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I WOULD LIKE TO BE CONSIDERED FOR A PLATFORM PRESENTATION? YES or NO Estimation of the Spatial Extent of Activation in Thalamic Deep Brain Stimulation John Doe 1, Scott E. Baker 2, Janice M. Holson 3 , Scott Adams 3 , and Wyatt M. Watson 1 Department of Biomedical Engineering, Duke University, Durham, NC 2 Department of Neurology, Cleveland Clinic Foundation, Cleveland, OH 3 Department of Neurosurgery, Cleveland Clinic Foundation, Cleveland, OH Successful treatment of movement disorders with deep brain stimulation (DBS) requires accurate electrode placement within the target brain area and the selection of appropriate stimulus parameters. The objectives of this research are first to quantify the effects of varying voltage and electrode positions (active contact on the DBS lead) on the responses to DBS, and second, to use these data to determine the spatial extent of neuronal activation during DBS. Nine subjects with essential tremor and DBS of the ventral intermediate nucleus of the thalamus, at least 3 months postimplant, were tested. Tremor amplitude and side-effect intensity were recorded at 5 to 10 voltages for each of the four electrode contacts in monopolar configuration with 90µs pulse width and 160 Hz frequency. In all subjects, the most ventral contact required the lowest voltage to elicit side effects, which invariably were paresthesias, while the most dorsal contact required the highest voltage to elicit side effects. The contact that required the least voltage for maximal tremor suppression varied across subjects. This variation could represent either a difference in the electrode location relative to the site of tremor suppression or anatomical variation in the site of tremor suppression itself. Stimulus (voltage) - Response (side effect intensity or tremor power) curves generated from the empirical data and the threshold (V th) - distance (r) relationship, defined as V th = V o + kr 2 (V o is the offset, k is the slope), were used to estimate the distance from the electrode to the anatomical targets that either caused SE or alleviated tremor. Model-based studies with r = 4 mm and k = 0.2 V/mm 2 indicated that in the presence of 10 percent noise, r and k were estimated with a mean error of 0.32 mm and 0.02 V/mm 2, respectively. These results indicate that the stimulus-response data can be used to estimate the distance to the neuronal elements responsible for the effects of DBS and, thus, can determine the spatial extent of stimulation. Understanding the effects of contact configuration on the clinical response to DBS and quantifying the spread of activation may improve electrode targeting and the techniques of stimulus parameter adjustment. This work was supported by NIH R01-NS-40894 and NIH T32-GM07535.