Generalization of prism adaptation

Prism exposure produces 2 kinds of adaptive response. Recalibration is ordinary strategic remapping of spatially coded movement commands to rapidly reduce performance error. Realignment is the extraordinary process of transforming spatial maps to bring the origins of coordinate systems into correspondence. Realignment occurs when spatial discordance signals noncorrespondence between spatial maps. In Experiment 1, generalization of recalibration aftereffects from prism exposure to postexposure depended upon the similarity of target pointing limb postures. Realignment aftereffects generalized to the spatial maps involved in exposure. In Experiment 2, the 2 kinds of aftereffects were measured for 3 test positions, one of which was the exposure training position. Recalibration aftereffects generalized nonlinearly, while realignment aftereffects generalized linearly, replicating Bedford (1989, 1993a) using a more familiar prism adaptation paradigm. Recalibration and realignment require methods for distinguishing their relative contribution to prism adaptation.     

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.     

Restricted adaptation to prism rearrangement

Changes in eye-foot and eye-hand coordination were measured following 20 min of squint prism viewing (alternate monocular viewing of the movements of each leg with the contralateral eye at 1-min intervals: prism base right for right eye and left for left eye). In different sessions, response changes were measured following the viewing of the left leg with the right eye (prism base right) for periods of i min interspersed with 1-min blank periods (periodic viewing). Sensorimotor changes following the alternate exposure condition were smaller and restricted to eye-foot responses.     

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.     

Intermanual transfer of prism adaptation

The authors measured intermanual transfer in participants (N = 48) whose exposed or unexposed right or left hand was tested 1st after participants experienced prismatic displacement. Test order did not affect either participants' performance during prismatic exposure or the usual aftereffects, but transfer occurred only when the authors tested the exposed right hand 1st. Transfer did not occur, and proprioceptive shift for the exposed left limb decreased when the authors tested the unexposed right limb 1st. The present results suggest that transfer occurs during testing for aftereffects of prism exposure, but not during prism exposure itself, as researchers have previously assumed. Results are consistent with those of previous research that has shown that limb control is lateralized in opposite hemispheres and that the left hemisphere contains a spatial map only for the right limb.     

Discriminative conditioning of prism adaptation

Two experiments were used to demonstrate that adaptation to ll-deg prism displacement can be conditioned to the stimuli associated with the goggles in which the prisms are housed. In Experiment 1 it was found that repeated alternation between a series of target-pointing responses while wearing prism goggles and a series of responses without prism goggles led to larger adaptive shift when S was tested with nondisplacing goggles than when tested without goggles. The results of Experiment 2 indicated that the adaptation revealed in the first experiment was primarily proprioceptive, rather than visual. Surprisingly, most Ss reported greater difficulty during the exposure period in overcoming the negative aftereffect than they did the prism-induced error.     

Cognitive factors and adaptation to auditory-visual discordance

Subjects pointed blindfolded at auditory targets before and after exposure to spatial conflict between the sound of a percussion instrument and images on a TV screen. Four experimental conditions were obtained by combining two levels of realism, in which sound was paired with either the image of the hands playing the instrument or with synchronously modulated light, and two levels of suggestion, in which a dummy loudspeaker from which the subjects had been told the sound would come was placed either in front of the TV screen or on top of the actual hidden loudspeaker. Adaptation occurred in all four conditions, but no difference between them was detected. These results confirm and extend the previous finding that auditory adaptation, unlike the impression of fusion (ventriloquism), is little influenced by cognitive factors.     

The role of acceleration information in prism adaptation

Two experiments were performed in which the acceleration component of limb movement information during prism exposure was manipulated, by controlling the trajectory and visibility of arm movement. When limb movements were confined to a lateral motion on a linear track, adaptation was evident when arm movement reversal at the end of the trajectory could be viewed (nonoccluded arm-movement reversal conditions). No adaptation occurred in the occluded arm-movement reversal condition. When movements were made on a curved track, adaptation was evident in both the nonoccluded and the occluded arm-movement reversal conditions. The results indicate that the acceleration component of reafferent stimulation may be critical in prism adaptation when no error information is available.     

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.     

Visual cues and perceived reachability

A rather consistent finding in studies of perceived (imagined) compared to actual movement in a reaching paradigm is the tendency to overestimate at midline. Explanations of such behavior have focused primarily on perceptions of postural constraints and the notion that individuals calibrate reachability in reference to multiple degrees of freedom, also known as the whole-body explanation. The present study examined the role of visual information in the form of binocular and monocular cues in perceived reachability. Right-handed participants judged the reachability of visual targets at midline with both eyes open, dominant eye occluded, and the non-dominant eye covered. Results indicated that participants were relatively accurate with condition responses not being significantly different in regard to total error. Analysis of the direction of error (mean bias) revealed effective accuracy across conditions with only a marginal distinction between monocular and binocular conditions. Therefore, within the task conditions of this experiment, it appears that binocular and monocular cues provide sufficient visual information for effective judgments of perceived reach at midline.     

Visual cues and perceived reachability

A rather consistent finding in studies of perceived (imagined) compared to actual movement in a reaching paradigm is the tendency to overestimate at midline. Explanations of such behavior have focused primarily on perceptions of postural constraints and the notion that individuals calibrate reachability in reference to multiple degrees of freedom, also known as the whole-body explanation. The present study examined the role of visual information in the form of binocular and monocular cues in perceived reachability. Right-handed participants judged the reachability of visual targets at midline with both eyes open, dominant eye occluded, and the non-dominant eye covered. Results indicated that participants were relatively accurate with condition responses not being significantly different in regard to total error. Analysis of the direction of error (mean bias) revealed effective accuracy across conditions with only a marginal distinction between monocular and binocular conditions. Therefore, within the task conditions of this experiment, it appears that binocular and monocular cues provide sufficient visual information for effective judgments of perceived reach at midline.