Behavioural robustness and the distributed mechanisms hypothesis

A current challenge in neuroscience and systems biology is to better understand properties that allow organisms to exhibit and sustain appropriate behaviours despite the effects of perturbations (behavioural robustness). There are still significant theoretical difficulties in this endeavour, mainly due to the context-dependent nature of the problem. Biological robustness, in general, is considered in the literature as a property that emerges from the internal structure of organisms, rather than being a dynamical phenomenon involving agent-internal controls, the organism body, and the environment. Our hypothesis is that the capacity for behavioural robustness is rooted in dynamical processes that are distributed between agent ‘brain’, body, and environment, rather than warranted exclusively by organisms’ internal mechanisms. Distribution is operationally defined here based on perturbation analyses. Evolutionary Robotics (ER) techniques are used here to construct four computational models to study behavioural robustness from a systemic perspective. Dynamical systems theory provides the conceptual framework for these investigations. The first model evolves situated agents in a goalseeking scenario in the presence of neural noise perturbations. Results suggest that evolution implicitly selects neural systems that are noise-resistant during coupling behaviour by concentrating search in regions of the fitness landscape that retain functionality for goal approaching. The second model evolves situated, dynamically limited agents exhibiting minimalcognitive behaviour (categorization task). Results indicate a small but significant tendency toward better performance under most types of perturbations by agents showing further cognitivebehavioural dependency on their environments. The third model evolves experience-dependent robust behaviour in embodied, one-legged walking agents. Evidence suggests that robustness is rooted in both internal and external dynamics, but robust motion emerges always from the systemin-coupling. The fourth model implements a historically dependent, mobile-object tracking task under sensorimotor perturbations. Results indicate two different modes of distribution, one in which inner controls necessarily depend on a set of specific environmental factors to exhibit behaviour, then these controls will be more vulnerable to perturbations on that set, and another for which these factors are equally sufficient for behaviours. Vulnerability to perturbations depends on the particular distribution. In contrast to most existing approaches to the study of robustness, this thesis argues that behavioural robustness is better understood in the context of agent-environment dynamical couplings, not in terms of internal mechanisms. Such couplings, however, are not always the full determinants of robustness. Challenges and limitations of our approach are also identified for future studies

Down-Conditioning of Soleus Reflex Activity using Mechanical Stimuli and EMG Biofeedback

Spasticity is a common syndrome caused by various brain and neural injuries, which can severely impair walking ability and functional independence. To improve functional independence, conditioning protocols are available aimed at reducing spasticity by facilitating spinal neuroplasticity. This down-conditioning can be performed using different types of stimuli, electrical or mechanical, and reflex activity measures, EMG or impedance, used as biofeedback variable. Still, current results on effectiveness of these conditioning protocols are incomplete, making comparisons difficult. We aimed to show the within-session task- dependent and across-session long-term adaptation of a conditioning protocol based on mechanical stimuli and EMG biofeedback. However, in contrast to literature, preliminary results show that subjects were unable to successfully obtain task-dependent modulation of their soleus short-latency stretch reflex magnitude

Can a Power Training Program Reduce Fall Risk Factors in Parkinson\u27s Disease?

Introduction: Frequent falls in Parkinson’s disease (PD) are likely partially due to impaired muscle function in PD (i.e. greater coactivation and decreased magnitude of activation in agonists) compared to older adults without PD. Reduced muscle strength and power (ability to generate force rapidly) are also risk factors and are likely occurring due to deficits in muscle parameters. Muscle parameters include: i) the amount of coactivation of antagonist muscles; ii) latency to onset of activation in agonist and antagonist muscles and; iii) the magnitude of activation of agonist and antagonist muscles. Rehabilitation should aim to improve impaired muscle parameters to reduce fall risk in PD. Therefore, two experiments were designed to address this gap in PD literature. Experiment one aimed to identify specific muscle parameters distinguishing fall status in PD, thus providing parameters that can be used to identify if a rehabilitation will be effective in reducing fall risk. Experiment two investigated whether power training (PWR) was more effective than strength training (ST) or a non-exercise control group (CTRL) at improving muscle parameters distinguishing fallers in experiment one. Methods: Experiment one - Forty-six individuals with PD were categorized based on fall status. A fall-like situation (lean and release) was used and electromyography (EMG) data was collected from muscles in both legs (stepping and stance leg): tibialis anterior (TA), lateral gastrocnemius (LG), biceps (BF) and rectus femoris (RF). Results: A Receiver Operating Characteristic (ROC) curve identified fallers vs. non-fallers by EMG measures in the stepping leg; an increased onset latency of LG and a greater TA activation. As well, in the stance limb, an increased coactivation of TA and a larger TA activation identified fallers. Experiment two- Forty-four individuals with PD were randomized to PWR or ST groups, and seventeen individuals with PD volunteered for the CTRL group. Training occurred twice weekly for 12-weeks, where PWR completed the concentric part of the movements rapidly. All groups completed the fall situation (at baseline, one to two weeks prior to the intervention, and one to two weeks after the intervention was complete) while muscle parameters were measured along with muscle strength and muscle power, disease severity and a weekly falls diaries. Results: No differences in muscle parameters were present at post-testing between groups. However, PWR and ST significantly improved muscle strength, and components of muscle power compared to CTRL. Disease severity was improved in PWR at post-testing. Conclusion: Muscle parameters distinguishing PD fallers were identified. As well, PWR and ST improved aspects of risk factors for falls similarly, providing two feasible rehabilitation strategies for PD