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Matthew Heath

This investigation tested the proposal that a “highly accurate” and temporally unstable stored target representation is available to the motor system for the online control of memory-guided reaches. Participants reached to a target that was: (a) visible during the response, (b) extinguished at movement onset, and (c) occluded for 0, 500, 1500 and 2,500 ms in advance of response cueing. Additionally, trials were performed with (i.e., limb visible) and without (i.e., limb occluded) vision of the reaching limb. Results showed that limb occluded trials undershot the target location in each target condition, and were characterized by a primarily offline mode of control. In contrast, limb visible trials showed a consistent level of endpoint accuracy for each target condition and elicited more online reaching corrections than limb occluded trials. It is therefore proposed that a reasonably accurate and temporally stable stored target representation can be combined with vision of the moving limb for the online control of memory-guided reaches.

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Matthew Heath and David A. Westwood

We investigated whether a representation of a visual target can be stored in memory and used to support the online control of reaching movements. To distinguish between the use of a stored target representation for movement planning versus online control, we employed a novel movement environment in which participants could not fully plan their action in advance of movement initiation; that is, the spatial mapping between the movement of a computer mouse and the on-screen movement of a cursor was randomly varied from trial to trial. As such, participants were required to use online control to reach the target position. Reaches were examined in full-vision and three memory-dependent conditions (0, 2, and 5 s of delay). Absolute constant error did not accumulate between full-vision and brief delay trials (i.e., the 0-s delay), suggesting a stored representation of the visual target can be used for online control of reaching given a sufficiently brief delay interval. Longer delay trials (2 and 5 s) were less accurate and more variable than brief delay trials; however, the residual accuracy of these memory-dependent actions suggests that the motor system may have access to a stored representation of the visual target for online control processes for upwards of 5 s following target occlusion.

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Matthew Heath, Kristina Neely and Olav Krigolson

The authors manipulated the availability of monocular and binocular vision during the constituent planning and control stages of a goal-directed reaching task. Furthermore, trials were completed with or without online limb vision to determine whether monocular- or binocular-derived ego-motion cues influence the integration of visual feedback for online limb corrections. Results showed that the manipulation of visual cues during movement planning did not influence planning times or overall kinematics. During movement execution, however, binocular reaches—and particularly those completed with online limb vision—demonstrated heightened endpoint accuracy and stability, a finding directly linked to the adoption of a feedback-based mode of reaching control (i.e., online control). In contrast, reaches performed with online monocular vision produced increased endpoint error and instability and demonstrated reduced evidence of feedback-based corrections (i.e., offline control). Based on these results, the authors propose that the combination of static (i.e., target location) and dynamic (i.e., the moving limb) binocular cues serve to specifically optimize online reaching control. Moreover, results provide new evidence that differences in the kinematic and endpoint parameters of binocular and monocular reaches reflect differences in the extent to which the aforementioned engage in online and offline modes of movement control.

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Matthew Heath, David A. Westwood and Gordon Binsted

The goal of the present investigation was to explore the putative contributions of feedforward- and feedback-based processes in the control of memory-guided reaching movements. Participants (N = 4) completed an extensive number of reaching movements (2700) to 3 midline targets (20, 30, 40 cm) in 6 visual conditions: full-vision, open-loop, and four memory-guided conditions (0, 200, 400, and 600 ms of delay). To infer limb control, we used a regression technique to examine the within-trial correspondence between the spatial position of the limb at peak acceleration, peak velocity, peak deceleration, and the ultimate movement endpoint. A high degree of within-trial correspondence would suggest that the final position of the limb was largely specified prior to movement onset and not adjusted during the action (i.e., feedforward control); conversely, a low degree of within-trial correspondence would suggest that movements were modified during the reaching trajectory (i.e., feedback control). Full-vision reaches were found to be more accurate and less variable than open-loop and memory-guided reaches. Moreover, full-vision reaches demonstrated only modest within-trial correspondence between the spatial position of the limb at each kinematic marker and the ultimate movement endpoint, suggesting that reaching accuracy was achieved by adjusting the limb trajectory throughout the course of the action. Open-loop and memory-guided movements exhibited strong within-trial correspondence between final limb position and the position of the limb at peak velocity and peak deceleration. This strong correspondence indicates that the final position of the limb was largely determined by processes that occurred before the reach was initiated; errors in the planning process were not corrected during the course of the action. Thus, and contrary to our previous findings in a video-based aiming task, it appears that stored target information is not extensively (if at all) used to modify the trajectory of reaching movements to remembered targets in peripersonal space.

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Jennifer Campbell, Stephanie Rossit and Matthew Heath

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 (R 2) 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 R 2 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.

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Scott J. Pedersen, Paul R. Surburg, Matthew Heath and David M. Koceja

The purpose was to investigate central and peripheral processing mechanisms through the use of electromyography (EMG) to determine differences between the performance of children with and without ADHD on a lower extremity choice response time task. Sixteen children with ADHD were tested on and off medication along with 19 children without ADHD. For premotor time, the comparison group performed significantly faster than children with ADHD. The longer latencies exhibited in central processing of children with ADHD were related to midline crossing inhibition (MCI). Medication improved the speed of processing for children with ADHD, but did not negate MCI.

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Olav Krigolson, Jon Bell, Courtney M. Kent, Matthew Heath and Clay B. Holroyd

We used the event-related potential (ERP) methodology to examine differences in neural processing between visually and memory-guided reaches. Consistent with previous findings (e.g., Westwood, Heath, & Roy, 2003), memory-guided reaches undershot veridical target location to a greater extent than their visually guided counterparts. Analysis of the ERP data revealed that memory-guided reaches were associated with reduced potentials over medial-frontal cortex at target presentation and following movement onset. Further, we found that the amplitudes of the potentials over medial-frontal cortex for visually and memory-guided reaches were significantly correlated with the peak accelerations and decelerations of the reaching movements. Our results suggest that memory-guided reaches are mediated by a motor plan that is generated while a target is visible, and then stored in memory until needed—a result counter to recent behavioral theories asserting that memory-guided reaches are planned just before movement onset via a stored, sensory-based target representation.