Variation in left posterior parietal-motor cortex interhemispheric facilitation following right parietal continuous theta-burst stimulation in healthy adults.
Spatial neglect is modeled on an imbalance of interhemispheric inhibition (IHI); however evidence is emerging that it may not explain neglect in all cases. The aim of this study was to investigate the IHI imbalance model of visual neglect in healthy adults, using paired pulse transcranial magnetic stimulation to probe excitability of projections from posterior parietal cortex (PPC) to contralateral primary motor cortex (M1) bilaterally. Motor-evoked potentials (MEPs) were recorded from the first dorsal interossei and facilitation was determined as ratio of conditioned to non-conditioned MEP amplitude. A laterality index reflecting the balance of excitability between the two hemispheres was calculated. A temporal order judgment task (TOJ) assessed visual attention. Continuous theta-burst stimulation was used to transiently suppress right parietal cortex activity and the effect on laterality and judgment task measured, along with associations between baseline and post stimulation measures. Stimulation had conflicting results on laterality, with most participants demonstrating an effect in the negative direction with no decrement in the TOJ task. Correlation analysis suggests a strong association between laterality direction and degree of facilitation of left PPC-to right M1 following stimulation (r=.902), with larger MEP facilitation at baseline demonstrating greater reduction (r=-.908). Findings indicate there was relative balance between the cortices at baseline but right PPC suppression did not evoke left PPC facilitation in most participants, contrary to the IHI imbalance model. Left M1 facilitation prior to stimulation may predict an individual's response to continuous theta-burst stimulation of right PPC.
Department of Rehabilitation and Aged Care, Flinders University, Adelaide, Australia; Department of Physiotherapy, Repatriation General Hospital, Adelaide, Australia.
Christopher
C
Killington
Department of Rehabilitation and Aged Care, Flinders University, Adelaide, Australia.
Christopher
C
Barr
Department of Psychology, University of South Australia, Australia.
Tobias
T
Loetscher
Discipline of Physiotherapy, Graduate School of Health, University of Technology Sydney, Sydney, Australia; Discipline of Physiotherapy, School of Health Sciences, Flinders University, Adelaide, Australia. Electronic address: Lynley.bradnam@uts.ed.au.
Lynley V
LV
Bradnam
D000328:Adult; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D007600:Judgment / Q000502:physiology; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology; D009483:Neuropsychological Tests; D010296:Parietal Lobe / Q000502:physiology*; D010468:Perceptual Disorders / Q000503:physiopathology; D013826:Theta Rhythm / Q000502:physiology; D013998:Time Perception / Q000502:physiology; D050781:Transcranial Magnetic Stimulation / Q000379:methods; D055815:Young Adult
interhemispheric inhibition; posterior parietal cortex; primary motor cortex; theta-burst stimulation