Abstract
Implicit sensorimotor adaptation has been proposed to be driven by error signals created by discrepancies between various sensory information sources. While proprioception has been proposed as a critical sensory source for the error signal driving adaptation, the role of the sensory prediction derived from the action-related efference copy has largely been neglected. Current task protocols do not typically separate afferent and efferent contributions; thus, their individual roles in implicit adaptation are poorly understood. The purpose of this study was to examine the effect of dissociating the afferent and efferent information available during implicit adaptation. We examined this through an isometric aiming task using task-irrelevant clamped feedback. Instead of performing actual reaching movements, participants moved a visual cursor towards radial targets by applying horizontal forces to a fixed handle at a central home location. During perturbation trials, the cursor followed an invariant path rotated relative to the target. Participants were instructed to ignore this task-irrelevant cursor feedback and continue to "reach" towards the target. We found that participants implicitly adapted in the isometric task, even when the hand never actually moved to the target. Moreover, the level of adaptation observed surpassed that of a typical clamped reaching paradigm by nearly twofold. This finding was confirmed in a secondary experiment where participants performed actual reaching movements and demonstrated significantly less adaptation. These findings suggest that while afferent proprioceptive feedback of hand position at the target location most likely plays a role in adaptation behaviour, it is not necessary to induce adaptation.
