1. A computer system for generating an estimated volume of activation (VOA), the system comprising: a computer processor configured to: for each of a plurality of neural elements that are in a vicinity of a leadwire: determine a parameter that characterizes the respective neural element; based on the determined parameter, select one or more of a plurality of parameters that are each stored in association with a respective threshold activation value: based on the selected one or more of the plurality of stored parameters and the associated one or more respective threshold activation values, set a respective threshold activation value of the respective neural element, wherein the respective neural element becomes activated at the respective threshold activation value and is not activated below the respective threshold activation value; and determine whether a user-input stimulation value to be applied to the leadwire meets the respective threshold activation value set for the respective neural element: and generate a surface structure corresponding to those of the neural elements whose respective threshold activation values have been determined to be met by the user-input stimulation value.