Extrinsic and intrinsic dynamics in visuomotor tracking

PhD ThesisHumans typically produce 2–3 submovements per second when tracking slow targets. This intermittency is altered by the addition of delays in sensory feedback suggesting that it is governed by extrinsic properties of the control loop. However, the motor cortex also exhibits an intrinsic rhythmicity at 2–3 Hz, which might influence the temporal structure of movements. This thesis examines how the interplay between extrinsic and intrinsic dynamics shapes the kinematics of tracking behaviour. I found that the dependence of submovement frequencies on extrinsic delays could be reproduced by a simple feedback controller model. This model predicted that submovements reflect frequencies at which visuomotor noise is exacerbated, and this was confirmed by perturbation experiments. However, these experiments also revealed a 2-3 Hz band-pass filtering of feedback responses irrespective of extrinsic delay. Further experimental evidence suggested this filter did not reflect properties of either visuomotor noise, the feedforward pathway, or visual processing. However, the filter exhibited features consistent with a state estimator required for optimal feedback control (OFC) in the presence of visual and motor noise. Finally, I sought evidence that this filter was implemented by motor cortical circuits. Multichannel local field potentials (LFPs) in the motor cortex of macaque monkeys were strongly correlated with submovements, at frequencies which depended on extrinsic delay. However, the dynamics of LFP cycles during submovements were independent of delay, and matched instead the properties of the state estimator in the OFC model. In summary, by combining human behavioural studies, computational modelling and monkey electrophysiology, I show how movement intermittency can be explained by the interplay of both extrinsic and intrinsic dynamics within an OFC framework. Moreover, I suggest that motor cortical rhythmicity reflects recurrent circuitry that combines sensory feedback with an internal dynamical model to form optimal estimates of required motor corrections.Indonesian Endowment Fund for Education (Lembaga Pengelola Dana Pendidikan Republik Indonesia) for supporting my PhD studies. This work would also not have been possible without the financial support of the Wellcome Trust, and the Medical Research Council

Submovements During Reaching Movements after Stroke

Neurological deficits after cerebrovascular accidents very frequently disrupt the kinematics of voluntary movements with the consequent impact in daily life activities. Robotic methodologies enable the quantitative characterization of specific control deficits needed to understand the basis of functional impairments and to design effective rehabilitation therapies. In a group of right handed chronic stroke survivors (SS) with right side hemiparesis, intact proprioception, and differing levels of motor impairment, we used a robotic manipulandum to study right arm function during discrete point-to-point reaching movements and reciprocal out-and-back movements to visual targets. We compared these movements with those of neurologically intact individuals (NI). We analyzed the presence of secondary submovements in the initial (i.e. outward) trajectory portion of the two tasks and found that the SS with severe impairment (F

Sub-movement organisation, pen pressure and muscle activity are modulated to precision demands in 2D tracking

The authors investigated how tracking performance, submovement organization, pen pressure and muscle activity in forearm and shoulder muscles were affected by target size in a 2D tracking task performed with a pen on a digitizer tablet. Twenty-six subjects took part in an experiment, in which either a small dot or a large dot was tracked, while it moved quasirandomly across a computer screen at a constant velocity of 2cm/s. The manipulation of precision level was successful, because mean distance to target and the standard deviation of this distance were significantly smaller with the small target than with the large target. With a small target, subjects trailed more behind the center of target and used submovements with larger amplitudes and of shorter duration, resulting in higher tracking accuracy. This change in submovement organization was accompanied by higher pen pressure, while at the same time muscle activity in the forearm extensors and flexors was increased, indicating higher endpoint stability. In conclusion, increased precision demands were accommodated by both a different organization of submovements and higher endpoint stability in a 2D tracking task performed with a pen on a digitizer tablet. © 2012 Copyright Taylor and Francis Group, LLC

Understanding Product Interest through Mouse-Cursor Tracking Analysis

With third-party cookies being banned, alternative methods to assess users’ interests online are necessary. We propose that analyzing mouse cursor movements can help address this need. Based on the response activation model, we hypothesize that interest in a product will decrease the user’s movement speed and increase the number of submovements. We conducted an online study that monitored users’ mouse movements while they were presented with several products and navigated to a button to indicate purchase intention (yes/no). Following this, participants ranked their interest in each product. Contrary to our prediction, we found that product interest increased speed and decreased the submovement count. This suggests that current theories and metrics for mouse cursor tracking are insufficient for predicting product interest. Further research is needed to develop reliable measures for gauging user interest in products

The Impact of Age and Physical Activity Level on Manual Aiming Performance

Older adults traditionally adapt their discrete aiming movements, thereby travelling a larger proportion of the movement under closed-loop control. As the beneficial impact of a physically active lifestyle in old age has been described for several aspects of motor control, we compared the aiming performance of young controls to active and sedentary older adults. To additionally determine the contribution of visual feedback, aiming movements were executed with and without saccades. Results showed only sedentary older adults adopted the typical movement changes, highlighting the impact of a physically active lifestyle on manual aiming in old age. In an attempt to reveal the mechanism underlying the movement changes, evidence for an age-related decline in force control was found, which in turn resulted in an adapted aiming strategy. Finally, prohibiting saccades did not affect older adults’ performance to a greater extent, suggesting they do not rely more on visual feedback than young controls. Keywords: aging, physical activity, manual aiming, eye movement

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

Identification and Retraining of Sensorimotor Deficits to Reduce Intention Tremor in Multiple Sclerosis

Multiple sclerosis (MS) affects approximately 1 in 1000 Americans and is a significant cause of disability in the United States. One significant contributor to disability in MS is intention tremor, which manifests as an oscillation about the endpoint of a goal-directed movement. A major challenge of treating intention tremor is that the underlying causes of tremor in MS are unknown. In this study, we describe a systems-level computational model and an experimental technique that parameterizes subject-specific deficits in sensory feedback control during goal-directed movements. We used this approach to characterize sensorimotor control and examine how sensory and motor processes are differentially impacted by age and MS. The specific aims of this study were to: 1) characterize age-related changes in sensorimotor control during goal-directed movements; 2) characterize deficits in sensorimotor control in individuals with multiple sclerosis; and 3) determine whether sensorimotor control deficits can be modified and intention tremor reduced using robot-assisted therapy. We show that age-related changes in movement control can be ascribed to increases in sensory noise, leading to slower and less accurate movements. In persons with MS, changes in movement control associated with intention tremor can be attributed to increases in visual response delay that are unaccounted for by predictive neuromotor control mechanisms. Finally, we show that training of goal-directed movements using carefully selected feedback delays can enable subjects to adapt to their increased visual delay, thereby reducing system instability and tremor. The results demonstrate that systems identification techniques provide an informative framework for investigating how neuromotor disease affects motor control and for developing individually targeted rehabilitation strategies to reduce motor disability

Investigation of Unintentional Movement in People with Cerebral Palsy to Improve Computer Target Aquisition

People with Cerebral Palsy (CP) have difficulty using computer pointing devices due to unintentional movement in their upper extremities. Fifty percent of people with CP have impaired arm-hand function which limits their ability to interface with pointing devices and effectively control cursor movement on the computer screen. This thesis involves two studies which utilize an Isometric Joystick in order to access the computer and complete target acquisition tasks. The first study titled "Quantification of Cursor Movement of People with Athetoid and Spastic Cerebral Palsy to Improve Target Acquisition," aims to guide real-time digital filter development for people with athetoid and spastic CP for target acquisition tasks. By investigating the cursor movement measures throughout the target acquisition trajectory we gained a better insight as to when and how to compensate for unintentional movement in people with CP. Results showed that both people with athetoid CP and spastic CP have more difficulty hovering over the target than they did moving to the target, indicating that filter development should focus on the hovering portion of the target acquisition task in order to improve target acquisition time. The second study titled "Customized Control for People with Athetosis and Dystonia to Improve Computer Access," aims to develop a method to prescribe appropriate switch/scanning control for people with athetosis and dystonia as well as to determine if customized switch/scanning control is more effective in completing icon selection tasks than the proportional isometric control. Results of this study suggest that switch/scanning control could be useful in moving on the most direct path to the target as shown by a significantly smaller percent distance error for customized control as compared to proportional isometric control (F(1,6) = 361.2, p < 0.01)