1. A computer-implemented method comprising: storing in a memory device a volume of tissue activation (VTA) data structure that is derived from analysis of a plurality of patients; receiving, by a computer processor, patient data for a given patient, the patient data representing an assessment of a patient condition; and evaluating, by the computer processor, the VTA data structure relative to the patient data to determine a target VTA for achieving a desired therapeutic effect for the given patient; wherein the VTA data structure comprises a plurality of statistical atlases, each of the plurality of statistical atlases comprising data that statistically characterizes a therapeutic effect for a plurality of VTAs acquired for the plurality of patients, and the plurality of statistical atlases are hierarchically organized according to one or more categories and a plurality of subcategories.