An Explicit Strategy Prevails When the Cerebellum Fails to Compute Movement Errors

In sensorimotor adaptation, explicit cognitive strategies are thought to be unnecessary because the motor system implicitly corrects performance throughout training. This seemingly automatic process involves computing an error between the planned movement and actual feedback of the movement. When explicitly provided with an effective strategy to overcome an experimentally induced visual perturbation, people are immediately successful and regain good task performance. However, as training continues, their accuracy gets worse over time. This counterintuitive result has been attributed to the independence of implicit motor processes and explicit cognitive strategies. The cerebellum has been hypothesized to be critical for the computation of the motor error signals that are necessary for implicit adaptation. We explored this hypothesis by testing patients with cerebellar degeneration on a motor learning task that puts the explicit and implicit systems in conflict. Given this, we predicted that the patients would be better than controls in maintaining an effective strategy assuming strategic and adaptive processes are functionally and neurally independent. Consistent with this prediction, the patients were easily able to implement an explicit cognitive strategy and showed minimal interference from undesirable motor adaptation throughout training. These results further reveal the critical role of the cerebellum in an implicit adaptive process based on movement errors and suggest an asymmetrical interaction of implicit and explicit processes

Human premotor areas parse sequences into their spatial and temporal features.

Skilled performance is characterized by precise and flexible control of movement sequences in space and time. Recent theories suggest that integrated spatio-temporal trajectories are generated by intrinsic dynamics of motor and premotor networks. This contrasts with behavioural advantages that emerge when a trained spatial or temporal feature of sequences is transferred to a new spatio-temporal combination arguing for independent neural representations of these sequence features. We used a new fMRI pattern classification approach to identify brain regions with independent vs integrated representations. A distinct regional dissociation within motor areas was revealed: whereas only the contralateral primary motor cortex exhibited unique patterns for each spatio-temporal sequence combination, bilateral premotor areas represented spatial and temporal features independently of each other. These findings advocate a unique function of higher motor areas for flexible recombination and efficient encoding of complex motor behaviours

The role of frontal cortical-basal ganglia circuits in simple and sequential visuomotor learning

Imaging, recording and lesioning studies implicate the basal ganglia and anatomically related regions of frontal cortex in visuomotor learning. Two experiments were conducted to elucidate the role of frontal cortex and striatum in visuomotor learning. Several tasks were used to characterize motor function including: a visuomotor reaction time (VSRT) task, measuring response speed and accuracy to luminance cues; simple stimulus-response (S-R) learning, measuring VSRT improvements when cues occurred in consistent locations over several trials; and a serial reaction time (SRT) task measuring motor sequence learning. SRT learning was characterized by incremental changes in reaction time (RT) when trained with the same sequence across daily sessions and by abrupt RT changes when switched to random sequence sessions. In experiment 1, rats with excitotoxic lesions in primary (M1) or secondary (M2) motor cortex, primary and secondary (M1M2) motor cortices, medial prefrontal cortex (mPF) or sham surgery were tested on these tasks. Cortical lesions slowed RT in the VSRT task but did not impair short- or long-term simple S-R learning. Cortical lesions increased RTs for the initial response of a 5-response sequence in the SRT task that was exacerbated when performing repeated (learned) sequences. All groups demonstrated visuomotor sequence learning including incremental changes in RTs for later responses in learned sequences that reversed abruptly when switched to random sequences. Rats in experiment 2 were given lesions in dorsolateral striatum, dorsomedial striatum, complete dorsal striatum, ventral striatum and sham surgery. Rats with ventral striatal lesions were unimpaired on any visuomotor task demonstrating shorter RTs than controls on most measures. Dorsomedial striatal lesions significantly impaired all VSRT performance measures. Striatal lesions had no effect on short or long-term simple S-R learning. Lesions involving dorsomedial striatum disrupted initiation of motor sequences in the SRT task. This impairment was exaggerated when performing well-learned sequences. Striatal lesions did not disrupt the incremental RT changes of later responses in the sequence indicative of motor learning. Results suggest that cortico-striatal circuits are involved in initiating learned motor sequences consistent with a role in motor planning. These circuits do not appear essential for acquisition or execution of learned visuomotor sequences

Swayed by sound: sonic guidance as a neurorehabilitation strategy in the cerebellar ataxias

Cerebellar disease leads to problems in controlling movement. The most common difficulties are dysmetria and instability when standing. Recent understanding of cerebellar function has expanded to include non -motor aspects such as emotional, cognitive and sensory processing. Deficits in the acquisition and processing of sensory information are one explanation for the movement problems observed in cerebellar ataxia. Sensory deficits result in an inability to make predictions about future events; a primary function of the cerebellum. A question therefore, is whether augmenting or replacing sensory information can improve motor performance in cerebellar disease. This question is tested in this thesis by augmenting sensory information through the provision of an auditory movement guide.A variable described in motor control theory (tau) was used to develop auditory guides that were continuous and dynamic. A reaching experiment using healthy individuals showed that the timing of peak velocity, audiomotor coordination accuracy, and velocity of approach, could be altered in line with the movement parameters embedded in the auditory guides. The thesis then investigated the use of these sonic guides in a clinical population with cerebellar disease. Performance on neurorehabilitation exercises for balance control was tested in twenty people with cerebellar atrophy, with and without auditory guides. Results suggested that continuous, predictive, dynamic auditory guidance is an effective way of improving iii movement smoothness in ataxia (as measured by jerk). In addition, generating and swaying with imaginary auditory guides was also found to increase movement smoothness in cerebellar disease.Following the tests of instantaneous effects, the thesis then investigated the longterm consequences on motor behaviour of following a two -month exercise with auditory guide programme. Seven people with cerebellar atrophy were assessed pre - and post -intervention using two measures, weight -shifting and walking. The results of the weight -shifting test indicated that the sonic -guide exercise programme does not initiate long -term changes in motor behaviour. Whilst there were minor, improvements in walking, because of the weight -shifting results, these could not be attributed to the sonic guides. This finding confirms the difficulties of motor rehabilitation in people with cerebellar disease.This thesis contributes original findings to the field of neurorehabilitation by first showing that on -going and predictive stimuli are an appropriate tool for improving motor behaviour. In addition, the thesis is the first of its kind to apply externally presented guides that convey continuous meaningful information within a clinical population. Finally, findings show that sensory augmentation using the auditory domain is an effective way of improving motor coordination in some forms of cerebellar disease