Pattern matching is assessed in retinotopic coordinates

We typically examine scenes by performing multiple saccades to different objects of interest within the image. Therefore, an extra-retinotopic representation, invariant to the changes in the retinal image caused by eye movements, might be useful for high-level visual processing. We investigate here, using a matching task, whether the representation of complex natural images is retinotopic or screen-based. Subjects observed two simultaneously presented images, made a saccadic eye movement to a new fixation point, and viewed a third image. Their task was to judge whether the third image was identical to one of the two earlier images or different. Identical images could appear either in the same retinotopic position, in the same screen position, or in totally different locations. Performance was best when the identical images appeared in the same retinotopic position and worst when they appeared in the opposite hemifield. Counter to commonplace intuition, no advantage was conferred from presenting the identical images in the same screen position. This, together with performance sensitivity for image translation of a few degrees, suggests that image matching, which can often be judged without overall recognition of the scene, is mostly determined by neuronal activity in earlier brain areas containing a strictly retinotopic representation and small receptive fields.