Adaptation to displaced vision: reafference is a special case of the cue-discrepancy hypothesis

Visual Discordance Cues Induce Prism Adaptation in Normal Monkeys

Contrary to previous reports, adaptation to laterally displaced visual input does require visual perception of the visuomotor mismatch. Using 4 rhesus monkeys as Ss, it was found that reaching errors induced by wearing 20 diopter wedge prisms remained at optically predicted magnitudes for 24 hr., provided that no visual misreaching cues were available. Unrestricted head movement did not provide such cues. However, terminal viewing of the prism induced reaching errors produced dramatic, rapid adaptation. Tactile and proprioceptive discordance cues alone, without visual feedback, were not corrective.

Short-Term Adaptation to Vertical Yoked Prisms

PURPOSE

The purpose of this study was to investigate short-term adaptation to vertical yoked prisms in visually normal subjects as well as to develop normative data for future comparative purposes in patients with acquired brain injury.

METHODS

A full-body enclosure containing a calibrated laser pointer was used to assess egocentric localization (i.e., one's perceived sense of "straight ahead") in total darkness. Fourteen visually normal subjects were tested in a counterbalanced manner under both experimental (20 pd base-down) and control (plano or zero power) test conditions with each test condition lasting 1 hour. A rating scale questionnaire was also administered to assess quantitatively subjective adaptation to the yoked prisms.

RESULTS

Group mean egocentric localization showed approximately 50% adaptation during the 1-hour test period. Subjective adaptation results were similar.

CONCLUSION

The findings suggest that rapid perceptually driven, sensorimotor adaptation occurred as assessed both objectively and subjectively in response to vertical yoked prisms. These results are similar to those found in other experiments involving short-term adaptation to optical rearrangements.

Prism adaptation to dynamic events

In the present study, we explored adaptation to prism-displaced dynamic and static events under conditions of minimal information. Many of our interactions with the world are dynamic and involve reaching for or intercepting moving objects. The consequences (or feedback) of those interactions entail the presence or absence of physical contact with the moving objects. In this study, humans learned, with only heptic feedback, to intercept optically displaced falling balls. To eliminate visual feedback, the falling balls disappeared behind an occluder (which systematically varied in size across groups) prior to either striking or missing a subject's hand. As occluder size decreased, adaptation increased. With minimum occluder sizes, the greatest adaptation occurred around the training position, and adaptation decreased as distance between training and testing positions increased. The results can best be described in terms of a generalization gradient centered around the training position. This generalization gradient was not present when subjects were trained with ecologically similar static arrays. Implications for models of adaptation are discussed.

The dominance of touch by vision: sometimes incomplete

It is generally agreed that, when the two senses are in conflict, vision dominates touch. In the series of experiments reported here the discrepancy between the visual and the haptic images was progressively increased. In each experiment half of the observes were adults and half children, and half of each group wore a glove which concealed the distortion produced by the lens. In experiment 1 the visual image of a haptic square was transformed by a week or strong cylindrical lens so that the sides were in the ratio 1:1.5 or 1:1.8. Gloved observers selected a comparison image close to the visual image. Bare-handed observers gave compromise judgements with the strong lens, but judgements similar to those of the gloved observers with the weak lens. In experiment 2 the square had a visual image which was either a diamond or a parallelogram, the parallelogram having angles of 70 degrees and 110 degrees. Observers described and drew their percepts, which were generally in accord with the visual image. There were no significant differences due to age, a finding contrary to that of Page and Locke, nor were there differences in experiment 2 due to the gloved-hand-bare-hand dimension. It is concluded (i) that repeated observations do not destroy the effect, (ii) tht considerable discrepancies between the visual and haptic images do not destroy the effect, but (iii) that feedback from a bare hand can reduce the effect when it is of considerable magnitude.

Dominance of touch by vision: generalization of the hypothesis to a tactually experienced population

It is now held that in conditions where there is conflicting sensory information vision dominates touch. The present study was designed to place boundary conditions on the generality of the hypothesis by comparing the performance of tactually experienced subjects - potters - with university students on a form-perception task in which a square was optically distorted so as to appear rectangular. Subjects examined the square haptically and visually, and matched it by touch or vision. Although potters were more accurate than students when not permitted vision, tactual judgements of both were dominated by vision when they were presented with conflicting information. In a supplementary study longer, more active exploration and emphasis on the necessity of matching by touch had no differential effect. In a third study it was found that a range effect produced by the comparison stimuli could explain the lesser accuracy of the students when they were making haptic judgements. The generality of the hypothesis that vision dominates touch is strengthened by our failure to refute it.