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.