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.

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.

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.

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.

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.

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.

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.

Exogenous testosterone increases men's perceptions of their own physical dominance.

Men's testosterone is associated with several constructs that are linked to dominance rank, such as risk-taking, mating success, and aggression. However, no study has directly tested the relationship between men's self-perceived dominance and testosterone using an experimental design. We employed a within-subjects, double-blind, placebo-controlled paradigm to assess whether testosterone influences men's self-perceived dominance. Exogenous testosterone or a placebo was administered to healthy adult men and self-perceptions of physical dominance were subsequently assessed by having participants select what they believed to be their true face from an array of images digitally manipulated in masculinity. Men picked a more masculine version of their own face after testosterone versus placebo--an effect that was particularly pronounced among men with relatively low baseline testosterone. These findings indicate that a single administration of testosterone can rapidly modulate men's perceptions of their own physical dominance, which may explain links between testosterone and dominance-related behaviors.

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.

Examining the Preferred Self-Controlled KR Schedules of Learners and Peers During Motor Skill Learning.

In many practical situations, learners are provided with feedback in the form of knowledge of results (KR) by a peer. However, when peers provide KR is currently unknown. When given the opportunity to request KR in a self-controlled manner, some participants have reported a preference for requesting KR after good performances. Alternatively, peers may provide KR in a different fashion. Subsequently, a discrepancy between the learner's desire to receive KR and when a peer provides KR may arise. In our study, peer- and self-controlled KR schedules were compared. Participants were peers who controlled KR (PC; 8), learners with peers (P-L; 8), or learners with self-control (SC; 8). Participants in the two learning groups (P-L and SC groups) completed a serial-timing task with a goal time of 2500 ms. Absolute error data on KR and no-KR trials along with self-reports indicate that participants with self-control preferred KR after good trials and peers preferred to provide KR after both good and bad trials equally. Results from the delayed retention test indicated that peer-controlled learners were more consistent (i.e., in terms of variable error) than the self-control group.

Online Vision as a Function of Real-Time Limb Velocity: Another Case for Optimal Windows.

The efficiency of online visuomotor processes was investigated by manipulating vision based on real-time upper limb velocity. Participants completed rapid reaches under two control (full vision, no vision) and three experimental visual window conditions. The experimental visual windows were early: 0.8-1.4 m/s, middle: above 1.4 m/s, and late: 1.4 to 0.8 m/s. The results indicated that endpoint consistency comparable to that of full-vision trials was observed when using vision from the early (43 ms) and middle (89 ms) windows, but vision from the middle window entailed a longer deceleration phase (i.e., a temporal cost). The late window was not useful to implement online trajectory amendments. This study provides further support for the idea of early visuomotor control, which may involve multiple online control processes during voluntary movement.

Body position differentially influences responses to exogenous and endogenous cues.

The influence of vestibular inputs on exogenous (Exp. 1) and endogenous (Exp. 2) orienting of visual attention was examined. The vestibular system was manipulated through a change in static body position. Participants engaged in an exogenous or endogenous response task while in a seated position, while lying in a prone position, and while in a prone position with their head down and neck flexed (HDNF). An attenuation of inhibition and facilitation effects during the exogenous task was observed in the HDNF position. However, responses to the cues remained similar in the endogenous task, irrespective of body position. The results reveal a potential dissociation between reflexive and volitional orienting of visual attention that is dependent on vestibular inputs.

Self-controlled KR schedules: does repetition order matter?

The impact of an experimenter-defined repetition schedule on the utility of a self-controlled KR context during motor skill acquisition was examined. Participants were required to learn three novel spatial-temporal tasks in either a random or blocked repetition schedule with or without the opportunity to control their KR. Results from the retention period showed that participants provided control over their KR schedule in a random repetition schedule demonstrated superior learning. However, performance measures from the transfer test showed that, independent of repetition schedule, learners provided the opportunity to control their KR schedule demonstrated superior transfer performance compared to their yoked counterparts. The dissociated impact of repetition schedule and self-controlled KR schedules on retention and transfer is discussed.

The utility of vision during action: multiple visuomotor processes?

Recently, D. Elliott et al. (2010) asserted that the current control phase of a movement could be segregated in multiple processes, including impulse and limb-target regulation processes. The authors aimed to provide further empirical evidence and determine some of the constraints that govern these visuomotor processes. In 2 experiments, vision was presented or withdrawn when limb velocity was above or below selected velocity criteria. The authors observed that vision provided between 0.8 and 0.9 m/s significantly improved impulse regulation processes while vision provided up to 1.1 m/s significantly increased limb-target regulation processes. These results lend support to D. Elliott et al. and provide evidence that impulse regulation and limb-target regulation can take place at different velocities during a movement.

Integration of alternating monocular samples during goal-directed aiming.

The current study examined the effect of interocular delay in a manual aiming task that required accurate end-point placement, but not precise control of a grip aperture. Participants aimed in binocular, monocular, or alternating monocular vision conditions. For the latter, 25ms monocular samples were provided to alternate eyes without delay (0ms), or a delay of 25 or 50ms. The interocular delay resulted in a longer movement time, caused by a longer time-to-peak and time-after-peak velocity, and a reduction in peak velocity. We suggest that the change in kinematics reflect a strategic response to preserve terminal aiming accuracy and variability when faced with an informational perturbation. These findings indicate that the response to the interocular delay between alternating monocular samples depends on the task-specific information used to control that behavior.

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.

Self-control of feedback during motor learning: accounting for the absolute amount of feedback using a yoked group with self-control over feedback.

A traditional control group yoked to a group that self-controls their reception of feedback receives feedback in the same relative and absolute manner. This traditional control group typically does not learn the task as well as the self-control group. Although the groups are matched for the amount of feedback they receive, the information is provided on trials in which the individual may not request feedback if he or she were provided the opportunity. Similarly, individuals may not receive feedback on trials for which it would be a beneficial learning experience. Subsequently, the mismatch between the provision of feedback and the potential learning opportunity leads to a decrement in retention. The present study was designed to examine motor learning for a yoked group with the same absolute amount of feedback, but who could self-control when they received feedback. Increased mental processing of error detection and correction was expected for the participants in the yoked self-control group because of their choice to employ a limited resource in the form of a decreasing amount of feedback opportunities. Participants in the yoked with self-control group committed fewer errors than the self-control group in retention and the traditional yoked group in both the retention and time transfer blocks. The results suggest that the yoked with self-control group was able to produce efficient learning effects and can be a viable control group for further motor learning studies.