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

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.

The study of the mechanisms of selective attention in the human brain is among the most active research areas in cognitive neuroscience. It is generally assumed that frontoparietal attentional control structures guide endogenous shifts of visual attention, and that activity within these control structures is causally responsible for selective attentional processing in visual areas of the brain. However, there have been few if any direct demonstrations for the existence of such causal links. The proposed research will pioneer the combination of EEG/ERP and TMS to uncover such interactions between higher-order attentional control processes and spatially selective modulations of visual information processing, and in particular to study the temporal dynamics of these interactions. TMS will be applied over putative frontal and parietal attentional control regions (frontal eye fields, posterior parietal cortex) to induce transient disruptions within these regions. EEG/ERP measures will then be used to assess the impact of these disruptions on mechanisms involved in attention. Effects of TMS on processes involved in the control of attention are studied by recording ERP components induced during covert spatial orienting that are sensitive to the direction of attention shifts. The impact of TMS on the attentional processing of visual stimuli in visual cortex and higher-order brain areas is investigated by contrasting visual ERP components and their modulation by visual-spatial attention under conditions where attentional control is or is not disrupted via TMS. Two experimental paradigms (attentional cueing and visual search) will be used, and behavioural measures are obtained in parallel with EEG/ERP measures. The applicant and his group have overcome the methodological problems arising when using interleaved TMS/EEG protocols, and are now able to combine EEG and TMS with minimal contamination and data loss. Combining EEG and TMS can yield insights into the causal mechanisms underlying attention that could not be obtained on the basis of a single method. Pioneering this combined-methods approach will also open up new ways to study other cognitive processes, and is therefore highly relevant for the neuroscientific community.