The multiple process model of goal-directed aiming/reaching: insights on limb control from various special populations.

Several years ago, our research group forwarded a model of goal-directed reaching and aiming that describes the processes involved in the optimization of speed, accuracy, and energy expenditure Elliott et al. (Psychol Bull 136:1023-1044, 2010). One of the main features of the model is the distinction between early impulse control, which is based on a comparison of expected to perceived sensory consequences, and late limb-target control that involves a spatial comparison of limb and target position. Our model also emphasizes the importance of strategic behaviors that limit the opportunity for worst-case or inefficient outcomes. In the 2010 paper, we included a section on how our model can be used to understand atypical aiming/reaching movements in a number of special populations. In light of a recent empirical and theoretical update of our model Elliott et al. (Neurosci Biobehav Rev 72:95-110, 2017), here we consider contemporary motor control work involving typical aging, Down syndrome, autism spectrum disorder, and tetraplegia with tendon-transfer surgery. We outline how atypical limb control can be viewed within the context of the multiple-process model of goal-directed reaching and aiming, and discuss the underlying perceptual-motor impairment that results in the adaptive solution developed by the specific group.

An optimal velocity for online limb-target regulation processes?

The utilization of visual information for the control of ongoing voluntary limb movements has been investigated for more than a century. Recently, online sensorimotor processes for the control of upper-limb reaches were hypothesized to include a distinct process related to the comparison of limb and target positions (i.e., limb-target regulation processes: Elliott et al. in Psychol Bull 136:1023-1044. doi: 10.1037/a0020958 , 2010). In the current study, this hypothesis was tested by presenting participants with brief windows of vision (20 ms) when the real-time velocity of the reaching limb rose above selected velocity criteria. One experiment tested the perceptual judgments of endpoint bias (i.e., under- vs. over-shoot), and another experiment tested the shifts in endpoint distributions following an imperceptible target jump. Both experiments revealed that limb-target regulation processes take place at an optimal velocity or "sweet spot" between movement onset and peak limb velocity (i.e., 1.0 m/s with the employed movement amplitude and duration). In contrast with pseudo-continuous models of online control (e.g., Elliott et al. in Hum Mov Sci 10:393-418. doi: 10.1016/0167-9457(91)90013-N , 1991), humans likely optimize online limb-target regulation processes by gathering visual information at a rather limited period of time, well in advance of peak limb velocity.

Effects of exogenous testosterone and mating context on men's preferences for female facial femininity.

Correlational research suggests that men show greater attraction to feminine female faces when their testosterone (T) levels are high. Men's preferences for feminine faces also seem to vary as a function of relationship context (short versus long-term). However, the relationship between T and preferences for female facial femininity has yet to be tested experimentally. In the current paper, we report the results of two experiments examining the causal role of T in modulating preferences for facial femininity across both short and long-term mating contexts. Results of Experiment 1 (within-subject design, n=24) showed that participants significantly preferred feminized versus masculinized versions of women's faces. Further, participants showed a stronger preference for feminine faces in the short versus the long-term context after they received T, but not after they received placebo. Post-hoc analyses suggested that this effect was driven by a lower preference for feminine faces in the long-term context when on T relative to placebo, and this effect was found exclusively for men who received placebo on the first day of testing, and T on the second day of testing (i.e., Order x Drug x Mating context interaction). In Experiment 2 (between-subject design, n=93), men demonstrated a significant preference for feminized female faces in the short versus the long-term context after T, but not after placebo administration. Collectively, these findings provide the first causal evidence that T modulates men's preferences for facial femininity as a function of mating context.

Intraspinal canal rod migration causing late-onset paraparesis 8 years after scoliosis surgery.

INTRODUCTION: Complete intraspinal canal rod migration with posterior bone reconstitution has never been described in the adolescent idiopathic scoliosis (AIS) population. We present an unusual but significant delayed neurological complication after spinal instrumentation surgery. CASE REPORT: A 24-year-old woman presented with lower limb weakness (ASIA D) 8 years after posterior instrumentation from T2 to L4 for AIS. CT scan and MRI demonstrated intra-canal rod migration with complete laminar reconstitution. The C-reactive protein was slightly elevated (fluctuated between 10 and 20 mg/l). Radiographs showed the convex rod had entered the spinal canal. The patient was taken into the operating room for thoracic spinal decompression and removal of the convex rod. This Cotrel-Dubousset rod, which had been placed on the convexity of the thoracic curve had completely entered the canal from T5 to T10 and was totally covered by bone with the eroded laminae entirely healed and closed. There was no pseudarthrosis. Intra-operatively, the fusion mass was opened along the whole length of this rod and the rod carefully removed and the spinal cord decompressed. The bacteriological cultures returned positive for Propionibacterium acnes. The patient recovered fully within 2 months post-operatively. CONCLUSION: We opine that the progressive laminar erosion with intra-canal rod migration resulted from mechanical and infectious-related factors. The very low virulence of the strain of Propionibacterium acnes is probably involved in this particular presentation where the rod was trapped in the canal, owing to the quite extensive laminar reconstitution.

Separating responses based on demand characteristics and phenomenological-spatial associations.

Previous researchers have found that participants associate higher frequencies with locations that are higher in space and lower frequencies with lower locations, creating a phenomenological-spatial association for the frequency of auditory tones. With such an association, the frequency of an auditory tone could potentially bias movements along multiple axes. This hypothesis was tested. In four experiments, nine frequencies (250-1,250 Hz) were binaurally presented to blindfolded participants (n = 10, 12, 20, & 9; M age = 22 yr.) who indicated the perceived location of the stimuli on a measurement scale oriented in the vertical, the horizontal (Experiment 1), or depth dimension (Experiment 2). In Experiment 3, participants were asked to indicate the perceived location of the frequencies on a two-dimensional vertical board located in front of them. In Experiment 4, participants indicated the perceived location in three-dimensional space. An optoelectronic device recorded at all locations. Analyses of constant error indicated a spatial association in the vertical, horizontal, and depth dimensions when responses were restricted to only one dimension (Experiments 1 & 2). Higher frequencies were perceived to be located higher, farther to the right, and farther away from the body than lower frequencies. However, this spatial association was only exhibited in the vertical dimension when the responses were unconstrained in two dimensions (vertical and horizontal; Experiment 3) and all three dimensions (Experiment 4). Although this spatial association is a robust phenomenon, it appears that the association only biases actions when indicating perceived locations in the vertical dimension during unconstrained responses.

Facilitation of ocular pursuit during transient occlusion of externally-generated target motion by concurrent upper limb movement.

Smooth pursuit during prolonged occlusion is improved in the presence of sensorimotor signals when tracking self-generated target motion. The current study investigated whether concurrent arm tracking of externally-generated target motion conveys a similar facilitation to ocular pursuit of transiently occluded constant velocity (Experiment 1) or accelerating (Experiment 2) targets. Velocity characteristics and occlusion duration were arranged in random or blocked order, thus permitting a novel examination of the contribution from sensorimotor signals and predictive processes acting within the ocular system during transient occlusion. Consistent with previous investigations, smooth pursuit decayed during transient occlusion; but eye velocity was higher when trials were presented in blocked compared to random order, particularly for positively accelerating targets. For fast, constant velocity targets, concurrent arm movement facilitated smooth pursuit during transient occlusion. Nevertheless, even with increased predictability regarding the upcoming target motion in blocked-order trials and the presence of sensorimotor signals from concurrent arm movement, eye velocity always remained less than target velocity during occlusion. This contrasted with the manual response, which attained velocity close to target velocity, whether in blocked or random conditions. These findings are discussed with reference to recent models of ocular pursuit that incorporate short-term and/or long-term prediction to account for target extrapolation during occlusion.

Stereotactic targeting and dose verification for age-related macular degeneration.

PURPOSE: Validation of the targeting and dose delivery of the IRay low voltage age-related macular degeneration treatment system. METHODS: Ten human cadaver eyes were obtained for this study and mounted in the IRay system. Using gel and vacuum, an I-Guide immobilization device was coupled to the eyes and radiochromic film was affixed to the posterior aspect of the globes. Three narrow x-ray beams were delivered through the pars plana to overlap on the predicted nominal fovea. A needle was placed through the center of the film's beam spot and into the eye to register the film and the inner retina. The process was performed three times for each of the ten eyes (30 simulated treatments; 90 individual beams). The globes were dissected to assess the targeting accuracy by measuring the distances from the needles to the fovea. The dose to the fovea was calculated from the radiochromic film. RESULTS: X-ray targeting on the retina averaged 0.6 +/- 0.4 mm from the fovea. Repeated treatments on the same eye showed a reproducibility of 0.4 +/- 0.4 mm. The optic nerve was safely avoided, with the 90% isodose edge of the beam spot between 0.4 and 2.6 mm from the edge of the optic disk. Measured dose matched that prescribed. CONCLUSIONS: This study provides confidence that the IRay, with an average accuracy of 0.6 mm and a precision of 0.4 mm, can reliably treat most AMD lesions centered on the fovea. With the exception of motion, all sources of error are included.

Between-person effects on attention and action: Joe and Fred revisited.

Previous study indicates that target-target inhibition of return (IOR) is not restricted to a single nervous system. Specifically, watching another person perform a goal-directed aiming movement engages similar inhibitory processes on a subsequent aiming attempt as if having performed the preceding movement oneself. This between-person effect has been attributed to the mirror neuron system. In the study reported here, we replicated this finding and examined the relative importance of automatic stimulus alerting events and action-observation by dissociating these two influences. This was done by having two people alternately perform sets of two aiming trials to the same equally probable targets. Under some experimental conditions, one or both of the performers moved to a non-illuminated target. In this way, we dissociated the stimulus and observed event under some between-person conditions. Although IOR was greatest when the stimulus and observed events were compatible, both contributed to the between-person inhibitory processes slowing the responses (Experiment 1). The impact of observing another person perform an aiming movement appears to have more to do with realizing a particular spatial goal than seeing the biological motion associated with achieving that goal (Experiment 2). Findings that both the illumination of a visual target signal and the observation of another person's action engage similar attention-action processes are consistent with action-based accounts of visual selective attention.

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.

The role of vision for online control of manual aiming movements in persons with autism spectrum disorders.

Recent studies suggest motor skills are not entirely spared in individuals with an autism spectrum disorder (ASD). Previous reports demonstrated that young adults with ASD were able to land accurately on a target despite increased temporal and spatial variability during their movement. This study explored how a group of adolescents and young adults with an ASD used vision and proprioception to land successfully on one of two targets. Participants performed eye movements and/or manual reaching movements, either with or without vision. Although eye movements were executed in a similar timeframe, participants with ASD took longer to plan and execute manual reaching movements. They also exhibited significantly greater variability during eye and hand movements, but were able to land on the target regardless of the vision condition. In general, individuals with autism used vision and proprioception. However, they took considerably more time to perform movements that required greater visual-proprioceptive integration.

Three-dimensional manual responses to unexpected target perturbations during rapid aiming.

The authors examined the execution of rapid aiming movements to targets that changed size and position. Participants viewed a medium-size target during movement preparation. The target became smaller or larger at movement initiation on size perturbation trials. The target moved closer or farther away from the home position on amplitude perturbation trials. The authors examined-in addition to several performance measures-the volume of 3-dimensional ellipsoids to quantify between-trials variability. In the size protocol, men executed movements in a similar manner irrespective of condition. In contrast, women exhibited less variability when target size increased. In the amplitude protocol, men moved inconsistently in the latter portion of the trajectory when targets became proximal. Men also failed to adjust for the perturbation on several trials. In comparison, women were more variable in the initial portions of the trajectory when the target became distal. Although men and women performed their movements in a similar duration, the trajectory and error analyses indicated different behaviors. Specifically, women made more rapid and efficient adjustments to their trajectories on the basis of concurrent visual information. In contrast, men executed movements on the basis of the visual information initially presented and then made adjustments, rather than gathering visual information and executing adjustments throughout the trajectory.

On Whether Task Experience of the Peer Differentially Impacts Feedback Scheduling and Skill Acquisition of a Learner.

Previous research has shown that peers without task experience provided knowledge of results (KR) as effectively as performers who self-controlled their own KR schedule (McRae et al., 2015). In the present experiment, a group of participants first practiced a motor task while self-controlling their KR during a defined acquisition period. Twenty-four hours after their last retention trial, these participants with motor experience then provided KR to a learner during their skill acquisition. Participants were required to learn a serial-timing task with a goal of 2,500 ms. Participants completed a defined acquisition period and then returned 24 h later for a retention test. In retention, learners who received KR from experienced peers were predicted to outperform learners who received KR from inexperienced peers. The results showed that performers learned the task similarly, independent of the peer's previous task experience. However, the peer groups differed in their frequency of providing KR to the learner and showed a discrepancy between their self-reported KR provision strategy and when they actually provided KR. The results have theoretical implications for understanding the impact of self-control in motor learning contexts.

Real-time manipulation of visual displacement during manual aiming.

This study examined the spatial and temporal limitations of the visual corrective process in the control of upper limb movements. Real-time calculation of kinematic data was used to trigger a prismatic displacement of the movement environment during manual aiming. Using an OptoTrak motion tracking system, a data acquisition unit, and a custom-made program, perturbations were triggered at peak acceleration, peak velocity, and the estimated time of peak deceleration. Movement outcome was significantly influenced only when the visual displacement occurred at peak acceleration. The results support models of visual control that posit that early visual information is required for accurate limb control.

Testosterone influences volitional, but not reflexive orienting of attention in human males.

The impact of testosterone (T) on the exogenous (Experiment 1) and endogenous (Experiment 2) orienting of visual attention in males was examined. Sixteen male participants completed both an exogenous and an endogenous cuing task on two separate days. About 2-3h prior to testing, either a placebo or a dose of T was administered. The inhibition of return (IOR) phenomenon was observed during the exogenous cuing task, but IOR was not influenced by T. During the endogenous task, participants demonstrated the expected cuing effects on both days. However, longer reaction time to invalid target locations was observed following T-administration. The manipulation of T-levels in males provides converging evidence of dissociation between reflexive and volitional orienting of attention.

Extending Energy Optimization in Goal-Directed Aiming from Movement Kinematics to Joint Angles.

Energy optimization in goal-directed aiming has been demonstrated as an undershoot bias in primary movement endpoint locations, especially in conditions where corrections to target overshoots must be made against gravity. Two-component models of upper limb movement have not yet considered how joint angles are organized to deal with the energy constraints associated with moving the upper limb in goal-directed aiming tasks. To address this limitation, participants performed aiming movements to targets in the up and down directions with the index finger and two types of rod extensions attached to the index finger. The rod extensions were expected to invoke different energy optimizing strategies in the up and down directions by allowing the distal joints the opportunity to contribute to end effector displacement. Primary movements undershot the farthest target to a greater extent in the downward direction compared to the upward direction, showing that movement kinematics optimize energy expenditure in consideration of the effects of gravity. As rod length increased, shoulder elevation was optimized in movements to the far-up target and elbow flexion was optimally minimized in movements to the far-down target. The results suggest energy optimization in the control of joint angles independent of the force of gravity.

The multiple process model of goal-directed reaching revisited.

Recently our group forwarded a model of speed-accuracy relations in goal-directed reaching. A fundamental feature of our multiple process model was the distinction between two types of online regulation: impulse control and limb-target control. Impulse control begins during the initial stages of the movement trajectory and involves a comparison of actual limb velocity and direction to an internal representation of expectations about the limb trajectory. Limb-target control involves discrete error-reduction based on the relative positions of the limb and the target late in the movement. Our model also considers the role of eye movements, practice, energy optimization and strategic behavior in limb control. Here, we review recent work conducted to test specific aspects of our model. As well, we consider research not fully incorporated into our earlier contribution. We conclude that a slightly modified and expanded version of our model, that includes crosstalk between the two forms of online regulation, does an excellent job of explaining speed, accuracy, and energy optimization in goal-directed reaching.

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.

The influence of advance information about target location and visual feedback on movement planning and execution.

This study was designed to determine if movement planning strategies incorporating the use of visual feedback during manual aiming are specific to individual movements. Advance information about target location and visual context was manipulated using precues. Participants exhibited a shorter reaction time and a longer movement time when they were certain of the target location and that vision would be available. The longer movement time was associated with greater time after peak velocity. Under conditions of uncertainty, participants prepared for the worst-case scenario. That is, they spent more time organizing their movements and produced trajectories that would be expected from greater open-loop control. Our results are consistent with hierarchical movement planning in which knowledge of the movement goal is an essential ingredient of visual feedback utilization.

Measuring Investment in Learning: Can Electrocardiogram Provide an Indication of Cognitive Effort During Learning?

Heart rate variability (i.e., low frequency:high frequency ratio) was measured to differentiate invested cognitive effort during the acquisition and retention of a novel task. Participants (12 male, M = 25.1 year, SD = 3.6; 12 female, M = 22.8 year, SD = 1.1) were required to produce Braille equivalents of English letter primes on a standardized keyboard in proactive or retroactive conditions (groups, each n = 12). The correct Braille response was either provided before (i.e., proactively) or after (i.e., retroactively) the participant's response. During acquisition, participants in the proactive group demonstrated shorter study time, greater recall success, and reported lower cognitive investment. Participants in the proactive and retroactive groups did not statistically differ in heart rate variability. For retention, the retroactive group showed greater recall success, lower perceived cognitive effort investment, and lower heart rate variability. The results highlight the usefulness of heart rate variability in discriminating the cognitive effort invested for a recently acquired skill.

Torque-onset determination: Unintended consequences of the threshold method.

BACKGROUND: Compared with visual torque-onset-detection (TOD), threshold-based TOD produces onset bias, which increases with lower torques or rates of torque development (RTD). PURPOSE: To compare the effects of differential TOD-bias on common contractile parameters in two torque-disparate groups. METHODS: Fifteen boys and 12 men performed maximal, explosive, isometric knee-extensions. Torque and EMG were recorded for each contraction. Best contractions were selected by peak torque (MVC) and peak RTD. Visual-TOD-based torque-time traces, electromechanical delays (EMD), and times to peak RTD (tRTD) were compared with corresponding data derived from fixed 4-Nm- and relative 5%MVC-thresholds. RESULTS: The 5%MVC TOD-biases were similar for boys and men, but the corresponding 4-Nm-based biases were markedly different (40.3±14.1 vs. 18.4±7.1ms, respectively; p<0.001). Boys-men EMD differences were most affected, increasing from 5.0ms (visual) to 26.9ms (4Nm; p<0.01). Men's visually-based torque kinetics tended to be faster than the boys' (NS), but the 4-Nm-based kinetics erroneously depicted the boys as being much faster to any given %MVC (p<0.001). CONCLUSIONS: When comparing contractile properties of dissimilar groups, e.g., children vs. adults, threshold-based TOD methods can misrepresent reality and lead to erroneous conclusions. Relative-thresholds (e.g., 5% MVC) still introduce error, but group-comparisons are not confounded.