Shifting visual attention in space: an electrophysiological analysis using high spatial resolution mapping

OBJECTIVES Evidence from cortical electrophysiology and functional imaging converges on the view that visual spatial selective attention results in a facilitation of early sensory processing in visual cortical structures. Little is known, however, about the neural control processes that lead to this facilitation. The present study was aimed at further investigating these control processes and their neural correlates by analyzing high spatial resolution maps of brain activity that were evoked by attention-directing cues, but occurred prior to presentation of the target stimulus. METHODS Subjects (n=14) were presented with central arrow cues that instructed them to attend covertly to either a left or right field location in order to compare two subsequent target stimuli simultaneously presented to the location. On half of the trials, targets were presented to the cued location, while in the other half, targets were presented to the opposite visual field location. Subjects had to respond via button press on 16% of the trials when target stimuli were identical. Event-related potentials (ERPs) were recorded from 92 scalp electrodes which allowed a sufficiently finegrained analysis of the regional specificity of the ERP components. RESULTS In response to the cues, an initial component over occipital-parietal electrode sites was consistent with an early involvement of the posterior-parietal cortex, perhaps in the initial step of attentional orienting. A second component over the lateral-prefrontal cortex is consistent with the voluntary control and maintenance of attention, a function known to be subserved by frontal cortical structures. A late component narrowly focussed over occipital-temporal electrode sites is most plausibly related to activation of parts of the ventral extrastriate cortex. CONCLUSIONS The data support the current view that voluntarily orienting visual attention in space leads to top-down modulations in cortical excitability of ventral extrastriate regions initiated by posterior-parietal and mediated by lateral-prefrontal cortical structures.