The frontal eye field (FEF) is one of several cortical regions thought to modulate sensory inputs. Moreover, several hypotheses suggest that the FEF can only modulate early visual areas in the presence of a visual stimulus. To test for bottom-up gating of frontal signals, we microstimulated subregions in the FEF of two monkeys and measured the effects throughout the brain with functional magnetic resonance imaging. The activity of higher-order visual areas was strongly modulated by FEF stimulation, independent of visual stimulation. In contrast, FEF stimulation induced a topographically specific pattern of enhancement and suppression in early visual areas, but only in the presence of a visual stimulus. Modulation strength depended on stimulus contrast and on the presence of distractors. We conclude that bottom-up activation is needed to enable topdown modulation of early visual cortex and that stimulus saliency determines the strength of this modulation.Contemporary hypotheses propose that feedback signals from areas in frontal and parietal cortex exert control over the processing of incoming visual information (1-5). Several models suggest that these signals are gated by bottom-up stimulation (6-9). In these models, feedback signals only influence neurons activated by visual input, just as has been observed for attentional effects, which are known to be strongest for neurons well driven by a visual stimulus (10)(11)(12). No causal evidence exists, however, to support these hypotheses, with the * To whom correspondence should be addressed. wim@nmr.mgh.harvard.edu. In our first experiment, the goal was to detect the functional consequences of EM of the FEF in the absence of a visual stimulus, using stimulation levels below those needed to evoke saccades. We first obtained anatomical ( fig. S1B) and behavioral evidence (Fig. 1A and fig. S2) in two monkeys that several chronically implanted microelectrodes were positioned in the FEF. Before each fMRI experiment, we stimulated these electrodes inside the MR scanner to determine the threshold needed to evoke saccades and to identify the saccade end point, or movement field (MF), of each FEF stimulation site (Fig. 1A and fig. S2). During the actual fMRI experiment, the monkeys carried out a passive fixation task while we alternated between epochs of no-EM and epochs of EM, at a stimulation level of 50% of the saccade-inducing amplitude. The use of this method in awake animals allowed us to titrate the stimulation to functionally relevant levels (14), an advantage compared to a previous study in anesthetized animals (15).The left column of Fig Our first experiment indicated that FEF-EM increased fMRI activity in higher-order visual areas known to be directly connected to the FEF. If feedback effects are gated by visual stimulation, however, one also predicts FEF-EM effects in visual areas separated from the FEF by multiple synapses, in the presence of a visual stimulus. In a second experiment, we therefore placed high-contrast, colored, moving gratings in the ...

It is generally assumed that sensitivity to different stimulus orientations is mapped in a globally equivalent fashion across primate visual cortex, at a spatial scale larger than that of orientation columns. However, some evidence predicts instead that radial orientations should produce higher activity than other orientations, throughout visual cortex. Here, this radial orientation bias was robustly confirmed using (1) human psychophysics, plus fMRI in (2) humans and (3) behaving monkeys. In visual cortex, fMRI activity was at least 20% higher in the retinotopic representations of polar angle which corresponded to the radial stimulus orientations (relative to tangential). In a global demonstration of this, we activated complementary retinotopic quadrants of visual cortex by simply changing stimulus orientation, without changing stimulus location in the visual field. This evidence reveals a neural link between orientation sensitivity and the cortical retinotopy, which have previously been considered independent.