The Corollary Discharge

Have you ever kneaded, or watched someone knead dough? If you've ever baked a pie, made cookies, or even your own homemade pasta noodles, then the answer is probably yes. Myself, I don't bake - but my mom does! And sometimes, I like to watch. Watching someone knead dough is pretty relaxing. 

When we watch someone knead dough, or really, do any sort of motion, corollary discharge takes effect in our eyes. The corollary discharge theory explains why, even when our head, body, and eyes are all stationary, we know that an object is moving. When we move our eye muscles, our brain sends motor signals to the eye, and another signal -- called the corollary discharge signal from which this theory got its name -- splits off from the motor signal. 

The corollary discharge signal, or CDS, reaches the comparator, and this tells the brain that the eye is moving. Now, when a stimulus moves across our retina, we know it's moving because the optic nerve in our eyes sends an image displacement signal or IDS to the comparator and brain.

What does this all imply? Well, if an object, for example the dough under your hands, is moving because someone is kneading it, then your eyes are sending image displacement signals to the comparator, which tells your brain, and it is perceived it as moving. If your eye is moving, then the CDS is sent instead of the IMS, and the comparator tells your brain that it is you moving, not the dough. But when the comparator gets both the CDS and the IDS, then it will tell your brain -- the dough is moving because your eyes are moving, thus, it's not moving at all. This is why we can look at stationary, non-moving scenes without feeling vertigo! 

If you're not convinced by the corollary discharge theory, try this trick out: look at any object in the room. Keep looking at it while you gently push back and forth on the side of your eyelid, and see what happens! 

REFERENCE: 

Goldstein, E. B. (2010). Sensation and Perception. Belmont, CA: Wadsworth, Cengage Learning.