Cerebellar modulation of human associative plasticity

Key point Increases in the strength of synaptic connections in the motor cortex (long term potentiation) can be induced in humans by repetitively pairing peripheral nerve stimuli and motor cortex transcranial magnetic stimuli given 21–25 ms apart – paired associative stimulation (PAS). This ‘associative plasticity’ effect has been assumed to relate to synchronicity between sensory input and motor output, with a similar mechanism proposed to underlie effects at all interstimulus intervals. Here we show that modulation of cerebellar activity using transcranial direct current stimulation can abolish associative plasticity in the motor cortex, but only for sensory/motor stimuli paired at 25 ms, not at 21.5 ms. The results indicate that human associative plasticity can be affected by cerebellar activity and that at least two different mechanisms are involved in the effects previously reported in studies using PAS at different inter‐stimulus intervals.