Representation and remapping of occluded objects in the activity of v4

How the visual brain represents perceptual objects and how it achieves stability of object representations despite eye movements and object movements is not well understood. Here we tested the hypothesis that the visual cortex uses grouping cells to combine elementary feature signals to larger entities representing objects (von der Heydt, Frontiers in Psychology 6, 1695, 2015). The activation of such a circuit we call a "proto-object". The proto-object hypothesis predicts that the system maintains the grouping cell activity and remaps it when the image of an object moves across the retina, thereby linking retinotopic feature signals to locations in external space. We studied single neuron activity in cortical areas V2 and V4 of non-human primates using a novel free-viewing paradigm in which the subject selectively fixates target objects in an array of objects. While the subject scans the array we briefly occlude some of the objects. Eye movement recordings showed that subjects did not hesitate to saccade to occluded objects and fixated them as accurately as visible objects. The neural recordings showed that in V4, but not in V2, occluded objects produced higher firing rates than occluded background. In the special case when a saccade moved the receptive field of a neuron from background to an occluded object, its firing rate increased despite the fact that the receptive field had never been visually stimulated. Interestingly, this internal activation was suppressed for the object that had been fixated before the saccade. We concluded that V4 neurons were activated by remapping of the proto-objects, except for the one corresponding to the last fixated object ("inhibition of return"). These findings are strong evidence for persisting internal representations that are being remapped with each saccade, as postulated by the proto-object hypothesis. Meeting abstract presented at VSS 2018