Differential effects of conventional transcranial direct current stimulation (tDCS) and high-definition transcranial direct current stimulation (HD-tDCS) of the cerebellum on offset analgesia

Background : Endogenous analgesic systems in the brain modulate pain perception. Offset analgesia (OA) describes the large decrease in perceived pain in response to a minor decrease in applied painful thermal stimulus. Here non-invasive brain stimulation (NIBS) of the cerebellum is used to probe OA. Methods : An OA protocol individualized to heat pain threshold (HPT) was applied via TSA-II (Medoc, Israel). Heat was ramped to HPT, with a transient temperature increase of HPT+1°C. NIBS interventions were applied prior to OA in 46 participants within a sham controlled repeated measures design. Cathodal cerebellar transcranial direct current stimulation (tDCS) and high-definition (4X1) transcranial direct current stimulation (HD-tDCS) were applied in separate experimental sessions to examine whether diffuse (tDCS) or focal (HD-tDCS) stimulation differentially modulates OA. Results : OA induced hypoanalgesia was robust, with 90% of responses showing a substantial drop in perceived pain (δVAS) following the 1°C fall in temperature, with an average VAS decrease of 38 in response to the 1°C fall in temperature. Cathodal cerebellar HD-tDCS enhances the analgesic impact of OA on four OA parameters (OA latency, VAS minimum, VAS mean and VAS 2nd max) relative to pre-stimulation. Conventional tDCS modulates two OA metrics relative to pre-stimulation (OA duration, VAS 2nd max) with an increase in OA duration following sham tDCS. Conclusion : There is a differential influence of conventional and high-definition cerebellar NIBS on OA. This is suggestive of cerebellar modulation of OA and highlights the importance of electrode montage in delineating the influence of the cerebellum in pain processing.