Abstract

The present study demonstrated that the magnitude of after-effect due to wedge prisms depends on the form of the visual feedback used to represent hand and target position in fast, targeted, transverse reaches. Trained human subjects made reaches with and without prisms in three visuomotor representations (VR): (1) the subject’s actual hand and targets (Direct), (2) a real-time video broadcast of hand and targets (Video), or (3) abstract, computer-generated targets and a cursor representing hand position (Cursor). A significant after-effect occurred in each VR. However, the magnitude of the after-effect was significantly different among VRs: the magnitude was greatest in Direct, smaller in Video and smallest in Cursor. A significant after-effect (carryover) also occurred when a subject prism-adapted reaches in one VR and then removed the prisms and made initial reaches in another VR. Our data showed that when reaches were prism-adapted in Direct and then prisms were removed, there was a large carryover to initial reaches in Video or Cursor (D→V and D→C). In contrast, when prisms were worn in Video and removed for reaches in Direct (V→D), there was a significantly smaller carryover than from both D→V and D→C. Finally, when prisms were worn in Cursor and removed for reaches in Direct (C→D), there was very little detectable carryover. Our results suggest that adaptation is context-dependent and that the magnitude of carryover is dependent on the VR in which adaptation occurred. Interpretations of adaptations made in abstract training and experimental conditions may be greatly affected by this finding.

Sheidt, R.A., Conditt, M.A., Secco, E.L., Mussa-Ivaldi, F.A. (2005), Interaction of visual and proprioceptive feedback during adaptation of human reaching movements, Journal of Neurophysiology, 93, 3200-3213.  

Abstract

People tend to make straight and smooth hand movements when reaching for an object. These trajectory features are resistant to perturbation, and both proprioceptive as well as visual feedback may guide the adaptive updating of motor commands enforcing this regularity. How is information from the two senses combined to generate a coherent internal representation of how the arm moves? Here we show that eliminating visual feedback of hand-path deviations from the straight-line reach (constraining visual feedback of motion within a virtual, "visual channel") prevents compensation of initial direction errors induced by perturbations. Because adaptive reduction in direction errors occurred with proprioception alone, proprioceptive and visual information are not combined in this reaching task using a fixed, linear weighting scheme as reported for static tasks not requiring arm motion. A computer model can explain these findings, assuming that proprioceptive estimates of initial limb posture are used to select motor commands for a desired reach and visual feedback of hand-path errors brings proprioceptive estimates into registration with a visuocentric representation of limb position relative to its target. Simulations demonstrate that initial configuration estimation errors lead to movement direction errors as observed experimentally. Registration improves movement accuracy when veridical visual feedback is provided but is not invoked when hand-path errors are eliminated. However, the visual channel did not exclude adjustment of terminal movement features maximizing hand-path smoothness. Thus visual and proprioceptive feedback may be combined in fundamentally different ways during trajectory control and final position regulation of reaching movements.

Redding, G.M., Wallace, B., Effects of pointing rate and availability of visual feedback on visual and proprioceptive components of prism adaptation, Journal of Motor Behavior, 24.3, 226-237. 

Abstract

When the limb becomes visible early in a pointing movement proprioceptive adaptation is greater than visual, but if visual feedback is delayed until the end of the movement the reverse is true. However, this effect occurs only if pointing rate is low. With high rates, adaptation is proprioceptive in nature regardless of feedback availability.

Different factors may determine the displacements in reaching that occur as a result of wearing prism glasses. Both visual and propioceptive factors are probably involved, but in previous studies, visual factos have been underemphasized. These experiments explored whether prism after-effects could be confined to a specific portion of the visual field. This would rule out a purely proprioceptive-motor hypothesis.

Subjects wore goggles over one eye with the other eye occluded. This allowed a monocular visual field of 60 degrees. Objects in the visual field were displaced by 22 degrees from their true position. Subjects were asked to look at the reflection of a target in a mirror so placed that the target appeared to lie on the horizontal surface of a table. The subject could mark the apparent position of the targets, but the mirror concealed his hands and marks, so subjects could not see or correct errors of localization. There were 3 phases of each experiment. The first was the pre-exposure phase in which the subject marked the apparent position of the target points. Then there was an exposure period of 1-min in which the subject with goggles on reached for a target and could see his active hand. When the exposure period was over, the goggles were removed and the subject repeated the same marking procedure as in pre-exposure. The difference in position between the pre-exposure and post-exposure markings served as a measure of the size of the Displacement after-effect.

Experiment I - adaptation to induced displacement in a limb seen through the prism, but not moved by the subject.
In the pre-exposure phase, subjects marked the apparent location of the target with the active hand first and then with the passive hand. During the exposure period, goggles were put on and the active hand was used to mark the target. In one condition, the passive hand was visible. In the other condition, the passive hand was held outside of the field of view. In the post-exposure phase, the subjects marked the location of the target with the passive hand and then the active hand to distinguish between visual and proprioceptive explainations for the DAE. Subjects showed a significant DAE when marking with the passive hand if it had been visible during the exposure period, but not if the hand had not been visible.

Experiments II & III - exposure of a limited retinal area to the prism to study the transfer of DAE between the central and peripheral regions of retina.
In the pre-exposure phase, subjects marked the location of three targets at 20 degree intervals across the visual field while fixating in the center of the field (so that peripheral areas of the retina were used when marking the lateral target points). In one exposure condition, the visual field through the prism was limited to 10 degrees. In the second condition, the goggles were masked to allow a 15 degree horizontal and a 10 degree vertical field at the periphery of the goggles' field, but there was also a pinhole in the center allowing the subject to hold fixation. The two conditions achieved differential stimulation of the central and peripheral retinal areas.
If subjects saw their hands moving in the central 10 degrees of the visual field, they showed equally large after-effects at all targets. If the subjects only saw their displaced hand in the periphery of the retina, the after-effects were greater on the exposed side of the field than on the other parts. Experiment III was the same except fixation was not required during the post-exposure phase. In this case, there was not difference in DAE for a central or lateral target.

The critical factor in the production of intermanual trasnfer of DAE was the presence of the passive hand in the visual field while the active hand was seen moving. When the passive hand was not in the field, there was no opportunity for a combined input to reach the comparator.