title: Structural Organization of the Corpus Callosum Predicts Attentional Shifts after Continuous Theta Burst Stimulation.
abstract: Repetitive transcranial magnetic stimulation (rTMS) applied over the right posterior parietal cortex (PPC) in healthy participants has been shown to trigger a significant rightward shift in the spatial allocation of visual attention, temporarily mimicking spatial deficits observed in neglect. In contrast, rTMS applied over the left PPC triggers a weaker or null attentional shift. However, large interindividual differences in responses to rTMS have been reported. Studies measuring changes in brain activation suggest that the effects of rTMS may depend on both interhemispheric and intrahemispheric interactions between cortical loci controlling visual attention. Here, we investigated whether variability in the structural organization of human white matter pathways subserving visual attention, as assessed by diffusion magnetic resonance imaging and tractography, could explain interindividual differences in the effects of rTMS. Most participants showed a rightward shift in the allocation of spatial attention after rTMS over the right intraparietal sulcus (IPS), but the size of this effect varied largely across participants. Conversely, rTMS over the left IPS resulted in strikingly opposed individual responses, with some participants responding with rightward and some with leftward attentional shifts. We demonstrate that microstructural and macrostructural variability within the corpus callosum, consistent with differential effects on cross-hemispheric interactions, predicts both the extent and the direction of the response to rTMS. Together, our findings suggest that the corpus callosum may have a dual inhibitory and excitatory function in maintaining the interhemispheric dynamics that underlie the allocation of spatial attention.
  
  
  CONCLUSIONS
  The posterior parietal cortex (PPC) controls allocation of attention across left versus right visual fields. Damage to this area results in neglect, characterized by a lack of spatial awareness of the side of space contralateral to the brain injury. Transcranial magnetic stimulation over the PPC is used to study cognitive mechanisms of spatial attention and to examine the potential of this technique to treat neglect. However, large individual differences in behavioral responses to stimulation have been reported. We demonstrate that the variability in the structural organization of the corpus callosum accounts for these differences. Our findings suggest novel dual mechanism of the corpus callosum function in spatial attention and have broader implications for the use of stimulation in neglect rehabilitation.
authors.affiliation: Department of Experimental Psychology, University of Oxford, Oxford OX1 3UD, United Kingdom, magdalena.chechlacz@psy.ox.ac.uk.
authors.forename: Magdalena
authors.identifier: http://orcid.org/0000-0003-1811-3946
authors.initials: M
authors.lastname: Chechlacz
authors.affiliation: Department of Experimental Psychology, University of Oxford, Oxford OX1 3UD, United Kingdom.
authors.forename: Glyn W
authors.initials: GW
authors.lastname: Humphreys
authors.affiliation: FMRIB Centre and.
authors.forename: Stamatios N
authors.initials: SN
authors.lastname: Sotiropoulos
authors.affiliation: Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford OX3 9UD, United Kingdom, and.
authors.forename: Christopher
authors.initials: C
authors.lastname: Kennard
authors.affiliation: Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford OX3 9UD, United Kingdom, and Gerontechnology and Rehabilitation Group, University of Bern, 3010 Bern, Switzerland.
authors.forename: Dario
authors.initials: D
authors.lastname: Cazzoli
mesh_terms: D000328:Adult; D001288:Attention / Q000502:physiology*; D001931:Brain Mapping; D003337:Corpus Callosum / Q000502:physiology*; D005133:Eye Movements; D005260:Female; D007839:Functional Laterality; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D008297:Male; D010775:Photic Stimulation; D011930:Reaction Time / Q000502:physiology; D013028:Space Perception / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation; D014794:Visual Fields / Q000502:physiology; D055815:Young Adult
keywords: TMS; corpus callosum; diffusion tractography; individual differences; neglect; spatial attention