Distinct learning, retention, and generalization patterns in de novo learning versus motor adaptation

People correct for movement errors when acquiring new motor skills (de novo learning) or adapting well-known movements (motor adaptation). While de novo learning establishes new control policies, adaptation modifies existing ones, and previous work have distinguished behavioral and underlying brain mechanisms for each motor learning type. However, it is still unclear whether learning in each type interferes with the other. In study 1, we use a within-subjects design where participants train with both 30° visuomotor rotation and mirror reversal perturbations, to compare adaptation and de novo learning respectively. We find no perturbation order effects, and find no evidence for differences in learning rates and asymptotes for both perturbations. Explicit instructions also provide an advantage during early learning in both perturbations. However, mirror reversal learning shows larger inter-participant variability and slower movement initiation. Furthermore, we only observe reach aftereffects following rotation training. In study 2, we incorporate the mirror reversal in a browser-based task, to investigate under-studied de novo learning mechanisms like retention and generalization. Learning persists across three or more days, substantially transfers to the untrained hand, and to targets on both sides of the mirror axis. Our results extend insights for distinguishing motor skill acquisition from adapting well-known movements.

Movement-goal relevant object shape properties act as poor but viable cues for the attribution of motor errors to external objects

When a context change is detected during motor learning, motor memories-internal models for executing movements within some context-may be created or existing motor memories may be activated and modified. Assigning credit to plausible causes of errors can allow for fast retrieval and activation of a motor memory, or a combination of motor memories, when the presence of such causes is detected. Features of the movement-context intrinsic to the movement dynamics, such as posture of the end effector, are often effective cues for detecting context change whereas features extrinsic to the movement dynamics, such as the colour of an object being moved, are often not. These extrinsic cues are typically not relevant to the motor task at hand and can be safely ignored by the motor system. We conducted two experiments testing if extrinsic but movement-goal relevant object-shape cues during an object-transport task can act as viable contextual cues for error assignment to the object, and the creation of new, object-shape-associated motor memories. In the first experiment we find that despite the object-shape cues, errors are primarily attributed to the hand transporting the object. In a second experiment, we find participants can execute differing movements cued by the object shape in a dual adaptation task, but the extent of adaptation is small, suggesting that movement-goal relevant object-shape properties are poor but viable cues for creating context specific motor memories.

Motor adaptation does not differ when a perturbation is introduced abruptly or gradually

People continuously adapt their movements to ever-changing circumstances, and particularly in skills training and rehabilitation, it is crucial that we understand how to optimize implicit adaptation in order for these processes to require as little conscious effort as possible. Although it is generally assumed that the way to do this is by introducing perturbations gradually, the literature is ambivalent on the effectiveness of this approach. Here, we tested whether there are differences in motor performance when adapting to an abrupt compared to a ramped visuomotor rotation. Using a within-subjects design, we tested this question under 3 different rotation sizes: 30-degrees, 45-degrees, and 60-degrees, as well as in 3 different populations: younger adults, older adults, and patients with mild cerebellar ataxia. We find no significant differences in either the behavioural outcomes, or model fits, between abrupt and gradual learning across any of the different conditions. Neither age, nor cerebellar ataxia had any significant effect on error-sensitivity either. These findings together indicate that error-sensitivity is not modulated by introducing a perturbation abruptly compared to gradually, and is also unaffected by age or mild cerebellar ataxia.

Adapting to visuomotor rotations in stepped increments increases implicit motor learning

Human motor adaptation relies on both explicit conscious strategies and implicit unconscious updating of internal models to correct motor errors. Implicit adaptation is powerful, requiring less preparation time before executing adapted movements, but recent work suggests it is limited to some absolute magnitude regardless of the size of a visuomotor perturbation when the perturbation is introduced abruptly. It is commonly assumed that gradually introducing a perturbation should lead to improved implicit learning beyond this limit, but outcomes are conflicting. We tested whether introducing a perturbation in two distinct gradual methods can overcome the apparent limit and explain past conflicting findings. We found that gradually introducing a perturbation in a stepped manner, where participants were given time to adapt to each partial step before being introduced to a larger partial step, led to ~ 80% higher implicit aftereffects of learning, but introducing it in a ramped manner, where participants adapted larger rotations on each subsequent reach, did not. Our results clearly show that gradual introduction of a perturbation can lead to substantially larger implicit adaptation, as well as identify the type of introduction that is necessary to do so.

Reduced feedback barely slows down proprioceptive recalibration

Introducing altered visual feedback of the hand produces quick adaptation of reaching movements. Our lab has shown that the associated shifts in estimates of the felt position of the hand saturate within a few training trials. The current study investigates whether the rapid changes in felt hand position that occur during classic visuomotor adaptation are diminished or slowed when training feedback is reduced. We reduced feedback by either providing visual feedback only at the end of the reach (terminal feedback) or constraining hand movements to reduce motor adaptation-related error signals such as sensory prediction errors and task errors (exposure). We measured changes as participants completed reaches with a 30° rotation, a -30° rotation, and clamped visual feedback, with these two "impoverished" training conditions, along with classic visuomotor adaptation training, while continuously estimating their felt hand position. Training with terminal feedback slightly reduced the initial rate of change in overall adaptation. However, the rate of change in hand localization, as well as the asymptote of hand localization shifts in both the terminal feedback group and the exposure training group was not noticeably different from those in the classic training group. Taken together, shifts in felt hand position are rapid and robust responses to sensory mismatches and are at best slightly modulated when feedback is reduced. This suggests that given the speed and invariance to the quality of feedback of proprioceptive recalibration, it could immediately contribute to all kinds of reach adaptation.NEW & NOTEWORTHY Reaching to targets with altered visual feedback about hand position leads to adaptation of movements as well as shifts in estimates of felt hand position. Felt hand position can shift in as little as one trial, and here we show that there is no noticeable reduction in speed when the feedback about movements is impoverished, indicating the robustness of the process of recalibrating felt hand position.

Multisensory visual-vestibular training improves visual heading estimation in younger and older adults

Self-motion perception (e.g., when walking/driving) relies on the integration of multiple sensory cues including visual, vestibular, and proprioceptive signals. Changes in the efficacy of multisensory integration have been observed in older adults (OA), which can sometimes lead to errors in perceptual judgments and have been associated with functional declines such as increased falls risk. The objectives of this study were to determine whether passive, visual-vestibular self-motion heading perception could be improved by providing feedback during multisensory training, and whether training-related effects might be more apparent in OAs vs. younger adults (YA). We also investigated the extent to which training might transfer to improved standing-balance. OAs and YAs were passively translated and asked to judge their direction of heading relative to straight-ahead (left/right). Each participant completed three conditions: (1) vestibular-only (passive physical motion in the dark), (2) visual-only (cloud-of-dots display), and (3) bimodal (congruent vestibular and visual stimulation). Measures of heading precision and bias were obtained for each condition. Over the course of 3 days, participants were asked to make bimodal heading judgments and were provided with feedback (“correct”/“incorrect”) on 900 training trials. Post-training, participants’ biases, and precision in all three sensory conditions (vestibular, visual, bimodal), and their standing-balance performance, were assessed. Results demonstrated improved overall precision (i.e., reduced JNDs) in heading perception after training. Pre- vs. post-training difference scores showed that improvements in JNDs were only found in the visual-only condition. Particularly notable is that 27% of OAs initially could not discriminate their heading at all in the visual-only condition pre-training, but subsequently obtained thresholds in the visual-only condition post-training that were similar to those of the other participants. While OAs seemed to show optimal integration pre- and post-training (i.e., did not show significant differences between predicted and observed JNDs), YAs only showed optimal integration post-training. There were no significant effects of training for bimodal or vestibular-only heading estimates, nor standing-balance performance. These results indicate that it may be possible to improve unimodal (visual) heading perception using a multisensory (visual-vestibular) training paradigm. The results may also help to inform interventions targeting tasks for which effective self-motion perception is important.

Measuring the double-drift illusion and its resets with hand trajectories

If a Gabor pattern drifts in one direction while its internal texture drifts in the orthogonal direction, its perceived position deviates further and further away from its true path. We first evaluated the illusion using manual tracking. Participants followed the Gabor with a stylus on a drawing tablet that coincided optically with the horizontal monitor surface. Their hand and the stylus were not visible during the tracking. The magnitude of the tracking illusion corresponded closely to previous perceptual and pointing measures indicating that manual tracking is a valid measure for the illusion. This allowed us to use it in a second experiment to capture the behavior of the illusion as it eventually degrades and breaks down in single trials. Specifically, the deviation of the Gabor stops accumulating at some point and either stays at a fixed offset or resets toward the veridical position. To report the perceived trajectory of the Gabor, participants drew it after the Gabor was removed from the monitor. Resets were detected and analyzed and their distribution matches neither a temporal nor a spatial limit, but rather a broad gamma distribution over time. This suggests that resets are triggered randomly, about once per 1.3 seconds, possible by extraneous distractions or eye movements.

Sensing hand position in Ehlers-Danlos syndrome

PURPOSE

To explore the effect of joint hypermobility on acuity, and precision, of hand proprioception.

MATERIALS AND METHODS

We compared proprioceptive acuity, and precision, between EDS patients and controls. We then measured any changes in their estimates of hand position after participants adapted their reaches in response to altered visual feedback of their hand. The Beighton Scale was used to quantify the magnitude of joint hypermobility.

RESULTS

There were no differences between the groups in the accuracy of estimates of hand location, nor in the visually induced changes in hand location. However, EDS patients' estimates were less precise when based purely on proprioception and could be partially predicted by Beighton score.

CONCLUSIONS

EDS patients are less precise at estimating their hand's location when only afferent information is available, but the presence of efferent signalling may reduce this imprecision. Those who are more hypermobile are more likely to be imprecise.

Differential contributions of implicit and explicit learning mechanisms to various contextual cues in dual adaptation

The ability to switch between different visuomotor maps accurately and efficiently is an invaluable feature to a flexible and adaptive human motor system. This can be examined in dual adaptation paradigms where the motor system is challenged to perform under randomly switching, opposing perturbations. Typically, dual adaptation doesn’t proceed unless each mapping is trained in association with a predictive cue. To investigate this, we first explored whether dual adaptation occurs under a variety of contextual cues including active follow-through movements, passive follow-through movements, active lead-in movements, and static visual cues. In the second experiment, we provided one group with a compensatory strategy about the perturbations (30° CW and 30° CCW rotations) and their relationships to each context (static visual cues). We found that active, but not passive, movement cues elicited dual adaptation. Expectedly, we didn’t find evidence for dual adaptation using static visual cues, but those in the Instruction group compensated by implementing aiming strategies. Then, across all experimental conditions, we explored the extent by which dual learning is supported by both implicit and explicit mechanisms, regardless of whether they elicited dual adaptation across all the various cues. To this end, following perturbed training, participants from all experiments were asked to either use or ignore the strategy as they reached without visual feedback. This Process Dissociation Procedure teased apart the implicit and explicit contributions to dual adaptation. Critically, we didn’t find evidence for implicit learning for those given instructions, suggesting that when explicit aiming strategies are implemented in dual adaptation, implicit mechanisms are likely not involved. Thus, by implementing conscious strategies, dual adaptation can be easily facilitated even in cases where learning would not occur otherwise.

Proprioceptive recalibration following implicit visuomotor adaptation is preserved in Parkinson's disease

Individuals with Parkinson's disease (PD) and healthy adults demonstrate similar levels of visuomotor adaptation provided that the distortion is small or introduced gradually, and hence, implicit processes are engaged. Recently, implicit processes underlying visuomotor adaptation in healthy individuals have been proposed to include proprioceptive recalibration (i.e., shifts in one's proprioceptive sense of felt hand position to match the visual estimate of their hand experienced during reaches with altered visual feedback of the hand). In the current study, we asked if proprioceptive recalibration is preserved in PD patients. PD patients tested during their "off" and "on" medication states and age-matched healthy controls reached to visual targets, while visual feedback of their unseen hand was gradually rotated 30° clockwise or translated 4 cm rightwards of their actual hand trajectory. As expected, PD patients and controls produced significant reach aftereffects, indicating visuomotor adaptation after reaching with the gradually introduced visuomotor distortions. More importantly, following visuomotor adaptation, both patients and controls showed recalibration in hand position estimates, and the magnitude of this recalibration was comparable between PD patients and controls. No differences for any measures assessed were observed across medication status (i.e., PD off vs PD on). Results reveal that patients are able to adjust their sensorimotor mappings and recalibrate proprioception following adaptation to a gradually introduced visuomotor distortion, and that dopaminergic intervention does not affect this proprioceptive recalibration. These results suggest that proprioceptive recalibration does not involve striatal dopaminergic pathways and may contribute to the preserved visuomotor adaptation that arises implicitly in PD patients.

Modeling inter-trial variability of pointing movements during visuomotor adaptation

Trial-to-trial variability during visuomotor adaptation is usually explained as the result of two different sources, planning noise and execution noise. The estimation of the underlying variance parameters from observations involving varying feedback conditions cannot be achieved by standard techniques (Kalman filter) because they do not account for recursive noise propagation in a closed-loop system. We therefore developed a method to compute the exact likelihood of the output of a time-discrete and linear adaptation system as has been used to model visuomotor adaptation (Smith et al. in PLoS Biol 4(6):e179, 2006), observed under closed-loop and error-clamp conditions. We identified the variance parameters by maximizing this likelihood and compared the model prediction of the time course of variance and autocovariance with empiric data. The observed increase in variability during the early training phase could not be explained by planning noise and execution noise with constant variances. Extending the model by signal-dependent components of either execution noise or planning noise showed that the observed temporal changes of the trial-to-trial variability can be modeled by signal-dependent planning noise rather than signal-dependent execution noise. Comparing the variance time course between different training schedules showed that the signal-dependent increase of planning variance was specific for the fast adapting mechanism, whereas the assumption of constant planning variance was sufficient for the slow adapting mechanisms.

Implicit motor learning within three trials

In motor learning, the slow development of implicit learning is traditionally taken for granted. While much is known about training performance during adaptation to a perturbation in reaches, saccades and locomotion, little is known about the time course of the underlying implicit processes during normal motor adaptation. Implicit learning is characterized by both changes in internal models and state estimates of limb position. Here, we measure both as reach aftereffects and shifts in hand localization in our participants, after every training trial. The observed implicit changes were near asymptote after only one to three perturbed training trials and were not predicted by a two-rate model's slow process that is supposed to capture implicit learning. Hence, we show that implicit learning is much faster than conventionally believed, which has implications for rehabilitation and skills training.

External error attribution dampens efferent-based predictions but not proprioceptive changes in hand localization

In learning and adapting movements in changing conditions, people attribute the errors they experience to a combined weighting of internal or external sources. As such, error attribution that places more weight on external sources should lead to decreased updates in our internal models for movement of the limb or estimating the position of the effector, i.e. there should be reduced implicit learning. However, measures of implicit learning are the same whether or not we induce explicit adaptation with instructions about the nature of the perturbation. Here we evoke clearly external errors by either demonstrating the rotation on every trial, or showing the hand itself throughout training. Implicit reach aftereffects persist, but are reduced in both groups. Only for the group viewing the hand, changes in hand position estimates suggest that predicted sensory consequences are not updated, but only rely on recalibrated proprioception. Our results show that estimating the position of the hand incorporates source attribution during motor learning, but recalibrated proprioception is an implicit process unaffected by external error attribution.

The effect of age on visuomotor learning processes

Knowing where our limbs are in space is essential for moving and for adapting movements to various changes in our environments and bodies. The ability to adapt movements declines with age, and age-related cognitive decline can explain a decreased ability to adopt and deploy explicit, cognitive strategies in motor learning. Age-related sensory decline could also lead to a reduced fidelity of sensory position signals and error signals, each of which can affect implicit motor adaptation. Here we investigate two estimates of limb position; one based on proprioception, the other on predicted sensory consequences of movements. Each is considered a measure of an implicit adaptation process and may be affected by both age and cognitive strategies. Both older (n = 38) and younger (n = 42) adults adapted to a 30° visuomotor rotation in a centre-out reaching task. We make an explicit, cognitive strategy available to half of participants in each age group with a detailed instruction. After training, we first quantify the explicit learning elicited by instruction. Instructed older adults initially use the provided strategy slightly less than younger adults but show a similar ability to evoke it after training. This indicates that cognitive explanations for age-related decline in motor learning are limited. In contrast, training induced much larger shifts of state estimates of hand location in older adults compared to younger adults. This is not modulated by strategy instructions, and appears driven by recalibrated proprioception, which is almost twice as large in older adults, while predictions might not be updated in older adults. This means that in healthy aging, some implicit processes may be compensating for other changes to maintain motor capabilities, while others also show age-related decline (data: https://osf.io/qzhmy).

Motor learning without moving: Proprioceptive and predictive hand localization after passive visuoproprioceptive discrepancy training

An accurate estimate of limb position is necessary for movement planning, before and after motor learning. Where we localize our unseen hand after a reach depends on felt hand position, or proprioception, but in studies and theories on motor adaptation this is quite often neglected in favour of predicted sensory consequences based on efference copies of motor commands. Both sources of information should contribute, so here we set out to further investigate how much of hand localization depends on proprioception and how much on predicted sensory consequences. We use a training paradigm combining robot controlled hand movements with rotated visual feedback that eliminates the possibility to update predicted sensory consequences (‘exposure training’), but still recalibrates proprioception, as well as a classic training paradigm with self-generated movements in another set of participants. After each kind of training we measure participants’ hand location estimates based on both efference-based predictions and afferent proprioceptive signals with self-generated hand movements (‘active localization’) as well as based on proprioception only with robot-generated movements (‘passive localization’). In the exposure training group, we find indistinguishable shifts in passive and active hand localization, but after classic training, active localization shifts more than passive, indicating a contribution from updated predicted sensory consequences. Both changes in open-loop reaches and hand localization are only slightly smaller after exposure training as compared to after classic training, confirming that proprioception plays a large role in estimating limb position and in planning movements, even after adaptation. (data: https://doi.org/10.17605/osf.io/zfdth, preprint: https://doi.org/10.1101/384941)

The effects of awareness of the perturbation during motor adaptation on hand localization

Awareness of task demands is often used during rehabilitation and sports training by providing instructions which appears to accelerate learning and improve performance through explicit motor learning. However, the effects of awareness of perturbations on the changes in estimates of hand position resulting from motor learning are not well understood. In this study, people adapted their reaches to a visuomotor rotation while either receiving instructions on the nature of the perturbation, experiencing a large rotation, or both to generate awareness of the perturbation and increase the contribution of explicit learning. We found that instructions and/or larger rotations allowed people to activate or deactivate part of the learned strategy at will and elicited explicit changes in open-loop reaches, while a small rotation without instructions did not. However, these differences in awareness, and even manipulations of awareness and perturbation size, did not appear to affect learning-induced changes in hand-localization estimates. This was true when estimates of the adapted hand location reflected changes in proprioception, produced when the hand was displaced by a robot, and also when hand location estimates were based on efferent-based predictions of self-generated hand movements. In other words, visuomotor adaptation led to significant shifts in predicted and perceived hand location that were not modulated by either instruction or perturbation size. Our results indicate that not all outcomes of motor learning benefit from an explicit awareness of the task. Particularly, proprioceptive recalibration and the updating of predicted sensory consequences appear to be largely implicit. (data: https://doi.org/10.17605/osf.io/mx5u2, preprint: https://doi.org/10.31234/osf.io/y53c2)

The fast contribution of visual-proprioceptive discrepancy to reach aftereffects and proprioceptive recalibration

Adapting reaches to altered visual feedback not only leads to motor changes, but also to shifts in perceived hand location; "proprioceptive recalibration". These changes are robust to many task variations and can occur quite rapidly. For instance, our previous study found both motor and sensory shifts arise in as few as 6 rotated-cursor training trials. The aim of this study is to investigate one of the training signals that contribute to these rapid sensory and motor changes. We do this by removing the visuomotor error signals associated with classic visuomotor rotation training; and provide only experience with a visual-proprioceptive discrepancy for training. While a force channel constrains reach direction 30o away from the target, the cursor representing the hand unerringly moves straight to the target. The resulting visual-proprioceptive discrepancy drives significant and rapid changes in no-cursor reaches and felt hand position, again within only 6 training trials. The extent of the sensory change is unexpectedly larger following the visual-proprioceptive discrepancy training. Not surprisingly the size of the reach aftereffects is substantially smaller than following classic visuomotor rotation training. However, the time course by which both changes emerge is similar in the two training types. These results suggest that even the mere exposure to a discrepancy between felt and seen hand location is a sufficient training signal to drive robust motor and sensory plasticity.

Time Course of Reach Adaptation and Proprioceptive Recalibration during Visuomotor Learning

Training to reach with rotated visual feedback results in adaptation of hand movements, which persist when the perturbation is removed (reach aftereffects). Training also leads to changes in felt hand position, which we refer to as proprioceptive recalibration. The rate at which motor and proprioceptive changes develop throughout training is unknown. Here, we aim to determine the timescale of these changes in order to gain insight into the processes that may be involved in motor learning. Following six rotated reach training trials (30° rotation), at three radially located targets, we measured reach aftereffects and perceived hand position (proprioceptive guided reaches). Participants trained with opposing rotations one week apart to determine if the original training led to any retention or interference. Results suggest that both motor and proprioceptive recalibration occurred in as few as six rotated-cursor training trials (7.57° & 3.88° respectively), with no retention or interference present one week after training. Despite the rapid speed of both motor and sensory changes, these shifts do not saturate to the same degree. Thus, different processes may drive these changes and they may not constitute a single implicit process.

Separating Predicted and Perceived Sensory Consequences of Motor Learning

During motor adaptation the discrepancy between predicted and actually perceived sensory feedback is thought to be minimized, but it can be difficult to measure predictions of the sensory consequences of actions. Studies attempting to do so have found that self-directed, unseen hand position is mislocalized in the direction of altered visual feedback. However, our lab has shown that motor adaptation also leads to changes in perceptual estimates of hand position, even when the target hand is passively displaced. We attribute these changes to a recalibration of hand proprioception, since in the absence of a volitional movement, efferent or predictive signals are likely not involved. The goal here is to quantify the extent to which changes in hand localization reflect a change in the predicted sensory (visual) consequences or a change in the perceived (proprioceptive) consequences. We did this by comparing changes in localization produced when the hand movement was self-generated (‘active localization’) versus robot-generated (‘passive localization’) to the same locations following visuomotor adaptation to a rotated cursor. In this passive version, there should be no predicted consequences of these robot-generated hand movements. We found that although changes in localization were somewhat larger in active localization, the passive localization task also elicited substantial changes. Our results suggest that the change in hand localization following visuomotor adaptation may not be based entirely on updating predicted sensory consequences, but may largely reflect changes in our proprioceptive state estimate.

Proprioceptive precision is impaired in Ehlers-Danlos syndrome

It has been suggested that people with Ehlers–Danlos syndrome (EDS), or other similar connective tissue disorders, may have proprioceptive impairments, the reason for which is still unknown. We recently found that EDS patients were less precise than healthy controls when estimating their felt hand’s position relative to visible peripheral reference locations, and that this deficit was positively correlated with the severity of joint hypermobility. We further explore proprioceptive abilities in EDS by having patients localize their non-dominant left hand at a greater number of workspace locations than in our previous study. Additionally, we explore the relationship between chronic pain and proprioceptive sensitivity. We found that, although patients were just as accurate as controls, they were not as precise. Patients showed twice as much scatter than controls at all locations, but the degree of scatter did not positively correlate with chronic pain scores. This further supports the idea that a proprioceptive impairment pertaining to precision is present in EDS, but may not relate to the magnitude of chronic pain.

Generalization patterns for reach adaptation and proprioceptive recalibration differ after visuomotor learning

Visuomotor learning results in changes in both motor and sensory systems (Cressman EK, Henriques DY. J Neurophysiol 102: 3505-3518, 2009), such that reaches are adapted and sense of felt hand position recalibrated after reaching with altered visual feedback of the hand. Moreover, visuomotor learning has been shown to generalize such that reach adaptation achieved at a trained target location can influence reaches to novel target directions (Krakauer JW, Pine ZM, Ghilardi MF, Ghez C. J Neurosci 20: 8916-8924, 2000). We looked to determine whether proprioceptive recalibration also generalizes to novel locations. Moreover, we looked to establish the relationship between reach adaptation and changes in sense of felt hand position by determining whether proprioceptive recalibration generalizes to novel targets in a similar manner as reach adaptation. On training trials, subjects reached to a single target with aligned or misaligned cursor-hand feedback, in which the cursor was either rotated or scaled in extent relative to hand movement. After reach training, subjects reached to the training target and novel targets (including targets from a second start position) without visual feedback to assess generalization of reach adaptation. Subjects then performed a proprioceptive estimation task, in which they indicated the position of their hand relative to visual reference markers placed at similar locations as the trained and novel reach targets. Results indicated that shifts in hand position generalized across novel locations, independent of reach adaptation. Thus these distinct sensory and motor generalization patterns suggest that reach adaptation and proprioceptive recalibration arise from independent error signals and that changes in one system cannot guide adjustments in the other.

Retention of proprioceptive recalibration following visuomotor adaptation

We have recently shown that visuomotor adaptation following reaches with a misaligned cursor not only induces changes in an individual's motor output, but their proprioceptive sense of hand position as well. Long-term changes are seen in motor adaptation; however, very little is known about the retention of changes in felt hand position. We sought to evaluate whether this recalibration in proprioception, following visuomotor adaptation, is sufficiently robust to be retained the following day (~24 h later), and if so, to determine its extent. Visuomotor adaptation was induced by having subjects perform reaches to visual targets using a cursor representing their unseen hand, which had been gradually rotated 45° counterclockwise. Motor adaptation and proprioceptive recalibration were determined by assessing subjects' reach aftereffects and changes in hand bias, respectively. We found that subjects adapted their reaches and recalibrated their sense of hand position following training with a misaligned cursor, as shown in Cressman and Henriques (J Neurophysiol 102:3505-3518, 2009). More importantly, subjects who showed proprioceptive recalibration in the direction of motor adaptation on Day 1 did retain changes in felt hand position and motor adaptation on Day 2. These findings suggest that in addition to motor changes, individuals are capable of retaining sensory changes in proprioception up to 24 h later.

Generalization of reach adaptation and proprioceptive recalibration at different distances in the workspace

Studies have shown that adapting one's reaches in one location in the workspace can generalize to other novel locations. Generalization of this visuomotor adaptation is influenced by the location of novel targets relative to the trained location such that reaches made to novel targets that are located far from the trained target direction (i.e., ~22.5°; Krakauer et al. in J Neurosci 20:8916-8924, 2000) show very little generalization compared to those that are closer to the trained direction. However, generalization is much broader when reaching to novel targets in the same direction but at different distances from the trained target. In this study, we investigated whether changes in hand proprioception (proprioceptive recalibration), like reach adaptation, generalize to different distances of the workspace. Subjects adapted their reaches with a rotated cursor to two target locations at a distance of 13 cm from the home position. We then compared changes in open-loop reaches and felt hand position at these trained locations to novel targets located in the same direction as the trained targets but either at a closer (10 cm) or at a farther distance (15 cm) from the home position. We found reach adaptation generalized to novel closer and farther targets to the same extent as observed at the trained target distance. In contrast, while changes in felt hand position were significant across the two novel distances, this recalibration was smaller for the novel-far locations compared to the trained location. Given that reach adaptation completely generalized across the novel distances but proprioceptive recalibration generalized to a lesser extent for farther distances, we suggest that proprioceptive recalibration may arise independently of motor adaptation and vice versa.

The cerebellum is not necessary for visually driven recalibration of hand proprioception

Decades of research have implicated both cortical and subcortical areas, such as the cerebellum, as playing an important role in motor learning, and even more recently, in predicting the sensory consequences of movement. Still, it is unknown whether the cerebellum also plays a role in recalibrating sensory estimates of hand position following motor learning. To test this, we measured proprioceptive estimates of static hand position in 19 cerebellar patients with local ischemic lesions and 19 healthy controls, both before and after reach training with altered visual feedback of the hand. This altered visual feedback, (30° cursor-rotation) was gradually introduced in order to facilitate reach adaptation in the patient group. We included two different types of training (in separate experiments): the typical visuomotor rotation training where participants had full volition of their hand movements when reaching with the cursor, and sensory exposure training where the direction of participants׳ hand movements were constrained and gradually deviated from the cursor motion (Cressman, E. K., Henriques, D. Y., 2010. Reach adaptation and proprioceptive recalibration following exposure to misaligned sensory input. J. Neurophysiol., vol. 103, pp. 1888-1895). We found that both healthy individuals and patients showed equivalent shifts in their felt hand position following both types of training. Likewise, as expected given that the cursor-rotation was introduced gradually, patients showed comparable reach aftereffects to those of controls in both types of training. The robust change in felt hand position across controls and cerebellar patients suggests that the cerebellum is not involved in proprioceptive recalibration of the hand.

Reach adaptation and proprioceptive recalibration following terminal visual feedback of the hand

We have shown that when subjects reach with continuous, misaligned visual feedback of their hand, their reaches are adapted and proprioceptive sense of hand position is recalibrated to partially match the visual feedback (Salomonczyk et al., 2011). It is unclear if similar changes arise after reaching with visual feedback that is provided only at the end of the reach (i.e., terminal feedback), when there are shorter temporal intervals for subjects to experience concurrent visual and proprioceptive feedback. Subjects reached to targets with an aligned hand-cursor that provided visual feedback at the end of each reach movement across a 99-trial training block, and with a rotated cursor over three successive blocks of 99 trials each. After each block, no cursor reaches, to measure aftereffects, and felt hand positions were measured. Felt hand position was determined by having subjects indicate the position of their unseen hand relative to a reference marker. We found that subjects adapted their reaches following training with rotated terminal visual feedback, yet slightly less (i.e., reach aftereffects were smaller), than subjects from a previous study who experienced continuous visual feedback. Nonetheless, current subjects recalibrated their sense of felt hand position in the direction of the altered visual feedback, but this proprioceptive change increased incrementally over the three rotated training blocks. Final proprioceptive recalibration levels were comparable to our previous studies in which subjects performed the same task with continuous visual feedback. Thus, compared to reach training with continuous, but altered visual feedback, subjects who received terminal altered visual feedback of the hand produced significant but smaller reach aftereffects and similar changes in hand proprioception when given extra training. Taken together, results suggest that terminal feedback of the hand is sufficient to drive motor adaptation, and also proprioceptive recalibration.

Visual targets aren't irreversibly converted to motor coordinates: eye-centered updating of visuospatial memory in online reach control

Counter to current and widely accepted hypotheses that sensorimotor transformations involve converting target locations in spatial memory from an eye-fixed reference frame into a more stable motor-based reference frame, we show that this is not strictly the case. Eye-centered representations continue to dominate reach control even during movement execution; the eye-centered target representation persists after conversion to a motor-based frame and is continuously updated as the eyes move during reach, and is used to modify the reach plan accordingly during online control. While reaches are known to be adjusted online when targets physically shift, our results are the first to show that similar adjustments occur in response to changes in representations of remembered target locations. Specifically, we find that shifts in gaze direction, which produce predictable changes in the internal (specifically eye-centered) representation of remembered target locations also produce mid-transport changes in reach kinematics. This indicates that representations of remembered reach targets (and visuospatial memory in general) continue to be updated relative to gaze even after reach onset. Thus, online motor control is influenced dynamically by both the external and internal updating mechanisms.

Intermanual transfer and proprioceptive recalibration following training with translated visual feedback of the hand

Reaching with visual feedback that is misaligned with respect to the actual hand's location leads to changes in reach trajectories (i.e., visuomotor adaptation). Previous studies have also demonstrated that when training to reach with misaligned visual feedback of the hand, the opposite hand also partially adapts, providing evidence of intermanual transfer. Moreover, our laboratory has shown that visuomotor adaptation to a misaligned hand cursor, either translated or rotated relative to the hand, also leads to changes in felt hand position (what we call proprioceptive recalibration), such that subjects' estimate of felt hand position relative to both visual and non-visual reference markers (e.g., body midline) shifts in the direction of the visuomotor distortion. In the present study, we first determined the extent that motor adaptation to a translated cursor leads to transfer to the opposite hand, and whether this transfer differs across the dominant and non-dominant hands. Second, we looked to establish whether changes in hand proprioception that occur with the trained hand following adaptation also transfer to the untrained hand. We found intermanual motor transfer to the left untrained (non-dominant) hand after subjects trained their right (dominant) hand to reach with translated visual feedback of their hand. Motor transfer from the left trained to the right untrained hand was not observed. Despite finding changes in felt hand position in both trained hands, we did not find similar evidence of proprioceptive recalibration in the right or left untrained hands. Taken together, our results suggest that unlike visuomotor adaptation, proprioceptive recalibration does not transfer between hands and is specific only to the arm exposed to the distortion.

Gaze-centered spatial updating in delayed reaching even in the presence of landmarks

Previous results suggest that the brain predominantly relies on a constantly updated gaze-centered target representation to guide reach movements when no other visual information is available. In the present study, we investigated whether the addition of reliable visual landmarks influences the use of spatial reference frames for immediate and delayed reaching. Subjects reached immediately or after a delay of 8 or 12s to remembered target locations, either with or without landmarks. After target presentation and before reaching they shifted gaze to one of five different fixation points and held their gaze at this location until the end of the reach. With landmarks present, gaze-dependent reaching errors were smaller and more precise than when reaching without landmarks. Delay influenced neither reaching errors nor variability. These findings suggest that when landmarks are available, the brain seems to still use gaze-dependent representations but combine them with gaze-independent allocentric information to guide immediate or delayed reach movements to visual targets.

The role of the cross-sensory error signal in visuomotor adaptation

Reaching to targets with misaligned visual feedback of the hand leads to changes in proprioceptive estimates of hand position and reach aftereffects. In such tasks, subjects are able to make use of two error signals: the discrepancy between the desired and actual movement, known as the sensorimotor error signal, and the discrepancy between visual and proprioceptive estimates of hand position, which we refer to as the cross-sensory error signal. We have recently shown that mere exposure to a sensory discrepancy in the absence of goal-directed movement (i.e. no sensorimotor error signal) is sufficient to produce similar changes in felt hand position and reach aftereffects. Here, we sought to determine the extent that this cross-sensory error signal can contribute to proprioceptive recalibration and movement aftereffects by manipulating the magnitude of this signal in the absence of volitional aiming movements. Subjects pushed their hand out along a robot-generated linear path that was gradually rotated clockwise relative to the path of a cursor. On all trials, subjects viewed a cursor that headed directly towards a remembered target while their hand moved out synchronously. After exposure to a 30° rotated hand-cursor distortion, subjects recalibrated their sense of felt hand position and adapted their reaches. However, no additional increases in recalibration or aftereffects were observed following further increases in the cross-sensory error signal (e.g. up to 70°). This is in contrast to our previous study where subjects freely reached to targets with misaligned visual hand position feedback, hence experiencing both sensorimotor and cross-sensory errors, and the distortion magnitude systematically predicted increases in proprioceptive recalibration and reach aftereffects. Given these findings, we suggest that the cross-sensory error signal results in changes to felt hand position which drive partial reach aftereffects, while larger aftereffects that are produced after visuomotor adaptation (and that vary with the size of distortion) are related to the sensorimotor error signal.

Functional magnetic resonance imaging adaptation reveals the cortical networks for processing grasp-relevant object properties

Grasping behaviors require the selection of grasp-relevant object dimensions, independent of overall object size. Previous neuroimaging studies found that the intraparietal cortex processes object size, but it is unknown whether the graspable dimension (i.e., grasp axis between selected points on the object) or the overall size of objects triggers activation in that region. We used functional magnetic resonance imaging adaptation to investigate human brain areas involved in processing the grasp-relevant dimension of real 3-dimensional objects in grasping and viewing tasks. Trials consisted of 2 sequential stimuli in which the object's grasp-relevant dimension, its global size, or both were novel or repeated. We found that calcarine and extrastriate visual areas adapted to object size regardless of the grasp-relevant dimension during viewing tasks. In contrast, the superior parietal occipital cortex (SPOC) and lateral occipital complex of the left hemisphere adapted to the grasp-relevant dimension regardless of object size and task. Finally, the dorsal premotor cortex adapted to the grasp-relevant dimension in grasping, but not in viewing, tasks, suggesting that motor processing was complete at this stage. Taken together, our results provide a complete cortical circuit for progressive transformation of general object properties into grasp-related responses.

When more is less: increasing allocentric visual information can switch visual-proprioceptive combination from an optimal to sub-optimal process

When reaching for an object in the environment, the brain often has access to multiple independent estimates of that object's location. For example, if someone places their coffee cup on a table, then later they know where it is because they see it, but also because they remember how their reaching limb was oriented when they placed the cup. Intuitively, one would expect more accurate reaches if either of these estimates were improved (e.g., if a light were turned on so the cup were more visible). It is now well-established that the brain tends to combine two or more estimates about the same stimulus as a maximum-likelihood estimator (MLE), which is the best thing to do when estimates are unbiased. Even in the presence of small biases, relying on the MLE rule is still often better than choosing a single estimate. For this work, we designed a reaching task in which human subjects could integrate proprioceptive and allocentric (landmark-relative) visual information to reach for a remembered target. Even though both of these modalities contain some level of bias, we demonstrate via simulation that our subjects should use an MLE rule in preference to relying on one modality or the other in isolation. Furthermore, we show that when visual information is poor, subjects do, indeed, combine information in this way. However, when we improve the quality of visual information, subjects counter-intuitively switch to a sub-optimal strategy that occasionally includes reliance on a single modality.

Reach endpoint errors do not vary with movement path of the proprioceptive target

Previous research has shown that reach endpoints vary with the starting position of the reaching hand and the location of the reach target in space. We examined the effect of movement direction of a proprioceptive target-hand, immediately preceding a reach, on reach endpoints to that target. Participants reached to visual, proprioceptive (left target-hand), or visual-proprioceptive targets (left target-hand illuminated for 1 s prior to reach onset) with their right hand. Six sites served as starting and final target locations (35 target movement directions in total). Reach endpoints do not vary with the movement direction of the proprioceptive target, but instead appear to be anchored to some other reference (e.g., body). We also compared reach endpoints across the single and dual modality conditions. Overall, the pattern of reaches for visual-proprioceptive targets resembled those for proprioceptive targets, while reach precision resembled those for the visual targets. We did not, however, find evidence for integration of vision and proprioception based on a maximum-likelihood estimator in these tasks.

Three-dimensional transformations for goal-directed action

Much of the central nervous system is involved in visuomotor transformations for goal-directed gaze and reach movements. These transformations are often described in terms of stimulus location, gaze fixation, and reach endpoints, as viewed through the lens of translational geometry. Here, we argue that the intrinsic (primarily rotational) 3-D geometry of the eye-head-reach systems determines the spatial relationship between extrinsic goals and effector commands, and therefore the required transformations. This approach provides a common theoretical framework for understanding both gaze and reach control. Combined with an assessment of the behavioral, neurophysiological, imaging, and neuropsychological literature, this framework leads us to conclude that (a) the internal representation and updating of visual goals are dominated by gaze-centered mechanisms, but (b) these representations must then be transformed as a function of eye and head orientation signals into effector-specific 3-D movement commands.

Proprioceptive recalibration following prolonged training and increasing distortions in visuomotor adaptation

Reaching with misaligned visual feedback of the hand leads to reach adaptation (motor recalibration) and also results in partial sensory recalibration, where proprioceptive estimates of hand position are changed in a way that is consistent with the visual distortion. The goal of the present study was to explore the relationship between changes in sensory and motor systems by examining these processes following (1) prolonged reach training and (2) training with increasing visuomotor distortions. To examine proprioceptive recalibration, we determined the position at which subjects felt their hand was aligned with a reference marker after completing three blocks of reach training trials with a cursor that was rotated 30° clockwise (CW) for all blocks, or with a visuomotor distortion that was increased incrementally across the training blocks up to 70°CW relative to actual hand motion. On average, subjects adapted their reaches by 16° and recalibrated their sense of felt hand position by 7° leftwards following the first block of reach training trials in which they reached with a cursor that was rotated 30°CW relative to the hand, compared to baseline values. There was no change in these values for the 30° training group across subsequent training blocks. However, subjects training with increasing levels of visuomotor distortion showed increased reach adaptation (up to 34° leftward movement aftereffects) and sensory recalibration (up to 15° leftwards). Analysis of motor and sensory changes following each training block did not reveal any significant correlations, suggesting that the processes underlying motor adaptation and proprioceptive recalibration occur simultaneously yet independently of each other.

Motor adaptation and proprioceptive recalibration

Goal-directed reaches are rapidly adapted after reaching with misaligned visual feedback of the hand. It has been suggested that reaching with misaligned visual feedback of the hand also results in proprioceptive recalibration (i.e., realigning proprioceptive estimates of hand position to match visual estimates). In this chapter, we review a series of experiments conducted in our lab which examine this proposal. We assessed proprioceptive recalibration by comparing subjects' estimates of the position at which they felt their hand was aligned with a reference marker (visual or proprioceptive) before and after aiming with a misaligned cursor that was typically rotated 30° clockwise (CW) with respect to the hand. In general, results indicated that subjects recalibrated proprioception such that their estimates of felt hand position were shifted in the same direction that they adapted their reaches. Moreover, proprioception was recalibrated to a similar extent of motor adaptation (∼30%), regardless of how the hand was positioned during the estimate trials (active or passive placement), the location or modality of the reference marker (visual or proprioceptive), the hand used during reach training (right or left), how the distortion was introduced (gradual or abrupt), and age (young or older subjects) and the magnitude of the visuomotor distortion introduced (30° or 50° or 70°). These results suggest that in addition to recalibrating the sensorimotor transformations underlying reaching movements, visuomotor adaptation results in partial proprioceptive recalibration.

Memory for proprioceptive and multisensory targets is partially coded relative to gaze

We examined the effect of gaze direction relative to target location on reach endpoint errors made to proprioceptive and multisensory targets. We also explored if and how visual and proprioceptive information about target location are integrated to guide reaches. Participants reached to their unseen left hand in one of three target locations (left of body midline, body midline, or right or body midline), while it remained at a target site (online), or after it was removed from this location (remembered), and also after the target hand had been briefly lit before reaching (multisensory target). The target hand was guided to a target location using a robot-generated path. Reaches were made with the right hand in complete darkness, while gaze was varied in one of four eccentric directions. Horizontal reach errors systematically varied relative to gaze for all target modalities; not only for visually remembered and online proprioceptive targets as has been found in previous studies, but for the first time, also for remembered proprioceptive targets and proprioceptive targets that were briefly visible. These results suggest that the brain represents the locations of online and remembered proprioceptive reach targets, as well as visual-proprioceptive reach targets relative to gaze, along with other motor-related representations. Our results, however, do not suggest that visual and proprioceptive information are optimally integrated when coding the location of multisensory reach targets in this paradigm.

Online action-to-perception transfer: only percept-dependent action affects perception

Perception self-evidently affects action, but under which conditions does action in turn influence perception? To answer this question we ask observers to view an ambiguous stimulus that is alternatingly perceived as rotating clockwise or counterclockwise. When observers report the perceived direction by rotating a manipulandum, opposing directions between report and percept ('incongruent') destabilize the percept, whereas equal directions ('congruent') stabilize it. In contrast, when observers report their percept by key presses while performing a predefined movement, we find no effect of congruency. Consequently, our findings suggest that only percept-dependent action directly influences perceptual experience.

Visuomotor adaptation and proprioceptive recalibration in older adults

Previous studies have shown that both young and older subjects adapt their reaches in response to a visuomotor distortion. It has been suggested that one's continued ability to adapt to a visuomotor distortion with advancing age is due to the preservation of implicit learning mechanisms, where implicit learning mechanisms include processes that realign sensory inputs (i.e. shift one's felt hand position to match the visual representation). The present study examined this proposal by determining if changes in sense of felt hand position (i.e. proprioceptive recalibration) follow visuomotor adaptation in older subjects. As well, we examined the influence of age on proprioceptive recalibration by comparing young and older subjects' estimates of the position at which they felt their hand was aligned with a visual reference marker before and after aiming with a misaligned cursor that was gradually rotated 30 degrees clockwise of the actual hand location. On estimation trials, subjects moved their hand along a robot-generated constrained pathway. At the end of the movement, a reference marker appeared and subjects indicated if their hand was left or right of the marker. Results indicated that all subjects adapted their reaches at a similar rate and to the same extent across the reaching trials. More importantly, we found that both young and older subjects recalibrated proprioception, such that they felt their hand was aligned with a reference marker when it was approximately 6 degrees more left (or counterclockwise) of the marker following reaches with a rotated cursor. The leftward shift in both young and older subjects' estimates was in the same direction and a third of the extent of adapted movement. Given that the changes in the estimate of felt hand position were only a fraction of the changes observed in the reaching movements, it is unlikely that sensory recalibration was the only source driving changes in reaches. Thus, we propose that proprioceptive recalibration combines with adapted sensorimotor mappings to produce changes in reaching movements. From the results of the present study, it is clear that changes in both sensory and motor systems are possible in older adults and could contribute to the preserved visuomotor adaptation.

Reach Adaptation and Proprioceptive Recalibration Following Exposure to Misaligned Sensory Input

Motor adaptation in response to a visuomotor distortion arises when the usual motor command no longer results in the predicted sensory output. In this study, we examined if exposure to a sensory discrepancy was sufficient on its own to produce changes in reaches and recalibrate the sense of felt hand position in the absence of any voluntary movements. Subjects pushed their hand out along a robot-generated fixed linear path (active exposure group) or were passively moved along the same path (passive exposure group). This fixed path was gradually rotated counterclockwise around the home position with respect to the path of the cursor. On all trials, subjects saw the cursor head directly to the remembered target position while their hand moved outwards. We found that after exposure to the visually distorted hand motion, subjects in both groups adapted their reaches such that they aimed ∼6° to the left of the intended target. The magnitude of reach adaptation was similar to the extent that subjects recalibrated their sense of felt hand position. Specifically the position at which subjects perceived their unseen hand to be aligned with a reference marker was the same as that to which they reached when allowed to move freely. Given the similarity in magnitude of these adaptive responses we propose that reach adaptation arose due to changes in subjects' sense of felt hand position. Moreover, results indicate that motor adaptation can arise following exposure to a sensory mismatch in the absence of movement related error signals.

Visuomotor adaptation and intermanual transfer under different viewing conditions

Does the brain use a separate internal model for cursor mechanics during visuomotor adaptation? We compared the amount of adaptation and transfer to the opposite arm when subjects reached the targets under different viewing conditions of the arm during reaching. If the brain forms separate models, we predict a difference in the amount of adaptation and transfer for each viewing condition. If the brain forms one model, we predict equivalent amounts of adaptation and transfer between the two hands for each viewing condition. Separate groups of subjects performed a reaching task with either a rotated view of cursor motion representing their unseen hand or a rotated view of their actual hand. The two groups were further divided so that the magnitude of the rotation was either 45 degrees or 75 degrees counter-clockwise. After adapting to the rotation with one hand, subjects reached the same targets under the same viewing condition but with the opposite hand. Similar amounts of adaptation and intermanual transfer were found across the different magnitudes of rotation and across patterns of hand-order. Our results suggest that the brain may not be learning a distinct model for cursor mechanics, or if it is, it must be equivalent or overlapping with the arm model.

Sensory Recalibration of Hand Position Following Visuomotor Adaptation

Goal-directed reaches are rapidly adapted following exposure to misaligned visual feedback of the hand. It has been suggested that these changes in reaches result in sensory recalibration (i.e., realigning proprioceptive estimates of hand position to match the visual estimates). In the current study we tested whether visuomotor adaptation results in recalibration of hand proprioception by comparing subjects' estimates of the position at which they felt their hand was aligned with a reference marker (visual or proprioceptive) before and after aiming with a misaligned cursor. The misaligned cursor was either translated or rotated to the right of the actual hand location. On the estimation trials, we did not allow subjects to freely move their hands into position. Instead, a robot manipulandum either passively positioned the hand ( experiments 1 and 2) or subjects moved their hand along a robot-generated constrained pathway ( experiments 3 and 4). We found that regardless of experimental manipulation, subjects' proprioceptive estimates of hand position were more biased to the left after visuomotor adaptation. The leftward shift in subjects' estimates was in the same direction and one third of the magnitude of the adapted movement. This suggests that in addition to recalibrating the sensorimotor transformations underlying reaching movements, visuomotor adaptation results in partial proprioceptive recalibration.

Visuomotor Adaptation Does Not Recalibrate Kinesthetic Sense of Felt Hand Path

Motor control relies on multiple sources of information. To estimate the position and motion of the hand, the brain uses both vision and body-position (proprioception and kinesthesia) senses from sensors in the muscles, tendons, joints, and skin. Although performance is better when more than one sensory modality is present, visuomotor adaptation suggests that people tend to rely much more on visual information of the hand to guide their arm movements to targets, even when the visual information and kinesthetic information about the hand motion are in conflict. The aim of this study is to test whether adapting hand movements in response to false visual feedback of the hand will result in the change or recalibration of the kinesthetic sense of hand motion. The advantage of this cross-sensory recalibration would ensure on-line consistency between the senses. To test this, we mapped participants' sensitivity to tilted and curved hand paths and then examined whether adapting their hand movements in response to false visual feedback affected their felt sense of hand path. We found that participants could accurately estimate hand path directions and curvature after adapting to false visual feedback of their hand when reaching to targets. Our results suggest that although vision can override kinesthesia to recalibrate arm motor commands, it does not recalibrate the kinesthetic sense of hand path geometry.

Interpreting ambiguous visual information in motor learning

Previous studies have shown that learning to reach accurately with an imposed visuomotor rotation requires a remapping of the relationship between vision and motor output. In this preliminary study, we examine how the brain works out the appropriate motor adjustments, in this case for both arms, based on visual images. Specifically, we investigate how visual errors seen while adapting reaches to visual targets affect the movements of both the trained and untrained hand. In our task subjects learned to make accurate reaches to targets in four visual feedback conditions: rotated 45 degrees, rotated 105 degrees, reversed left to right and rotated 45 degrees plus reversed. In all conditions the rotation was applied to the subject's feedback of their hand and not the targets. In the reversed and rotated-reversed condition, when the subject used their right hand, the feedback looked like their left hand (and vice versa). After a training period with one hand (e.g., right) subjects were tested with the opposite hand (e.g., left) on the same task. We predicted that after reaching with the right hand with reversed visual feedback the control of the left arm would also be altered-more so than after learning an equal-sized adjustment to right-arm reaching with a rotated, but non-reversed, view of their hand movements. Our results showed that people were able to learn the visuomotor adaptation with reversed visual feedback, but more interestingly, that learning occurred for the untrained hand as well for the reversed conditions alone. Here, vision alone--when it resembles the image of the opposite hand--led to improved initial performance for this opposite, untrained arm when reaching in a similar task. The brain seems to take advantage of reversed visual feedback of the arm to adjust the motor commands to the untrained arm in a way that facilitates transfer of the adaptation from one arm to the other.

Updating visual memory across eye movements for ocular and arm motor control

Remembered object locations are stored in an eye-fixed reference frame, so that every time the eyes move, spatial representations must be updated for the arm-motor system to reflect the target's new relative position. To date, studies have not investigated how the brain updates these spatial representations during other types of eye movements, such as smooth-pursuit. Further, it is unclear what information is used in spatial updating. To address these questions we investigated whether remembered locations of pointing targets are updated following smooth-pursuit eye movements, as they are following saccades, and also investigated the role of visual information in estimating eye-movement amplitude for updating spatial memory. Misestimates of eye-movement amplitude were induced when participants visually tracked stimuli presented with a background that moved in either the same or opposite direction of the eye before pointing or looking back to the remembered target location. We found that gaze-dependent pointing errors were similar following saccades and smooth-pursuit and that incongruent background motion did result in a misestimate of eye-movement amplitude. However, the background motion had no effect on spatial updating for pointing, but did when subjects made a return saccade, suggesting that the oculomotor and arm-motor systems may rely on different sources of information for spatial updating.

Reference frame conversions for repeated arm movements

The aim of this study was to further understand how the brain represents spatial information for shaping aiming movements to targets. Both behavioral and neurophysiological studies have shown that the brain represents spatial memory for reaching targets in an eye-fixed frame. To date, these studies have only shown how the brain stores and updates target locations for generating a single arm movement. But once a target's location has been computed relative to the hand to program a pointing movement, is that information reused for subsequent movements to the same location? Or is the remembered target location reconverted from eye to motor coordinates each time a pointing movement is made? To test between these two possibilities, we had subjects point twice to the remembered location of a previously foveated target after shifting their gaze to the opposite side of the target site before each pointing movement. When we compared the direction of pointing errors for the second movement to those of the first, we found that errors for each movement varied as a function of current gaze so that pointing endpoints fell on opposite sides of the remembered target site in the same trial. Our results suggest that when shaping multiple pointing movements to the same location the brain does not use information from the previous arm movement such as an arm-fixed representation of the target but instead mainly uses the updated eye-fixed representation of the target to recalculate its location into the appropriate motor frame.