Vector subtraction using visual and extraretinal motion signals: a new look at efference copy and corollary discharge theories.

The question as to how the visual motion generated during eye movements can be 'canceled' to prevent an apparent displacement of the external world has a long history. The most popular theories (R. W. Sperry, 1950; E. von Holst & H. Mittelstaedt, 1950) lack specifics concerning the neural mechanisms involved and their loci. Here we demonstrate that a form of vector subtraction can be implemented in a biologically plausible way using cosine distributions of activity from visual motion sensors and from an extraretinal source such as a pursuit signal. We show that the net result of applying an 'efference copy/corollary discharge signal' in the form of a cosine distribution is a motion signal that is equivalent to that produced by vector subtraction. This vector operation provides a means of 'canceling' the effect of eye movements. It enables the extraretinal generated image motion to be correctly removed from the combined retinal-extraretinal motion, even in cases where the two motions do not share the same direction. In contrast to the established theories (efference copy and corollary discharge), our new model makes specific testable predictions concerning the location (the MT-MST/VIP areas) and nature of the eye-rotation cancellation stage (neural-based vector subtraction).