Properties of intermodal transfer after dual visuo- and auditory-motor adaptation

Previous work documented that sensorimotor adaptation transfers between sensory modalities: When subjects adapt with one arm to a visuomotor distortion while responding to visual targets, they also appear to be adapted when they are subsequently tested with auditory targets. Vice versa, when they adapt to an auditory-motor distortion while pointing to auditory targets, they appear to be adapted when they are subsequently tested with visual targets. Therefore, it was concluded that visuomotor as well as auditory-motor adaptation use the same adaptation mechanism. Furthermore, it has been proposed that sensory information from the trained modality is weighted larger than sensory information from an untrained one, because transfer between sensory modalities is incomplete. The present study tested these hypotheses for dual arm adaptation. One arm adapted to an auditory-motor distortion and the other either to an opposite directed auditory-motor or visuomotor distortion. We found that both arms adapted significantly. However, compared to reference data on single arm adaptation, adaptation in the dominant arm was reduced indicating interference from the non-dominant to the dominant arm. We further found that arm-specific aftereffects of adaptation, which reflect recalibration of sensorimotor transformation rules, were stronger or equally strong when targets were presented in the previously adapted compared to the non-adapted sensory modality, even when one arm adapted visually and the other auditorily. The findings are discussed with respect to a recently published schematic model on sensorimotor adaptation.     

Implicit motor sequence learning is not represented purely in response locations

This study employed a novel variant of the serial reaction time task, focused on sequencing one element of movement—direction. During the task a repeated pattern of alternating directions (right–left–right, etc.) was embedded in the stimuli, and there was no series of response locations. Responses were made via two effector systems: single-finger responding (necessitates lateral arm movements between response keys), and four-fingered responding (4 individual fingers on 4 individual keys; requires no lateral arm movement). The sequence of directions was only learned by participants who performed lateral movements during training, indicating that learning was contingent on the particular motor effector used. Participants with low levels of sequence awareness displayed the same pattern of results.     

Motor adaptation and manual transfer: Insight into the persistent nature of sensorimotor representations

It is well known that sensorimotor memories are built and updated through experience with objects. These representations are useful to anticipatory and feedforward control processes that preset grip and load forces during lifting. When individuals lift objects with qualities that are not congruent with their memory-derived expectations, feedback processes adjust motor plans to achieve successful lifts and contribute to the updating of the stored representations. The two experiments presented examine motor adaptation to an illusory size–weight lifting task, and the transfer of this motor adaptation to the unexposed hand. In Experiment 1, performers acquired motor adaptation with their right hand and transfer was measured on their left hand. In Experiment 2, adaptation was acquired with the left hand and transfer was measured on the right hand. In order to investigate the persistence of sensorimotor memories, these experiments measure adaptation, retention, and transfer after 15 min and 24 h delay periods. Both experiments confirm that experience with objects leads to adaptation of force scaling processes, that these adaptations transcend effector and are persistent. The results are discussed in terms favouring interpretations that describe motor adaptations to illusion as being centrally available.     

Transfer of adaptation between ocular saccades and arm movements

Previous studies found little or no transfer of adaptation from reactive saccades to arm pointing movements, which suggests that the two motor systems rely on distinct adaptive mechanisms. However, this conclusion is based on experiments about the adaptation of response amplitudes, which is known to follow somewhat different principles than the adaptation of response directions. In the present study, we therefore investigate whether adapting the direction of reactive saccades will transfer to arm movements. We also test transfer in the opposite direction, from the arm to the eyes. Participants executed aimed saccades or arm movements from a central starting point towards visual targets in the participants' frontal plane. Targets were presented in eight possible locations along a circle of 20 cm radius about the starting point; each remained for 200 ms in one position, and was then displaced along the circle by -15 degrees . Participants from group E adapted to these double-stepped targets while executing eye movements, and were then tested for transfer while executing arm movements. The reciprocal design was used in participants from group A. Adaptive change in group A was about 14 degrees , while in group E it was only about 7 degrees . Transfer of adaptation was substantial, and was more pronounced when using the arm (i.e., eye-to-arm transfer in group E) rather than the eyes (i.e., arm-to-eye transfer in group A). Strong aftereffects were yielded in both groups. This pattern of findings implies that the adaptive change observed in our study was mainly based on recalibration rather than on cognitive strategies (strong aftereffects), that eyes and arm had access to a common adaptive mechanism (substantial transfer), and that the arm had better access than the eyes (larger adaptation and transfer when using the arm). When considering this outcome along with the available literature, it appears that arm and eyes may rely sometimes on a common and sometimes on distinct adaptive mechanisms, depending on the adapted parameter and on the nature of the motor task.     

Interocular transfer of visual aftereffects

A number of the well-known visual after-effects of adaptation exhibit interocular transfer, so that presentation of an adaptation figure to one eye produces a temporary change in the performance of the nonadapted eye. This outcome is usually attributed to the involvement of binocular visual neurons that respond to stimulation of either eye. The fact that interocular transfer is incomplete (i.e., the transferred aftereffect is smaller in magnitude than that induced and measured in the same eye) is routinely cited as evidence for the involvement of monocular neurons. This article critically examines these two interpretations, which are developed in terms of a neural model of interocular transfer. No evidence, logical or empirical, was obtained for rejecting the model. Our analysis further shows that the model must assume some type of pooling process that operates over all tested neurons, both adapted and unadapted. Finally, general implications of the interocular transfer model are discussed, the aim being to delimit the conclusions that may be drawn from interocular transfer experiments.     

Distinct consolidation outcomes in a visuomotor adaptation task: Off-line leaning and persistent after-effect

Consolidation is a time-dependent process responsible for the storage of information in long-term memory. As such, it plays a crucial role in motor learning. In two experiments, we sought to determine whether one’s performance influences the outcome of the consolidation process. We used a visuomotor adaptation task in which the cursor moved by the participants was rotated 30° clockwise. Thus, participants had to learn a new internal model to compensate for the rotation of the visual feedback. The results indicated that when participants showed good adaptation in the first session, consolidation resulted in a persistent after-effect in a no-rotation transfer test; they had difficulty returning to their normal no-rotation internal model. However, when participants showed poor adaptation in the first session, consolidation led to significant off-line learning (between sessions improvement) but labile after-effects. These observations suggest that distinct consolidation outcomes (off-line learning and persistent after-effect) may occur depending on the learner’s initial performance.     

Alternating Adaptation of Eye and Hand Movements to Opposite Directed Double Steps

Eye and hand movements can adapt to a variety of sensorimotor discordances. Studies on adaptation of movement directions suggest that the oculomotor and the hand motor system access the same adaptive mechanism related to the polarity of a discordance, because concurrent adaptations to opposite directed discordances strongly interfere. The authors scrutinized whether participants adapt their hand and eye movements to opposite directions (clockwise/counterclockwise) when both motor systems are alternatingly exposed to opposite directed double steps, and whether such adaptation is influenced by the allocation of effector to adaptation direction. The results showed that hand and eye movements adapted to opposite directions, but adaptation was biased to the counterclockwise direction. Aftereffects emerged nearly unbiased and independently for both motor systems. The authors conclude that the oculomotor and the hand motor system use independent mechanisms when they adapt to opposite polarities, although they interact during adaptation or concurrent performance.     

Through the inverting glass: first-person observations on spatial vision and imagery

Experience with inverting glasses reveals key factors of spatial vision. Interpretations of the literature based on the metaphor of a “visual image” have raised the question whether visual experience with inverting glasses remains inverted or whether it may turn back to normal after adaptation to the glasses. Here, I report on my experience with left/right inverting glasses and argue that a more fine-grained sensorimotor analysis can resolve the issue. Crucially, inverting glasses introduce a conflict at the very heart of spatial vision. At first, the experience of visual direction grounded in head movements differs from visual experience grounded in eye movements. During adaptation, this difference disappears, and one may learn to see without conflict where objects are located (this took me 123 h of practice). The momentary experience became once again integrated within the larger flow of visual exploration involving head movements, a change of experience that was abrupt and comparable to a Gestalt switch. The resulting experience remains different from normal vision, and I argue that this difference can be understood in sensorimotor terms. I describe how adaptation to inverting glasses is further reflected in mental imagery, supporting the idea that imagery is grounded in sensorimotor engagement with the environment as well.     

The Effects of Brain Lateralization on Motor Control and Adaptation

ABSTRACT

Lateralization of mechanisms mediating functions such as language and perception is widely accepted as a fundamental feature of neural organization. Recent research has revealed that a similar organization exists for the control of motor actions, in that each brain hemisphere contributes unique control mechanisms to the movements of each arm. The authors review present research that addresses the nature of the control mechanisms that are lateralized to each hemisphere and how they impact motor adaptation and learning. In general, the studies suggest an enhanced role for the left hemisphere during adaptation, and the learning of new sequences and skills. The authors suggest that this specialization emerges from a left hemisphere specialization for predictive control—the ability to effectively plan and coordinate motor actions, possibly by optimizing certain cost functions. In contrast, right hemisphere circuits appear to be important for updating ongoing actions and stopping at a goal position, through modulation of sensorimotor stabilization mechanisms such as reflexes. The authors also propose that each brain hemisphere contributes its mechanism to the control of both arms. They also discuss the potential advantages of such a lateralized control system.     

Forward modeling allows feedback control for fast reaching movements

Delays in sensorimotor loops have led to the proposal that reaching movements are primarily under pre-programmed control and that sensory feedback loops exert an influence only at the very end of a trajectory. The present review challenges this view. Although behavioral data suggest that a motor plan is assembled prior to the onset of movement, more recent studies have indicated that this initial plan does not unfold unaltered, but is updated continuously by internal feedback loops. These loops rely on a forward model that integrates the sensory inflow and motor outflow to evaluate the consequence of the motor commands sent to a limb, such as the arm. In such a model, the probable position and velocity of an effector can be estimated with negligible delays and even predicted in advance, thus making feedback strategies possible for fast reaching movements. The parietal lobe and cerebellum appear to play a crucial role in this process. The ability of the motor system to estimate the future state of the limb might be an evolutionary substrate for mental operations that require an estimate of sequelae in the immediate future.     

Interlimb differences in visuomotor and dynamic adaptation during targeted reaching in children

While a number of studies have focused on movement (a)symmetries between the arms in adults, less is known about movement asymmetries in typically developing children. The goal of this study was to examine interlimb differences in children when adapting to novel visuomotor and dynamic conditions while performing a center-out reaching task. We tested 13 right-handed children aged 9–11 years old. Prior to movement, one of eight targets arranged radially around the start position was randomly displayed. Movements were made either with the right (dominant) arm or the left (nondominant) arm. The children participated in two experiments separated by at least one week. In one experiment, subjects were exposed to a rotated visual display (30° about the start circle); and in the other, a 1 kg mass (attached eccentrically to the forearm axis). Each experiment consisted of three blocks: pre-exposure, exposure and post-exposure. Three measures of task performance were calculated from hand trajectory data: hand-path deviation from the straight target line, direction error at peak velocity and final position error. Results showed that during visuomotor adaptation, no interlimb differences were observed for any of the three measures. During dynamic adaptation, however, a significant difference between the arms was observed at the first cycle during dynamic adaptation. With regard to the aftereffects observed during the post-exposure block, direction error data indicate considerably large aftereffects for both arms during visuomotor adaptation; and there was a significant difference between the arms, resulting in substantially larger aftereffects for the right arm. Similarly, dynamic adaptation results also showed a significant difference between the arms; and post hoc analyses indicated that aftereffects were present only for the right arm. Collectively, these findings indicate that the dominant arm advantage for developing an internal model associated with a novel visuomotor or dynamic transform, as previously shown in young adults, may already be apparent at 9 to 11-year old children.     

Transfer of short-term motor learning across the lower limbs as a function of task conception and practice order

Interlimb transfer of motor learning, indicating an improvement in performance with one limb following training with the other, often occurs asymmetrically (i.e., from non-dominant to dominant limb or vice versa, but not both). In the present study, we examined whether interlimb transfer of the same motor task could occur asymmetrically and in opposite directions (i.e., from right to left leg vs. left to right leg) depending on individuals’ conception of the task. Two experimental conditions were tested: In a dynamic control condition, the process of learning was facilitated by providing the subjects with a type of information that forced them to focus on dynamic features of a given task (force impulse); and in a spatial control condition, it was done with another type of information that forced them to focus on visuomotor features of the same task (distance). Both conditions employed the same leg extension task. In addition, a fully-crossed transfer paradigm was used in which one group of subjects initially practiced with the right leg and were tested with the left leg for a transfer test, while the other group used the two legs in the opposite order. The results showed that the direction of interlimb transfer varied depending on the condition, such that the right and the left leg benefited from initial training with the opposite leg only in the spatial and the dynamic condition, respectively. Our finding suggests that manipulating the conception of a leg extension task has a substantial influence on the pattern of interlimb transfer in such a way that the direction of transfer can even be opposite depending on whether the task is conceived as a dynamic or spatial control task.     

Mental representations of magnitude and order: A dissociation by sensorimotor learning

Numbers and spatially directed actions share cognitive representations. This assertion is derived from studies that have demonstrated that the processing of small- and large-magnitude numbers facilitates motor behaviors that are directed to the left and right, respectively. However, little is known about the role of sensorimotor learning for such number–action associations. In this study, we show that sensorimotor learning in a serial reaction-time task can modify the associations between number magnitudes and spatially directed movements. Experiments 1 and 3 revealed that this effect is present only for the learned sequence and does not transfer to a novel unpracticed sequence. Experiments 2 and 4 showed that the modification of stimulus–action associations by sensorimotor learning does not occur for other sets of ordered stimuli such as letters of the alphabet. These results strongly suggest that numbers and actions share a common magnitude representation that differs from the common order representation shared by letters and spatially directed actions. Only the magnitude representation, but not the order representation, can be modified episodically by sensorimotor learning.     

Difference in sensorimotor adaptation to horizontal and vertical mirror distortions during ballistic arm movements

When learning a novel motor task, the sensorimotor system must develop new strategies to efficiently control the limb(s) involved, and this adaptation appears to be developed through the construction of a behavioral map known as an 'internal model'. A common method to uncover the mechanisms of adaptation and reorganization processes is to expose the system to new environmental conditions, typically by introducing visual or mechanical distortions. The present study investigated the adaptation mechanisms of the human sensorimotor system to horizontal and vertical mirror distortions (HMD and VMD) during the execution of fast goal-directed arm movements. Mirror distortions (MDs) were created by means of virtual visual feedback on a computer screen while the movement was executed on a graphics tablet. Twenty healthy adult participants were recruited and assigned to one of two groups of 10 people each. Tests were divided in two subsequent blocks of five trials. The first block consisted of trials with no mirror distortion (NMD), while the second block was recorded when exposing one group to HMD and the other to VMD. Both MDs resulted in kinematic changes: during the tests with the MDs the participants did not reach the performance level found at the NMD test. Motor performance during HMD appeared to be globally better than during VMD and the adaptation process to VMD appeared to be slower than to HMD, but data interpretation was hampered by large within-participant and between-participant variability. In-depth analyses of the data revealed that most of the motor performance information was contained in the direction of movement. The data supported the idea that the internal model for HMD was already partially built.     

Exploiting redundancy for flexible behavior: unsupervised learning in a modular sensorimotor control architecture

Autonomously developing organisms face several challenges when learning reaching movements. First, motor control is learned unsupervised or self-supervised. Second, knowledge of sensorimotor contingencies is acquired in contexts in which action consequences unfold in time. Third, motor redundancies must be resolved. To solve all 3 of these problems, the authors propose a sensorimotor, unsupervised, redundancy-resolving control architecture (SURE_REACH), based on the ideomotor principle. Given a 3-degrees-of-freedom arm in a 2-dimensional environment, SURE_REACH encodes 2 spatial arm representations with neural population codes: a hand end-point coordinate space and an angular arm posture space. A posture memory solves the inverse kinematics problem by associating hand end-point neurons with neurons in posture space. An inverse sensorimotor model associates posture neurons with each other action-dependently. Together, population encoding, redundant posture memory, and the inverse sensorimotor model enable SURE_REACH to learn and represent sensorimotor grounded distance measures and to use dynamic programming to reach goals efficiently. The architecture not only solves the redundancy problem but also increases goal reaching flexibility, accounting for additional task constraints or realizing obstacle avoidance. While the spatial population codes resemble neurophysiological structures, the simulations confirm the flexibility and plausibility of the model by mimicking previously published data in arm-reaching tasks.     

Development of Force Adaptation During Childhood

Humans learn to make reaching movements in novel dynamic environments by acquiring an internal motor model of their limb dynamics. Here, the authors investigated how 4- to 11-year-old children (N = 39) and adults (N = 7) adapted to changes in arm dynamics, and they examined whether those data support the view that the human brain acquires inverse dynamics models (IDM) during development. While external damping forces were applied, the children learned to perform goal-directed forearm flexion movements. After changes in damping, all children showed kinematic aftereffects indicative of a neural controller that still attempted to compensate the no longer existing damping force. With increasing age, the number of trials toward complete adaptation decreased. When damping was present, forearm paths were most perturbed and most variable in the youngest children but were improved in the older children. The findings indicate that the neural representations of limb dynamics are less precise in children and less stable in time than those of adults. Such controller instability might be a primary cause of the high kinematic variability observed in many motor tasks during childhood. Finally, the young children were not able to update those models at the same rate as the older children, who, in turn, adapted more slowly than adults. In conclusion, the ability to adapt to unknown forces is a developmental achievement. The present results are consistent with the view that the acquisition and modification of internal models of the limb dynamics form the basis of that adaptive process.     

How generative drawing affects the learning process: An eye‐tracking analysis

Generative drawing is a learning strategy in which students draw illustrations while reading a text to depict the content of the lesson. In two experiments, students were asked to generate drawings as they read a scientific text or read the same text on influenza with author‐provided illustrations (Experiment 1) or to generate drawings or write verbal summaries as they read (Experiment 2). An examination of students' eye movements during learning showed that students who engaged in generative drawing displayed more rereadings of words, higher proportion of fixations on the important words, higher rate of transitions between words and workspace, and higher proportion of transitions between important words and workspace than students given a text lesson with author‐generated illustrations (Experiment 1) or students who were asked to write a summary (Experiment 2). These findings contribute new evidence to guide theories for explaining how generative drawing affects learning processes.     

Learning and transfer of a relative phase pattern and a joint amplitude ratio in a rhythmic multijoint arm movement

According to the coordination dynamics perspective, one can characterize the learning of novel relative phase patterns as the formation of a stable attractor in the coordination landscape of the order parameter relative phase. The authors examined 18 participants' learning and transfer of a 90 degrees relative phase pattern and a 0.6-joint-amplitude ratio between the elbow and wrist. Variability in the relative phasing and the joint amplitude ratio between the elbow and wrist decreased with practice. Positive transfer of the 90 degrees relative phase pattern was not dependent on the learning arm (dominant or nondominant). Positive transfer of the joint amplitude ratio was dependent on the learning arm and the direction of transfer. The results demonstrated that relative phase is an order parameter that characterizes the coordination dynamics of learning and transferring multijoint arm movements, and they provide preliminary evidence that joint amplitude ratios act as order parameters in the learning and transfer of multijoint arm movements.     

A sensorimotor account of vision and visual consciousness

Many current neurophysiological, psychophysical, and psychological approaches to vision rest on the idea that when we see, the brain produces an internal representation of the world. The activation of this internal representation is assumed to give rise to the experience of seeing. The problem with this kind of approach is that it leaves unexplained how the existence of such a detailed internal representation might produce visual consciousness. An alternative proposal is made here. We propose that seeing is a way of acting. It is a particular way of exploring the environment. Activity in internal representations does not generate the experience of seeing. The outside world serves as its own, external, representation. The experience of seeing occurs when the organism masters what we call the governing laws of sensorimotor contingency. The advantage of this approach is that it provides a natural and principled way of accounting for visual consciousness, and for the differences in the perceived quality of sensory experience in the different sensory modalities. Several lines of empirical evidence are brought forward in support of the theory, in particular: evidence from experiments in sensorimotor adaptation, visual "filling in," visual stability despite eye movements, change blindness, sensory substitution, and color perception.