Online control of discrete action following visual perturbation

Awareness of Visual Offset Reduces but Does Not Eliminate Joint Repositioning Errors in Virtual Reality

The present study investigated the effect of visual offset (visuo-proprioceptive mismatch) in joint repositioning task in a three-dimensional virtual reality (VR) environment when participants were instructed to ignore vision. Twenty-five physically healthy young individuals performed shoulder joint position sense test. Repositioning accuracy was tested under two visual conditions, accurate and offset visions, and two instructions, no guidance or ignore vision. In accurate vision trials, the virtual hand of the tested limb seen in VR was congruent with where the participant placed their hand. In the offset vision condition, the virtual hand was seen 8° above or below their actual hand in the vertical plane. Repositioning error (i.e. constant error) in offset vision trials was lower when the participants were instructed to ignore vision compared to when no instruction about the visual offset was given (p < 0.001). However, constant error in offset vision trials was larger than accurate vision trials when the participants tried to ignore vision in both visual conditions (p < 0.001). Our results suggest that humans may be able to down-weight vision to some extent by conscious effort, while the influence of vision is difficult to eliminate when vision is present.

The impact of subclinical neck pain on goal-directed upper limb movement in the horizontal plane

Subclinical neck pain (SCNP) refers to recurrent neck pain and/or stiffness for which individuals have not yet sought treatment. Prior studies have shown that individuals with SCNP have altered cerebellar processing that exhibits an altered body schema. The cerebellum also plays a vital role in upper limb reaching movements through refining internal models and integrating sensorimotor information. However, the impact of SCNP on these processes has yet to be examined in the context of a rapid goal-directed aiming response that relies on feedforward and feedback processes to guide the limb to the target. To address this, SCNP and control participants performed goal-directed upper limb movements with the dominant and non-dominant hands using light and heavy styli in the horizontal plane. The results show greater peak accelerations in SCNP participants using the heavy stylus. However, there were no other group differences seen, possibly due to the fact that reaching behavior predominantly relies on vision such that any proprioceptive deficits seen in those with SCNP can be compensated. This study illustrates the robust compensatory nature of the CNS when performing end-effector reaching tasks, suggesting studies altering visual feedback may be needed to see the full impact of SCNP on upper limb aiming.

Visuomotor Behaviour in Amblyopia: Deficits and Compensatory Adaptations

Amblyopia is a neurodevelopmental visual disorder arising from decorrelated binocular experience during the critical periods of development. The hallmark of amblyopia is reduced visual acuity and impairment in binocular vision. The consequences of amblyopia on various sensory and perceptual functions have been studied extensively over the past 50 years. Historically, relatively fewer studies examined the impact of amblyopia on visuomotor behaviours; however, research in this area has flourished over the past 10 years. Therefore, the aim of this review paper is to provide a comprehensive review of current knowledge about the effects of amblyopia on eye movements, upper limb reaching and grasping movements, as well as balance and gait. Accumulating evidence indicates that amblyopia is associated with considerable deficits in visuomotor behaviour during amblyopic eye viewing, as well as adaptations in behaviour during binocular and fellow eye viewing in adults and children. Importantly, due to amblyopia heterogeneity, visuomotor development in children and motor skill performance in adults may be significantly influenced by the etiology and clinical features, such as visual acuity and stereoacuity. Studies with larger cohorts of children and adults are needed to disentangle the unique contribution of these clinical characteristics to the development and performance of visuomotor behaviours.

Evaluation of the Leap Motion Controller during the performance of visually-guided upper limb movements

Kinematic analysis of upper limb reaching provides insight into the central nervous system control of movements. Until recently, kinematic examination of motor control has been limited to studies conducted in traditional research laboratories because motion capture equipment used for data collection is not easily portable and expensive. A recently developed markerless system, the Leap Motion Controller (LMC), is a portable and inexpensive tracking device that allows recording of 3D hand and finger position. The main goal of this study was to assess the concurrent reliability and validity of the LMC as compared to the Optotrak, a criterion-standard motion capture system, for measures of temporal accuracy and peak velocity during the performance of upper limb, visually-guided movements. In experiment 1, 14 participants executed aiming movements to visual targets presented on a computer monitor. Bland-Altman analysis was conducted to assess the validity and limits of agreement for measures of temporal accuracy (movement time, duration of deceleration interval), peak velocity, and spatial accuracy (endpoint accuracy). In addition, a one-sample t-test was used to test the hypothesis that the error difference between measures obtained from Optotrak and LMC is zero. In experiment 2, 15 participants performed a Fitts’ type aiming task in order to assess whether the LMC is capable of assessing a well-known speed-accuracy trade-off relationship. Experiment 3 assessed the temporal coordination pattern during the performance of a sequence consisting of a reaching, grasping, and placement task in 15 participants. Results from the t-test showed that the error difference in temporal measures was significantly different from zero. Based on the results from the 3 experiments, the average temporal error in movement time was 40±44 ms, and the error in peak velocity was 0.024±0.103 m/s. The limits of agreement between the LMC and Optotrak for spatial accuracy measures ranged between 2–5 cm. Although the LMC system is a low-cost, highly portable system, which could facilitate collection of kinematic data outside of the traditional laboratory settings, the temporal and spatial errors may limit the use of the device in some settings.

THE SEX DIFFERENCE IN ROD BALANCING: CONFIRMATION OF THE DIFFERENCE AND A TEST OF THREE HYPOTHETICAL EXPLANATIONS

Previous studies have shown that men can balance a dowel rod on the index finger for a longer time than women can. The factors that account for the difference are unknown, but the difference may be attributable either to a difference in whole-body agility or a difference in the use of visual cues. Three experiments involving a total of 62 adult women with a mean age of 21.2 yr. (SD=3.8) and 62 adult men with a mean age of 21.9 yr. (SD=6.6) tested these potential explanations. Experiment 1 replicated the sex difference and assessed the relevance of whole-body agility by comparing standing and seated conditions. Experiments 2 and 3 explored the role of rod length and visual fixation point, respectively. Each experiment yielded a significant sex difference, but the difference was not affected by the participant's posture, the length of the rod, or the fixation point. Possible alternative explanations for the difference include differences in (1) the speed of processing degree of visual tilt; (2) arm mass, which affects the inertia of the balancing system; and (3) experience in open-skill sports.

Tailoring reach-to-grasp to intended action: the role of motor practice

Motor learning results from repeated exposure to the same movement and allows a mover to increase movement optimality. Typically, this has only been considered in single-step movements. In sequential movements, an initial reach movement is tailored to the demands of the onward movement. However, the exact role of motor practice in the tailoring to onward task demands is unknown. Eighteen adults performed blocks of 15 movements; each movement consisted of a reach phase and an onward phase (the object was placed in a tight-fitting hole, placed in a loose-fitting hole or thrown). Simple practice effects were seen; for the reach phase, the amount of time spent decelerating decreased over trials, and for the onward phase, the accuracy of the place/throw movements increased over trials. Furthermore, approximately 30 % of variance in the practice effect of the onward phase could be explained by the practice effect in the reach phase. Therefore, we suggest that the changes in the reach phase are directly linked to the changes in the efficiency of action and that this is necessary but not sufficient for explaining the calibration of the onward action.

125 years of perceptual-motor skill research

This article celebrates the contribution that the American Journal of Psychology (AJP) has made to the area of perceptual-motor skill over its 125-year history. We highlight the articles published in AJP and trace the technical and theoretical developments that stem from this groundbreaking work. Included in our overview are AJP articles on the excitability of the motor system, motor learning, adaptation to visual rearrangement, the ecological approach to perception and action, and the measurement of human handedness. We conclude by identifying a number of areas associated with perceptual-motor skill where AJP continues to make an important contribution.

Adaptation to mirror-reversed vision is based on directionally tuned modules

Sensorimotor adaptation to rotated visual feedback is thought to be achieved by directionally tuned modules. Here we scrutinize whether adaptation to reversed vision utilizes similar mechanisms. Specifically, we hypothesize that adaptive transfer to unpracticed target directions is determined by the superposition of neighboring modules. One group of subjects adapted to a left-right reversal of visual feedback, which requires a 180°, ±90°, or no change of response direction, depending on target position. Two groups of control subjects adapted to a 180° and to a 90° rotation of visual feedback. We found that adaptation to a left-right reversal is less efficient than adaptation to rotations requiring the same adaptive change, and attribute this decrement to an overlap of neighboring modules. We further found that transfer to unpracticed targets is well predicted by a simple Gaussian model. From this we conclude that adaptation to a left-right reversal emerges in a regional and interdependent fashion, and can be modeled as overlapping Gaussian tuned processes.

Visual regulation of manual aiming: a comparison of methods

Visual regulation of upper limb movements occurs throughout the trajectory and is not confined to discrete control in the target area. Early control is based on the dynamic relationship between the limb, the target, and the environment. Despite robust outcome differences between protocols involving visual manipulations, it remains difficult to identify the kinematic events that characterize these differences. In this study, participants performed manual aiming movements with and without vision. We compared several traditional approaches to movement analysis with two new methods of quantifying online limb regulation. As expected, participants undershot the target and their movement endpoints were more variable when vision was not available. Although traditional measures such as reaction time, time after peak velocity, and the presence of discontinuities in acceleration were sensitive to the visual manipulation, measures quantifying the trial-to-trial spatial variability throughout the trajectory were the most effective in isolating the time course of online regulation.

Effects of anisometropic amblyopia on visuomotor behavior, part 2: visually guided reaching

PURPOSE

The effects of impaired spatiotemporal vision in amblyopia on visuomotor skills have rarely been explored in detail. The goal of this study was to examine the influences of amblyopia on visually guided reaching.

METHODS

Fourteen patients with anisometropic amblyopia and 14 control subjects were recruited. Participants executed reach-to-touch movements toward targets presented randomly 5° or 10° to the left or right of central fixation in three viewing conditions: binocular, monocular amblyopic eye, and monocular fellow eye viewing (left and right monocular viewing for control subjects). Visual feedback of the target was removed on 50% of the trials at the initiation of reaching.

RESULTS

Reaching accuracy was comparable between patients and control subjects during all three viewing conditions. Patients' reaching responses were slightly less precise during amblyopic eye viewing, but their precision was normal during binocular or fellow eye viewing. Reaching reaction time was not affected by amblyopia. The duration of the acceleration phase was longer in patients than in control subjects under all viewing conditions, whereas the duration of the deceleration phase was unaffected. Peak acceleration and peak velocity were also reduced in patients.

CONCLUSIONS

Amblyopia affects both the programming and the execution of visually guided reaching. The increased duration of the acceleration phase, as well as the reduced peak acceleration and peak velocity, might reflect a strategy or adaptation of feedforward/feedback control of the visuomotor system to compensate for degraded spatiotemporal vision in amblyopia, allowing patients to optimize their reaching performance.

Optimising speed and energy expenditure in accurate visually directed upper limb movements

Traditional models of speed-accuracy relations and limb control are steady-state models that fail to consider the learning history and strategic approach of the performer. Work from this laboratory indicates that a performer adjusts his/her behaviour from trial-to-trial to optimise not only the speed and accuracy of performance, but also energy expenditure. Because some errors have greater temporal and energy costs than others, most performers execute movements that are prepared such that potential errors are of minimal expense. The trajectories and subsequent endpoint distributions of rapid aiming movements depend on advance knowledge about the availability of afferent information for online control, as well as the costs associated with undershooting or overshooting the target position with the initial impulse. With practice, a performer is able to reduce the trial-to-trial variability associated with goal-directed movement through more consistent movement planning processes and more rapid online control. Part of the optimisation process is related to the development of an internal model of performance against which early afferent feedback can be evaluated. This framework for examining speed, accuracy and energy expenditure in goal-directed reaching can be used to help understand the breakdown of efficient limb control due to fatigue, ageing and pathology.

The number of discernible colors in natural scenes

The number of colors discernible by normal trichromats has been estimated for the idealized object-color solid. How well these estimates apply to natural scenes is an open question, as it is unknown how much their colors approach the theoretical limits. The aim of this work was to estimate the number of discernible colors based on a database of hyperspectral images of 50 natural scenes. The color volume of each scene was computed in the CIELAB color space and was analyzed using the CIEDE2000 color-difference formula. It was found that the color volume of the set of natural scenes was about 30% of the theoretical maximum for the full object-color solid, and it corresponded to a number of about 2.3 million discernible colors. Moreover, when the lightness dimension was ignored, only about 26,000 (1%) could be perceived as different colors. These results suggest that natural stimuli may be more constrained than expected from the analysis of the theoretical limits.

Using 1/f noise to examine planning and control in a discrete aiming task

The present study used 1/f noise to examine how spatial, physical, and timing constraints affect planning and control processes in aiming. Participants moved objects of different masses to different distances at preferred speed (Experiment 1) and as quickly as possible (Experiment 2). Power spectral density, standardized dispersion, rescaled range, and an autoregressive fractionally integrated moving average (ARFIMA) model selection procedure were used to quantify 1/f noise. Measures from all four analyses were in reasonable agreement, with more ARFIMA (long-range) models selected at peak velocity in Experiment 1 and fewer selected at peak velocity in Experiment 2. There also was a nonsignificant trend where, at preferred speed, of those participants who showed 1/f noise, more tended to show 1/f noise at peak velocity, when planning and control would overlap most. This trend disappeared for fast movements, where planning and control would have less time to overlap. Summing short-range processes at different timescales can produce 1/f-like noise. As planning is a slower-moving process and control faster, present results suggest that, with enough time for both planning and control, 1/f noise in aiming may arise from a similar summation of processes. Potential limitations of time series length in the present task are discussed.