Spatial distribution of attention and inter-hemispheric competition.

Considerable evidence has demonstrated functional asymmetry in spatial attention between the left and right hemispheres. In the present study, we aimed to examine the theoretical models of spatial attention by considering distribution and inter-hemispheric competition in neurologically healthy participants. Participants searched for a green circle target among green diamond non-targets in the presence or absence of a red singleton. Assuming that the salient singleton would increase the activation of the corresponding hemisphere, we manipulated the sides of the singleton visual fields and target visual fields. When the salient singleton was presented to the right visual field, target detection was faster for left visual field targets than for right visual field targets. In contrast, when the salient singleton was presented to the left visual field, target detection time was equivalent for left and right visual field targets. These results suggest that when the perceptually salient singleton acts as an activator, distribution of attention differs depending on the activated hemisphere induced by inter-hemispheric competition. These findings are in line with Kinsbourne's opponent processor theory.

1-Hz Repetitive Transcranial Magnetic Stimulation over the Posterior Parietal Cortex Modulates Spatial Attention.

Lesion and neuroimaging studies have suggested that regions in the posterior parietal cortex (PPC) are involved in visual spatial attention. The aim of this study was to investigate the potential effects on spatial attention resulting from a transient parietal impairment induced by 1-Hz repetitive transcranial magnetic stimulation (rTMS). We examined 50 healthy subjects using the attention network test (ANT) after first applying rTMS to right or left PPC. The right parietal rTMS, but not left PPC rTMS, caused a significant slowing in the mean reaction time (RT) to target presentation following a spatial cue during the ANT test. There were no significant effects of rTMS on mean RT under the no-cue, center-cue, and double-cue conditions, or for each flanker type among the experimental groups. Moreover, after rTMS to the right PPC, test subjects displayed deficits in networks related to alerting and orienting, whereas they exhibited improvement following rTMS to the left PPC. These findings indicate that the right PPC serves an important function in spatial orienting and the alerting activities. We interpreted the enhancement in alerting and spatial orienting function following low-frequency rTMS of left PPC as reflecting a disinhibition of right PPC via an inter-hemispheric inhibition account.