title: Cerebellar transcranial magnetic stimulation: the role of coil geometry and tissue depth.
abstract: BACKGROUND
  While transcranial magnetic stimulation (TMS) coil geometry has important effects on the evoked magnetic field, no study has systematically examined how different coil designs affect the effectiveness of cerebellar stimulation.
  
  
  OBJECTIVE
  The depth of the cerebellar targets will limit efficiency. Angled coils designed to stimulate deeper tissue are more effective in eliciting cerebellar stimulation.
  
  
  METHODS
  Experiment 1 examined basic input-output properties of the figure-of-eight, batwing and double-cone coils, assessed with stimulation of motor cortex. Experiment 2 assessed the ability of each coil to activate cerebellum, using cerebellar-brain inhibition (CBI). Experiment 3 mapped distances from the scalp to cerebellar and motor cortical targets in a sample of 100 subjects' structural magnetic resonance images.
  
  
  RESULTS
  Experiment 1 showed batwing and double-cone coils have significantly lower resting motor thresholds, and recruitment curves with steeper slopes than the figure-of-eight coil. Experiment 2 showed the double-cone coil was the most efficient for eliciting CBI. The batwing coil induced CBI only at higher stimulus intensities. The figure-of-eight coil did not elicit reliable CBI. Experiment 3 confirmed that cerebellar tissue is significantly deeper than primary motor cortex tissue, and we provide a map of scalp-to-target distances.
  
  
  CONCLUSIONS
  The double-cone and batwing coils designed to stimulate deeper tissue can effectively stimulate cerebellar targets. The double-cone coil was found to be most effective. The depth map provides a guide to the accessible regions of the cerebellar volume. These results can guide coil selection and stimulation parameters when designing cerebellar TMS studies.
authors.affiliation: Department of Physical Medicine and Rehabilitation, Johns Hopkins University, Baltimore, MD, USA; Behavioural Brain Sciences, School of Psychology, University of Birmingham, Birmingham, UK. Electronic address: rhardwi1@jhmi.edu.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Hardwick
authors.affiliation: Behavioural Brain Sciences, School of Psychology, University of Birmingham, Birmingham, UK.
authors.forename: Elise
authors.initials: E
authors.lastname: Lesage
authors.affiliation: Behavioural Brain Sciences, School of Psychology, University of Birmingham, Birmingham, UK.
authors.forename: R Chris
authors.initials: RC
authors.lastname: Miall
mesh_terms: D000293:Adolescent; D000328:Adult; D002531:Cerebellum / Q000502:physiology*; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D012535:Scalp / Q000502:physiology; D050781:Transcranial Magnetic Stimulation / Q000295:instrumentation* / Q000379:methods*; D055815:Young Adult
keywords: Batwing coil; Cerebellar brain inhibition; Cerebello brain inhibition; Cerebellum; Deep TMS; Double cone coil; Figure-of-eight coil; TMS; TMS coil geometry