Cortical Control of Human Upright Stance

This dissertation examined, for the first time, differences between young and elderly volunteers in cortical representations of human posture control during (1) quiet stance with normal and altered sensory stimulation, (2) biomechanical perturbations, and (3) dual tasking. The primary focus of the first part was to monitor changes in cortical activity when unexpectedly altering the sensory conditions of upright stance, such as switching from stable (eyes open, fixed support surface) to less-stable (eyes closed, sway-referenced support surface) conditions (experiment 1). Our results demonstrate increased cortical activations in delta (0.2-4Hz) and gamma (30-50 Hz) oscillations, primarily over central-frontal, central and central parietal cortices during challenging postural conditions. While increased delta rhythms were observed in both groups during challenging sensory conditions, elderly individuals also showed increased gamma band activity over sensorimotor and parietal cortices, when compared to the younger group. Correlation analyses also suggest that increased cerebral activity became more relevant to the control of Center of Mass dynamics when upright stance was threatened, especially in the elderly group. The second part studied compensatory postural responses to unexpected perturbations while simultaneously recording Electroencephalography, Electromyography, and Center of Mass dynamics (experiment 2). Our results also suggest that, rather than motor system malfunctioning, impairments in perceptual processing of sensory afferences forms the basis of prolonged postural responses to perturbed stance conditions in non-faller older adults. In general, our results are not only consistent with previous reports suggesting involvement of cerebral cortices in human upright stance control, but also extend them by showing ageing related cortical activity modulations during challenging postural tasks. The third part focused on performance changes in posture control-cognition dual tasking as well as cortical representations of these performance changes both in cognitive and posture control tasks (experiment 3). Postural and cognitive data analyses showed that elderly people had no performance deficits during single postural task conditions, but decreased cognitive performance even during challenging single cognitive tasks. Dual tasking analyses indicated that working memory impairments in the elderly group can be observed when a challenging cognitive task is performed in any postural condition, while postural control performance differences only became significant during dual tasking with challenging postural and cognitive task conditions. EEG analyses showed increased delta, theta and gamma oscillations, primarily over frontal, central-frontal, central and central-parietal cortices during challenging dual tasking conditions. While delta oscillations are more responsive to challenging postural conditions, theta rhythms are found to be changing as a function of cognitive task difficulty in both groups, with more pronounced increases in the young subjects. These results, in general, indicate that elderly subjects may adopt a non-automated conscious control strategy and prioritize postural performance over cognitive performance to maintain upright stance only when the cognitive load is low. High cognitive loads, on the other hand, dramatically increase postural sway, thus the risk of falling, in the elderly people. Regarding the cortical basis of age related performance differences during dual tasking conditions, EEG analyses suggest that while increased theta over frontal and central-frontal cortices may underlie the cortical correlates of high level cognitive computations including encoding and retrieval, delta oscillations, in general, maybe underlie cortical monitoring of changes in postural state of the body when sensory conditions of upright stance is compromised.