Assessing and defining explicit processes in visuomotor adaptation

Updating of Implicit Adaptation Processes through Erroneous Numeric Feedback

There is debate about how implicit and explicit processes interact in sensorimotor adaptation, implicating how error signals drive learning. Target error information is thought to primarily influence explicit processes, therefore manipulations to the veracity of this information should impact adaptation but not implicit recalibration (i.e. after-effects). Thirty participants across three groups initially adapted to rotated cursor feedback. Then we manipulated numeric target error through knowledge of results (KR) feedback, where groups practised with correct or incorrect (+/-15°) numeric KR. Participants adapted to erroneous KR, but only the KR + 15 group showed augmented implicit recalibration, evidenced by larger after-effects than before KR exposure. In the presence of sensory prediction errors, target errors modulated after-effects, suggesting an interaction between implicit and explicit processes.

Aged Patients With Severe Small Vessel Disease Exhibit Poor Bimanual Coordination During the Anti-Phase Horizontal Line Drawing Task

The present study explores whether SVD affects bimanual coordination, which is easier to detect than by conventional, MRI-based methods. We tested nine severe SVD patients, eight non-severe (i.e., moderate or mild) SVD patients, eleven healthy age-matched controls, and eight young adults. They were grouped according to Fazekas scale and by age. Participants performed horizontal line drawings with both hands simultaneously on two pen tablets. The movements consisted of rhythmic patterns where participants used both hands to draw horizontal lines in anti-phase on two pen tablets. Each participant underwent a series of neuropsychiatric assessments. Results showed that SVD patients exhibited in each hand smaller horizontal movement amplitudes with variability larger compared to the healthy age-matched controls. Only movement amplitudes appeared to decrease significantly with severity of SVD. Interestingly, we found no relevant differences between the age-matched, elderly controls and the young controls. Therefore, this effect appeared indicative of SVD. The variability of the lines orthogonal to the horizontal lines of the left, non-dominant hand differed only between the severe SVD group and the other groups. Furthermore, partial correlations demonstrated that the mean horizontal movement amplitude of the left hand was positively associated with the clock drawing test score, and the inter-manual asynchrony of the horizontal movements was positively associated with the Trail Making Test-B time. These results indicated that SVD patients show poor bimanual coordination, as reflected by spatial features such as movement amplitudes and variabilities, and abnormal bimanual coordination was associated with executive dysfunction.

Implicit Adaptation Processes Promoted by Immediate Offline Visual and Numeric Feedback

In adaptation learning, visual feedback impacts how adaptation proceeds. With concurrent feedback, a more implicit/feedforward process is thought to be engaged, compared to feedback after movement, which promotes more explicit processes. Due to discrepancies across studies, related to timing and type of visual feedback, we isolated these conditions here. Four groups (N = 52) practiced aiming under rotated feedback conditions; feedback was provided concurrently, immediately after movement (visually or numerically), or visually after a 3 s delay. All groups adapted and only delayed feedback attenuated implicit adaptation as evidenced by post-practice after-effects. Contrary to some suggestions, immediately presented offline and numeric feedback resulted in implicit after-effects, potentially due to comparisons between feedforward information and seen or imagined feedback.

Improved proprioception does not benefit visuomotor adaptation

Visuomotor adaptation arises when reaching in an altered visual environment, where one's seen hand position does not match their felt (i.e., proprioceptive) hand position in space. Here, we asked if proprioceptive training benefits visuomotor adaptation, and if these benefits arise due to implicit (unconscious) or explicit (conscious strategy) processes. Seventy-two participants were divided equally into 3 groups: proprioceptive training with feedback (PTWF), proprioceptive training no feedback (PTNF), and Control (CTRL). The PTWF and PTNF groups completed passive proprioceptive training, where a participant's hand was moved to an unknown reference location and they judged the felt position of their unseen hand relative to their body midline on every trial. The PTWF group received verbal feedback with respect to their response accuracy on the middle 60% of trials, whereas the PTNF did not receive any feedback during training. The CTRL group did not complete proprioceptive training and instead sat quietly during this time. Following proprioceptive training or time delay, all three groups reached when seeing a cursor that was rotated 30° clockwise relative to their hand motion. The experiment ended with participants completing a series of no-cursor reaches to assess implicit and explicit adaptation. Results indicated that the PTWF group improved the accuracy of their sense of felt hand position following proprioceptive training. However, this improved proprioceptive acuity (i.e., the accuracy of their sense of felt hand) did not benefit visuomotor adaptation, as all three groups showed similar visuomotor adaptation across rotated reach training trials. Visuomotor adaptation arose implicitly, with minimal explicit contribution for all three groups. Together, these results suggest that passive proprioceptive training does not benefit, nor hinder, the extent of implicit visuomotor adaptation established immediately following reach training with a 30° cursor rotation.

The influence of awareness on implicit visuomotor adaptation

It is well documented that reaches are adapted when reaching with a visuomotor distortion (i.e., rotated cursor feedback). Less clear is the influence of awareness on visuomotor adaptation, where awareness encompasses knowledge of the changes in one's reaches and the visuomotor distortion itself. In the current experiment, we asked if awareness governs the magnitude of implicit (i.e., unconscious) visuomotor adaptation achieved, independent of how the distortion is introduced (i.e., abruptly vs. gradually introduced visuomotor distortion), and hence initial errors experienced. Participants were divided into two groups that differed with respect to how the visuomotor distortion was introduced (i.e., Abrupt vs. Gradual Groups) and reached in a virtual environment where a cursor on the screen misrepresented the position of their hand. Participants completed three blocks of 150 reach training trials in the following order: aligned cursor feedback (baseline), rotated cursor feedback (adaptation) and aligned cursor feedback (washout). For the Abrupt Group, the cursor was immediately rotated 45° clockwise (CW) relative to hand motion in the adaptation block, whereas in the Gradual Group, the 45° cursor rotation was gradually introduced over adaptation trials. Following reach training, participants' awareness of changes in their reaches and the visuomotor distortion were established based on a drawing task, where participants drew the path their hand took to get the cursor on target, as well as a post-experiment questionnaire. Participants were subsequently divided into the following 3 groups: Abrupt-Aware (n = 16), Gradual-Aware (n = 11) and Gradual-Unaware (n = 14). Results revealed that errors differed for the Gradual-Unaware Group at the end of adaptation training compared to the Gradual-Aware Group and at the start of the washout block compared to the Abrupt-Aware Group. Errors in the two aware groups did not differ from each other. These results suggest that awareness may lead to reduced implicit adaptation, regardless of the size of initial errors experienced.

Assessing explicit processes does not influence the magnitude of implicit processes

Implicit (unconscious) and explicit (strategy) processes have been shown to contribute to visuomotor adaptation. Current methods, such as the Process Dissociation Procedure (PDP) and the Verbal Report Framework (VRF), simultaneously evaluate both implicit and explicit contributions to visuomotor adaptation. It is unclear whether the act of assessing explicit adaptation leads to variations in the magnitude of implicit adaptation observed. To address this question, four groups of participants adapted their reaches to a 40° clockwise visuomotor rotation. Implicit and explicit adaptation were assessed in a PDP-IE group and a VRF-IE group following 3 blocks of rotated reach training trials. In contrast, only implicit adaptation was assessed at the same time points for a PDP-I group and VRF-I group. Results indicated a similar magnitude of implicit adaptation regardless of whether explicit adaptation was assessed or not. Thus, assessing explicit adaptation simultaneously with implicit adaptation following reach adaptation does not influence the magnitude of implicit adaptation established via the PDP and VRF methods.