Abstract

In daily life, we frequently execute reaching movements, for example to be able to grasp our mobile phone. The processing of movement direction is fundamental to efficiently reach the target object. Many neurophysiological studies reported neuronal populations selective for movement direction in many regions of the monkey brain. In my thesis, I investigated which areas in the human brain show directional selectivity. Moreover, I measured to what extent directional selective regions are sensitive to changes in other movement parameters, like the type of motor act and movement amplitude. In three functional magnetic resonance imaging (fMRI) experiments, participants were adapted to execute reaching movements in the adaptation direction. Occasionally, test trials were presented. Test trials differed from adaptation trials in movement direction only, or in movement direction as well as in another movement parameter (Experiment 1 and 2: type of motor act; Experiment 3: movement amplitude). By comparing the fMRI signal in conditions where only movement direction was manipulated with conditions where also other movement parameters changed, we were able to measure sensitivity of directionally tuned neuronal populations to these additional movement parameters. Multiple regions in the human visuomotor system showed selectivity for movement direction. This selectivity was modulated by the type of motor act to varying degrees, with the largest effect in M1 and the smallest modulation in the parietal reach region. Moreover, directional selectivity was clearly sensitive also to changes in movement amplitude. These results extend the current knowledge on the representation of actions from monkey physiology to the human brain and furthermore may have important practical implications for restoring lost motor functions in tetraplagic patients.