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

BackgroundOffset 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) is used to modulate the cerebellum, since the cerebellum is known to signal sensory prediction errors and is implicated in pain processing. MethodsAn OA protocol individualized to heat pain threshold (HPT) was applied via TSA-II (Medoc, Israel). NIBS interventions were applied prior to OA. Cathodal cerebellar transcranial direct current stimulation (tDCS) and high-definition (4X1) transcranial direct current stimulation (HD-tDCS) were applied to 46 healthy participants within a sham controlled repeated measures design to examine whether diffuse or focal stimulation differentially modulates OA. ResultsOA induced hypoalgesia was robust, with 90% of responses showing a drop in perceived pain ({delta}VAS) > 10 following the 1{degrees}C fall in temperature. This OA response was augmented following a protocol with sham and focal cathodal cerebellar stimulation on four OA parameters (OA latency, VAS minimum, VAS mean and VAS 2nd max) relative to pre-stimulation. This effect was differential to the protocol with sham and conventional tDCS where two OA metrics altered (OA duration, VAS 2nd max). ConclusionOA enhancement via cathodal cerebellar NIBS may involve both a placebo effect and sustaining a noxious sensory prediction error. Understanding how the cerebellum is involved in OA could enhance therapies for pain patients.