Processing errors is a major requirement for behavioral adaptation. While it has been assumed that the basal ganglia play an important role in initiating these processes. , the role of the striatal microstructure for these processes remains to be uncovered. Previous studies in basal ganglia diseases could not elucidate the relevance of the striatal medium spiny neuron (MSN) microstructure unambiguously because structural alterations occur together with alterations in various neurotransmitter systems.We present and examine a possible model that allows the examination of MSN dysfunction unbiased by other modulations, i.e. a case of 'benign hereditary chorea' (BHC) in comparison to healthy controls. We apply event-related potentials (ERPs) to uncover the underlying neurophysiological mechanisms underlying post-error behavioral adaptation. The BHC patient revealed a smaller error-related negativity (ERN) together with almost absent behavioral adaptation after an error and generally more error-prone behavior. Performance monitoring processes unrelated to errors, as well as response inhibition processes, were not affected in the BHC patient. The results suggest that the striatal MSN microstructural integrity is more important for error-related behavioral adaptation than for other response monitoring processes unrelated to errors.