title: Inter-subject Variability in Electric Fields of Motor Cortical tDCS.
abstract: BACKGROUND
  The sources of inter-subject variability in the efficacy of transcranial direct current stimulation (tDCS) remain unknown. One potential source of variations is the brain's electric field, which varies according to each individual's anatomical features.
  
  
  OBJECTIVE
  We employed an approach that combines imaging and computational modeling to quantitatively study the extent and primary causes of inter-subject variation in tDCS electric fields.
  
  
  METHODS
  Anatomically-accurate models of the head and brain of 24 males (age: 38.63 ± 11.24 years) were constructed from structural MRI. Finite-element method was used to computationally estimate the electric fields for tDCS of the motor cortex. Surface-based inter-subject registration of the electric field and functional MRI data was used for group level statistical analysis.
  
  
  RESULTS
  We observed large differences in each individual's electric field patterns. However, group level analysis revealed that the average electric fields concentrated in the vicinity of the primary motor cortex. The variations in the electric fields in the hand motor area could be characterized by a normal distribution with a standard deviation of approximately 20% of the mean. The cerebrospinal fluid (CSF) thickness was the primary factor influencing an individual's electric field, thereby explaining 50% of the inter-individual variability, a thicker layer of CSF decreasing the electric field strength.
  
  
  CONCLUSIONS
  The variability in the electric fields is related to each individual's anatomical features and can only be controlled using detailed image processing. Age was found to have a slight negative effect on the electric field, which might have implications on tDCS studies on aging brains.
authors.affiliation: Department of Computer Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan. Electronic address: laakso.ilkka@nitech.ac.jp.
authors.forename: Ilkka
authors.initials: I
authors.lastname: Laakso
authors.affiliation: Laboratory of Psychology, Hamamatsu University School of Medicine, Shizuoka 431-3192, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Tanaka
authors.affiliation: School of Life Sciences, The Graduate University for Advanced Studies, Kanagawa 240-0193, Japan; Division of Cerebral Integration, National Institute for Physiological Sciences, Aichi 444-8585, Japan.
authors.forename: Soichiro
authors.initials: S
authors.lastname: Koyama
authors.affiliation: Department of Computer Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan.
authors.forename: Valerio
authors.initials: V
authors.lastname: De Santis
authors.affiliation: Department of Computer Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan.
authors.forename: Akimasa
authors.initials: A
authors.lastname: Hirata
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D058256:Brain Waves; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D065908:Transcranial Direct Current Stimulation / Q000379:methods*
keywords: Electric field; Finite-element method; Inter-subject variability; Motor cortex; tDCS