Fusion of visuo-ocular and vestibular signals in arm motor control

Prediction in the Vestibular Control of Arm Movements

The contribution of vestibular signals to motor control has been evidenced in postural, locomotor, and oculomotor studies. Here, we review studies showing that vestibular information also contributes to the control of arm movements during whole-body motion. The data reviewed suggest that vestibular information is used by the arm motor system to maintain the initial hand position or the planned hand trajectory unaltered during body motion. This requires integration of vestibular and cervical inputs to determine the trunk motion dynamics. These studies further suggest that the vestibular control of arm movement relies on rapid and efficient vestibulomotor transformations that cannot be considered automatic. We also reviewed evidence suggesting that the vestibular afferents can be used by the brain to predict and counteract body-rotation-induced torques (e.g., Coriolis) acting on the arm when reaching for a target while turning the trunk.

Exploratory movement generates higher-order information that is sufficient for accurate perception of scaled egocentric distance

Body movement influences the structure of multiple forms of ambient energy, including optics and gravito-inertial force. Some researchers have argued that egocentric distance is derived from inferential integration of visual and non-visual stimulation. We suggest that accurate information about egocentric distance exists in perceptual stimulation as higher-order patterns that extend across optics and inertia. We formalize a pattern that specifies the egocentric distance of a stationary object across higher-order relations between optics and inertia. This higher-order parameter is created by self-generated movement of the perceiver in inertial space relative to the illuminated environment. For this reason, we placed minimal restrictions on the exploratory movements of our participants. We asked whether humans can detect and use the information available in this higher-order pattern. Participants judged whether a virtual object was within reach. We manipulated relations between body movement and the ambient structure of optics and inertia. Judgments were precise and accurate when the higher-order optical-inertial parameter was available. When only optic flow was available, judgments were poor. Our results reveal that participants perceived egocentric distance from the higher-order, optical-inertial consequences of their own exploratory activity. Analysis of participants’ movement trajectories revealed that self-selected movements were complex, and tended to optimize availability of the optical-inertial pattern that specifies egocentric distance. We argue that accurate information about egocentric distance exists in higher-order patterns of ambient energy, that self-generated movement can generate these higher-order patterns, and that these patterns can be detected and used to support perception of egocentric distance that is precise and accurate.

Biases in the perception of self-motion during whole-body acceleration and deceleration

Several studies have investigated whether vestibular signals can be processed to determine the magnitude of passive body motions. Many of them required subjects to report their perceived displacements offline, i.e., after being submitted to passive displacements. Here, we used a protocol that allowed us to complement these results by asking subjects to report their introspective estimation of their displacement continuously, i.e., during the ongoing body rotation. To this end, participants rotated the handle of a manipulandum around a vertical axis to indicate their perceived change of angular position in space at the same time as they were passively rotated in the dark. The rotation acceleration (Acc) and deceleration (Dec) lasted either 1.5 s (peak of 60°/s², referred to as being “High”) or 3 s (peak of 33°/s², referred to as being “Low”). The participants were rotated either counter-clockwise or clockwise, and all combinations of acceleration and deceleration were tested (i.e., AccLow-DecLow; AccLow-DecHigh; AccHigh-DecLow; AccHigh-DecHigh). The participants’ perception of body rotation was assessed by computing the gain, i.e., ratio between the amplitude of the perceived rotations (as measured by the rotating manipulandum’s handle) and the amplitude of the actual chair rotations. The gain was measured at the end of the rotations, and was also computed separately for the acceleration and deceleration phases. Three salient findings resulted from this experiment: (i) the gain was much greater during body acceleration than during body deceleration, (ii) the gain was greater during High compared to Low accelerations and (iii) the gain measured during the deceleration was influenced by the preceding acceleration (i.e., Low or High). These different effects of the angular stimuli on the perception of body motion can be interpreted in relation to the consequences of body acceleration and deceleration on the vestibular system and on higher-order cognitive processes.

Visuo-vestibular interaction: predicting the position of a visual target during passive body rotation

Following body rotation, optimal updating of the position of a memorized target is attained when retinal error is perceived and corrective saccade is performed. Thus, it appears that these processes may enable the calibration of the vestibular system by facilitating the sharing of information between both reference frames. Here, it is assessed whether having sensory information regarding body rotation in the target reference frame could enhance an individual's learning rate to predict the position of an earth-fixed target. During rotation, participants had to respond when they felt their body midline had crossed the position of the target and received knowledge of result. During practice blocks, for two groups, visual cues were displayed in the same reference frame of the target, whereas a third group relied on vestibular information (vestibular-only group) to predict the location of the target. Participants, unaware of the role of the visual cues (visual cues group), learned to predict the location of the target and spatial error decreased from 16.2 to 2.0°, reflecting a learning rate of 34.08 trials (determined from fitting a falling exponential model). In contrast, the group aware of the role of the visual cues (explicit visual cues group) showed a faster learning rate (i.e. 2.66 trials) but similar final spatial error 2.9°. For the vestibular-only group, similar accuracy was achieved (final spatial error of 2.3°), but their learning rate was much slower (i.e. 43.29 trials). Transferring to the Post-test (no visual cues and no knowledge of result) increased the spatial error of the explicit visual cues group (9.5°), but it did not change the performance of the vestibular group (1.2°). Overall, these results imply that cognition assists the brain in processing the sensory information within the target reference frame.

Influence of Feedback Modality on Sensorimotor Adaptation: Contribution of Visual, Kinesthetic, and Verbal Cues

In 4 studies, the authors tested the contributions of visual, kinesthetic, and verbal knowledge of results to the adaptive control of reaching movements toward visual targets. The same apparatus was used in all experiments, but the procedures differed in the sensory modality of the feedback that participants (N s = 5, 5, 6, and 6, respectively, in Experiments 1, 2, 3, and 4) received about their performances. Using biased visual, proprioceptive, or verbal feedback, the authors introduced a 5 degrees shift in the visuomanual relationship. Results showed no significant difference in the final amount of adaptation to the mismatch: On average, participants adapted to 79% of the perturbation. That finding is consistent with the view that adaptation is a multisensory, highly flexible process whose efficiency does not depend on the sensory channel conveying the error signal.

Vestibular system may provide equivalent motor actions regardless of the number of body segments involved in the task

The vestibulospinal system likely plays an essential role in motor equivalence--the ability to reach the desired motor goal despite intentional or imposed changes in the number of body segments involved in the task. To test this hypothesis, we compared the ability of healthy subjects and patients with unilateral vestibular lesions (surgical acoustic neuroma resection 0.6 to 6.7 yr before the study) to maintain either the same hand position or the same trajectory of within arm reach movements while flexing the trunk, in the absence of vision. In randomly selected trials, the trunk motion was prevented by an electromagnetic device. Healthy subjects were able to preserve the hand position or trajectory by modifying the elbow and shoulder joint rotations in a condition-dependent way, at a minimal latency of about 60 ms after the trunk movement onset. In contrast, six of seven patients showed deficits in the compensatory angular modifications at least in one of two tasks so that 30-100% of the trunk displacement was not compensated and thus influenced the hand position or trajectory. Results suggest that vestibular influences evoked by the head motion during trunk flexion play a major role in maintaining the consistency of arm motor actions in external space despite changes in the number of body segments involved. Our findings also suggest that despite long-term plasticity in the vestibular system and related neural structures, unilateral vestibular lesion may reduce the capacity of the nervous system to achieve motor equivalence.

Coordination between postural and movement controls: effect of changes in body mass distribution on postural and focal component characteristics

Whole-body reaching movements are accomplished through a combination of anticipatory postural adjustments (APAs) and focal movements. Two different modes of central organization is usually proposed for this coordination: first, a single-process control, where the APAs and the focal movements would share a common command; second, where the APAs and the focal movements would be independently controlled through parallel commands (dual-process control). Here, we investigated which one of these modes of control could better explain the coordination between the trunk and the upper limb for standing subjects reaching for a target located beyond arm's length. This was done evaluating the effect of changing the APAs settings on the arm movement. The APAs modification was achieved by shifting the subject's centre of mass prior to the focal movement onset; this was done by adding an asymmetric load on either side of the head (a control condition with the load fixed centrally at the top of the head was also performed). As it changed the body mass distribution, the muscular torques and the orientation of the head inertia tensor, it is assumed that the addition of the asymmetric load led to a change in the APAs. Analyses indeed showed that both the initial head and trunk displacement towards the supporting side (during the unloading of the moving leg) were smaller when the load was fixed on the side of the supporting leg than when it was fixed on the side of the moving leg. However, changing the initial conditions, and therefore the APAs settings, had no significant effect on the path and kinematics of the focal hand movement. Therefore, subjects cancelled out the effect of the trunk motion on the hand-in-space motion through compensatory arm movements. These results support the dual-process control hypothesis for the postural and the focal components.