The neural basis of visual-spatial attention: A combined TMS/ERP investigation

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At any waking moment, our sensory organs receive a large amount of information about external events. However, the human brain only processes a small fraction of this information to a stage where it becomes accessible to conscious awareness. We often describe this selection of sensory information by referring to ?attention?: We are aware of those stimuli that we decide to attend to, but fail to notice other stimuli when our attention is directed elsewhere. How does the brain select and process those stimuli that we decide to attend to, and what is the fate of other, currently unattended information? Recent neuroscientific research has uncovered brain regions in the frontal and parietal lobes that are activated when observers direct their attention to specific regions of their sensory environment. It has been suggested that these brain regions play an important part in the selection of information. We now also know that shifts of attention result in activity changes in parts of the brain that analyse visual information. What we do not know is how these different aspects of attention are related. It is often assumed that activity in frontal and parietal brain regions represents ?attentional control signals? that inform visual brain regions which stimuli are currently relevant, so that these stimuli can be preferentially processed. We will investigate this by applying transcranial magnetic stimulation (TMS) to attentional control areas in the frontal and parietal lobe while observers are performing an attentional task, and simultaneously record EEG activity from other parts of the brain (such as the visual cortex). TMS briefly disrupts the operation of specific brain areas by creating a ?virtual lesion?. We want to find out how such ?virtual lesions? induced when TMS is applied to attentional control regions can impair the ability of the brain to select and process visual events. We expect to find that TMS disrupts the ability to attend to specific visual stimuli, and that this leads to different patterns of EEG responses to these stimuli. Such disruptions of attention by TMS may be similar to the attentional deficits that are observed in many neurological patients after stroke. These attentional impairments are often very disabling, but are hard to rehabilitate. We hope that our research will help to provide new insights into the brain mechanisms that are affected in these patients, so that new interventions may be developed in the future.