Gaze locations affect explicit process but not implicit process during visuomotor adaptation

Imagery practice of motor skills without conscious awareness?: a commentary to Frank et al

Modifications of imagined sensory consequences will not benefit overt performance when they cannot be transformed into motor outflow that produces them. With physical practice, the acquisition of internal models of motor transformations is largely based on prediction errors that are absent in imagery practice. What can imagery practice nevertheless contribute to transformation learning? Explicit, strategic adjustments to novel transformations should be possible. This appears less likely for implicit adjustments. Are there variants of imagery practice that can produce adjustments without conscious awareness of the transformation and/or the resultant movement changes?

The role of explicit knowledge in compensating for a visuo-proprioceptive cue conflict

It is unclear how explicit knowledge of an externally imposed mismatch between visual and proprioceptive cues of hand position affects perceptual recalibration. The Bayesian causal inference framework might suggest such knowledge should abolish the visual and proprioceptive recalibration that occurs when individuals perceive these cues as coming from the same source (their hand), while the visuomotor adaptation literature suggests explicit knowledge of a cue conflict does not eliminate implicit compensatory processes. Here we compared visual and proprioceptive recalibration in three groups with varying levels of knowledge about the visuo-proprioceptive cue conflict. All participants estimated the position of visual, proprioceptive, or combined targets related to their left index fingertip, with a 70 mm visuo-proprioceptive offset gradually imposed. Groups 1, 2, and 3 received no information, medium information, and high information, respectively, about the offset. Information was manipulated using instructional and visual cues. All groups performed the task similarly at baseline in terms of variance, weighting, and integration. Results suggest the three groups recalibrated vision and proprioception differently, but there was no difference in variance or weighting. Participants who received only instructional cues about the mismatch (Group 2) did not recalibrate less, on average, than participants provided no information about the mismatch (Group 1). However, participants provided instructional cues and extra visual cues of their hands during the perturbation (Group 3) demonstrated significantly less recalibration than other groups. These findings are consistent with the idea that instructional cues alone are insufficient to override participants' intrinsic belief in common cause and reduce recalibration.

Imprecise perception of hand position during early motor adaptation

Localizing one's body parts is important for movement control and motor learning. Recent studies have shown that the precision with which people localize their hand places constraints on motor adaptation. Although these studies have assumed that hand localization remains equally precise across learning, we show that precision decreases rapidly during early motor learning. In three experiments, healthy young participants (n = 92) repeatedly adapted to a 45° visuomotor rotation for a cycle of two to four reaches, followed by a cycle of two to four reaches with veridical feedback. Participants either used an aiming strategy that fully compensated for the rotation (experiment 1), or always aimed directly at the target, so that adaptation was implicit (experiment 2). We omitted visual feedback for the last reach of each cycle, after which participants localized their unseen hand. We observed an increase in the variability of angular localization errors when subjects used a strategy to counter the visuomotor rotation (experiment 1). This decrease in precision was less pronounced in the absence of reaiming (experiment 2), and when subjects knew that they would have to localize their hand on the upcoming trial, and could thus focus on hand position (experiment 3). We propose that strategic reaiming decreases the precision of perceived hand position, possibly due to attention to vision rather than proprioception. We discuss how these dynamics in precision during early motor learning could impact on motor control and shape the interplay between implicit and strategy-based motor adaptation.NEW & NOTEWORTHY Recent studies indicate that the precision with which people localize their hand limits implicit visuomotor learning. We found that localization precision is not static, but decreases early during learning. This decrease is pronounced when people apply a reaiming strategy to compensate for a visuomotor perturbation and is partly resistant to allocation of attention to the hand. We propose that these dynamics in position sense during learning may influence how implicit and strategy-based motor adaption interact.

Implicit Adaptation Is Modulated by the Relevance of Feedback

Given that informative and relevant feedback in the real world is often intertwined with distracting and irrelevant feedback, we asked how the relevancy of visual feedback impacts implicit sensorimotor adaptation. To tackle this question, we presented multiple cursors as visual feedback in a center-out reaching task and varied the task relevance of these cursors. In other words, participants were instructed to hit a target with a specific task-relevant cursor, while ignoring the other cursors. In Experiment 1, we found that reach aftereffects were attenuated by the mere presence of distracting cursors, compared with reach aftereffects in response to a single task-relevant cursor. The degree of attenuation did not depend on the position of the distracting cursors. In Experiment 2, we examined the interaction between task relevance and attention. Participants were asked to adapt to a task-relevant cursor/target pair, while ignoring the task-irrelevant cursor/target pair. Critically, we jittered the location of the relevant and irrelevant target in an uncorrelated manner, allowing us to index attention via how well participants tracked the position of target. We found that participants who were better at tracking the task-relevant target/cursor pair showed greater aftereffects, and interestingly, the same correlation applied to the task-irrelevant target/cursor pair. Together, these results highlight a novel role of task relevancy on modulating implicit adaptation, perhaps by giving greater attention to informative sources of feedback, increasing the saliency of the sensory prediction error.

Effect of repeated explicit instructions on visuomotor adaptation and intermanual transfer

The aim of the present study was to investigate the effect of repeated explicit instructions on visuomotor adaptation, awareness, and intermanual transfer. In a comprehensive study design, 48 participants performed center-out reaching movements before and during exposure to a 60° rotation of visual feedback. Awareness and intermanual transfer were then determined. Twelve participants each were assigned to one of the following adaptation conditions: gradual adaptation, sudden adaptation without instructions, sudden adaptation with a single instruction before adaptation, and sudden adaptation with multiple instructions before and during adaptation. The explicit instructions explained the nature of the visual feedback perturbation and were given using an illustration of a clock face. Analysis of adaptation indices revealed neither increased nor decreased adaptation after repeated instructions compared with a single instruction. In addition, we found significant group differences for the awareness index, with lower awareness after gradual adaptation than after sudden, instructed adaptation. Our data also show increased initial adaptation in aware participants; regardless of whether awareness was developed independently or with instruction. Intermanual transfer did not differ between groups. However, we found a significant correlation between the awareness and intermanual transfer indices. We conclude that the magnitude of the explicit process cannot be further increased by repeated instruction and that intermanual transfer appears to be largely related to the explicit adaptation process.

Delay of gaze fixation during reaching movement with the non-dominant hand to a distant target

The present study examined the effects of hand and task difficulty on eye-hand coordination related to gaze fixation behavior (i.e., fixating a gaze to the target until reach completion) in single reaching movements. Twenty right-handed young adults made reaches on a digitizer, while looking at a visual target and feedback of hand movements on a computer monitor. Task difficulty was altered by having three target distances. In a small portion of trials, visual feedback was randomly removed at the target presentation. The effect of a moderate amount of practice was also examined using a randomized trial schedule across target-distance and visual-feedback conditions in each hand. The results showed that the gaze distances covered during the early reaching phase were reduced, and the gaze fixation to the target was delayed when reaches were performed with the left hand and when the target distance increased. These results suggest that when the use of the non-dominant hand or an increased task difficulty reduces the predictability of hand movements and its sensory consequences, eye-hand coordination is modified to enhance visual monitoring of the reach progress prior to gaze fixation. The randomized practice facilitated this process. Nevertheless, variability of reach trajectory was more increased without visual feedback for right-hand reaches, indicating that control of the dominant arm integrates more visual feedback information during reaches. These results together suggest that the earlier gaze fixation and greater integration of visual feedback during right-hand reaches contribute to the faster and more accurate performance in the final reaching phase.

Functional Use of Eye Movements for an Acting System

Movements of the eyes assist vision and support hand and body movements in a cooperative way. Despite their strong functional coupling, different types of movements are usually studied independently. We integrate knowledge from behavioral, neurophysiological, and clinical studies on how eye movements are coordinated with goal-directed hand movements and how they facilitate motor learning. Understanding the coordinated control of eye and hand movements can provide important insights into brain functions that are essential for performing or learning daily tasks in health and disease. This knowledge can also inform applications such as robotic manipulation and clinical rehabilitation.

A condition that produces sensory recalibration and abolishes multisensory integration

We examined the influence of extended exposure to a visuomotor rotation, which induces both motor adaptation and sensory recalibration, on (partial) multisensory integration in a cursor-control task. Participants adapted to a 30° (adaptation condition) or 0° (control condition) visuomotor rotation by making center-out movements to remembered targets. In subsequent test trials of sensory integration, they made center-out movements with variable visuomotor rotations and judged the position of hand or cursor at the end of these movements. Test trials were randomly embedded among twice the number of maintenance trials with 30° or 0° rotation. The biases of perceived hand (or cursor) position toward the cursor (or hand) position were measured. We found motor adaptation together with proprioceptive and visual recalibrations in the adaptation condition. Unexpectedly, multisensory integration was absent in both the adaptation and control condition. The absence stemmed from the extensive experience of constant visuomotor rotations of 30° or 0°, which probably produced highly precise predictions of the visual consequences of hand movements. The frequently confirmed predictions then dominated the estimate of the visual movement consequences, leaving no influence of the actual visuomotor rotations in the minority of test trials. Conversely, multisensory integration was present for sensed hand positions when these were indirectly assessed from movement characteristics, indicating that the relative weighting of discrepant estimates of hand position was different for motor control. The existence of a condition that abolishes multisensory integration while keeping sensory recalibration suggests that mechanisms that reduce sensory discrepancies (partly) differ between integration and recalibration.

Visual and proprioceptive recalibrations after exposure to a visuomotor rotation

Adaptation to a visuomotor rotation in a cursor-control task is accompanied by proprioceptive recalibration, whereas the existence of visual recalibration is uncertain and has even been doubted. In the present study, we tested both visual and proprioceptive recalibration; proprioceptive recalibration was not only assessed by means of psychophysical judgments of the perceived position of the hand, but also by an indirect procedure based on movement characteristics. Participants adapted to a gradually introduced visuomotor rotation of 30° by making center-out movements to remembered targets. In subsequent test trials, they made center-out movements without visual feedback or observed center-out motions of a cursor without moving the hand. In each test trial, they judged the endpoint of hand or cursor by matching the position of the hand or of a visual marker, respectively, moving along a semicircular path. This path ran through all possible endpoints of the center-out movements. We observed proprioceptive recalibration of 7.3° (3.1° with the indirect procedure) and a smaller, but significant, visual recalibration of 1.3°. Total recalibration of 8.6° was about half as strong as motor adaptation, the adaptive shift of the movement direction. The evidence of both proprioceptive and visual recalibration was obtained with a judgment procedure that suggests that recalibration is restricted to the type of movement performed during exposure to a visuomotor rotation. Consequently, identical physical positions of the hand can be perceived differently depending on how they have been reached, and similarly identical positions of a cursor on a monitor can be perceived differently.

Eye movements do not play an important role in the adaptation of hand tracking to a visuomotor rotation

Adapting hand movements to changes in our body or the environment is essential for skilled motor behavior. Although eye movements are known to assist hand movement control, how eye movements might contribute to the adaptation of hand movements remains largely unexplored. To determine to what extent eye movements contribute to visuomotor adaptation of hand tracking, participants were asked to track a visual target that followed an unpredictable trajectory with a cursor using a joystick. During blocks of trials, participants were either allowed to look wherever they liked or required to fixate a cross at the center of the screen. Eye movements were tracked to ensure gaze fixation as well as to examine free gaze behavior. The cursor initially responded normally to the joystick, but after several trials, the direction in which it responded was rotated by 90°. Although fixating the eyes had a detrimental influence on hand tracking performance, participants exhibited a rather similar time course of adaptation to rotated visual feedback in the gaze-fixed and gaze-free conditions. More importantly, there was extensive transfer of adaptation between the gaze-fixed and gaze-free conditions. We conclude that although eye movements are relevant for the online control of hand tracking, they do not play an important role in the visuomotor adaptation of such tracking. These results suggest that participants do not adapt by changing the mapping between eye and hand movements, but rather by changing the mapping between hand movements and the cursor’s motion independently of eye movements.

NEW & NOTEWORTHY Eye movements assist hand movements in everyday activities, but their contribution to visuomotor adaptation remains largely unknown. We compared adaptation of hand tracking under free gaze and fixed gaze. Although our results confirm that following the target with the eyes increases the accuracy of hand movements, they unexpectedly demonstrate that gaze fixation does not hinder adaptation. These results suggest that eye movements have distinct contributions for online control and visuomotor adaptation of hand movements.

Dissociable cognitive strategies for sensorimotor learning

Computations underlying cognitive strategies in human motor learning are poorly understood. Here we investigate such strategies in a common sensorimotor transformation task. We show that strategies assume two forms, likely reflecting distinct working memory representations: discrete caching of stimulus-response contingencies, and time-consuming parametric computations. Reaction times and errors suggest that both strategies are employed during learning, and trade off based on task complexity. Experiments using pressured preparation time further support dissociable strategies: In response caching, time pressure elicits multi-modal distributions of movements; during parametric computations, time pressure elicits a shifting distribution of movements between visual targets and distal goals, consistent with analog re-computing of a movement plan. A generalization experiment reveals that discrete and parametric strategies produce, respectively, more localized or more global transfer effects. These results describe how qualitatively distinct cognitive representations are leveraged for motor learning and produce downstream consequences for behavioral flexibility.

Parallel learning processes of a visuomotor adaptation task in a changing environment

During the control of reaching movements, a key contribution of the visual system is the localization of relevant environmental targets. In motor adaptation processes, the visual evaluation of effector motor behavior enables learning from errors, which demands continuous visual attentional focus. However, most current adaptation paradigms include static targets; therefore, when a learning situation develops in a highly variable environment and there is a double demand for visual resources (environment and motor performance), the evolution of learning processes is unknown. In order to understand how learning processes evolve in a variable environment, a video game task was designed in which subjects were asked to manage a 60° counterclockwise-rotated cursor to capture descending targets with initially unpredictable trajectories. During the task, the cursor and eye movements were recorded to dissect visuomotor coordination. We observed that the pursuit of the targets conditioned a predominant and continuous visual inspection of the environment instead of the rotated cursor. As learning progressed, subjects exhibited a linear reduction in directional error and selected a motor strategy based on the degree of reward, which improved the performance. These results suggest that when the environment demands high visual attention, error-based and reinforced motor learning processes are implemented simultaneously, thus enabling efficient predictive behavior.

Using gaze behavior to parcellate the explicit and implicit contributions to visuomotor learning

Successful motor performance relies on our ability to adapt to changes in the environment by learning novel mappings between motor commands and sensory outcomes. Such adaptation is thought to involve two distinct mechanisms: an implicit, error-based component linked to slow learning and an explicit, strategic component linked to fast learning and savings (i.e., faster relearning). Because behavior, at any given moment, is the resultant combination of these two processes, it has remained a challenge to parcellate their relative contributions to performance. The explicit component to visuomotor rotation (VMR) learning has recently been measured by having participants verbally report their aiming strategy used to counteract the rotation. However, this procedure has been shown to magnify the explicit component. Here we tested whether task-specific eye movements, a natural component of reach planning, but poorly studied in motor learning tasks, can provide a direct readout of the state of the explicit component during VMR learning. We show, by placing targets on a visible ring and including a delay between target presentation and reach onset, that individual differences in gaze patterns during sensorimotor learning are linked to participants' rates of learning and their expression of savings. Specifically, we find that participants who, during reach planning, naturally fixate an aimpoint rotated away from the target location, show faster initial adaptation and readaptation 24 h later. Our results demonstrate that gaze behavior cannot only uniquely identify individuals who implement cognitive strategies during learning but also how their implementation is linked to differences in learning. NEW & NOTEWORTHY Although it is increasingly well appreciated that sensorimotor learning is driven by two separate components, an error-based process and a strategic process, it has remained a challenge to identify their relative contributions to performance. Here we demonstrate that task-specific eye movements provide a direct read-out of explicit strategies during sensorimotor learning in the presence of visual landmarks. We further show that individual differences in gaze behavior are linked to learning rate and savings.

Visuomotor Prediction Errors Modulate EEG Activity Over Parietal Cortex

The parietal cortex is thought to be involved in visuomotor adaptation, yet it remains unclear whether it is specifically modulated by visuomotor prediction errors (i.e. PEs; mismatch between the predicted and actual visual consequences of the movement). One reason for this is that PEs tend to be associated with task errors, as well as changes in motor output and visual input, making them difficult to isolate. Here this issue is addressed using electroencephalography. A strategy (STR) condition, in which participants were instructed on how to counter a 45° visuomotor rotation, was compared to a condition in which participants had adapted to the rotation (POST). Both conditions were matched for task errors and movement kinematics, with the only difference being the presence of PEs in STR. Results revealed strong parietal modulations in current source density and low theta (2–4 Hz) power shortly after movement onset in STR vs. POST, followed by increased alpha/low beta (8–18 Hz) power during much of the post-movement period. Given recent evidence showing that feedforward and feedback information is respectively carried by theta and alpha/beta oscillations, the observed power modulations may reflect the bottom-up propagation of PEs and the top-down revision of predictions.

Eye-Hand Coordination during Visuomotor Adaptation with Different Rotation Angles: Effects of Terminal Visual Feedback

This study examined adaptive changes of eye-hand coordination during a visuomotor rotation task under the use of terminal visual feedback. Young adults made reaching movements to targets on a digitizer while looking at targets on a monitor where the rotated feedback (a cursor) of hand movements appeared after each movement. Three rotation angles (30°, 75° and 150°) were examined in three groups in order to vary the task difficulty. The results showed that the 30° group gradually reduced direction errors of reaching with practice and adapted well to the visuomotor rotation. The 75° group made large direction errors of reaching, and the 150° group applied a 180° reversal shift from early practice. The 75°and 150° groups, however, overcompensated the respective rotations at the end of practice. Despite these group differences in adaptive changes of reaching, all groups gradually adapted gaze directions prior to reaching from the target area to the areas related to the final positions of reaching during the course of practice. The adaptive changes of both hand and eye movements in all groups mainly reflected adjustments of movement directions based on explicit knowledge of the applied rotation acquired through practice. Only the 30° group showed small implicit adaptation in both effectors. The results suggest that by adapting gaze directions from the target to the final position of reaching based on explicit knowledge of the visuomotor rotation, the oculomotor system supports the limb-motor system to make precise preplanned adjustments of reaching directions during learning of visuomotor rotation under terminal visual feedback.

Effects of Reliability and Global Context on Explicit and Implicit Measures of Sensed Hand Position in Cursor-Control Tasks

In a cursor-control task in which the motion of the cursor is rotated randomly relative to the movement of the hand, the sensed directions of hand and cursor are mutually biased. In our previous study, we used implicit and explicit measures of the bias of sensed hand direction toward the direction of the cursor and found different characteristics. The present study serves to explore further differences and commonalities of these measures. In Experiment 1, we examined the effects of different relative reliabilities of visual and proprioceptive information on the explicitly and implicitly assessed bias of sensed hand direction. In two conditions, participants made an aiming movement and returned to the start position immediately or after a delay of 6 s during which the cursor was no longer visible. The unimodal proprioceptive information on final hand position in the delayed condition served to increase its relative reliability. As a result, the bias of sensed hand direction toward the direction of the cursor was reduced for the explicit measure, with a complementary increase of the bias of sensed cursor direction, but unchanged for the implicit measure. In Experiment 2, we examined the influence of global context, specifically of the across-trial sequence of judgments of hand and cursor direction. Both explicitly and implicitly assessed biases of sensed hand direction did not significantly differ between the alternated condition (trial-to-trial alternations of judgments of hand and cursor direction) and the blocked condition (judgments of hand or cursor directions in all trials). They both substantially decreased from the alternated to the randomized condition (random sequence of judgments of hand and cursor direction), without a complementary increase of the bias of sensed cursor direction. We conclude that our explicit and implicit measures are equally sensitive to variations of coupling strength as induced by the variation of global context in Experiment 2, but are differently sensitive to variations of the relative reliabilities as induced by our additional unimodal proprioceptive information in Experiment 1.

Evidence for distinct brain networks in the control of rule-based motor behavior

Reach guidance when the spatial location of the viewed target and hand movement are incongruent (i.e., decoupled) necessitates use of explicit cognitive rules (strategic control) or implicit recalibration of gaze and limb position (sensorimotor recalibration). In a patient with optic ataxia (OA) and bilateral superior parietal lobule damage, we recently demonstrated an increased reliance on strategic control when the patient performed a decoupled reach (Granek JA, Pisella L, Stemberger J, Vighetto A, Rossetti Y, Sergio LE. PLoS One 8: e86138, 2013). To more generally understand the fundamental mechanisms of decoupled visuomotor control and to more specifically test whether we could distinguish these two modes of movement control, we tested healthy participants in a cognitively demanding dual task. Participants continuously counted backward while simultaneously reaching toward horizontal (left or right) or diagonal (equivalent to top-left or top-right) targets with either veridical or rotated (90°) cursor feedback. By increasing the overall neural load and selectively compromising potentially overlapping neural circuits responsible for strategic control, the complex dual task served as a noninvasive means to disrupt the integration of a cognitive rule into a motor action. Complementary to our previous results observed in patients with optic ataxia, here our dual task led to greater performance deficits during movements that required an explicit rule, implying a selective disruption of strategic control in decoupled reaching. Our results suggest that distinct neural processing is required to control these different types of reaching because in considering the current results and previous patient results together, the two classes of movement could be differentiated depending on the type of interference.