Neural Correlate of Object-Based Selection in Area V4

Abstract

Single unit studies of attention in monkeys have identified competitive circuits in extrastriate cortex that could mediate selection of one stimulus over another. While these studies show that attention operates by resolving competition, they used stimuli atseparate locations, confounding selection of objects with selection of spatial locations. To resolve this, we recorded responses of V4 neurons to two spatially superimposed transparent surfaces, one of which was delayed in onset. The surfaces were defined by patterns of dots that rotated rigidly around a common center. One set of dots was of the neuron's preferred color and the other was of an isoluminant non-preferred color. Human psychophysics using the same type of stimuli found that the delayed onset of one surface exogenously cues attention to that surface and suppresses processing of the other surface for several hundred milliseconds. Consistent with this, neurons in area V4 were preferentially driven by the delayed surface. Using superimposed surfaces ruled out spatial selection. But is this selection object-based? If it is, the selection should survive moving the superimposed surfaces through space. When the appearance of one of the two surfaces was delayed outside the neuron's receptive field and both surfaces then moved into the RF, the pair response was still preferentially driven by the delayed surface. Neurophysiological and functional imaging studies have shown that endogenously directing attention to the color or motion of a stimulus preferentially processes it throughout the visual field. We tested for feature-based selection by using placing two surfaces within the RF and two outside of the RF. When the delayed surface appeared within the RF, the results were similar to the first experiment, i.e. the delayed surface was preferentially processed. If this effect were the result of global color-based selection, thenthe same effect should be seen when the delayed surface appeared outside the RF. This effect was not seen, hence the selection was not of the color of the surface but of the surface itself. These results show that competitive circuits in V4 are not limited to mediating competition between spatial locations, but also select objects. These circuits are a likely neural substrate for object-based attention.