Research ReportThe contribution of visual feedback to visuomotor adaptation: How much and when?
Introduction
Unusual force fields and optical transformations such as those created by mirrors, prisms or computer-generated rotations create problems for an inexperienced person attempting a target-directed aiming task. For example, unusual forces can push the person off course and visual rotations typically cause people to move in the wrong direction. However, after attempting the task a few times a person typically learns to deal with the altered environment: their movement paths become straight with bell-shaped velocity profiles, almost identical to those originally produced in normal conditions. The person is said to adapt to the altered environment. Experiments have shown that adaptation typically involves changes in the feedforward motor commands that take the alteration into account and compensate for it. Such changes in the commands can be interpreted as demonstrating that the process of adaptation involves creating or updating an internal model (e.g., Kawato, 1999). In the case of altered optical transformations, the model takes the form of a visuomotor map that transforms visual information into motor commands (Cunningham, 1989). Adaptation to a visual rotation then involves adjusting the visuomotor map so as to compensate for the magnitude and direction of rotation.
The adaptation process is driven by sensory feedback information about the discrepancy between the intended movement and the actual movement (the error). The visual and somatosensory systems are the most important sources of such information and normally both systems are likely to contribute to adaptation. However, it is known that vision alone can sometimes be sufficient for adaptation (Ghez et al., 1995) and somatosensory information can be sufficient for adaptation to novel force environments (Lackner and Dizio, 1994, Tong et al., 2002, Scheidt et al., 2005). In the case of altered visuomotor environments (e.g., those induced by prisms or rotations of feedback on a computer display), visual feedback concerning task performance is necessary for adaptation of aiming movements, when the success of the movement can only be determined visually. A recent study by Mazzoni and Krakauer (2006) indicated that in an out-and-back movement of a cursor, visual feedback in the first 100 ms of the movement, and a cursor depicting the reversal point of the movement, was sufficient to allow adaptation. This suggests that continuous visual feedback of the cursor position is not necessary for visuomotor adaptation, at least in dynamic visuomotor tasks (see also Krakauer et al., 1999, Miall et al., 2004).
It is clear that visual feedback about various features of performance can be provided to a person in an aiming task. The most natural and obvious situation is one in which the person is able to see themselves move the working point as they attempt to acquire the target (complete concurrent feedback). Various restrictions can be introduced that allows a person to see only some of their performance (e.g., Mazzoni and Krakauer, 2006). An alternative is to deny a person visual feedback during performance but provide it after completion of the task. Depending upon the information actually provided, this type of feedback is called knowledge of results (KR) or knowledge of performance (KP). In KR only feedback about the outcome is provided: in an aiming task it might show the relative position of target and aiming device at completion. In KP, feedback about the movement is given: in an aiming task it might show the path taken to the target.
We sought to investigate how different types of visual feedback influence adaptation to a visual rotation. In particular, we asked whether the type of visual feedback (complete concurrent feedback or post-trial feedback) would affect how participants learned to compensate for the rotation. When aiming at a target in the presence of a visual rotation, it is possible to reach the target by moving in a direction that exactly cancels out the rotation. For example, if the rotation is 60° clockwise and the target is straight ahead, a movement directed 60° to the left will be in the direction of the target. Moving in the appropriate direction could be the result of a change in the visuomotor map. Altenatively, the person might learn the cognitive strategy of always aiming in a direction 60° to the left of the target. The method of interspersing occasional catch trials (Shadmehr and Mussa-Ivaldi, 1994), in which no rotation (perturbation) is applied, amongst a sequence of rotation trials is unable to distinguish between these two possibilities because in either case it would result in identical behaviour. For this reason we adopted a different approach. If participants were informed as to whether or not there was a visual rotation, for example with a colour cue, then the cognitive strategy of always aiming in a direction 60° to the left of the target, could be adopted when appropriate. In this case, we would expect little or no errors when participants return to a non-rotated environment. If, however, learning a 60° rotation results in a change in the visuomotor map, a return to a non-rotated environment would result in aiming errors in the opposite direction of the rotation. We employed an isometric aiming task in which torques are applied to a fixed manipulandum and converted into movements of a cursor on a computer screen (Shemmell et al., 2005). This task removes potential complications due to the muscular and skeletal degrees of freedom available to participants in unconstrained reaching tasks, and eliminates the effects of anisotropic viscous and inertial properties of the limb (Pellegrini and Flanders, 1996).
Section snippets
Results
Participants produced isometric torques to move a cursor towards visual targets, presented on a computer screen (Fig. 1). Two groups of participants received continuous visual feedback of the cursor position (i.e., concurrent feedback, CF). One of these groups produced feedback modifications to correct errors (CF-FB), while one group only made feedforward responses (CF-FF). Two groups were provided with post-trial (PF) knowledge (feedback) of task performance (PF-KP) or task result (PF-KR)
Discussion
We investigated the effects of the type of visual feedback (concurrent or post-trial feedback) and of task instruction (“modify path” versus “do not modify path”) on participants' ability to adapt to a novel visuomotor rotation in an isometric target acquisition task. All groups of participants were able to compensate for the visual rotation to a similar degree over the course of training. The compensation had a major feedforward component as the angular error in the initial direction of
Participants
Thirty-two self-reported right-handed participants (18 male, 14 female, 19–44 years) took part in this study. All participants gave informed consent to the procedures, which were approved by the Medical Ethics Committee of the University of Queensland, and conformed to the Declaration of Helsinki.
Apparatus
Participants sat in a height-adjustable chair 65 cm from a computer screen, positioned at eye level. The right arm was placed in a padded brace with the elbow flexed at 90° and the forearm in a neutral
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