Abstract The time taken to choose a response appropriate for a cue increases with the number of potential response options (Hick's law). We used this temporal constraint of choice behavior in event-related fMRI experiments to identify brain areas carrying out the translation from a cue stimulus to response: Since the process takes longer with more alternatives, the activity of the areas would increase in correlation with the number of alternatives (NA) available at the time of choice decision. During a choice saccade task in which one target is selected among many alternatives based on the color of a cue in the center, the frontal eye fields (FEF) and intraparietal sulcus (IPS) were activated. Within these regions, signals were correlated with NA at the medial part of the FEF and the posterior part of the IPS, but not at the lateral FEF and anterior IPS. This suggests a functional differentiation within each cortical region. Furthermore, given the separate fronto-parietal projections linking sub-regions that are alike in NA-correlation, cognitive steps for choice decision may occur in different networks of the sub-regions: i.e., the stimulus–response translation step in a network of the posterior IPS and medial FEF, and response execution in the anterior IPS/lateral FEF network.