Trunk rotation and handedness modulate cortical activation in neglect-associated regions during temporal order...

The rotation of the trunk around its vertical midline could be shown to bias visuospatial temporal judgments towards targets in the hemifield ipsilateral to the trunk orientation and to improve visuospatial performance in patients with visual neglect. However, the underlying brain mechanisms are not well understood. Therefore, the goal of the present study was to investigate the neural effects associated with egocentric midplane shifts under consideration of individual handedness. We employed a visuospatial temporal order judgment (TOJ) task in healthy right- and left-handed subjects while their trunk rotation was varied. Participants responded by a saccade towards the stimulus perceived first out of two stimuli presented with different stimulus onset asynchronies (SOA). Apart from gaze behavior, BOLD-fMRI responses were measured using functional magnetic resonance imaging (fMRI). Based on findings from spatial neglect research, analyses of fMRI-BOLD responses were focused on a bilateral fronto-temporo-parietal network comprising Brodmann areas 22, 39, 40, and 44, as well as the basal ganglia core nuclei (caudate, putamen, pallidum). We observed an acceleration of saccadic speed towards stimuli ipsilateral to the trunk orientation modulated by individual handedness. Left-handed participants showed the strongest behavioral and neural effects, suggesting greater susceptibility to manipulations of trunk orientation. With respect to the dominant hand, a rotation around the vertical trunk midline modulated the activation of an ipsilateral network comprising fronto-temporo-parietal regions and the putamen with the strongest effects for saccades towards the hemifield opposite to the dominant hand. Within the investigated network, the temporo-parietal junction (TPJ) appears to serve as a region integrating sensory, motor, and trunk position information. Our results are discussed in the context of gain modulatory and laterality effects.