Pierre-Michel Bernier

Human Posterior Parietal Cortex Flexibly Determines Reference Frames for Reaching Based on Sensory Context

Current models of sensorimotor transformations emphasize the dominant role of gaze-centered representations for reach planning in the posterior parietal cortex (PPC). Here we exploit fMRI repetition suppression to test whether the sensory modality of a target determines the reference frame used to define the motor goal in the PPC and premotor cortex. We show that when targets are defined visually, the anterior precuneus selectively encodes the motor goal in gaze-centered coordinates, whereas the parieto-occipital junction, Brodman Area 5 (BA 5), and PMd use a mixed gaze- and body-centered representation. In contrast, when targets are defined by unseen proprioceptive cues, activity in these areas switches to represent the motor goal predominantly in body-centered coordinates. These results support computational models arguing for flexibility in reference frames for action according to sensory context. Critically, they provide neuroanatomical evidence that flexibility is achieved by exploiting a multiplicity of reference frames that can be expressed within individual areas.

Inferring online and offline processing of visual feedback in target-directed movements from kinematic data

Vision plays an important role in the planning and execution of target-directed aiming movements. In this review, we highlight the limitations that exist in detecting visual regulation of limb trajectories from traditional kinematic analyses such as the identification of discontinuities in velocity and acceleration. Alternative kinematic analyses that involve examining variability in limb trajectories to infer visual control processes are evaluated. The basic assumption underlying these methods is that noise exists in the neuromotor system that subsequently leads to variability in motor output. This leads to systematic relations in limb trajectory variability at different stages of the movement that are altered when trajectories are modified during movement execution. Hence, by examining the variability in limb trajectories and correlations of kinematic variables throughout movement for vision and no vision conditions, the contribution of visual feedback in the planning and control of movement can be determined.

Is proprioception calibrated during visually guided movements

In the present study, a visuomotor adaptation paradigm was used to determine whether proprioception is calibrated during visually guided movements. A full vision group (FV) was given vision of a cursor representing hand position and was asked to aim towards visible targets. A no vision group (NV) performed the same task without vision of the cursor and was given knowledge of results (KR) after movement completion. A directional bias was introduced between the location of the cursor and the location of the hand in an adaptation phase, which resulted in a deviation to the right of the intended target. Of interest was whether participants would still show rightward deviations in a post-test series of trials in which vision was removed (aftereffects). The NV group presented strong aftereffects. However, the FV group only showed modest aftereffects early in the post-test, which rapidly decayed over the course of the post-test. Further analyses showed that the presence of those early aftereffects was due to an offline influence of vision on movement planning.

Direct Evidence for Cortical Suppression of Somatosensory Afferents during Visuomotor Adaptation

Upon exposure to novel visuomotor relationships, the information carried by visual and proprioceptive signals becomes discrepant, often disrupting motor execution. It has been shown that degradation of the proprioceptive sense (arising either from disease or experimental manipulation) enhances performance when drawing with mirror-reversed vision. Given that the central nervous system can exert a dynamic control over the transmission of afferent signals, reducing proprioceptive inflow to cortical areas could be part of the normal adaptive mechanisms deployed in healthy humans upon exposure to novel visuomotor environments. Here we address this issue by probing the transmission of somatosensory afferents throughout the course of adaptation to a visuomotor conflict, by recording median nerve somatosensory evoked potentials. We show that early exposure to tracing with mirror-reversed vision is accompanied by substantial proprioceptive suppression occurring in the primary somatosensory cortex (S1). This proprioceptive gating is gradually alleviated as performance increases with adaptation, returning to baseline levels. Peripheral and spinal evoked potentials were not modulated throughout, suggesting that the gating acted to reduce cortico-cortico excitability directly within S1. These modulations provide neurophysiological evidence for flexibility in sensory integration during visuomotor adaptation, which may functionally serve to reduce the sensory conflict until the visuo-proprioceptive mapping is updated.

Effector selection precedes reach planning in the dorsal parietofrontal cortex

Experimental evidence and computational modeling suggest that target selection for reaching is associated with the parallel encoding of multiple movement plans in the dorsomedial posterior parietal cortex (dmPPC) and the caudal part of the dorsal premotor cortex (PMdc). We tested the hypothesis that a similar mechanism also accounts for arm selection for unimanual reaching, with simultaneous and separate motor goal representations for the left and right arms existing in the right and left parietofrontal cortex, respectively. We recorded simultaneous electroencephalograms and functional MRI and studied a condition in which subjects had to select the appropriate arm for reaching based on the color of an appearing visuospatial target, contrasting it to a condition in which they had full knowledge of the arm to be used before target onset. We showed that irrespective of whether subjects had to select the arm or not, activity in dmPPC and PMdc was only observed contralateral to the reaching arm after target onset. Furthermore, the latency of activation in these regions was significantly delayed when arm selection had to be achieved during movement planning. Together, these results demonstrate that effector selection is not achieved through the simultaneous specification of motor goals tied to the two arms in bilateral parietofrontal cortex, but suggest that a motor goal is formed in these regions only after an arm is selected for action.

Sensorimotor adaptation in response to proprioceptive bias

Studies investigating visuo-motor adaptation typically introduce sensory conflicts by manipulating visual information (prisms, cursor gains). The purpose of the present study was to determine whether similar adaptation would be observed when a conflict is created through distortion of the proprioceptive sense, rather than through visual distortion. We used a coordinated movement task that required participants to release thumb and index finger at a specific elbow angle during passive elbow extension. Participants could not see their arm, but were shown a cursor representing the forearm on a video screen. In the proprioceptive group, a sensory conflict was introduced by vibrating the biceps brachii muscle, introducing a discrepancy of approximately 7.5° between the proprioceptively perceived and visually perceived elbow angle. In the visual group, a conflict of similar magnitude was obtained by introducing a gain of 7.5° to the cursor with respect to forearm position. Adaptation was assessed by the presence of plastic changes in release elbow angles following a period of exposure to the sensory conflict (i.e., aftereffects). Both groups showed high accuracy during exposure despite the sensory conflicts. More importantly, the visual group presented large and persistent aftereffects, while the proprioceptive group presented none. We suggest that the proprioceptive group’s lack of adaptation was due to the artificial muscle spindle activity resulting from vibration, which prevented visual and proprioceptive signals to be merged into a common frame of reference.

Determinants of Offline Processing of Visual Information for the Control of Reaching Movements

The authors investigated the use of visual feedback as a form of knowledge of results (KR) for the control of rapid (200-250 ms) reaching movements in 40 participants. They compared endpoint accuracy and intraindividual variability of a full-vision group (FV) with those of no-vision groups provided with KR regarding (a) the endpoint in numerical form, (b) the endpoint in visual form, or (c) the endpoint and the trajectory in visual form (DEL). The FV group was more accurate and less variable than were the no-vision groups, and the analysis of limb trajectory variability indicated that their superior performance resulted primarily from better movement planning rather than from online visual processes. The FV group outperformed the DEL group even though both groups were obtaining the same amount of spatial visual information from every movement. That finding suggests that the effectiveness with which visual feedback is processed offline is not a simple function of the amount of visual information available, but depends on how that information is presented.

Neural correlates for task-relevant facilitation of visual inputs during visually-guided hand movements

Vision is a powerful source of information for controlling movements, especially fine actions produced by the hand that require a great deal of accuracy. However, the neural processes that enable vision to enhance movement accuracy are not well understood. In the present study, we tested the hypothesis that the cortical sensitivity to visual inputs increases during a spatially-constrained hand movement compared to a situation where visual information is irrelevant to the task. Specifically, we compared the cortical visual-evoked potentials (VEPs) in response to flashes (right visual hemifield) recorded while participants followed the outline of an irregular polygon with a pen (i.e., tracing), with VEPs recorded when participants simply kept the pen still. This tracing task was chosen specifically because it requires many different visual processes (e.g., detection of line orientation, motion perception, visuomotor transformation) to be completed successfully. The tracing and resting tasks were performed with normal vision and also with mirror-reversed vision, thereby increasing task difficulty when tracing. We predicted that the sensitivity to visual inputs would be enhanced (i.e. greater VEPs) during tracing and that this increase in response sensitivity would be greater when tracing was performed with mirror-reversed vision. In addition, in order to investigate the existence of a link between the sensitivity to visual inputs and the accuracy with which participants traced the shape, we assigned participants to high performer (HP) or low performer (LP) groups according to their tracing performance in the condition with mirror-reversed visual feedback. Source analyses revealed that, for both groups, the sensitivity to visual inputs of the left occipital and MT/MST regions increased when participants traced the shape as compared to when they were resting. Also, for both groups of participants, the mirror-reversed vision did not affect the amplitude of the cortical response to visual inputs but increased the latencies of the responses in the occipital, temporal, and parietal regions. However, the HP group showed cortical responses that largely differed from those displayed by the LP group. Specifically, the HP group demonstrated movement-related increases of visual sensitivity in regions of the visual cortex that were not observed in the LP group. These increased responses to visual inputs were evidenced in the posterior inferior parietal, temporal-occipital, and inferior-temporal regions. Overall, our results are in line with the assertion that increasing the sensitivity to visual inputs serves to promote relevant visual information for the different processes involved during visually-guided hand movements. Our results also suggest that maintaining accurate hand tracing movements in the presence of discrepant visual and somatosensory feedback requires additional perceptual and spatial information processing that is tightly linked to visual inputs.

On the Link between Sensorimotor Adaptation and Sensory Recalibration

Our environment is constantly changing, as is the state of our own musculoskeletal system. This imposes a permanent adaptive pressure on the sensorimotor system, which must be updated across development and aging to allow smooth interactions with the world. Consider the case of having to reach an

Attenuation of visual reafferent signals in the parietal cortex during voluntary movement

It is well established that the cortical processing of somatosensory and auditory signals is attenuated when they result from self-generated actions compared with external events. This phenomenon is thought to result from an efference copy of motor commands used to predict the sensory consequences of an action through a forward model. The present work examined whether attenuation also takes place for visual reafferent signals from the moving limb during voluntary reaching movements. To address this issue, EEG activity was recorded in a condition in which visual feedback of the hand was provided in real time and compared with a condition in which it was presented with a 150-ms delay, thus creating a mismatch between the predicted and actual visual consequences of the movement. Results revealed that the amplitude of the N1 component of the visual event-related potential evoked by hand visual feedback over the parietal cortex was significantly smaller when presented in real time compared with when it was delayed. These data suggest that the cortical processing of visual reafferent signals is attenuated when they are correctly predicted, likely as a result of a forward model.