Effects of error size and error distribution on mechanisms of sensory motor adaptation

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The human motor system is subject to natural changes of the relations between muscle activation and its effects, due to growth, aging, and disease. Despite these changes, motor performance can be maintained over long periods of life thanks to the adaptive capabilities of neuronal movement control which allows compensation for these changes. In the simplest case, motor adaptation is characterized by an exponential decay of the motor error, the time constant of which is independent of the motor error and its history. However, many previous studies suggest that such a linear model only approximates the dynamics of real adaptation processes and that 1) the decay of the motor error may be described by a superposition of exponentials with different time constants, 2) the error decay depends non-linearly on the size of the motor error, and 3) the time constants prove to be time-variant on closer observation. Theoretical considerations showed that the time constant depends on both error sensitivity and retention. Correspondingly, previous explanations of the time dependence of the adaptation mechanisms suggested that error sensitivity or retention is itself (similar to the motor command) the result of a dynamic process driven by the temporal sequence of motor errors. Thus, these models consider not only the adaptation of the motor command but also a kind of meta-adaptation by which the system is able to adjust error sensitivity and retention of the primary adaptation process according to the actual conditions. Since previous studies focused on either error sensitivity or on retention, there is a lack of studies investigating both of these characteristics of the primary adaptation process in parallel.The main objective of this project is the experimental observation of the time courses of error sensitivity and retention factor as a function of error size during a single adaptation block or a sequence of adaptation blocks. In two subprojects these investigations will be performed 1) in saccade amplitude adaptation induced by intrasaccadic target displacement and 2) in reach adaptation to rotated cursor feedback. The common question is whether differences of error sensitivity and retention factor across subjects and across different conditions follow a certain pattern (either same or opposite differences) or whether these two features of motor adaptation are independent of each other. Differences across subjects mean differences within a group of healthy control subjects, as well as group differences between cerebellar patients and healthy controls. A particular focus of the study is the role of the cerebellum in the mentioned meta-adaptations.