Left: Types of injections and referencing schemes tested in this study, with Transversal(T) and Longitudinal (L) injections performed with either a Single-ring (S) or Double-ring (D) reference, leading to four possible configurations (TS, LS, TD, LD). Right: diagram showing the electrode spacing configurations tested in this study over either transversal or longitudinal injections, from 1-off to 6-off spacing.