Some Neurophysiological Effects of Cerebellar Stimulation in Man

This paper presents the results of neurophysiological studies of the effects of cerebellar stimulation on H reflexes, late reflexes, blink reflexes, evoked potentials and EEG patterns in 18 human subjects (Male 13, Female 5, Age 25.8+/-10.0, Epileptic 9, Cerebral Palsy 9). In addition to the effects of cerebellar stimulation on the H reflex studies on soleus we assessed V1 and V2, "late" responses (Upton et al., 1971), cortical somatosensory evoked potentials (SSEP) after median nerve stimulation, and visual evoked responses (VER) after flash stimulation. Experiments were extended to assess recovery curves of all the potentials and we examined the effects of changes on the rate or voltage of cerebellar stimulation. Cerebellar stimulation was inhibitory to all the responses except the visual evoked potentials. Serial studies in five patients produced consistent results. Preoperative and postoperative results were compared in two patients with no significant difference in the results in the absence of cerebellar stimulation. Ipsilateral cerebellar stimulation (CS) had the greatest inhibitory effects on H, V1 and V2 responses in the arm and leg whereas contralateral CS produced the greatest effects on cortical SSEPs. There was a greater bilateral effect on SSEPs and reflex responses after right CS than left CS and this may be the first indication of "dominance" in the cerebellar hemispheres. Cerebellar stimulation in patients on diphenylhydantoin produced minimal effects on SSEP's and this observation has led to further studies in patients taking diphenylhydantoin. Recovery of amplitude of the reflex and cortical responses took eight to 30 minutes after one minute of cerebellar stimulation. Serial CS of one minute on and one minute off produced increasing inhibition of SSEP's and reflexes for up to five stimulations. Recovery after cessation of cerebellar stimultion was associated with rebound excitation in six patients, the rebound being noted in the amplitude of H reflexes and SSEP's as well as in the frequency of paroxysmal spike and wave discharges in the EEG. The correlation of the results of such quantitative neurophysiological tests with clinical improvement may allow prediction of clinical improvement may allow prediction of clinical results after cerebellar stimulation. These techniques have already been used to measure the threshold of stimulation and may allow optimal stimulation characteristics to be assessed. The prolonged neurophysiological effects of stimulation may allow the use of maximum effective intervals between optimal epochs of stimulation so that any cerebellar damage can be minimized.