title: Paired associative stimulation of left and right human motor cortex shapes interhemispheric motor inhibition based on a Hebbian mechanism.
abstract: This study was designed to examine whether corticocortical paired associative stimulation (cc-PAS) can modulate interhemispheric inhibition (IHI) in the human brain. Twelve healthy right-handed volunteers received 90 paired transcranial stimuli to the right and left primary motor hand area (M1(HAND)) at an interstimulus interval (ISI) of 8 ms. Left-to-right cc-PAS (first pulse given to left M1(HAND)) attenuated left-to-right IHI for one hour after cc-PAS. Left-to-right cc-PAS also increased corticospinal excitability in the conditioned right M1(HAND). These effects were not seen in an asymptomatic individual with callosal agenesis. Additional experiments showed no changes in left-to-right IHI or corticospinal excitability when left-to-right cc-PAS was given at an ISI of 1 ms or at multiple ISIs in random order. At the behavioral level, left-to-right cc-PAS speeded responses with the left but not right index finger during a simple reaction time task. Right-to-left cc-PAS (first pulse given to right M1(HAND)) reduced right-to-left IHI without increasing corticospinal excitability in left M1(HAND). These results provide a proof of principle that cc-PAS can induce associative plasticity in connections between the targeted cortical areas. The efficacy of cc-PAS to induce lasting changes in excitability depends on the exact timing of the stimulus pairs suggesting an underlying Hebbian mechanism.
authors.affiliation: Department of Neurosciences, Psychiatry and Anaesthesiology, University of Messina, Sicily, Italy.
authors.forename: V
authors.initials: V
authors.lastname: Rizzo
authors.forename: H S
authors.initials: HS
authors.lastname: Siebner
authors.forename: F
authors.initials: F
authors.lastname: Morgante
authors.forename: C
authors.initials: C
authors.lastname: Mastroeni
authors.forename: P
authors.initials: P
authors.lastname: Girlanda
authors.forename: A
authors.initials: A
authors.lastname: Quartarone
mesh_terms: D000328:Adult; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D055815:Young Adult