, 2005, Ungerleider et al., 2008 and Pouget et al., 2009). In this sense, V4 is well positioned for integrating top-down influences with information about stimuli from the bottom-up direction. Causal Interactions between Frontal and Visual Cortical Areas? Although imaging and neuropsychological studies strongly suggested that feedback signals from fronto-parietal cortex interact with sensory signals in visual areas such as V4, it has been

difficult to prove a causal link between activity in frontal (or parietal) cortex and modulation of visually driven Pomalidomide order activity. One area in prefrontal cortex that has been proposed as a source of top-down influence is the frontal eye fields (FEF), a cortical area responsible for directing eye movements. During overt attention, FEF initiates circuits which direct the center of gaze toward salient objects. During covert attention, similar neuronal mechanisms may be at play (which has led to the “pre-motor theory of attention”) ( Corbetta et al., 1998, Corbetta, 1998, Hoffman and Subramaniam, 1995, Kustov and Robinson, 1996, Moore et al., 2003, Moore and

Armstrong, 2003, Moore and Fallah, 2001, Moore and Fallah, Screening Library purchase 2004, Nobre et al., 2000 and Rizzolatti et al., 1987). If so, then FEF should play a causal role in directing attention and in influencing V4 activity. Currently, the only evidence of causal influences from FEF comes from studies of spatial attention. Moore and colleagues mafosfamide provided the first elegant evidence showing such a causal link (Moore

and Fallah, 2004). By using microstimulation in FEF, they showed a causal relationship between altered activity in the FEF and spatially specific enhanced visual representations within V4. Second, they showed that microstimulation in FEF increased perceptual abilities at the stimulated visuotopic locations. More recently, using fMRI methods, Ekstrom and colleagues examined the effect of electrical microstimulation in FEF on visually driven responses in V4 and other extrastriate cortical areas of behaving monkeys (Ekstrom et al., 2008 and Ekstrom et al., 2009). They found that voxels in V4 which showed the strongest enhancement of fMRI activity caused by FEF microstimulation were not the voxels with the strongest visual responses, but rather adjacent voxels. In fact, strongly visually driven voxels themselves were unaffected or even suppressed by FEF microstimulation. These results led them to test whether effects of electrical stimulation on visually driven activity in V4 would be stronger in the presence of “distractor” stimuli. Without distractors, electrical stimulation increased fMRI activity in V4. With distractors (which normally cause a decrease in activity), the activity in V4 voxel increased substantially beyond the effect without distractors. These results are consistent with neurophysiological studies that show stronger enhancement in the presence of competitive distractors.