, 2003, Bisley and Goldberg, 2010, Craighero et al., 1999, Gitelman et al., 1999 and Moore et al., 2003); such feedback may target local groups of neurons. In contrast, most features are represented by neurons that are dispersed throughout cortex. Attending to these features would require a mechanism that does not rely on topographic organization. selleck inhibitor One possibility is that attention to such features is only be possible through learning and longer-term plasticity (Wolfe et al., 2004), and all forms of attention may require topographic organization. Perhaps because attention to topographically

organized features is more natural, most neurophysiological studies have focused on attention to topologically organized features, most notably motion direction in the middle temporal area (Albright, 1984, Martinez-Trujillo and Treue, 2004 and Sally et al., 2009). Over blocks of behavioral trials, the attentional modulation of either behavior or neuronal responses depends largely on the details see more of the behavioral paradigm chosen by experimenters. However, cognitive states such as attention inevitably fluctuate from

trial-to-trial, even within a task condition. We showed recently that the responses of populations of sensory neurons can be used to detect trial-to-trial fluctuations in spatial attention that are predictive of psychophysical performance (Cohen and Maunsell, 2010). These spontaneous attentional fluctuations found can provide hints about the mechanisms mediating feature and spatial attention. For example, if feature attention relies on spatial attention to affect behavior (Kwak and Egeth, 1992 and Nissen and Corkin, 1985), then fluctuations in feature attention might either covary with fluctuations in spatial attention

or else have little effect on behavior relative to fluctuations in spatial attention. Fluctuations in attention can also be used to determine whether either form of attention acts selectively on local groups of neurons by examining the extent to which fluctuations in feature or spatial attention are coordinated across cortex. We investigated whether spatial and feature attention employ common or unique mechanisms by analyzing the responses of populations of neurons in visual area V4 in both cerebral hemispheres. We found many qualitative and quantitative similarities between the two types of attention, including their effects on local populations of neurons and the extent to which they could be estimated on individual trials from the responses of a few dozen neurons, suggesting that they employ similar neuronal mechanisms. However, we found that unlike spatial attention, which targets spatially localized groups of neurons in V4, feature attention selectively comodulates neurons located far apart, even in opposite hemispheres.