Whether general principles can explain the layouts of cortical maps remains unresolved. In primary visual cortex of ferret, the relationships between the maps of visual space and response features are predicted by a "dimension-reduction" model. The representation of visual space is anisotropic, with the elevation and azimuth axes having different magnification. This anisotropy is reflected in the orientation, ocular dominance, and spatial frequency domains, which are elongated such that their directions of rapid change, or high-gradient axes, are orthogonal to the high-gradient axis of the visual map. The feature maps are also strongly interdependent-their high-gradient regions avoid one another and intersect orthogonally where essential, so that overlap is minimized. Our results demonstrate a clear influence of the visual map on each feature map. In turn, the local representation of visual space is smooth, as predicted when many features are mapped within a cortical area.

Natural scenes typically contain multiple objects that are unique in different stimulus dimensions so that an object with feature contrast to surrounding objects draws attention and pops out. Furthermore, if we have previous knowledge about the dimension in which a target object differs from the surrounding objects, we will attend to that dimension and more easily detect the target. Our aims here were to elucidate neural mechanisms underlying this type of attention by recording neuronal activities from area V4 and to investigate how visual signals encoding feature contrast between objects are modulated by attention specific to a particular dimension. To accomplish this, we trained monkeys to do a multidimensional visual search task in which two singleton stimuli, unique in the color or shape dimension, were presented with four other identical stimuli. The monkeys had to search for the singleton stimulus that was unique in the instructed dimension while the search dimension was switched between shape and color. We found that individual V4 neurons carry visual signals encoding feature contrast in either shape or color, and this signal is modulated depending on the search dimension. Population responses to the target singleton stimulus were significantly higher than to others, regardless of the search dimension. In most V4 neurons, however, significant response increases occurred only when one particular singleton stimulus was the target. These findings suggest that interaction between bottom-up signals encoding feature contrast between stimuli and top-down signals encoding search dimension occurs in V4 and facilitates adaptive selection of targets in a complex visual environment.