Identification of the target neuronal elements in electrical deep brain stimulation

The aim of this study is to identify the primary neuronal target elements in electrical deep‐brain stimulation, taking advantage of the difference in strength–duration time constant (τsd) of large myelinated axons (≈ 30–200 µs), small axons (≈ 200–700 µs) and cell bodies and dendrites (≈ 1–10 ms). Strength–duration data were measured in patients suffering from Parkinson's disease or essential tremor and treated by high‐frequency stimulation in the ventral intermediate thalamic nucleus or the internal pallidum. Threshold voltages for the elimination of tremor were determined at various pulsewidths and a pulse rate of 130 pulses per second. The τsd was calculated using Weiss's linear approximation. Its mean value was 64.6 ± 25.4 µs (SD) for the thalamic nucleus and 75.3 ± 25.5 µs for the internal pallidum. Corrections to the mean values were made because the τsd values were based on voltage–duration measurements using polarizable electrodes. Apart from this systematic error, a resolution error, due to the relatively large increment steps of the pulse amplitude, was taken into account, resulting in mean τsd estimates of 129 and 151 µs for the thalamic nucleus and the internal pallidum, respectively. It is concluded that the primary targets of stimulation in both nuclei are most probably large myelinated axons.