7Background. Understanding neural selectivity is essential for optimizing medical applications of deep brain 8 stimulation (DBS). We previously showed that modulation of the DBS waveform can induce changes in 9 orientation-based selectivity, and that lengthening of DBS pulses or directional segmentation can reduce 10 preferential selectivity for large axons. In this work, we sought to answer a simple, but important question: how 11 do the size and shape of the contact influence neural selectivity?
13Methods. We created multicompartment neuron models for several axon diameters and used finite element 14 modeling with standard-sized cylindrical leads to determine the effects on changing contact size and shape on 15 axon activation profiles and volumes of tissue activated. Contacts ranged in size from 0.04 to 16 mm 2 , 16 compared with a standard size of 6 mm 2 .
18Results. We found that changes in contact size induce substantial changes in orientation-based selectivity in 19 the context of a cylindrical lead, and rectangular shaping of the contact can alter this selectivity. Smaller 20 contact sizes were more effective in constraining neural activation to small, nearby axons representative of 21 grey matter. However, micro-scale contacts enable only limited spread of neural activation before exceeding 22 standard charge density limitations; further, energetic efficiency is optimized by somewhat larger contacts.
24Interpretations. Small-scale contacts are optimal for constraining stimulation in nearby grey matter and 25 avoiding orientation-selective activation. However, given charge density limitations and energy inefficiency of 26 micro-scale contacts, our results suggest that contacts about half the size of those on segmented clinical leads 27 may optimize efficiency and charge density limitation avoidance. 28 29 Introduction 30 Recently, the deep brain stimulation (DBS) field has advanced in a number of directions with the goal of 31 maximizing therapeutic benefit while minimizing side effects. Numerous reports have been published 32 describing the modulation of DBS pulse width to induce changes in axon size selectivity 1-4 . At the same time, 33 advancements have been made in DBS technology and programming. Segmented electrodes have been 34 proposed and built, enabling the directional steering of stimulation to varying degrees of precision 2,5-7 , and 35 optimization algorithms have been created and validated 8-11 , often with the goal of avoidance of maximizing 36 stimulation within a specified target, such as the subthalamic nucleus, or avoidance of other targets, such as 37 the internal capsule. Further, several groups have investigated modifications in charge balancing and leading 38 pulse polarity to optimize neural selectivity 12,13 . Finally, several groups have investigated methods of 39 generating orientation-specific selectivity, whether through the use of multiple electrodes 14,15 , multiple contact 40 cathode configurations 16 , or alterations in the charge balancing pulse or leading pulse polarity ...