Top-down influences of spatial attention in visual cortex.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see Review of Bressler et al. It has been known for decades that neu-rons in visual cortex respond more vigorously when attention is directed to their receptive field. In recent years, substantial progress has been made toward understanding the neural mechanisms involved in controlling spatial attention. Many studies have shown that activity increases in several regions outside visual cortex when subjects select a spatial location for further processing. Two regions in particular , the frontal eye fields (FEFs) and the intraparietal sulcus (IPS), are consistently activated during top-down control of attention. Considerable evidence suggests that the FEFs and IPS are responsible for the changes in visual cortical activity observed during spatial attention. The most compelling evidence comes from studies recording from neurons in monkey cortex. However, demonstrating these influences in humans under physiological conditions is difficult, in part due to inherent technical limitations of the primary neu-roimaging methodology, functional magnetic resonance imaging (fMRI). Because of its poor temporal resolution, fMRI analyses typically show areas that are co-activated during a task, but not how they influence each other. Recently, several methods of inferring functional connections between brain regions have been advanced for use with fMRI data, including Granger causality (Roebroeck et al., 2005). Granger causal-ity is a concept that was developed by Clive Granger in the 1960s for use in economic forecasting. The premise is that if prior states of process A can improve predictions of the current state of process B, then A may cause B. This definition of causality, based on temporal precedence and predictability, is called Granger cau-sality to distinguish it from other definitions of causality. Applied to fMRI data, Granger causal-ity can be used to test the influence of one brain area (source) on another (target). Because the signal measured by fMRI is blurred in time, prior states of the target area are likely to have predictive value for its current state. The critical test of Granger causality is to what extent does adding the prior state of the source brain region improve the ability to predict the response of the target area from its own prior states. …