Neural Control of Walking: Role of the Cerebellum

Walking is a relatively easy task; we can walk without thinking about how to do it. While we are able to walk without thinking about it, from a control perspective, walking is definitely not a simple task. It needs to be stable and efficient in a broad range of different conditions. Insights into the neural control of gait are important to learn us about why specific individuals (patients, elderly) experience difficulties with walking, in general or under specific conditions. With this knowledge rehabilitation and fall prevention strategies can be optimized.While it is known that there is an important role for the spinal circuitry in the neural control of gait, multiple supraspinal structures have been suggested to contribute substantially as well. Specifically, based on common gait deficits in cerebellar patients, the cerebellum can be expected to be important in the neural control of gait. So far this has been evaluated in humans, mainly during unperturbed, steady state walking. An important next step is to study the role of the cerebellum in the control of gait corrections in reaction to perturbations. In those conditions, the cerebellum can be expected to be even more involved in the control of gait, because of its function in comparing expected and real sensory signals.The objective of this thesis was to increase the understanding of the roles of the cerebellum in the control of gait corrections in reaction to perturbations, and of the localization of these functions within the cerebellum. We evaluated the role of the cerebellum in dynamic gait stability, in cutaneous reflex modulation during gait and in locomotor adaptation, all important features of neural control of gait in non-steady state conditions.In the first part, we performed three studies, focusing on the relationship between cutaneous reflex modulation and gait stability, and on the role of the cerebellum in these features. First, we studied cutaneous reflexes during backward walking in healthy controls and observed a prominent phase-dependent reflex modulation during this task. Next, we addressed the potential role of the cerebellum in the control of cutaneous reflexes. Cutaneous reflex modulation was similar between healthy controls and patients with focal cerebellar lesions, but the latter appeared less able to attenuate reflexes to self-induced stimuli. This suggests that the cerebellum is not primarily involved in cutaneous reflex modulation but that it could act in attenuation of self-induced reflex responses. The latter role in locomotion would be consistent with the common view that the cerebellum predicts sensory consequences of movement. Furthermore, biceps femoris muscle activity during the single stance phases was increased in the patient group compared to the controls. This increased activation was likely related to a co-activation strategy to reduce instability of gait. This was supported by findings in our third study, where we evaluated dynamic gait stability in patients with focal cerebellar lesions and in healthy controls. The short-term maximum Lyapunov exponent was higher in cerebellar patients, indicating reduced dynamic gait stability. Furthermore, while step width was increased and self-selected overground walking speed was decreased in the patient group while other spatio-temporal gait parameters were similar. Patients with the largest lesions in the vermis displayed the least stable gait pattern.In the second part, we focused on split-belt walking which, in the past decade, has become a popular paradigm to study the role of the cerebellum in locomotor adaptation. First, we evaluated split-belt adaptation in healthy controls and mildly ataxic patients with focal cerebellar lesions. We observed that during the split-belt adaptation experiment, patients and healthy controls globally displayed similar changes in gait parameters. However, a group difference was observed in the aftereffect of the Stance Time Symmetry: during the early phase of the post-adaptation period the relative stance times were more asymmetric for the patient group than for the control group. Patients who walked with more asymmetric relative stance times were more likely to have lesions in vermal lobules VI and Crus II. In the final study, we assessed the role of somatosensory perception in the control of split-belt walking. We observed that participants who were less able to perceive differences between belt speeds, initially walked with more asymmetric stance times during split-belt walking. This is in line with our general view that load and stretch information are important in the neural control of gait.In conclusion, the cerebellum appears important in the control in dynamic gait stability. Furthermore, our data suggests that the cerebellum is not primarily involved in cutaneous reflex modulation but that it could act in attenuation of self-induced reflex responses. Our results demonstrated that mildly ataxic cerebellar patients show no deficits in split-belt adaptation but exhibit differences in the post-adaptation period. Finally, the observed relations between speed-difference perception and gait asymmetry during split-belt walking confirmed the importance of proprioceptive information in gait control.<w:latentstyles deflockedstate="false" defunhidewhenused="true"