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 (tau(sd)) of large myelinated axons ( approximately 30-200 micros), small axons ( approximately 200-700 micros) and cell bodies and dendrites ( approximately 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 tau(sd) was calculated using Weiss's linear approximation. Its mean value was 64.6+/-25.4 micros (SD) for the thalamic nucleus and 75.3+/-25.5 micros for the internal pallidum. Corrections to the mean values were made because the tau(sd) 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 tau(sd) estimates of 129 and 151 micros 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.