, 2005 and Martinez-Trujillo and Treue, 2004). Neurons in area V4, for example, show enhanced responses to stimuli within LY294002 purchase their receptive fields (RFs) during visual search when they contain a color or shape feature that is shared with the searched-for target (Chelazzi et al., 2001), even when the animal is planning an eye movement (and, thus, directing spatial attention) to another stimulus

in the search array (Bichot et al., 2005). Thus, feature-selective attentional enhancement appears to occur in parallel across the visual field representations of extrastriate visual areas and presumably helps guide the eyes to searched-for targets. Although extrastriate neuronal responses are modulated by feature attention, to our knowledge, the source of the top-down feedback that biases responses in favor of the attended feature is unknown. During spatial attention, there is evidence that the response enhancement with attention observed in extrastriate visual areas results from top-down feedback from areas such as the frontal eye field (FEF) and lateral intraparietal area (LIP) (Desimone and Duncan, 1995, Gregoriou et al., 2009, Kastner and Ungerleider, 2000 and Serences and Boynton, 2007). Electrical stimulation of the FEF causes enhancement BMS-354825 chemical structure of V4 responses and activation

of the cortex measured by fMRI, similar to what is found during spatial attention (Ekstrom et al., 2008 and Moore and Armstrong, 2003), and neurons Metalloexopeptidase in the FEF and V4 synchronize their activity with each other in the gamma frequency range during spatial attention (Gregoriou et al., 2009). However, to our knowledge, whether these areas play the similar role during feature-based attention is still unknown. Like neurons in area V4, neurons in the FEF and LIP also show enhanced responses

to targets (or distracters that share features with the targets) compared to dissimilar distracters in their RFs, even when these stimuli are not selected for the next saccade during visual search (Bichot and Schall, 1999 and Ipata et al., 2009). This suggests that the responses of FEF and LIP neurons to stimuli in their RFs are influenced by the target features in parallel across the visual field, independently of spatial attention. However, the target stimuli used in these studies were fixed, at least within the same session, raising the possibility that the parallel effects of target features on responses arose from learning effects rather than flexible feature attention mechanisms. Learning effects on target responses have been found in prior studies in the FEF (Bichot et al., 1996). Indeed, one recent study of FEF neurons with a target that changed from trial to trial during visual search found that cells exhibited a serial shift of spatial attention effects from one stimulus to another in the search array, rather than parallel, feature attention effects (Buschman and Miller, 2009).