Corticospinal activation confounds cerebellar effects of posterior fossa stimuli

Abstract

ObjectiveTo investigate the efficacy of magnetic stimulation over the posterior fossa (PF) as a non-invasive assessment of cerebellar function in man.MethodsWe replicated a previously reported conditioning-test paradigm in 11 healthy subjects. Transcranial magnetic stimulation (TMS) at varying intensities was applied to the PF and motor cortex with a 3, 5 or 7 ms interstimulus interval (ISI), chosen randomly for each trial. Surface electromyogram (EMG) activity was recorded from two intrinsic hand muscles and two forearm muscles. Responses were averaged and rectified, and MEP amplitudes were compared to assess whether suppression of the motor output occurred as a result of the PF conditioning pulse.ResultsCortical MEPs were suppressed following conditioning-test ISIs of 5 or 7 ms. No suppression occurred with an ISI of 3 ms. PF stimuli alone also produced EMG responses, suggesting direct activation of the corticospinal tract (CST).ConclusionsCST collaterals are known to contact cortical inhibitory interneurones; antidromic CST activation could therefore contribute to the observed suppression of cortical MEPs.SignificancePF stimulation probably activates multiple pathways; even at low intensities it should not be regarded as a selective assessment of cerebellar function unless stringent controls can confirm the absence of confounding activity in other pathways.

Cerebellar transcranial magnetic stimulation: The role of coil type from distinct manufacturers

“…For each double-cone coil tested, the stimulator output intensity was set to participant's maximum tolerated intensity (MTI). To avoid potential artifacts caused by antidromic stimulation of the pyramidal tract itself [31], we first assessed the brainstem threshold. We then asked participants to pre-activate their right FDI by lifting the index finger and searched if stimulation evoked MEPs in either hand in 3 out of 6 pulses.…”

Contribution of transcranial magnetic stimulation to assessment of brain connectivity and networks

“…For instance, if the conditioning pulse intensity is too high it could activate the corticospinal track antidromically with the consequent collision of descendent volleys coming from the TMS pulse over M1 (Fisher et al, 2009). To avoid this potential confound, it is important to first determine the TMS intensity required to elicit MEPs from the posterior fossa (brainstem threshold) and then use a conditioning pulse intensity of at least 5% below the brainstem threshold to assesses CBI.…”

Laterality Differences in Cerebellar–Motor Cortex Connectivity

“…We assessed cerebellar excitability using a previously described protocol that measures cerebellar-M1 (brain) inhibition (CBI) (Ugawa et al 1995;Ugawa 1999;Pinto and Chen 2001;Daskalakis et al 2004;Galea et al 2009;Schlerf et al 2012). To avoid potential artifacts caused by antidromic stimulation of the pyramidal tract itself (Fisher et al 2009), we first assessed the brainstem threshold. To this end, we searched for MEPs by stimulating directly at the midline (over the inion) with a double cone coil (diameter 110 mm) with the stimulator current directed downward (Ugawa, Uesaka et al 1994).…”