FIG. 8.

The extent of GPi and GPe efferent activation depended in different ways on the electrode contact and stimulation amplitude. A: activation of GPe fibers of passage exhibited a nonuniform spatial profile with the highest percentage activation occurring when stimulating through contact 2. B: GPi neuronal output during DBS, especially at −2 V, exhibited a distinct spatial tuning with maximum activation at contact 1. Population responses are shown for DBS voltages at −2 V (black) and −5 V (gray). Activation was defined as driving axonal output ≥80% of the stimulation frequency. C: activation ratios of GPi to GPe output were more pronounced at lower stimulation voltages such that at contact 3, a greater percentage of GPe efferents than GPi efferents were excited. On increasing the stimulus intensity to −5 V, activation ratios showed less variability and were >1 across all contacts.