Trunk rotation affects temporal order judgments with direct saccades: Influence of handedness

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

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.

We examined the effect of trunk rotation on brain responses in neglect-associated areas.•

Trunk-related BOLD-fMRI activation patterns depend on handedness.

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They were modulated most during trunk rotation contralateral to the dominant hand.

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Trunk rotation and saccade direction show interaction effects at TPJ.

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TPJ serves as a region integrating sensory, motor, and trunk position information.

Biased temporal order judgments in chronic neglect influenced by trunk position

Numerous studies have reported that temporal order perception is biased in neurological patients with extinction and neglect. These individuals tend to perceive two objectively simultaneous stimuli as occurring asynchronously, with the ipsilesional item being perceived as appearing prior to the contralesional item. Likewise, they report that two stimuli occurred simultaneously in situations where the contralesional item is presented substantially prior to the ipsilesional item. Therefore, they exhibit a biased point of subjective simultaneity (PSS). Here we demonstrate that the magnitude of this effect is modulated by the relative position of the stimuli with respect to the patient's trunk. This effect was only observed in patients who still exhibited neglect symptoms, and neither the pathological bias nor substantial modulation were observed in individuals who had recovered from neglect, those who never had neglect or neurologically healthy controls. Crucially, our design kept the retinal and head-centered coordinates of these stimuli constant, providing a pure measure for the influence of egocentric trunk position. This finding emphasizes the interaction of egocentric spatial position on the temporal symptoms observed in these individuals.