Neuroimaging studies suggest that the cerebellum contributes to human cognitive processing, particularly procedural learning. This type of learning is often described as implicit learning and involves automatic, associative, and unintentional learning processes. Our aim was to investigate whether cerebellar transcranial direct current stimulation (tDCS) influences procedural learning as measured by the serial reaction time task (SRTT), in which subjects make speeded key press responses to visual cues. A preliminary modeling study demonstrated that our electrode montage (active electrode over the cerebellum with an extra-cephalic reference) generated the maximum electric field amplitude in the cerebellum. We enrolled 21 healthy subjects (aged 20-49 years). Participants did the SRTT, a visual analogue scale and a visual attention task, before and 35 min after receiving 20-min anodal and sham cerebellar tDCS in a randomized order. To avoid carry-over effects, experimental sessions were held at least 1 week apart. For our primary outcome measure (difference in RTs for random and repeated blocks) anodal versus sham tDCS, RTs were significantly slower for sham tDCS than for anodal cerebellar tDCS (p = 0.04), demonstrating that anodal tDCS influenced implicit learning processes. When we assessed RTs for procedural learning across the one to eight blocks, we found that RTs changed significantly after anodal stimulation (interaction "time" × "blocks 1/8": anodal, p = 0.006), but after sham tDCS, they remained unchanged (p = 0.094). No significant changes were found in the other variables assessed. Our finding that anodal cerebellar tDCS improves an implicit learning type essential to the development of several motor skills or cognitive activity suggests that the cerebellum has a critical role in procedural learning. tDCS could be a new tool for improving procedural learning in daily life in healthy subjects and for correcting abnormal learning in neuropsychiatric disorders.

Modeling approach reveals that during cerebellar tDCS the current spread to other structures outside the cerebellum is unlike to produce functional effects.

Two neuromodulatory techniques based on applying direct current (DC) non-invasively through the skin, transcranial cerebellar direct current stimulation (tDCS) and transcutaneous spinal DCS, can induce prolonged functional changes consistent with a direct influence on the human cerebellum and spinal cord. In this article we review the major experimental works on cerebellar tDCS and on spinal tDCS, and their preliminary clinical applications. Cerebellar tDCS modulates cerebellar motor cortical inhibition, gait adaptation, motor behaviour, and cognition (learning, language, memory, attention). Spinal tDCS influences the ascending and descending spinal pathways, and spinal reflex excitability. In the anaesthetised mouse, DC stimulation applied under the skin along the entire spinal cord may affect GABAergic and glutamatergic systems. Preliminary clinical studies in patients with cerebellar disorders, and in animals and patients with spinal cord injuries, have reported beneficial effects. Overall the available data show that cerebellar tDCS and spinal tDCS are two novel approaches for inducing prolonged functional changes and neuroplasticity in the human cerebellum and spinal cord, and both are new tools for experimental and clinical neuroscientists. Abbreviations CBI, cerebello-brain inhibition; cerebellar tDCS, transcranial cerebellar direct current stimulation; MEP, motor-evoked potential; PAS, paired associative stimulation; rTMS, repetitive transcranial magnetic stimulation; SCI, spinal cord injury; spinal tDCS, transcutaneous spinal direct current stimulation; TMS, transcranial magnetic stimulation.Alberto Priori is the head of the Clinical Center for Neurostimulation, Neurotechnology and Movement Disorders at the Fondazione IRCCS Ca' Granda Ospedale Maggiore di Milano and professor of neurology at the University of Milan Italy. He pioneered research on the effect of DC on the human motor cortex in the 1990s and more recently led his research group in studying how constant electric fields influence the human cerebellum and spinal cord. Matteo Ciocca is a neurologist and clinical research fellow. His research activity assesses the effects of spinal cord DC stimulation in healthy humans. Marta Parazzini is an engineer. Her research activity deals with the computational modeling of the interactions between electromagnetic field and biological system. She developed the first computational modeling studies specifically designed for cerebellar and spinal cord DC stimulation. Maurizio Vergari is a neurophysiology technician, who dealt with the technical aspects of cerebellar and spinal cord direct current stimulation since the first experiments. Roberta Ferrucci is a psychologist, postdoctoral fellow at the University of Milan. She began her research work by developing the technique for cerebellar DC stimulation and the cognitive and behavioural experimental protocols to assess its effects.

TRT was equally effective with sound generator or open ear hearing aids: they gave basically identical, statistically indistinguishable results.