Inconsistent and conflicting results about hand reaction time (RT) asymmetries have led to controversies over lateralization and hemispheric specialization for visuomotor transformations. In this paper, we propose a methodological solution to separate the central and peripheral processes that may be differently involved in simple visuomotor asymmetries. We provide empirical data from a simple RT task that suggests that fractionating RT into motor and premotor components with respect to a change in the electromyographic activity is necessary to disentangle motor and premotor lateralization patterns. These results call for a refinement of classical studies of visuomotor transformations using electrophysiological correlates of embedded but unrelated asymmetries rather than overt psychophysical measurements alone.