Binocular and monocular depth cues in online feedback control of 3D pointing movement

Superior performance by two new methods in identifying the online reaction time of reaching movements

Reaching movements can be redirected during their progress to handle unexpected visual changes, such as a change in target location. It is important to know when these redirections start, i.e., the online reaction time (oRT), but this information is not readily evident since redirections are embedded within a time-varying baseline movement that differs from trial to trial. The one previous study that evaluated the performance of different oRT identification methods utilized simulated redirections with the exact same onset, rather than a range of onsets as would be typically encountered. We addressed this gap by utilizing batches of "hybrid" trials with temporal spread in their oRTs. Each hybrid trial combined a sampled baseline movement with an idealized corrective response. Two new methods had the most accurate identification of online reaction times: 1) a threshold-aligned grand mean regression, and 2) a template-based approach we term the canonical correction search. The threshold-aligned grand mean regression is simple to implement and effective. The canonical correction search is a more complex procedure but arguably better linked to the underlying response. Applying the two methods to a published dataset revealed more delayed oRTs than was previously reported along with new information such as the width of oRT distributions. Taken together, our results demonstrate the utility of two new methods for dissecting corrective action from ongoing movement.NEW & NOTEWORTHY Advancing our understanding of visual feedback control requires methods that accurately identify the onset of corrective action. We developed a modified regression approach and a template-based approach to identify the online reaction time of single-reaching movements. Both outperform previous methods when challenged by temporal jitter in the response onset and increased background noise.

Characteristic of Motor Control in Three-Dimensional Circular Tracking Movements during Monocular Vision

Analysis of visually guided tracking movements is an important component of understanding human visuomotor control system. The aim of our study was to investigate the effects of different target speeds and different circular tracking planes, which provide different visual feedback of depth information, on temporal and spatial tracking accuracy. In this study, we analyze motor control characteristic of circular tracking movements during monocular vision in three-dimensional space using a virtual reality system. Three parameters in polar coordinates were analyzed: ΔR, the difference in the distance from the fixed pole; Δθ, the difference in the position angle; and Δω, the difference in the angular velocity. We compare the accuracy of visually guided circular tracking movements during monocular vision in two conditions: (1) movement in the frontal plane relative to the subject that requires less depth information and (2) movement in the sagittal plane relative to the subject that requires more depth information. We also examine differences in motor control at four different target speeds. The results show that depth information affects both spatial and temporal accuracy of circular tracking movement, whereas target speed only affects temporal accuracy of circular tracking movement. This suggests that different strategies of feedforward and feedback controls are performed in the tracking of movements.

No Vertical Visual Field Asymmetry in Online Control: Evidence from Reaching in Depth

We sought to determine whether a putative lower-visual field (loVF) advantage for projections to the visuomotor networks of the dorsal visual pathway influences online reaching control. Participants reached to 3-dimensional depth targets presented in the loVF and upper-visual field (upVF) in binocular and monocular visual conditions, and when online vision was available (i.e., closed-loop) or unavailable (i.e., open-loop). To examine the degree to which responses were controlled online we computed the proportion of variance (R²) explained by the spatial position of the limb at distinct stages in the reaching trajectory relative to a response's ultimate movement endpoint. Results showed that binocular and closed-loop reaches exhibited shorter movement times and more online corrections (i.e., smaller R² values) than their monocular and open-loop counterparts. Notably, however, loVF and upper-visual field reaches exhibited equivalent performance metrics across all experimental conditions. Accordingly, results provide no evidence of a loVF advantage for online reaching control to 3-dimensional targets.

The role of somatosensory input in target localization during binocular and monocular viewing while performing a high precision reaching and placement task

Binocular vision provides the most accurate and precise depth information; however, many people have impairments in binocular visual function. It is possible that other sensory inputs could be used to obtain reliable depth information when binocular vision is not available. However, it is currently unknown whether depth information from another modality improves target localization in depth during action execution. Therefore, the goal of this study was to assess whether somatosensory input improves target localization during the performance of a precision placement task. Visually normal young adults (n = 15) performed a bead threading task during binocular and monocular viewing in two experimental conditions where needle location was specified by 1) vision only, or 2) vision and somatosensory input, which was provided by the non-dominant limb. Performance on the task was assessed using spatial and temporal kinematic measures. In accordance with the hypothesis, results showed that the interval spent placing the bead on the needle was significantly shorter during monocular viewing when somatosensory input was available in comparison to a vision only condition. In contrast, results showed no evidence to support that somatosensory input about the needle location affects trajectory control. These findings demonstrate that the central nervous system relies predominately on visual input during reach execution, however, somatosensory input can be used to facilitate the performance of the precision placement task.

Grasping in absence of feedback: systematic biases endure extensive training

Reach-to-grasp movements performed without visual and haptic feedback of the hand are subject to systematic inaccuracies. Grasps directed at an object specified by binocular information usually end at the wrong distance with an incorrect final grip aperture. More specifically, moving the target object away from the observer leads to increasingly larger undershoots and smaller grip apertures. These systematic biases suggest that the visuomotor mapping is based on inaccurate estimates of an object's egocentric distance and 3D structure that compress the visual space. Here we ask whether the appropriate visuomotor mapping can be learned through an extensive exposure to trials where haptic and visual feedback of the hand is provided. By intermixing feedback trials with test trials without feedback, we aimed at maximizing the likelihood that the motor execution of test trials is positively influenced by that of preceding feedback trials. We found that the intermittent presence of feedback trials both (1) largely reduced the positioning error of the hand with respect to the object and (2) affected the shaping of the hand before the final grasp, leading to an overall more accurate performance. While this demonstrates an effective transfer of information from feedback trials to test trials, the remaining biases indicate that a compression of visual space is still taking place. The correct visuomotor mapping, therefore, could not be learned. We speculate that an accurate reconstruction of the scene at movement onset may not actually be needed. Instead, the online monitoring of the hand position relative to the object and the final contact with the object are sufficient for a successful execution of a grasp.

Stereopsis and amblyopia: A mini-review

Amblyopia is a neuro-developmental disorder of the visual cortex that arises from abnormal visual experience early in life. Amblyopia is clinically important because it is a major cause of vision loss in infants and young children. Amblyopia is also of basic interest because it reflects the neural impairment that occurs when normal visual development is disrupted. Amblyopia provides an ideal model for understanding when and how brain plasticity may be harnessed for recovery of function. Over the past two decades there has been a rekindling of interest in developing more effective methods for treating amblyopia, and for extending the treatment beyond the critical period, as exemplified by new clinical trials and new basic research studies. The focus of this review is on stereopsis and its potential for recovery. Impaired stereoscopic depth perception is the most common deficit associated with amblyopia under ordinary (binocular) viewing conditions (Webber & Wood, 2005). Our review of the extant literature suggests that this impairment may have a substantial impact on visuomotor tasks, difficulties in playing sports in children and locomoting safely in older adults. Furthermore, impaired stereopsis may also limit career options for amblyopes. Finally, stereopsis is more impacted in strabismic than in anisometropic amblyopia. Our review of the various approaches to treating amblyopia (patching, perceptual learning, videogames) suggests that there are several promising new approaches to recovering stereopsis in both anisometropic and strabismic amblyopes. However, recovery of stereoacuity may require more active treatment in strabismic than in anisometropic amblyopia. Individuals with strabismic amblyopia have a very low probability of improvement with monocular training; however they fare better with dichoptic training than with monocular training, and even better with direct stereo training.

Binocular advantage for prehension movements performed in visually enriched environments requiring visual search

The purpose of this study was to examine the role of binocular vision during a prehension task performed in a visually enriched environment where the target object was surrounded by distractors/obstacles. Fifteen adults reached and grasped for a cylindrical peg while eye movements and upper limb kinematics were recorded. The complexity of the visual environment was manipulated by varying the number of distractors and by varying the saliency of the target. Gaze behavior (i.e., the latency of the primary gaze shift and frequency of gaze shifts prior to reach initiation) was comparable between viewing conditions. In contrast, a binocular advantage was evident in performance accuracy. Specifically, participants picked up the wrong object twice as often during monocular viewing when the complexity of the environment increased. Reach performance was more efficient during binocular viewing, which was demonstrated by shorter reach reaction time and overall movement time. Reaching movements during the approach phase had higher peak velocity during binocular viewing. During monocular viewing reach trajectories exhibited a direction bias during the acceleration phase, which was leftward during left eye viewing and rightward during right eye viewing. This bias can be explained by the presence of esophoria in the covered eye. The grasping interval was also extended by ~20% during monocular viewing; however, the duration of the return phase after the target was picked up was comparable across viewing conditions. In conclusion, binocular vision provides important input for planning and execution of prehension movements in visually enriched environments. Binocular advantage was evident, regardless of set size or target saliency, indicating that adults plan their movements more cautiously during monocular viewing, even in relatively simple environments with a highly salient target. Nevertheless, in visually-normal adults monocular input provides sufficient information to engage in online control to correct the initial errors in movement planning.

The effect of sensory uncertainty due to amblyopia (lazy eye) on the planning and execution of visually-guided 3D reaching movements

BACKGROUND

Impairment of spatiotemporal visual processing in amblyopia has been studied extensively, but its effects on visuomotor tasks have rarely been examined. Here, we investigate how visual deficits in amblyopia affect motor planning and online control of visually-guided, unconstrained reaching movements.

METHODS

Thirteen patients with mild amblyopia, 13 with severe amblyopia and 13 visually-normal participants were recruited. Participants reached and touched a visual target during binocular and monocular viewing. Motor planning was assessed by examining spatial variability of the trajectory at 50-100 ms after movement onset. Online control was assessed by examining the endpoint variability and by calculating the coefficient of determination (R(2)) which correlates the spatial position of the limb during the movement to endpoint position.

RESULTS

Patients with amblyopia had reduced precision of the motor plan in all viewing conditions as evidenced by increased variability of the reach early in the trajectory. Endpoint precision was comparable between patients with mild amblyopia and control participants. Patients with severe amblyopia had reduced endpoint precision along azimuth and elevation during amblyopic eye viewing only, and along the depth axis in all viewing conditions. In addition, they had significantly higher R(2) values at 70% of movement time along the elevation and depth axes during amblyopic eye viewing.

CONCLUSION

Sensory uncertainty due to amblyopia leads to reduced precision of the motor plan. The ability to implement online corrections depends on the severity of the visual deficit, viewing condition, and the axis of the reaching movement. Patients with mild amblyopia used online control effectively to compensate for the reduced precision of the motor plan. In contrast, patients with severe amblyopia were not able to use online control as effectively to amend the limb trajectory especially along the depth axis, which could be due to their abnormal stereopsis.