Large Evoked Responses and Synchronisation Recorded over the Cerebellum in Human Subjects in Response to Postural Perturbations.

In this study we recorded EEG changes over the cerebellum and leg muscle EMG in response to externally applied, stereotyped postural perturbations during standing. Ten healthy adults underwent mechanical perturbations delivered at the shoulders, applied by a torque motor. There was a standing force of 1.17 N and maximum additional force of 18.8 N. Five intermingled conditions were used: four consisted of backwards pulls (75-465 ms duration) and one a "let go" condition (200 ms). EEG was recorded using a cerebellar-extended montage with detailed analyses at the midline Iz electrode. Lower limb EMG (tibialis anterior and soleus) and accelerometry were collected concurrently. Posterior perturbations consistently evoked early postural EMG responses with inhibition of soleus and excitation in tibialis anterior (mean latency 90.2-94.2 ms). Associated with this was a prominent early biphasic cerebellar potential at Iz (P80-N100; mean amplitude approximately 40 µV peak to peak). The properties of this response - its positive onset, location and amplitude - are consistent with Purkinje neuron discharge. RMS averages for EEG activity above 30 Hz revealed patterns localised to the posterior fossa. Responses at Iz consisted of multiple bursts of excitation and inhibition followed by inhibition when the longer perturbations reduced. Single-trial analyses revealed biphasic potentials with short inter-peak intervals (~ 5 ms) synchronised to the excitatory bursts. We conclude that postural perturbations evoke a short latency potential over the cerebellum, likely to represent Purkinje neuron discharge due to climbing fibre input, in turn followed by synchronisation and further facilitation. Our findings support the view that synchronised climbing fibre activity occurs in response to significant sensory events and may facilitate compensatory motor responses.