Cerebellar theta burst stimulation impairs eyeblink classical conditioning

Theta burst stimulation (TBS) protocols of repetitive transcranial magnetic stimulation (rTMS) have after-effects on excitability of motor areas thought to be due to LTP- and LTD-like processes at cortical synapses. The present experiments ask whether, despite the low intensities of stimulation used and the anatomy of the posterior fossa, TBS can also influence the cerebellum. Acquisition and retention of eyeblink classical conditioning (EBCC) was examined in 30 healthy volunteers after continuous theta burst stimulation (cTBS) over the right cerebellar hemisphere. In subjects who received cerebellar cTBS, conditioned responses were fewer and their onsets were earlier (in the last half of the acquisition blocks) than those from control subjects. There was, however, no effect of cerebellar cTBS on the re-acquisition of EBCC in another session of EBCC 7–10 days later. There was also no effect of cerebellar cTBS on the re-acquisition of EBCC in subjects not naive to EBCC when the stimulation was delivered immediately before a re-acquisition session. Control experiments verified that suppressive effects of cTBS on EBCC were not due to changes in motor cortical excitability or sensory disturbance caused by cTBS. Based on previous EBCC studies in various cerebellar pathologies, our data are compatible with the hypothesis that cerebellar cTBS has a focal cerebellar cortical effect, and are broadly in line with data from studies of EBCC in various animal models. These results confirm that cerebellar TBS has measurable effects on the function of the cerebellum, and indicate it is a useful non-invasive technique with which to explore cerebellar physiology and function in humans.