, 2006, Nobre et al., 2006, Mirabella et al., 2007 and Wegener et al., 2008). In contrast to feature tagging of objects, sorting or classifying of objects on

the basis of one elemental feature dimension (e.g., color of fruit or shape of fruit, Figure 7A) requires that the unity of perceptual objects be broken down. This type of feature attention was directly examined in a study in which the activity of V4 neurons was recorded while an animal was attending to either the color or the orientation feature dimension of colored oriented bars (four possible colors and four possible orientations) ( Mirabella et al., 2007). Monkeys were trained to turn a response lever to the Cyclopamine solubility dmso left in response to two of the four colors (red and blue) and two of the four orientations (0° and 45°), and to the right in response to the other two colors (yellow and green) and two orientations (90° and 135°). Monkeys responded (left or right) to color-orientation pairings. To perform the task correctly, the monkeys had to selectively attend to the feature dimension that was cued, while ignoring the other feature dimension. The study

reported that responses of V4 neurons to otherwise identical stimuli are modulated depending on the task cue ( Figure 7B); Forskolin remarkably, the selected task-relevant features were “selected” into one of two behaviorally relevant response categories (left versus right). This type of task-dependent neuronal response grouping provides the first evidence that network associations in V4 can be directed, not only Ketanserin by sensory-defined features, but also by top-down motor output categories. Neuronal firing rate may not be the only means by which attentional signals are mediated. Recent findings suggest that feature-based attention may also act by increasing synchronization among the neurons selective for the relevant features, particularly in the gamma-band (35–70 hz) frequency range (Bichot et al.,

2005, Taylor et al., 2005 and Womelsdorf and Fries, 2007). In a visual search task that contained an array of objects defined by both color and shape, Bichot et al. (2005) showed that gamma band oscillations occurred more frequently when attended targets fell in the receptive field (both initially and prior to eye movements), suggesting a role for synchrony in feature-guided serial and parallel search. Such enhancements in gamma band oscillation have also been reported to occur during spatial attention tasks (Fries et al., 2001). Furthermore, other studies report that attentional modulation leads to decreased firing rate synchronization in V4, and proposed this as a way to reduce correlated noise and thus enhance signal-to-noise (Mitchell et al., 2009 and Cohen and Maunsell, 2009). Participation of enhanced versus decreased correlation may be cell type specific. Mitchell et al.