoria with convergence excess as confirmed by Sheard’s and Percival’s criteria. Such changes in oculomotor parameters support adaptive behaviour linked with telestereoscopic viewing.

Agreement between clinical and laboratory methods assessing tonic and cross-link components of accommodation and vergence.

Several tests are available to optometrists for investigating accommodation and vergence. This study sought to investigate the agreement between clinical and laboratory methods and to clarify which components are actually measured when tonic and cross‐link of accommodation and vergence are assessed.

The Role of Vertical Disparity in Distance and Depth Perception as Revealed by Different Stereo-Camera Configurations:

Vertical binocular disparity is a source of distance information allowing the portrayal of the layout and 3D metrics of the visual space. The role of vertical disparity in the perception of depth, size, curvature, or slant of surfaces was revealed in several previous studies using cue conflict paradigms. In this study, we varied the configuration of stereo-cameras to investigate how changes in the horizontal and vertical disparity fields, conflicting with the vergence cue, affect perceived distance and depth. In four experiments, observers judged the distance of a cylinder displayed in front of a large fronto-parallel surface. Experiment 1 revealed that the presence of a background surface decreases the uncertainty in judgments of distance, suggesting that observers use the relative horizontal disparity between the target and the background as a cue to distance. Two other experiments showed that manipulating the pattern of vertical disparity affected both distance and depth perception. When vertical disparity specified a nearer distance than vergence (convergent cameras), perceived distance and depth were underestimated as compared with the condition where vertical disparity was congruent with vergence cues (parallel cameras). When vertical disparity specified a further distance than vergence, namely an infinite distance, distance and depth were overestimated. The removal of the vertical distortion lessened the effect on perceived distance. Overall, the results suggest that the vertical disparity introduced by the specific camera configuration is mainly responsible for the effect. These findings outline the role of vertical disparity in distance and depth perception and support the use of parallel cameras for designing stereograms.

The initial effects of hyperstereopsis on visual perception in helicopter pilots flying with see-through helmet-mounted displays

Abstract In see-through helmet-mounted displays (HMDs), image-intensifier tubes are located on the sides of the helmet. This HMD design increases the effective interpupillary distance and exposes the observer to hyperstereopsis. The modification of three-dimensional space perception by hyperstereopsis in laboratory studies is well known. Little is known, however, about its effects in real, operational conditions. The present study investigated the effects of hyperstereopsis on visual perception in helicopter pilots during the first flight of a night-flight training program using a see-through HMD. Fifteen pilots’ percepts of distance and height under hyperstereoscopic viewing were assessed using a double-distance estimation task in standard flight circuits. In addition, two questionnaires were used to characterize the perceptual effects induced by hyperstereoscopic viewing during the first night flight, and to determine whether any aftereffects were still present during the 24-h period following the flight. Height and distance estimation errors were observed. On average, pilots tended to underestimate the near plane relative to the farther plane in the double-distance estimation task; this is the inverse of what is commonly observed under natural viewing. We ascribed this effect to a non-linear decrease in the hyperstereopsis effect with viewing distance. Six pilots experienced sensory illusions resembling a distortion of apparent visual space: the impression of “sinking into a hole” when landing, of objects appearing closer than they actually were, and reduced in size. Four pilots cited landing as the most difficult situation encountered during the flight. Only minor complaints were reported for the 24-h period following the flight. The results of this field study represent a step towards characterizing the perceptual and operational impact of this display technology in helicopter pilots during initial exposure.

Stereoscopic Viewing Can Induce Changes in the CA/C Ratio

PURPOSE Stereoscopic displays challenge the neural cross-coupling between accommodation and vergence by inducing a constant accommodative demand and a varying vergence demand. Stereoscopic viewing calls for a decrease in the gain of vergence accommodation, which is the accommodation caused by vergence, quantified by using the convergence-accommodation to convergence (CA/C) ratio. However, its adaptability is still a subject of debate. METHODS Cross-coupling (CA/C and AC/A ratios) and tonic components of vergence and accommodation were assessed in 12 participants (27.5 ± 5 years, stereoacuity better than 60 arc seconds, 6/6 acuity with corrected refractive error) before and after a 20-minute exposure to stereoscopic viewing. During stimulation, vergence demand oscillated from 1 to 3 meter angles along a virtual sagittal line in sinusoidal movements, while accommodative demand was fixed at 1.5 diopters. RESULTS Results showed a decreased CA/C ratio (-10.36%, df = 10, t = 2.835, P = 0.018), with no change in the AC/A ratio (P = 0.090), tonic vergence (P = 0.708), and tonic accommodation (P = 0.493). CONCLUSIONS These findings demonstrated that the CA/C ratio can exhibit adaptive adjustments. The observed nature and amount of the oculomotor modification failed to compensate for the stereoscopic constraint.

Adaptive estimates of egocentric distance from vergence in telestereoscopic viewing: Von Hofsten's model revisited

von Hofsten proposed a model for explaining how the perception of egocentric distance is affected by vergence angle. According to his model, binocular distance estimation relies on the difference between vergence response and rest vergence, rather than on absolute vergence angle. We applied this model to the data obtained in a study on adaptation to telestereoscopic viewing. Such an optical distortion, found in some novel stereo displays, calls for a change in the relationship between vergence and perceived distance, as well as in the level of rest vergence. We sought to investigate any changes in the model coefficients during and after exposure. The model testing corroborated the observation of distinct visual adaptive mechanisms, for the two different exposure conditions of the study.

Adaptation to alterations of apparent distance in stereoscopic displays: From lab to hyperstereopsis

The modification of binocular cues in some stereoscopic-display designs potentially alters apparent distances. Although further work is needed to completely clarify the determinants of adaptation to stereoscopic displays, our results reveal that such adaptation can occur, and involves oculomotor, perceptual and motor changes.