Adaptation to displaced vision: Evidence for prolonged after-effects

It was found that brief periods of exposure to visual rearrangement elicit aftereffects that are still of considerable magnitude 24 h later. Despite the existence of these after-effects, visuomotor coordination does not appear to be disrupted between test periods, suggesting that these after-effects may be similar to some of the recently described situation-dependent visual after-effects.     

Adaptation to displaced vision: Evidence for transfer of adaptation and long-lasting aftereffects

Aftereffects following a single exposure to visual rearrangement last for 48 h and longer. Following multiple spaced exposures to visual rearrangement, aftereffects persisted for at least 2 weeks. Over the course of the multiple exposures, subjects showed diminished scatter when pointing without sight of hand to visual targets but did not show diminished constant errors. Adaptation achieved in one visuomotor transferred nearly perfectly to a test situation involving somewhat different movement demands.     

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

Subjects were exposed to a prismatically displaced view of their actively-moved right hand which was optically “stopped”; they achieved as much adaptation in this condition as in one in which they were allowed full “reafferent” stimulation. This provides further evidence against Held's “reafference“ hypothesis.     

Adaptation to prism-displaced vision: The importance of target-pointing

Two experiments investigated the hypothesis that the experience of manually pointing at visual targets enhances motoric adaptation to prism-displaced vision. Experiment 1 indicated that when adaptation was measured by means of redirected pointing behavior (negative aftereffect) it varied directly with the specificity of the target, the least adaptation occurring when no target was available. This relationship was not observed when adaptation was measured in terms of a shift in the felt position of the prism-exposed hand (proprioceptive shift). Experiment 2 demonstrated that after double the prism-exposure trials used in Experiment 1, target-pointing experience continued to enhance adaptation (as indexed by both types of adaptation measure). In both experiments negative aftereffect was significantly larger than proprioceptive shift for all experimental conditions and the two measures were not correlated. These latter two findings cast doubt on Harris’s notion that negative aftereffect is entirely the result of altered position sense.     

Adaptation to vertical displacement of the visual direction

Sixty Ss wore vertically displacing wedge prisms and adapted by looking at their feet for 10 min. Half of them did this while standing and the others in supine position. The latter condition produced adaptation measurable with a visual-motor test and with a test of egocentric localization, but on a purely motoric test no adaptation was apparent. Standing during the adaptation period produced no effect.     

Allocation of attention and the locus of adaptation to displaced vision.

Experimental subjects were exposed to prism-induced visual displacement of a target whose location was correctly given by proprioceptive-kinesthetic information. Control subjects were exposed alternately to visual displacement or proprioceptive-kinesthetic location information. During the adaptation period, experimental subjects in the visual attention condition performed a localization task that directed them to attend selectively to the visual modality; experimental subjects in the proprioceptive attention condition attended selectively to the proprioceptive modaltiy; control subjects performed the task on the basis of the available modality. Measures of adaptation and aftereffect were secured separately in each of the two modalities. These confirmed the predictions that the shifts in the experimental conditions would be confirmed to localization tests dependent on the unattended modality and that control subjects would not exhibit adaptation. We proposed that allocation of attention determines situational dominance and that dominance determines the locus of adaptation. The findings were compared to those reported by Canon (1970) and were applied to a reassessment of the "visual capture" phenomenon.     

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.     

Simultaneous visuomotor adaptation to optical tilt and displacement

Change in visuomotor direction and orientation was measured following simultaneous exposure to optical displacement and tilt. Adaptation to both transforms simultaneously was not different from adaptation to each transform separately. These results are consistent with previous work involving purely visual change, and suggest that the two kinds of adaptation involve independent processes for locus-specific and relational analysis.

     

Adaptation to field displacement during head movement unrelated to the constancy of visual direction

Adaptation to vertical field displacements dependent on head turning about a vertical axis was demonstrated under two conditions, rapid training with 100 head movements and 1-h-long training with continuous head turning. The effect of rapid training was measured with the slant estimation method. Adaptation after the longer training was ascertained by comparing the uncertainty ranges for apparent target immobility before and after the adaptation period. Adaptation to field displacements in directions parallel to the plane of the head rotation obtained under corresponding conditions was also measured and found to be somewhat greater than adaptation to vertical field displacements. The result of work by Wallach and Frey that adaptation to field displacement in the direction with the head rotation is greater than to displacement against it was corroborated. While the previous result had, been obtained with rapid adaptation and with the slant estimation method, we confirmed it with 1-h training and by measuring the uncertainty ranges before and after the adaptation period.     

Adaptation to visual displacement with active and passive limb movements: effect of movement frequency and predictability of movement

Three experiments were performed to evaluate the influence of active and passive limb movements on adaptation to visual displacement. Over a wide frequency range (0·5-1·25 Hz) with constant amplitude, 30°, significant adaptation was achieved with active and passive movements. When arm movement frequency was constant at 1·0 Hz but amplitude of movement was varied, less adaptation was achieved for both active and passive movements than when amplitude was held constant. Even at a frequency above that of most naturally occurring limb movements, 1·67 Hz, and with variable amplitude motion, significant adaptation was achieved with active and passive limb movements. These findings emphasize the importance of visual-proprioceptive discordances for adaptation to visual displacement when only sight of the hand is permitted. Significant differences did not appear between the active and passive movement conditions in any of the experiments.     

Saccadic suppression of displacement is strongest in central vision

Perception of target displacement is severely degraded if the displacement occurs during a saccadic eye movement, but the variation of this effect across the visual field is unknown. A small target was displaced from a starting point at the midline, or 10 deg to the right or left, while the eye made a saccade from the 10 deg right position to the 10 deg left position. Saccades were detected and the target displaced on line. Assessed with a signal detection measure, suppression was stronger in central vision than in more peripheral locations for all three subjects. Leftward and rightward displacements yielded equal thresholds. The results complement the findings of others to reveal a picture of perceptual events during saccades, with both deeper saccadic suppression and faster correction of spatial values (the correspondences between retinal position and perceived egocentric direction), favouring more accurate spatial processing in central vision than in the periphery.     

The effects of task and practice on search for invisibly displaced objects

The ability to successfully search for an invisibly displaced object is frequently assumed to relate to the emergence of mental representation at around 2 years of age. However, little is known about what is actually being measured in the typical “stage 6” object permanence search task. Two studies examined the effects of task factors and practice on invisible displacements and systematic search tasks to show that simple variations in ways of hiding an object change the child's ability to find it. Experiment 1 compared two common methods of presenting invisible displacements items to 36 children, approximately 2 years of age. One task was significantly easier than the other, despite the fact that they contained identical items that varied only in their order of presentation. Experiment 2 tested the effects of short-term practice on 18- and 24-month-olds' success on invisible displacements trials and on two systematic search tasks. Results indicate that too little practice inhibited performance in 18-month-olds, but that practice beyond an optimum level did not further affect their scores. In contrast, 24-month-olds continued to benefit from increased practice until they reached the maximum possible score. The findings are interpreted to mean that invisible displacements tasks may require only sensorimotor search strategies rather than representation. The findings also have implications for (a) research that attempts to relate object permanence to other variables and (b) programs that use object permanence as an assessment or screening measure.     

Prismatic displacement of vision induces transient changes in the timing of eye-hand coordination

Eye-hand coordination was investigated during a task of finger pointing toward visual targets viewed through wedge prisms. Hand and eye latencies and movement times were identical during the control condition and at the end of prism exposure. A temporal reorganization of eye and hand movements was observed during the course of adaptation. During the earlier stage of prism exposure, the time gap between the end of the eye saccade and the onset of hand movement was increased from a control time of 23 to 68 msec. This suggests that a time-consuming process occurred during the early prism-exposure period. The evolution of this time gap was correlated with the evolution of pointing errors during the early stage of prism exposure, in such a way that both measures increased at the onset of prism exposure and decreased almost back to control values within about 10 trials. However, spatial error was not entirely corrected, even late in prism exposure when the temporal organization of eye and hand had returned to baseline. These data suggest that two different adaptive mechanisms were at work: a rather short-term mechanism, involved in normal coordination of spatially aligned eye and hand systems, and a long-term mechanism, responsible for remapping spatially misaligned systems. The former mechanism can be strategically employed to quickly optimize accuracy in a situation involving misalignment, but completely adaptive behavior must await the slower-acting latter mechanism to achieve longterm spatial alignment.     

Similarity and categorization: from vision to touch

Even though human perceptual development relies on combining multiple modalities, most categorization studies so far have focused on the visual modality. To better understand the mechanisms underlying multisensory categorization, we analyzed visual and haptic perceptual spaces and compared them with human categorization behavior. As stimuli we used a three-dimensional object space of complex, parametrically-defined objects. First, we gathered similarity ratings for all objects and analyzed the perceptual spaces of both modalities using multidimensional scaling analysis. Next, we performed three different categorization tasks which are representative of every-day learning scenarios: in a fully unconstrained task, objects were freely categorized, in a semi-constrained task, exactly three groups had to be created, whereas in a constrained task, participants received three prototype objects and had to assign all other objects accordingly. We found that the haptic modality was on par with the visual modality both in recovering the topology of the physical space and in solving the categorization tasks. We also found that within-category similarity was consistently higher than across-category similarity for all categorization tasks and thus show how perceptual spaces based on similarity can explain visual and haptic object categorization. Our results suggest that both modalities employ similar processes in forming categories of complex objects.