Searching for object pointers in the visual cortex.

ABSTRACT

We perceive objects as permanent and stable despite frequent occlusions and eye movements, but their representation in the visual cortex is neither permanent nor stable. Feature selective cells respond only as long as objects are visible, and their responses depend on eye position. We explored the hypothesis that the system maintains object pointers that provide permanence and stability. Pointers should send facilitatory signals to the feature cells of an object, and these signals should persist across temporary occlusions and remap to compensate for image displacements caused by saccades. Here, we searched for such signals in monkey areas V2 and V4 (Macaca mulatta). We developed a new paradigm in which a monkey freely inspects an array of objects in search for reward while some of the objects are being occluded temporarily by opaque drifting strips. Two types of objects were used to manipulate attention. The results were as follows. 1) Eye movements indicated a robust representation of location and type of the occluded objects; 2) in neurons of V4, but not V2, occluded objects produced elevated activity relative to blank condition; 3) the elevation of activity was reduced for objects that had been fixated immediately before the current fixation ('inhibition of return'); and 4) when attended, or when the target of a saccade, visible objects produced enhanced responses in V4, but occluded objects produced no modulation. Although results 1-3 confirm the hypothesis, the absence of modulation under occlusion is not consistent. Further experiments are needed to resolve this discrepancy.NEW & NOTEWORTHY The way we perceive objects as permanent contrasts with the short-lived responses of visual cortical neurons. A theory postulates pointers that give objects continuity, predicting a class of neurons that respond not only to visual objects but also when an occluded object moves into their receptive field. Here, we tested this theory with a novel paradigm in which a monkey freely scans an array of objects while some of them are transiently occluded.