An analysis of variables affecting acquisition and decay of adaptation to lateral prismatic displacement

The study was designed to test current theories of prism adaptation to lateral prismatic displacement by investigating importance of sight of the body, active movement, and magnitude of displacement. An attempt was made to uncover adaptation effects due to information by providing appropriate controls for proprioceptive-kinesthetic and visual, asymmetrical-stimulation effects. Three dependent variables - visual egocentric localization, visual-target pointing (hand unseen), and nonvisual hand-to-hand localization permitted estimation of visual, visuomotor, and nonvisual adaptation, respectively. One hundred and eighty male college students served as subjects. Each was assigned to one of 18 treatment groups at random. Monocular, base-left, prism-viewing conditions (to 20 or 30-pd. for 40-minutes) included: (a) passive viewing of the target-pointing test apparatus, (b) passive viewing of the feet, (c) passive viewing of the arm to be tested, and (d) tracking a moving spot of light contingent upon active-movement of the arm to be tested. Subjects assigned to visual-asymmetry control groups viewed their limbs passively positioned in those spatial locations where subjects in passive-viewing, prism-exposure groups saw their limbs. Subjects in motorasymmetry control groups were treated similarly, except that their eyes were closed during the 40-minutes 'exposure' period. Subjects in other normal-viewing control groups were treated identically to subjects in the prismviewing groups, except that they wore plane-glass goggles during 'exposure'. Test measurements were made before exposure, immediately after exposure, and after 5 - and 10 - minutes of readaptation experience. Half of the subjects within each of the 18 treatment groups underwent active readaptation; the remainder underwent passive readaptation. In the passive readaptation condition, S sat quietly in the dark (head held fixed by the biteplate of the test apparatus). In the active readaptation condition, was required to sit on a chair in a hallway adjacent to the experimental room and to observe his self-produced leg movements and to move his body. Significant, but small amounts of visual (15-27%) and visuomotor (9-20%) initial postexposure adaptive shifts were obtained across all prism-exposure conditions with no significant differences between them. Sight of the passive limbs was associated with visuomotor adaptation that could not be attributed to prolonged asymmetrical stimulation. Relevance of asymmetry as opposed to 'information' (e.g., from intersensory discordance), however, might account for postexposure adaptive shifts in visual egocentric localization following sight of the limbs. Since amounts of visual and visuomotor adaptation obtained in the passive-arm, visual-asymmetry condition were comparable to those obtained in the apparatus-viewing condition, it is likely that the optical rotation variable, identified by McLaughlin et al. (1966), was responsible for both adaptive shifts in these two conditions. Both visual and visuomotor adaptation following exposure to visual reafference from self-produced arm movements could not be attributed either to asymmetry or to a 'correction effect'. There were no differences in either visual or visuomotor adaptation associated with prism magnitude. The only significant shifts to be obtained on the test for nonvisual (arm) adaptation were immediate postexposure adaptive shifts following 0 - and 20 - pd. active-movement exposure. No differences in either visual or visuomotor readaptation were attributable to type of readaptation experience. Decay of visuomotor adaptation was greater when ^pointed to a test target positioned 14.5° to his right than when the visual target was positioned 14.5° to his left. There was significantly greater persistence of visual adaptation, which suggests that conditioning of adaptation to features of the testing situation took place. Results of this investigation fail to offer complete support to any current theory of adaptation, although it is suggested that prism adaptation is due to an informational-inconsistency comparing process. No attempt was made to specify the nature of the adaptive change (s) that occurred.