First-Trial Adaptation to Prism Exposure

Terminal target-pointing error on the 1st trial of exposure to optical displacement is usually less than that expected from the optical displacement magnitude. Such 1st trial adaptation was confirmed in 2 experiments (N = 48 students in each) comparing pointing toward optically displaced targets and toward equivalent physically displaced targets (no optical displacement), with visual feedback delayed until movement completion. First-trial performance could not be explained by ordinary target undershoot, online correction, or reverse optic flow information about true target position and was unrelated to realignment aftereffects. Such adaptation might be an artifact of the asymmetry of the structured visual field produced by optical displacement, which induces a felt head rotation opposite to the direction of the displacement, thereby reducing the effective optical displacement.     

First-trial adaptation to prism exposure: artifact of visual capture

Terminal target-pointing error on the 1st trial of exposure to optical displacement is usually less than is expected from the optical displacement magnitude. The authors confirmed 1st-trial adaptation in the task of pointing toward optically displaced targets while visual feedback was delayed until movement completion. Measurement of head-shoulder posture while participants (N = 24) viewed the optically displaced field revealed that their shoulders felt turned in the direction opposite to the displacement (visual capture), accounting for all but about 4% to 10% of 1st-trial adaptation. First-trial adaptation was unrelated to realignment aftereffects. First-trial adaptation is largely an artifact of the asymmetry of the structured visual field produced by optical displacement, which induces a felt body rotation, thereby reducing the effective optical displacement.     

Displacement in depth: Representational momentum and boundary extension

Memory for targets moving in depth and for stationary targets was examined in five experiments. Memory for targets moving in depth was displaced behind the target with slower target velocities (longer ISIS and retention intervals) and beyond the target with faster target velocities (shorter ISIS and retention intervals), and the overall magnitude of forward displacement for motion in depth was less than the overall magnitude of forward displacement for motion in the picture plane. Memory for stationary targets was initially displaced away from the observer, but memory for smaller stationary targets was subsequently displaced toward the observer and memory for larger stationary targets was subsequently displaced away from the observer; memory for the top or bottom edge of a stationary target was displaced inside the target perimeter. The data are consistent with Freyd and Johnson's (1987) two-component model of the time course of representational momentum and with Intraub et al.'s (1992) two-component model of boundary extension.     

Adaptation to displaced vision: Amount of optical displacement and practice

The influence of two variables, length of exposure and amount of optical distortion, on adaptation to displaced vision was examined. The extent of adaptation was positively related to number of trials in a task involving spatial localization of a target displaced by a wedge prism. A substantial adaptation (38%) was produced after only two trials. The adaptation was also positively related to degree of optical displacement, except at the highest level used. The findings are discussed in terms of availability of information about the discrepancy between vision and task.     

Spatial memory averaging, the landmark attraction effect, and representational gravity

The effect of a large stationary landmark on memory for the location of a stationary target was examined. Memory for a stationary target was displaced toward the landmark, and targets that were larger, further from, or above the landmark exhibited greater magnitudes of displacement. Displacement was generally larger when the landmark vanished prior to judgment than when the landmark was visible during judgment. Memory for stationary targets offset from the major vertical or horizontal cardinal axis of the landmark was also displaced toward that cardinal axis. The data support the hypotheses that spatial memory averaging of the locations of a target and landmark occurs, and that this averaging may be combined with representational gravity in determining the remembered position of a stationary target. Received: 17 May 1999 / Accepted: 8 February 2000     

Target distance and adaptation in distance perception in the constancy of visual direction

Hay and Sawyer recently demonstrated that the constancy of visual direction (CVD) also operates for near targets. A luminous spot in the dark, 40 cm from the eyes, was perceived as stationary when S nodded his head. This implies that CVD takes target distance, as well as head rotation, into account as a stationary environment is perceived during head movements. Distance is a variable in CVD because, during a turning or nodding of the head, the eyes become displaced relative to the main target direction, the line between the target and the rotation axis of the head. This displacement of the eyes during head rotation causes an additional change in the target direction, i.e., a total angular change greater than the angle of the head rotation. The extent of this additional angular displacement is greater the nearer the target. We demonstrated that the natural combination of accommodation and convergence can supply the information needed by the nervous system to compensate for this additional target displacement. We also found that wearing glasses that alter the relation between these oculomotor adjustments and target distance produces an adaptation in CVD. An adaptation period of 1.5 h produced a large adaptation effect. This effect was not entirely accounted for by an adaptation in distance perception. Measurements of the alteration between oculomotor cues and registered distance with two kinds of tests for distance perception yielded effects significantly smaller than the effect measured with the CVD test. We concluded that the wearing of the glasses had also produced an adaptation within CVD.     

Control of Rapid Arm Movements When Target Position Is Altered During Saccadic Suppression

This experiment examined whether rapid arm movements can be corrected in response to a change in target position that occurs just prior to movement onset, during saccadic suppression of displacement. Because the threshold of retinal input reaches its highest magnitude at that time, displacement of the visual target of a saccade is not perceived. Subjects (N = 6) were instructed to perform very rapid arm movements toward visual targets located 16, 20, and 24 degrees from midline (on average, movement time was 208 ms). On some trials the 20 degrees target was displaced 4 degrees either to the right or to the left during saccadic suppression. For double-step trials, arm movements did not deviate from their original trajectory. Movement endpoints and movement structure (i.e., velocity-and acceleration-time profiles) were similar whether or not target displacements occurred, showing the failure of proprioceptive signals or internal feedback loops to correct the arm trajectory. Following this movement, terminal spatially oriented movements corrected the direction of the initial movement (as compared with the single-step control trials) when the target eccentricity decreased by 4 degrees. Subjects were unaware of these spatial corrections. Therefore, spatial corrections of hand position were driven by the goal level of the task, which was updated by oculomotor corrective responses when a target shift occurred.     

Targets of the memorized position and visual contexts]

The goal of this study was to determine whether a sensorimotor or cognitive encoding is used to encode a target position and save it into iconic memory. The methodology consisted of disrupting a manual aiming movement to a memorized visual target by displacing the visual field containing the target. The nature of the encoding was inferred from the nature and the size of the errors relative to a control. The target was presented either centrally or in the right periphery. Participants moved their hand from the left to the right of fixation. Black and white vertical stripes covered the whole visual field. The visual field was either stationary throughout the trial or was displaced to the right or left at the extinction of the target or at the start of the hand movement. In the latter case, the displacement of the visual field obviously could only be taken into account by the participant during the gesture. In this condition, our hypothesis was that the aiming error would follow the direction of visual field displacement. Results showed three major effects: (1) Vision of the hand during the gesture improved the final accuracy; (2) visual field displacement produced an underestimation of the target distance only when the hand was not visible during the gesture and was always in the same direction displacement; and (3) the effect of the stationary structured visual field on aiming precision when the hand was not visible depended on the distance to the target. These results suggest that a stationary structured visual field is used to support the memory of the target position. The structured visual field is more critical when the hand is not visible and when the target appears in peripheral rather than central vision. This suggests that aiming depends on memory of the relative peripheral position of the target (allocentric reference). However, in the present task, cognitive encoding does not maintain the "position" of the target in memory without reference to the environment. The systematic effect of the visual field displacement on the manual aiming suggests that the role of environmental reference frames in memory for position is not well understood. Some studies, in particular those of Giesbrecht and Dixon (1999) and Glover and Dixon (2001), suggested differing roles of the environment in the retention of the target position and the control of aiming movements toward the target. The present observations contribute to understanding the mechanism involved in locating and grasping objects with the hand.     

Changes in straight-ahead eye position during adaptation to wedge prisms

If S is instructed to look straight ahead before adapting to laterally displaced vision, he does so without noticeable error. After adapting, however, in response to the same instruction, he may rotate his eyes as much as 8° toward the the displaced visual target. This is the change in judgment of the direction of gaze which Helmholtz identified in 1867 as an important physiological mechanism in adaptation to prisms. It leads to more accurate reaching behavior by causing S to make a visual judgment that the target is closer to straight ahead than it was when he first looked through the prisms. This type of adaptive change (change in judgment of the direction of gaze, oculomotor change) can be measured either by manual judgments (difference between successive “straight ahead” and “visual target” judgments) or by changes in straight-ahead eye position. It may be described as a parametric adjustment in the oculomotor control system, and is closely analogous to the eye movement which subserves the recovery of binocular fusion in prism vergence.     

Adaptation to visual and proprioceptive rearrangement: Origin of the differential effectiveness of active and passive movements

In Experiment 1, subjects exposed to a discordance between the visual and ”proprioceptive” locations of external targets were found to exhibit aftereffects when later pointing without sight of their hands at visual targets. Aftereffects occur both when the discordance is introduced in the traditional fashion by displacing the visual locations of targets and when the proprioceptive locations of targets are displaced. These observations indicate that there is nothing unique about the visual rearrangement paradigm—the crucial factor determining whether adaptation will be elicited is the presence of a discordance in the positional information being conveyed over two different sensory modalities. In a second experiment, the effectiveness of active and passive movements in eliciting adaptation was studied using an experimental paradigm in which subjects were exposed to a systematic discordance between the visual and proprioceptive locations of external targets without ever being permitted sight of their hands; a superiority of active movements was observed, just as is usually found in visual rearrangement experiments in which sight of the hand is permitted. Evidence is presented that the failure of passive movements to elicit adaptation is related to a deterioration in accuracy of position sense information during passive limb movement.     

Error processing in pointing at randomly feedback-induced double-step stimuli

Two experiments were conducted to determine the spatial and temporal organization of the arm trajectory in human subjects as they pointed to single- and double-step target displacements. Subjects pointed either without (Experiment 1) or with (Experiment 2) vision of their moving hand throughout the trial. In both experiments, target perturbation occurring in double-step trials was clearly perceived by the subjects and was randomly introduced either at the onset or at peak velocity of hand movement. Regardless of whether or not visual reafference from the pointing hand was available, subjects corrected the trajectory of their moving hand to accommodate the double-step. Moreover, asymmetrical velocity profiles were observed for responses to both types of target, with or without vision of the moving hand. The acceleration phase was a fixed pattern independent of the type of step stimulation. However, a clear dissociation, both in the deceleration phase and accuracy of responses to double-step targets, emerged according to the timing of target perturbation. When targets were perturbed at the onset of hand movement, subjects modulated the deceleration phase of their response to compensate for 88 to 100% of the second target displacement. In contrast, when targets were perturbed at peak velocity of hand movement, subjects were unable to modulate the deceleration phase adequately and compensated for only 20 to 40% of the perturbation. These results suggest that motor error is dynamically evaluated during the acceleration phase of a movement toward a perturbed target, allowing amendments to the trajectory to be performed during the deceleration phase. This main corrective process appears to be basically independent of visual reafference from the moving hand.     

Control of Trajectory Modifications in Target-Directed Reaching

Human reaching movements to fixed and displaced visual targets were recorded and compared with simulated movements generated by using a two-joint arm model based on the equilibrium-point (EP) hypothesis (lambda model) of motor control (Feldman, 1986). The aim was to investigate the form of central control signals underlying these movements. According to this hypothesis, movements result from changes in control variables that shift the equilibrium position (EP) of the arm. At any time, muscle activations and forces will depend on the difference between the arm's EP and its actual position and on the limb's velocity. In this article, we suggest that the direction of EP shift in reaching is specified at the hand level, whereas the rate of EP shift may be specified at the hand or joint level. A common mechanism underlying reaching to fixed and displaced targets is proposed whereby the EP of the hand shifts in a straight line toward the present target. After the target is displaced, the direction of the hand EP shift is modified toward the second target. The results suggest that the rate of shift of the hand EP may be modified for movements in different parts of the work space. The model, with control signals that vary in a simple fashion over time, is able to generate the kinematic patterns observed empirically.     

Correction processes in early reaching: Reaches by infants to prismatically displaced targets

Summary A comparison was made between trajectories of reaches to visual targets by 6-months-old infants, with and without lateral prismatic displacement. In the No Prism condition, the major element of the movement veered towards the target, confirming the results of Mathew and Cook (1990). In the Prism condition, the prism did not affect hand position or movement direction at the beginning of the major element, but a lateral displacement was evident by the end of the element. This is taken as evidence that shifts in trajectory during the element were not due to visual guidance. It is suggested instead that these shifts may reflect correction based upon somatic feedback about position in relation to the visually specified goal of the movement. Evidence that the hand approached the virtual target supports the suggestion that any visual guidance was confined to the approach phase of the reach. Nevertheless, it appears that visual guidance was operating at this stage, since movement direction was displaced away from the virtual target and towards the real target.     

Representational momentum in spatial hearing

The final position of a moving visual object usually appears to be displaced in the direction of motion. We investigated this phenomenon, termed representational momentum, in the auditory modality. In a dark anechoic environment, an acoustic target (continuous noise or noise pulses) moved from left to right or from right to left along the frontal horizontal plane. Listeners judged the final position of the target using a hand pointer. Target velocity was 8 degrees s(-1) or 16 degrees s(-1). Generally, the final target positions were localised as displaced in the direction of motion. With presentation of continuous noise, target velocity had a strong influence on mean displacement: displacements were stronger with lower velocity. No influence of sound velocity on displacement was found with motion of pulsed noise. Although these findings suggest that the underlying mechanisms may be different in the auditory and visual modality, the occurrence of displacements indicates that representational-momentum-like effects are not restricted to the visual modality, but may reflect a general phenomenon with judgments of dynamic events.     

A structural dynamic of form: Displacements in memory for the size of an angle

Memory for the angular size of a chevron (V) shaped target was examined in four experiments. When the target was stationary, memory was displaced inwards (i.e., towards a smaller angle), and the magnitude of displacement increased with increases in absolute angle size. When the target moved vertically or horizontally, memory was displaced inwards, but the effect of absolute angle size was weakened, and displacement was not influenced by whether the direction of motion and the direction in which the angle pointed were the same or different. When the target expanded or contracted (i.e., increased or decreased in angular size), memory for expanding targets was displaced inwards more than was memory for contracting targets, and displacement was not influenced by whether motion was coherent or incoherent. Implications of the data for the possibility of dynamic aspects of mental representation based on the shape of a stimulus are discussed.     

Control of Trajectory Modifications in Target-Directed Reaching

Human reaching movements to fixed and displaced visual targets were recorded and compared with simulated movements generated by using a two-joint arm model based on the equilibrium-point (EP) hypothesis (λ model) of motor control (Feldman, 1986). The aim was to investigate the form of central control signals underlying these movements. According to this hypothesis, movements result from changes in control variables that shift the equilibrium position (EP) of the arm. At any time, muscle activations and forces will depend on the difference between the arm's EP and its actual position and on the limb's velocity. In this article, we suggest that the direction of EP shift in reaching is specified at the hand level, whereas the rate of EP shift may be specified at the hand or joint level. A common mechanism underlying reaching to fixed and displaced targets is proposed whereby the EP of the hand shifts in a straight line toward the present target. After the target is displaced, the direction of the hand EP shift is modified toward the second target. The results suggest that the rate of shift of the hand EP may be modified for movements in different parts of the work space. The model, with control signals that vary in a simple fashion over time, is able to generate the kinematic patterns observed empirically.     

Apparent displacement with a monocular prism differs from optical displacement

This experiment showed that phoria-induced displacement adds to or subtracts from prism-induced displacement. A near stimulus (25 cm) was apparently displaced more than the optical displacement when the base of a prism was out and less when the base was in. In contrast, a far stimulus (200 cm) was displaced less when the base was out and more when the base was in. Moreover, the between-subjects variability of the apparent displacement was greater with monocular than with binocular viewing. Some implications for studies on monocular prism adaptation are discussed.     

Vernier offset produced by rotary target motion

Two separated colinear lines appear displaced from colinearity when either the target or the subject’s head is rotated in a frontal plane. The direction of perceived offset is reversed for opposite directions of rotation. The present experiments prove that the effect depends on some property of the visual system that is responsive to stimulus motion per se and is not manifested in the response to stationary targets. Two mechanisms which may be responsible for the rotation-contingent effect are considered: (1) An induction mechanism based on the dynamics of induced tilt or of figural aftereffect displacement. (2) A mechanism based on variation of visual latency with stimulus energy/time.     

Adaptation and counteradaptation to complex optical distortion

The optical distortion caused by wearing a facemask in water magnifies the angular size of objects and reduces their optical distance. However, objects generallyappear to be further than their optical distance, with the result that points in the left part of the visual field are apparently displaced to the left, and those on the right to the right. Experiments on hand-eye coordination under water showed that adaptation to one aspect of the distortion produced some counteradaptation to complementary aspects: adaptation to distance produced increased lateral distortion, and adaptation to one side of the lateral distortion produced increased distortion on the opposite side. Nevertheless, “trading” was incomplete, and some overall adaptation of the visual metric occurred.     

The constancy of object orientation: Compensation for ocular rotation

When a thin horizontal line is displaced, either left or right of straight ahead, or when a vertical line is displaced up or down, systematic changes occur in the binocular disparity associated with the target. In threeexperiments, Ss matched the orientation of displaced targets with a variable comparison line. Estimates of apparent displacement with a pointing technique were also made. Since head position was fixed, apparent displacement was mediated by the angle of ocular rotation. Near perfect matches were made with vertical targets, but horizontal targets produced errors suggestive of underestimation of apparent displacement. However, the pointing data did not yield clear evidence for this view. Control data denien the possible role of the induced effect (IE) in matching the horizontal targets, and the result were discussed in the context of orientation constancy based upon compensation for displacement.