Encoding of sensory prediction errors in the human cerebellum.

A central tenet of motor neuroscience is that the cerebellum learns from sensory prediction errors. Surprisingly, neuroimaging studies have not revealed definitive signatures of error processing in the cerebellum. Furthermore, neurophysiologic studies suggest an asymmetry, such that the cerebellum may encode errors arising from unexpected sensory events, but not errors reflecting the omission of expected stimuli. We conducted an imaging study to compare the cerebellar response to these two types of errors. Participants made fast out-and-back reaching movements, aiming either for an object that delivered a force pulse if intersected or for a gap between two objects, either of which delivered a force pulse if intersected. Errors (missing the target) could therefore be signaled either through the presence or absence of a force pulse. In an initial analysis, the cerebellar BOLD response was smaller on trials with errors compared with trials without errors. However, we also observed an error-related decrease in heart rate. After correcting for variation in heart rate, increased activation during error trials was observed in the hand area of lobules V and VI. This effect was similar for the two error types. The results provide evidence for the encoding of errors resulting from either the unexpected presence or unexpected absence of sensory stimulation in the human cerebellum

Dynamic modulation of cerebellar excitability for abrupt, but not gradual, visuomotor adaptation

The cerebellum is critically important for error driven adaptive motor learning, as evidenced by the fact that cerebellar patients do not adapt well to sudden predictable perturbations. However, recent work has shown that cerebellar patients adapt much better if the perturbation is gradually introduced. Here we explore physiological mechanisms that underlie this distinction between abrupt and gradual motor adaptation in humans. We used Transcranial Magnetic Stimulation (TMS) to evaluate whether neural mechanisms within the cerebellum contribute to either process during a visuomotor reach adaptation. When a visuomotor rotation was introduced abruptly, cerebellar excitability changed early in learning, and approached baseline levels near the end of the adaptation block. However, we observed no modulation of cerebellar excitability when we presented the visuomotor rotation gradually during learning. Similarly, we did not observe cerebellar modulation during trial-by-trial adaptation to random visuomotor displacements or during reaches without perturbations. This suggests that the cerebellum is most active during the early-phases of adaptation when large perturbations are successfully compensated

The cerebellum and motor learning

During our daily lives, we make thousands of movements. When we stop and consider that doing something as ordinary as reaching for a glass of juice involves the precise sequential contraction of dozens of muscles simply to move our hand, we appreciate the immense problem that our brains are solving. If we then recognize that both the world and the body are constantly changing, the accuracy with which we move becomes quite staggering. Moving with such proficiency requires the motor system to be continuously learning and adapting. A host of neural structures are important for this behavior. One remarkable part of this system is the cerebellum, or "little brain": a phylogenetically ancient neural structure, containing over half of the neurons in the human central nervous system. Damage to this structure results in a loss of coordination, with marked impairments in the control of eye movements, the timing of simple rhythmic movements, and most intriguingly the ability to adjust well-learned motor skills. The aim of this dissertation is to explore the processes of motor control and learning, with a special emphasis on the functional contribution of the cerebellum. Following a short introduction (Chapter 1), empirical evidence is provided from two classes of behavior. Chapter 2 deals with the production of rhythmic movements in a population of patients with cerebellar pathology. Chapters 3 through 5 involve the production of goal-directed reaching movements, carefully investigating the representation and correction of errors through the combined use of psychophysics, brain imaging, and patient studies. In Chapter 2, patients with cerebellar pathology are observed to be impaired when producing rhythmic movements, particularly when the movements contain a distinct event that can be used to determine the performance error. In Chapter 3, we observe that by reshaping a target region, we can predictably impact the correction of movement errors during reaching movements toward that target. In Chapter 4, we provide physiological evidence of the representation of movement errors within the cerebellum, an effect only observed when appropriate measures are taken to factor out the effects of changes in heart rate. In Chapter 5, we show that patients with cerebellar pathology are impaired in adjusting their movements to counteract a visual perturbation, and furthermore suggest that this impairment is equivalent whether the perturbation is applied suddenly or gradually. Taken together, this work demonstrates that we learn to make better movements by rapidly evaluating our movements with respect to our goals, and correcting any mistakes with the help of the cerebellum

The Cerebellum and Motor Learning

During our daily lives, we make thousands of movements. When we stop and consider that doing something as ordinary as reaching for a glass of juice involves the precise sequential contraction of dozens of muscles simply to move our hand, we appreciate the immense problem that our brains are solving. If we then recognize that both the world and the body are constantly changing, the accuracy with which we move becomes quite staggering. Moving with such proficiency requires the motor system to be continuously learning and adapting. A host of neural structures are important for this behavior. One remarkable part of this system is the cerebellum, or "little brain": a phylogenetically ancient neural structure, containing over half of the neurons in the human central nervous system. Damage to this structure results in a loss of coordination, with marked impairments in the control of eye movements, the timing of simple rhythmic movements, and most intriguingly the ability to adjust well-learned motor skills.The aim of this dissertation is to explore the processes of motor control and learning, with a special emphasis on the functional contribution of the cerebellum. Following a short introduction (Chapter 1), empirical evidence is provided from two classes of behavior. Chapter 2 deals with the production of rhythmic movements in a population of patients with cerebellar pathology. Chapters 3 through 5 involve the production of goal-directed reaching movements, carefully investigating the representation and correction of errors through the combined use of psychophysics, brain imaging, and patient studies.In Chapter 2, patients with cerebellar pathology are observed to be impaired when producing rhythmic movements, particularly when the movements contain a distinct event that can be used to determine the performance error. In Chapter 3, we observe that by reshaping a target region, we can predictably impact the correction of movement errors during reaching movements toward that target. In Chapter 4, we provide physiological evidence of the representation of movement errors within the cerebellum, an effect only observed when appropriate measures are taken to factorout the effects of changes in heart rate. In Chapter 5, we show that patients with cerebellar pathology are impaired in adjusting their movements to counteract a visual perturbation, and furthermore suggest that this impairment is equivalent whether the perturbation is applied suddenly or gradually.Taken together, this work demonstrates that we learn to make better movements by rapidly evaluating our movements with respect to our goals, and correcting any mistakes with the help of the cerebellum