passive arm movements*

Transcription

1 Arm-body adaptation with passive arm movements* JOHN S. BAILY Laboratory of Experimental Psychology University of Sussex, Falmer, Brighton, England Two experiments investigated the aftereffects of pointing with passive movements during exposure to 15-deg laterally displacing wedge pnsms. Experiment 1 compared exposure with passive and active supported movements when the aftereffects were measured with the arm still in the passive movement device. Following passive exposure and active supported exposure, 5.6 and 7.9 deg, respectively, of arm-body adaptation were measured. In Experiment 2, the passive and active supported exposure tasks were compared with a third task in which similar movements were made with the arm fully supported by the muscles. Aftereffects were measured with active test movements. The amount of arm-body adaptation measured following passive exposure was decreased to 2.8 deg, and following active supported exposure it was decreased to 2.8 deg, while following exposure with normal active movements, 5.3 deg of arm-body adaptation was found. The results suggest that when the arm remains outstretched during prism exposure, adaptation is specific to this extended posture. When a person practices pointing with one arm at a target viewed through wedge prisms (termed an exposure task), an adaptive change in spatial coordination takes place between the trained (or exposed) arm and the body (Harris, 1963b). This will be termed arm-body adaptation. It has been shown that a systematic change in proprioceptive perception for the exposed arm occurs in arm-body adaptation (Harris, 1963a; Craske, 1966), although the significance of this proprioceptive recalibration has not been established with certainty. Baily (1972) has recently argued that adapted behavior is mediated directly by the proprioceptive change only when arm movements are slow and under current proprioceptive feedback control. The question of whether adaptation occurs when the arm is moved passively during exposure has been the subject of several experimental studies. This is a matter of considerable importance for theories of prism adaptation, for if arm-body adaptation does occur with passive exposure it must presumably be due solely to a proprioceptive change. Held and Hein (1958) found that adaptation did not take place when S simply watched his arm being passively moved to and fro across the visual field. Held and Freedman (1963) argued from this and related data that prism adaptation involves setting new values for certain *This work was supported by a grant from the Medical Research Council for work on sensorimotor adaptation. I wish to thank Professor N. S. Sutherland for advice and discussion, and M. Shrimpton and C. Chiltem for assistance with building apparatus. constants in the system which relates visual feedback with motor outflow. This theory ignores the importance of proprioception in motor control. Howard and Templeton (1966) argued that Held's position was unwarranted because Held used exposure tasks of passive movement which were weak as training methods, in the sense that they did not force S to utilize the discordant information which was available. Templeton, Howard, and Lowman (1966) found that adaptation did occur with passive arm movements when the exposure task required S to judge when his passively moved unseen arm was pointing at a visual target, visual feedback from the arm being given only at the end of each exposure trial. This result suggests that proprioceptive recalibration can result purely from making visual-proprioceptive matching judgments. Some evidence for adaptation with passive arm movements was found by Pick and Hay (1965); Singer and Day (1966a, b) claimed to show that passive exposure gave rise to as much adaptation as active exposure when the exposure task involved spatial judgments. A serious criticism of all these studies is that none of them employed tests which showed whether adaptation actually involved a change in spatial coordination between the exposed arm and the body. The adaptation with passive arm movement found in these experiments could have been due to eye-head adaptation, which can take place in the absence of any 0 movement or visual feedback from viewing parts of the body through prisms (Rock, Goldberg, & Mack, 1966). Experiment 1 was designed to investigate whether or not arm-body adaptation occurs with passive exposure. The method was based on the finding of Wilkinson (1968) that the size of the arm-body component in the total adaptive shift in ipsilateral eye-hand coordination following prism exposure in a pointing task can be derived by measuring eye-hand coordination with both arms and subtracting the contralateral aftereffect from the ipsilateral aftereffect. EXPERIMENT 1 Experiment 1 employed a 2 by 2 factorial design similar to that used by Pick and Hay (1965). The passive movement exposure task was compared with an equivalent task in which the arm was placed in the trolley used for the passive movement condition and moved by muscle action (active supported movement). Two groups of S8 were run in each condition, in one group the pre- and posttests were performed with passive movements and in the other with active supported movements. The exposed arm remained in the trolley throughout exposure and testing. Method Subjects. Forty experimentally naive Ss (20 male, 20 female) took part in the experiment. Sixteen were university undergraduates, while the rest were school children aged They were divided into four groups, each containing equal numbers from the two populations and equal numbers of males and females. All Ss were right-handed. Apparatus. The apparatus has been described in detail elsewhere (Baily, 1972). To measure eye-hand coordination, Ss pointed below dim target lights (test targets) viewed in darkness, with no visual feedback from the limbs. Direction of pointing was recorded in degrees of arc relative to the median plane with an automated system when S touched down on the marking surface with contacts attached to the index fingers. Two bars at waist height were provided for the hands to rest upon. In Experiment 1, the right arm was supported throughout exposure and testing in an extended horizontal posture by the trolley, the distal end of which ran on wheels over the marking surface. In passive movement, the trolley was driven at a rate of 1 deg/sec by a small reversible motor which was operated by S, using two foot switches. In active supported.movement, the motor was disengaged with a clutch and the trolley was moved by the action of the arm and shoulder muscles. The right index Perception & Psychophysics, 1972, Vol. 12 (1A) Copyright 1972, Psychonomic Society, Austin, Texas 39

2 finger lay outstretched in a trough, and its position was registered at the end of a response by a solenoid contacting the marking surface when S operated a switch with his left hand. The level of neuromuscular activity in the passively moved arm was not monitored electromyographically. The criterion of passive movement adopted in this and in previous experiments by other authors is that the limb is moved by an external agent while the muscles are in a relatively relaxed state, not that the limb shows total neuromuscular silence. Two 15-deg displacement wedge prisms were mounted base right on a hinging panel in a mask which covered S's face. The size of the visual field through the prisms was 30 deg in diam. The target for the exposure task (the exposure target) consisted of a thin vertical 3-in. rod of Perspex with a dim light source at one end. It was at the same level as the test targets, and during exposure was positioned 15 deg right so as to appear straight ahead when viewed through the base-right prisms. Viewed in the dark, the exposure target appeared as a luminous bar, and with the houselights on, as a white strip. The exposure target was fixed to the center of an oblong panel suspended from a boom pivoted above the head and was placed well to the left of 8 during testing. The three strips of the marking surface 14, 15, and 16 deg right were connected to a buzzer which sounded when 8 hit one of these strips during exposure. Head position was stabilized during exposure by 8 keeping his face pressed firmly against the eyepieces of the mask. During testing, 8 was instructed to keep his head still and aligned with his body. Test tasks. Before and after prism exposure, eye-hand coordination was tested with the left and right arms on one test target, which was located straight ahead. When pointing with the left arm, 8 first raised the hand from its rest bar to a position 3 in. below his chin and then started to move his hand away from his body. As the elbow joint was extended, the arm was moved at the shoulder so that the hand oscillated from side to side in an increasing arc (zeroing out). Once the arm was straight, it was moved slowly from side to side in a series of oscillations of decreasing arc about the position which felt directly below the test target (zeroing in). The size of the oscillation at the start of zeroing in was around 30 deg. Having zeroed in under the test target, the arm was lowered until the index finger touched the marking surface. At the end of the response, the hand was returned to its rest bar. The Ss learned to make the zero-out/zero-in movement with a time course of 5-10 sec during a pretraining session immediately before the experiment. Ss first learned the appropriate spatial structure of the movement in the apparatus with the houselights on and were later given practice in pointing below a range of test targets positioned at 15-deg intervals, with 2-sec visual feedback given after each response. The right arm was tested in the trolley. Two groups of Ss performed the tests with passive movement. During the 15-min pretraining session, Ss learned to use the trolley to point below test targets in the dark with terminal visual feedback while keeping the muscles of the arm and shoulder as relaxed as possible. Frequent reminders about keeping relaxed were given. The other two groups performed the tests with active supported movement. They were trained to move the arm at the same speed as in passive movement. Before each right arm test, the trolley was pushed by E to a start position between 15 and 30 deg left or right of the test target. At these times, E was able to check to see that Ss in the passive movement conditions were keeping their right arms relaxed. The movement strategy was to approach the target continuously from the start position, to carryon until the arm was felt to have overshot, and then to go back to what felt to be the correct position, When the left arm was tested, the right arm was positioned by E 45 deg right. The tests were performed in darkness. After each test, the houselights were switched on to allow E to make the adjustments for the next test. At these times, S was instructed to close his eyes, and a hood was placed across the face mask so that if he inadvertently opened his eyes, he could see no part of his body or the apparatus. Performance of the test tasks was not paced. Exposure tasks. During exposure, 8 pointed repeatedly with the right arm below the exposure target, using either passive (two groups) or active supported movements (two groups). The movements were made without visual feedback, but when S pressed the switch with his left hand to register the right arm's position at the end of each response, the houselights were automatically switched on for 2 sec to provide terminal visual feedback. 8tart positions during exposure were between 15 and 30 deg alternatively left and right of the exposure target. The movement strategy was the same as for the right arm test movements. The 8 was instructed to position his arm so that when the houselights came on, he could see his index finger below the exposure target. In addition, if the arm was positioned correctly, a buzzer would sound. Performance was not paced during exposure. During the short periods between the pretests and exposure and between exposure and the posttests, the right arm was positioned approximately straight ahead. Experimental design. Eye-hand coordination was tested five times with the left arm and five times with the right arm before and after exposure. Exposure consisted of 25 pointing responses. During testing, the left and right arms were used in alternation. Half the Ss in each group began the pre- and posttests with the left arm, and the other half with the right. Right arm start positions were to the right of the test target in three of the pretests and in two of the posttests, so that if there was bias towards the start position, this would tend to decrease the size of the measured ipsilateral aftereffects rather than increase them. Mean changes in eye-hand coordination were derived by calculating the difference between the means of the pre- and posttest observations in the two tests for each S, and then determining the mean shifts in coordination in the two tests in the four experimental conditions. Shifts to the right were in the adaptive direction. Results Performance during exposure. In both exposure tasks, performance was initially in error approximately by the amount expected from the optical displacement. The mean direction of the first response in passive exposure was 2.8 deg right (of the median axis), and in active supported exposure was 0.4 deg right. A criterion of five consecutive trials within a range of ±4 deg of the exposure target was adopted to compare the two tasks in terms of rate of adaptation. One 8 in passive exposure did not reach criterion. The mean number of trials to criterion for the other 19 Ss was The mean number of trials to criterion in active supported exposure was The rate of adaptation in the two tasks was not significantly different (Mann-Whitney test). Aftereffects. The means for the preand posttests and the mean changes in eye-hand coordination (aftereffects) found in the four experimental conditions are shown in Table 1. The results of the 8 who failed to reach criterion were included because inspection of the aftereffect data showed that this 8 did not differ from other Ss in the same condition. The aftereffects for the two arms were 40 Perception & Psychophysics, 1972, Vol. 12 (la)

3 analyzed separately in two two-way analyses of variance in which exposure movement and test movement were the main factors. The analysis of variance for the nonexposed arm showed that neither factor nor their interaction was significant. The mean contralateral aftereffect for the four conditions was 1.5 deg, a significant shift, t( 39) = 5.37, P <.001. The mean size of the mean contralateral aftereffects in the two conditions of exposure will be taken as a measure of the amount of eye-hand (or head-body) adaptation that occurred (Wilkinson, 1968). The analysis of variance for the exposed arm showed that the ipsilateral aftereffects of active supported exposure were significantly larger than the ipsilateral aftereffects of passive exposure, F(1,36) = 9.12, p <.01. Although the ipsilateral a ftereffects measured with active supported tests were larger than the ipsilateral aftereffects measured with passive tests, this difference was not significant. Subtraction of the contralateral aftereffects from the mean ipsilateral aftereffects gives a value of 5.6 deg for the amount of arm-body adaptation found following passive exposure and 7.9 deg for the amount following active supported exposure. To determine whether the difference between passive and active supported exposure was due to differences in the rate of decay of the ipsilateral aftereffects, the aftereffects were plotted test by test (see Fig. 1). It is evident that the size of the ipsilateral aftereffects of both exposure conditions increased somewhat during the postexposure period. Figure 1 shows also that the ipsilateral aftereffects in both exposure conditions were affected by start position, being larger when the arm started from a position to the right of the test target. This bias was also found in the pretests and during exposure. CONTROL EXPERIMENT In addition to exposure to laterally displaced vision, other factors such as postural asymmetry of the right arm might have affected the ipsilateral aftereffects without affecting the contralateral aftereffects. To test for the presence of such factors, a control experiment was run in which 10 new Ss went through a procedure similar to the passive exposure/passive test group of Experiment 1 (the group that showed the smallest ipsilateral aftereffect) but without visual displacement. The exposure target was positioned 15 deg right, and terminal visual feedback was given. The mask was not positioned over S's face. The S was told to keep his head aligned with his body during exposure and testing, but head position was not otherwise stabilized. The results of the control experiment (see Table 2) showed that eye-hand coordination shifted with both arms towards the left (a nonadaptive shift) by approximately equal amounts. The contralateral shift was not significantly larger than the ipsilateral shift (Wilcoxon test). The overall shift to the left when the data for the two arms were combined was significant (Wilcoxon test, p <.001). Since both arms were affected equally, it is justified to assume that in Experiment 1 the exposed and nonexposed arms were not differently affected by factors other than exposure to laterally displaced vision. It might be argued that as the nonadaptive shift found in the control experiment would have decreased the size of the aftereffects measured in Experiment 1, this value should be added to the contralateral and ipsilateral aftereffects of prism Table 1 Results of Experiment 1 (in Degrees) Showing Shifts in Eye-Hand Coordination With Exposed and Nonexposed Arms N onexposed Arm exposure. However, the nonadaptive tendency may have been peculiar to the conditions of the control experiment. Visual asymmetry during exposure may have biased the apparent visual median plane to the right, or a head-body postural aftereffect may have resulted from a tendency for Ss to tum their heads to the right in order to fixate the exposure target. But, as both arms were equally affected, correcting the aftereffects would not make any difference to the values for arm-body Exposed Arm Pretest Posttest Shift Pretest Posttest Shift P Exposure/P Test 0.5 R 1.6 R R 11.2 R +6.4 P Exposure/AS Test 0.6 L 1.3R R 11.5 R +7.9 AS Exposure/P Test 0.3 L 1.3R R 12.6 R +8.7 AS Exposure/AS Test 0.8 L 0.7 R R 12.3 R No terrp = passive movement. AS active supported movement; L = mean response to left of median axis, R = to right; + = shift in eye hand coordination in adaptive direction..,.. 10 m 8., '" ~... u 6 ẉ... w tr ẉ <l 2 '" active supported exposure passive exposure POST TESTS Fig. 1. Ipsilateral aftereffects in Experiment 1. In Posttests 2 and 4, the start position was to the right of the test target; in Posttests 1,3, and 5, it was to the left. adaptation derived in Experiment 1 by sub t r a c t i n g the contralateral aftereffects from the ipsilateral aftereffects. Discussion The results of Experiment 1 provide the first direct evidence to show that a substantial amount of arm-body adaptation can be found following prism exposure with passive arm movements. The results of the control experiment show that this adaptation did not contain a component due to a postural aftereffect. As discussed in the introduction, Held and Hein (1958) found that exposure with passive arm movements did not lead to adaptation when the limb was continuously visible and no targets were employed. In such a task, information about the discordance between vision and proprioception is available to S, but the task does not require S to utilize this information. The training method employed in Experiment 1 required S to match his disparate visual and proprioceptive inputs on each exposure trial. It appears that this matching procedure led to a recalibration of the arm's proprioceptive input which decreased the discordance between vision and proprioception. If this interpretation is valid, it also appears reasonable to suppose that in exposure tasks which involve slow active movements pointing at a target with terminal visual feedback, arm-body adaptation again consists of a proprioceptive change which is the result of visual-proprioceptive matching. Exposure tasks of this type have been used by McLaughlin (e.g., McLaughlin & Bower, 1965) and also by Baily (1972). The finding that the ipsilateral Perception & Psychophysics, 1972, Vol. 12 (la) 41

4 Table 2 Results (In Deerees) of Control Experiment ShowinC Shifts In Eye-Hand Coordination With Exposed and Nonexposed Arms Pretest O.2R Nonexposed Arm Posttest 2.4 L Shift -2.6 Note-L $ mean response to left of median axis, R $ to right, - $ shift in eye-hand coordination in nonadaptiue direction. aftereffect of active supported exposure was significantly larger than the ipsilateral aftereffect of passive exposure seems to support the argument that active movement per se is an important determinant of adaptation (Held & Freedman, 1963). However, this difference may have been an experimental artifact. The data show that in the passive pretests, the right arm was biased more to the right than in the active supported pretests. This differential bias was also found in exposure, when the initial error on the first response in passive exposure was smaller than in active supported exposure. Less proprioceptive recalibration may have been required in the passive exposure task for performance to become accurate, with the result that there was a smaller aftereffect. Thus, the results of Experiment 1 do not provide conclusive evidence that there is a difference in the amount of adaptation resulting from equivalent active and passive exposure tasks. EXPERIMENT 2 Baily (1972) measured ipsilateral and contralateral aftereffects following performance of an exposure task with passive arm movements similar to that used in Experiment 1. The aftereffects were measured with zero-out/zero-in pointing movements. Although some arm-body adaptation was found with these slow tests following passive exposure, the amount found was rather small, only 2.6 deg. Experimen t 1 shows that a considerably larger amount of arm-body adaptation can be found following passive exposure when the arm remains in the trolley during testing. Baily (1972) suggested that the aftereffect of passive exposure may be conditioned to tactual cues associated with the trolley. Experiment 2 investigated this suggestion by comparing the passive and active supported exposure tasks used in Experiment 1 with a third exposure task in which spatially similar movements were made with the arm fully supported by the muscles. This will be termed normal active movement. Aftereffects were measured with zero-out/zero-in test movements. It was expected that more arm-body adaptation would be found Pretest 4.8 R Exposed Arm Posttest 2.6R Shift -2.2 following exposure with normal active movements than with passive or active supported movements. Experiment 2 also measured the lateral transfer of arm-body adaptation resulting from the three conditions of exposure to test targets 30 deg either side of the exposed direction. Wide lateral transfer is one of the properties of arm-body adaptation (Harris, 1963b; Baily, 1972). These tests were run as a check to see that the arm-body adaptation produced by the exposure tasks in Experiment 2 also showed this characteristic. Method Subjects. Thirty experimentally naive Ss (15 male, 15 female), all of whom were school children, aged 17-19, took part. They were divided into three groups, each group containing an equal number of males and females. All Ss were right-handed. Apparatus. The apparatus was the same as that used for Experiment I, with the addition of a horizontal platform which could be placed between the marking surface and 8's chest in order to support the right arm in an extended horizontal posture in the normal active exposure condition. Test tasks. Eye-hand coordination was tested for both arms with zero-out/zero-in movements. When not responding, the hands were kept on their respective rest bars. During testing, the trolley was removed from the apparatus. Eye-hand coordination was tested for the left arm with one test target, which was located straight ahead. Eye-hand coordination with the right arm was tested with three test targets, located 30 deg left, straight ahead, and 30 deg right. Exposure tasks. The passive and active supported exposure tasks were the same as those for Experiment 1. In the normal active exposure task, 8 rested his right arm in a horizontal outstretched posture on the platform. Movement in this condition was similar in terms of speed and spatial structure to movement in the passive and active supported conditions. Before each response, E guided the arm to a start position between 15 and 30 deg to the left or right of the exposure target. At the start of each response, S raised his arm an inch or two above the platform, moved it slowly toward the exposure target, overshot once, and went back to what felt to be the correct position. The S then lowered his arm until the index finger touched the marking surface, when 2 sec of visual feedback (and auditory feedback if the response was accurate) was given. While the houselights were on, S kept his arm supported above the platform, with only the index finger touching the marking surface. When the lights went out, the arm was rested on the platform until guided to the next start position. Experimental design. Each test of eye-hand coordination was given three times before and three times after exposure. Exposure consisted of 25 responses. The left arm was always used first in each block of four tests, followed by the three right arm tests, the six possible permutations of test target presentation being used in each session. Results Performance during exposure. The mean direction of the first response in passive exposure was 0.7 deg left, in active supported exposure was 1.6 deg left, and in normal active exposure was 0.4 deg left of the median axis. The same criterion was adopted as in Experiment 1 to compare the rate of adaptation in the three tasks. Criterion was reached by 9 of the 10 Ss in passive exposure, with a mean of 13.0 trials; by 9 of the 10 Ss in active supported exposure, with a mean of 10.1 trials; and by all Ss in normal active exposure, with a mean of 11.7 trials. The rate of adaptation in the three tasks did not differ significantly (Mann-Whitney U test). Aftereffects. Changes in eye-hand coordination found in the three experimental conditions are shown in Fig. 2. The data of the two Ss who failed to reach criterion did not differ from other Ss in the same conditions and was therefore included. The data for the nonexposed arm were submitted to a two-way analysis of variance in which exposure condition and pre-/posttest measures were the main factors. Neither factor nor the interaction were significant. The mean shift in contralateral eye-hand coordination of 1.0 deg was significant, however, when tested directly, t(29) = 2.68, p <.02. The size of the mean contralateral aftereffects in the three conditions of exposure will be taken as a measure of the amount of eye-head (or head-body) adaptation that occurred. The aftereffects for the exposed arm were submitted to a two-way analysis of variance in which positions of the test targets and exposure condition were the main factors. Only 42 Perception & Psychophysics, 1972, Vol. 12 (la)

5 exposure condition was significant, F(2,81) = 11.00, p <.01. Differences between the levels of the ipsilateral aftereffects found in the three conditions were also tested with t tests, in which the data for the three target positions were combined. There was no difference between the mean ipsilateral aftereffects of passive and active supported exposure, while the mean ipsilateral aftereffect of normal active exposure was significantly larger than the mean ipsilateral aftereffects of passive exposure, t(58) = 4.71, p <.001, and of active supported exposure, t(58) = 2.83, p <.01. Subtracting the mean contralateral aftereffects from the mean ipsilateral aftereffects found in each condition gives the following values for the measured with zero-out/zero-in tests: 2.8 deg following passive exposure; 2.8 deg following active supported exposure; and 5.3 deg following normal active exposure. The wide lateral transfer of the ipsilateral aftereffect in each case shows that generalized changes in arm-body coordination were involved. This amount of lateral transfer agrees with the results of previous experiments (Baily, 1972). DISCUSSION Following passive exposure, 5.6 deg of arm-body adaptation was found when the arm was tested in the trolley (Experiment 1), while only 2.8 deg was found with zero-out/zero-in test movements (Experiment 2). Following active supported exposure, the amount of arm-body adaptation measured likewise decreased from 7.9 to 2.8 deg. In Experiment 2, there were no differences between the aftereffects of active supported and passive exposure. This is possibly associated with the fact that the difference in the initial error in the two tasks was smaller than in Experiment 1. A significantly larger (5.3 deg) was found in zero-out/zero-in tests when spatially similar exposure responses were made with normal active movements. These results could be interpreted to support the hypothesis that when the arm is in the trolley during exposure, the aftereffect is (to some extent) conditioned to the trolley. However, a different factor may account for the decrement in the aftereffects of passive and active supported exposure when measured with zero-out/zero-in tests. This is suggested by.the finding that the measured following normal active exposure in Experiment 2 was considerably smaller than the amount measured with the arm in the trolley I- u w u, 6 u, J) w (I) 4 G- ~ 0:: L. W 01 I- Q) u, '0 ~ 2 <t o 0 30 left straight - ahead 30 rignt TEST TARGET POSITION ~ exposed arm. normal active exposure o exposed arm, active supported exposure o exposed arm. passive exposure non- exposed arm, normal active exposure non-exposed arm, active supported exposure non- exposed arm, passive exposure Fig. 2. Changes in eye-hand coordination in Experiment 2. following active supported exposure in Experiment 1. This apparent decrement could be related to the fact that the exposed arm remained outstretched throughout exposure and testing in Experiment 1, while it was returned to a resting position after exposure and before each test trial in Experiment 2. In the latter case, arm-body adaptation might be subject to a special kind of interference effect. For example, when the arm remains outstretched throughout exposure, proprioceptive recalibration may be specific to this extended posture. When the arm is flexed and placed in a position near the body, S may receive conflicting kinesthetic inputs which lead to a decrease in the amount of proprioceptive recalibration.shown when the arm is replaced in an extended posture during testing. If interference from the test task was the cause of a decrement in the aftereffect, the interference must have been of a highly specific kind. This is shown by the finding in Experiment 2 that the ipsilateral aftereffects in the three conditions transferred fully to positions 30 deg either side of the exposed direction. Again, in Experiment 1, the absence of an interaction effect between exposure movement and test movement indicates that there was no interference when exposure and test tasks involved different movements. The interference hypothesis predicts that more arm-body adaptation would have been measured following normal active exposure in Experiment 2 if the arm had remained outstretched between exposure and testing and between the test trials. The results of Experiment 1 suggest that 7.9 deg of arm-body adaptation would have been measured with this type of test. If this is a valid estimate, then interference from the test task decreased the measured following normal active exposure by 2.6 deg, A decrement of 2. 6 deg due to interference is sufficient to explain why the 5.6 deg of arm-body adaptation following passive exposure was reduced to 2.8 deg when measured with zero-out/zero-in test movements. It does not explain how the 7.9 deg of arm-body adaptation following active supported exposure was reduced to 2.8 deg, for this leaves 2.5 deg unaccounted for. This might be a measure of a component of arm-body adaptation conditioned to the trolley, but little confidence can be held in this suggestion. Experiment 2 does not provide unequivocal evidence for interference between exposure and test tasks, but this is perhaps the most reasonable explanation of why Baily (1972) obtained unexpectedly small aftereffects with zero-out/zero-in tests following passive exposure. REFERENCES BAILY. J. S. 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