A (fascinating) litmus test for human retino- vs. non-retinotopic processing.

In human vision, the optics of the eye map neighboring points of the environment onto neighboring photoreceptors in the retina. This retinotopic encoding principle is preserved in the early visual areas. Under normal viewing conditions, due to the motion of objects and to eye movements, the retinotopic representation of the environment undergoes fast and drastic shifts. Yet, perceptually our environment appears stable suggesting the existence of non-retinotopic representations in addition to the well-known retinotopic ones. Here, we present a simple psychophysical test to determine whether a given visual process is accomplished in retino- or non-retinotopic coordinates. As examples, we show that visual search and motion perception can occur within a non-retinotopic frame of reference. These findings suggest that more mechanisms than previously thought operate non-retinotopically. Whether this is true for a given visual process can easily be found out with our "litmus test."