The contribution of imbalance between interhemispheric parietal – motor facilitation and visual neglect in healthy adults.

Author: Christopher Killington

Killington, Christopher, 2017 The contribution of imbalance between interhemispheric parietal – motor facilitation and visual neglect in healthy adults., Flinders University, School of Health Sciences

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Spatial neglect is a common disorder following stroke, characterised by a failure to acknowledge stimuli in contralesional space. One neurophysiological model to explain neglect is based on an imbalance of interhemispheric inhibition (IHI); however, evidence is emerging that the IHI imbalance model may not explain neglect in all cases. The aim of this study was to investigate the IHI imbalance model of neglect in healthy adults, using paired-pulse transcranial magnetic stimulation (TMS) to probe excitability of projections from the posterior parietal cortex (PPC) to the contralateral primary motor cortex (M1) bilaterally.

Motor-evoked potentials (MEPs) were recorded from the first dorsal interossei muscle of each hand. The excitability of the PPC to M1 projections was determined by MEP facilitation, determined as a ratio of conditioned to non-conditioned MEP amplitude. A laterality index (LI) reflecting the balance of excitability between the two hemispheres was then calculated. A temporal order judgement task and a landmark task assessed visual attention. To evoke a visual neglect-like response in healthy adults, continuous theta-burst stimulation was used to transiently suppress right parietal cortex activity. The effect on laterality and visual tasks was assessed, along with associations between baseline and post-stimulation measures.

Stimulation had conflicting results on the LI, with most participants demonstrating a non-significant effect in the negative direction, with no decrement in the temporal order judgement or landmark task. The negative shift in LI reflected a small rightward shift in hemispheric balance. Correlation analysis suggested a strong association between laterality direction and degree of facilitation of left PPC to right M1 following stimulation (r = 0.902).

The findings indicate that there was relative balance between the cortices at baseline, but that right PPC suppression did not evoke left PPC facilitation in most participants, contrary to predictions of the IHI imbalance model. The degree of left M1 facilitation prior to stimulation may predict an individual’s response to continuous theta-burst stimulation of right PPC.

Keywords: Paired pulse transcranial magenetic stimulation, Hemispheric rivalry model of neglect, Interhemispheric imbalance

Subject: Health Sciences thesis

Thesis type: Masters
Completed: 2017
School: School of Health Sciences
Supervisor: Stacey George