Converging evidence from neuropsychological and neuroimaging studies suggests that the ability to maintain an alert, ready-to-respond state is mediated by a network of right-hemisphere frontal and parietal cortical areas. This right lateralization may help to explain why visuospatial hemineglect, a cluster of deficits in detecting and responding to contralesional stimuli, is more common and persistent after right-hemisphere lesions. Indeed, it has been hypothesized that this asymmetry reflects a direct, functional link between alertness and spatial attention. In the present study, we investigated whether a pharmacologically induced increase in alertness would influence lateral bias in healthy people. Eighteen healthy participants were each given placebo or the psychostimulant drugs methylphenidate 40 mg or modafinil 400 mg on separate days and completed an hour-long version of the spatially sensitive landmark task. For those participants who demonstrated the expected alerting effect of modafinil, there was a significant Condition by Time interaction, consistent with the effects of the drug resisting time-on-task rightward drifts in spatial bias in the placebo condition. In contrast, no effect of methylphenidate on spatial bias was observed. These results suggest that spatial bias may be modulated by a psychostimulant-induced increase in alertness, supporting the hypothesis of a direct, functional link between right-hemisphere systems controlling alertness and visuospatial attention.

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