Visual attention: Insights from brain imaging

Attention has a central function in the construction of every visual experience. This review considers the contributions of functional neuroimaging to our understanding of what visual attention is and how it works. It addresses four principle questions:What is the locus of attentional selection? The debate concerning early versus late selection is evaluated in light of recent findings. New imaging evidence indicates that attention affects neural activity not only in extrastriate cortex, but also at the first stage of cortical information processing in primary visual cortex. What exactly gets selected by attention? Behavioural data show that selection can operate at the level of spatial locations, visual features or objects. Imaging data provide support for all three types of visual selection, with attention modulating activity in areas specialized for processing the attended attributes. How does attention affect the neural response to a stimulus? Attentional modulation of neural activity can reflect a multiplicative gain of the sensory response, or an additive increase in baseline activity. Although most imaging results are consistent with a gain mechanism, there is now evidence for top-down baseline shifts: attention can increase neural activity in extrastriate and even striate cortex in the absence of a stimulus. Where do attentional signals come from? Recent evidence indicates that the source of attentional modulation stems from a fronto-parietal attention network. Several areas of this network participate in many kinds of attentional processing, including spatial orienting, eye movements, nonspatial selection and attention in non-visual modalities.