An examination into the influence of observed gaze cues on motor output. / Richardson and colleagues (2013) demonstrated oculo-manual spatial interference by finding that the finger trajectory in a vertical tapping task deviated toward the direction of a concurrent saccade. It was proposed that the entrainment of the hand to the eyes was in part a function of generalized motor planning. Human action observation research has shown that cortical motor planning is also active during action observation (e.g. Buccino et al. 2001; Decety et al. 1997), which can lead to other forms of spatial interference (Kilner et al 2003). We hypothesized that because motor planning subserves both observation and execution of action, simply observing the horizontal saccades of another person would cause sufficient recruitment of oculomotor planning structures, that would result in finger tap trajectory deviations toward the direction of the observed saccade (but would not do so in a non-biological observation control condition).19 participants performed 24 trials of vertical finger taps under three different visual conditions. They were required to: a) saccade horizontally between targets; b) fixate on a biological stimulus (i.e. a video of horizontally saccading human eyes); or c) fixate on a non-biological control stimulus (horizontally moving black dots) while tapping their finger to an auditory metronome beat presented at a 750ms intervals. Results from the saccading condition replicate Richardson et al’s (2013) entrainment effect. That is, finger taps deviated to the left when participants saccaded left, and to the right when executed with a rightward saccade. Contrary to expectations however, there was no entrainment induced by observing either the biological stimulus or the control stimulus. This suggests that competing motor plans (eyes and hands) are necessary to induce interference. Further, simply observing eye movements do not recruit the same oculomotor planning networks as action execution. / Thesis / Master of Science in Kinesiology

The control of object manipulation tasks involves the close interplay of predictive and reactive control mechanisms. For example, when lifting an object, people typically predict the weight based on object size and material as well as sensorimotor memory obtained from previous lifts of the object. When lifting objects with a precision grip, people increase vertical load force to a target level that slightly exceeds the predicted weight. When the object is heavier than expected, the mismatch between expected and actual tactile signals associated with lift-off triggers a corrective action within ~100 ms, that involves probing increases in load force that continue until the object is lifted. Here we investigated whether this correction action can be adaptively influenced by experience. Participants repeatedly lifted an object that was instrumented with force sensors to measure the forces applied by the fingertips, with weight that could be varied without the knowledge of the participant. In 80% of trials, the weight was set to 2 N and, in different blocks of 110 trials, the remaining 20 % of trials (2 trials randomly selected from each successive 10 trials) was set to either 4 or 6 N. We found that the rate of change of the reflexively triggered increase in load force that occurred in the 4 or 6 N trials, scaled with the additional weight. That is, following the initial increase in load force to ~2 N, the subsequent increase in load force was more rapid for the 6 N object than the 4 N object. In contrast, the onset time of the reactive increase in load force was independent of the additional weight. Finally, this adaptation of reactive load force control took place quickly and was evident after only a few lifts of the heavier weight. These results indicate that the reactive increases in load force that occur when a lifted object is heavier than expected can be adapted and tuned, to refine behavior. This further suggests that multiple predictions can be generated about object weight when lifting. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2013-02-02 13:34:20.533

<p>Stability is the ability of a system to reject noise and maintain or return to the desired movement pattern and is an important feature of a motor system. In contrast, maneuverability is the ability of a system to transition between different motor states. A system that prioritizes stability inhibits its ability to transition between different motor states in a dexterous fashion. Since stability and maneuverability are opposing characteristics of a system, stability could be traded off to increase maneuverability. This study focuses on isometric finger force production, and its goals were to identify whether (1) the amount of information available about an upcoming motor transition influences the reduction in stability of total isometric force produced by the fingers, (2) stability reduction was correlated with greater maneuverability, i.e., less time for initiating a change in the total force, (3) the amount of stability reduction is correlated across tasks with different amount of information regarding the upcoming force changes, and (4) the times required to change force correlated across tasks with different informational content. </p> <p>Twenty-nine young adults (17 women; age, 23.3 ± 4.3 years) participated in this study and completed three different finger force tasks. For each task, the participants modulated the total pressing force produced by the four fingers of their right hand to track a target presented on a computer screen. In each task, participants began by producing a consistent (10% of their maximum voluntary contraction, MVC) background force with their fingers. In the Steady task, the target remained stationary and participants knew the target would not move. In the Reaction Time (RT) task, the target moved randomly in the vertical direction and participants knew that this could happen at any point in time. In the Self-paced task, participants started producing a background force and then produced a quick increase in total force using a predefined target that was displayed at the beginning of the trial, and visible throughout the trial. </p> <p>The uncontrolled manifold analysis was used to assess the stability of the total force during each task. This assessment was performed when the participants produced the same force (10% MVC), but expected different upcoming force changes, and had different amount of information about these upcoming force changes. This analysis yielded a stability index, and measures of the variance structure in the finger forces, computed across multiple repetitions. The reaction time and the movement time in the RT and the Self-paced tasks, respectively, was computed to quantify maneuverability. </p> <p>In contrast to previous findings and our expectations, the stability index was not statistically different for the Steady, RT, and Self-paced tasks, meaning that stability of the total force was not reduced in response to the mere expectation of an upcoming change in total force. However, the stability index reduced immediately before individuals changed their total force in the Self-paced tasks, which supports findings from previous studies. The stability modulation between the Steady and RT tasks did not correlate with the RT, and the stability modulation between the Steady and Self-paced tasks did not correlate with the movement time. Therefore, this study did not reveal a stability-maneuverability trade-off in isometric finger force production tasks. The movement time for the RT and Self-paced tasks were also not correlated. However, the novel finding of this study was that participants changed stability similarly for the RT and Self-paced tasks.</p> <p>Finally, the variance components obtained from the uncontrolled manifold analysis were higher in the RT task compared to the Steady task, consistent with previous reports. In fact, the increase in the performance error (greater variability in total force) while expecting to change total force in uncertain conditions (RT tasks) is the most striking and consistent result across multiple similar studies. This result indicates that despite the inconsistent results regarding the stability index, the performance of the current task (producing a constant total force) is hampered by the uncertainty and the expectation of upcoming changes in total force.</p> <p>It is likely that the stability-maneuverability trade-off is not essential for young, healthy adults in manual force production tasks. Investigations that include participants across the lifespan will shed light on this relation and help identify whether it plays a salient role in understanding loss of manual dexterity with healthy aging. </p>

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Many of the problems associated with the planning and execution of human arm trajectories are illuminated by planning and control strategies which have been developed for robotic manipulators. This comparison may provide explanations for the predominance of straight line trajectories in human reaching and pointing movements, the role of feedback during arm movement, as well as plausible compensatory mechanisms for arm dynamics.