When moving our upper-limb towards a single target, movement times are typically shorter than when movement to a second target is required. This is known as the one-target advantage. Most studies that have demonstrated the one-target advantage have employed separate trial blocks for the one- and two-segment movements. To test if the presence of the one-target advantage depends on advance knowledge of the number of segments, the present study investigated whether the one-target advantage would emerge under different trial orders/sequences. One- and two-segment responses were organized in blocked (i.e., 1-1-1, 2-2-2), alternating (i.e., 1-2-1-2-1-2), and random (i.e., 1-1-2-1-2-2) trial sequences. Similar to previous studies, where only blocked schedules have typically been utilized, the one-target advantage emerged during the blocked and alternate conditions, but not in the random condition. This finding indicates that the one-target advantage is contingent on participants knowing the number of movement segments prior to stimulus onset.

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In order to maximize the precise completion of voluntary actions, humans can theoretically utilize both visual and proprioceptive information to plan and amend ongoing limb trajectories. Although vision has been thought to be a more dominant sensory modality, research has shown that sensory feedback may be processed as a function of its relevance and reliability. As well, theoretical models of voluntary action have suggested that both vision and proprioception can be used to prepare online trajectory amendments. However, empirical evidence regarding the use of proprioception for online control has come from indirect manipulations from the sensory feedback (i.e., without directly perturbing the afferent information; e.g., visual–proprioceptive mismatch). In order to directly assess the relative contributions of visual and proprioceptive feedback to the online control of voluntary actions, direct perturbations to both vision (i.e., liquid crystal goggles) and proprioception (i.e., tendon vibration) were implemented in two experiments. The first experiment employed the manipulations while participants simply performed a rapid goal-directed movement (30 cm amplitude). Results from this first experiment yielded no significant evidence that proprioceptive feedback contributed to online control processes. The second experiment employed an imperceptible target jump to elicit online trajectory amendments. Without or with tendon vibration, participants still corrected for the target jumps. The current study provided more evidence of the importance of vision for online control but little support for the importance of proprioception for online limb–target regulation mechanisms.

Robotic guidance has been employed with limited effectiveness in neurologically intact and patient populations. For example, our lab has effectively used robotic guidance to acutely improve movement smoothness of a discrete trajectory without influencing movement endpoint distributions. The purpose of the current study was to investigate the efficacy of combining robotic guidance and unassisted trials in the learning of a golf putting task. Participants completed a pre-test, an acquisition phase, and an immediate and delayed (24-hour) post-test. During the pre-test, kinematic data from the putter was converted into highly accurate, consistent, and smooth trajectories delivered by a robot arm. During acquisition, three groups performed putts towards three different targets with robotic guidance on either 0%, 50%, or 100% of acquisition trials. Only the 50% guidance group statistically reduced both the ball endpoint distance and variability between the pre-test and the immediate or 24-hr post-test. The results of the 50% guidance group yielded seminal evidence that combining both unassisted and robotic guidance trials (i.e., mixed practice) could facilitate at least short-term motor learning for a golf putting task. Such work is relevant to incorporating robotic guidance in sport skills and other practical areas (e.g., rehabilitation).