The information supplied to our retinas via the photons that get through our pupils is surprisingly sparse. It’s mostly a murky blur. The visual processing region of our brain is where the real work is done, where the coarse raw data supplied by our eyes is converted into the complex sense of sight. Essentially, our brains are constantly and frantically editing everything that hits our retinas, to make it presentable.
Impressive as it is, this system isn’t perfect. Converting the crude retinal information to complex visual perception is an elaborate process and it takes time. That’s why we get things like the Pinna illusion (pictured), where complex rings of edged squares appear to be rotating in opposite directions if you stare into the centre of the image and move your head forward, or back.
Exactly why this happens was unknown for quite some time, but a recent study appears to have worked it out. In a nutshell, our brains have multiple systems for recognising and processing visible motion, but the one that recognises, and rules out, illusory motion takes 15 milliseconds longer to have an effect than the processes that say, “looks like motion to me, so that’s what we’ll see”.
It may not sound like much, but 15 milliseconds is a long time at the neurophysiological level. It’s certainly long enough for us to ‘see’ motion in the Pinna illusion, thanks to our less-discerning but faster motion-sensing parts of our brain.