Optical flow

The late James J Gibson, an American psychologist, developed what he called an ecological theory of perception based on the key concept of optical flow. When you're moving forwards there's an optical flow away from a point in the direction you're aiming and the relative speed of this flow can tell you whether or not one thing is in front of another. The nearer the object the faster the flow. 

He used this to analyse various forms of animal behaviour. For example when a sea bird dives it has to choose the point at which to pull in its wings. Too early and it loses control over the latter bit of its flight, too late and it runs the risk of damaging them. It turned out that the faster a bird was diving the higher above the sea it would pull in its wings. Since the rate of optical flow gets bigger if you're travelling faster or if you are closer, Gibson put forward the hypothesis that the bird simply pulled its wings in when the rate of flow reached a critical value. Now, whilst this matches the observed behaviour it doesn't necessarily mean that this is what the bird is doing all it means is that the hypothesis can live to fight another day.

Another more familiar example concerns a driver's braking behaviour in a stream of traffic. How does he or she judge how quickly to slow down so that they end up a not unreasonable distance behind the car in front. Gibson's argument here was that if you took the car in front's brake lights as a reference the entire behaviour could be explained as simply adjusting your own braking so that the rate at which the brake lights appeared to get further apart was held constant. Until you've stopped of course.

Allied with the concept of visual flow was the more ecological concept of an affordance. When a bird is flying through the world it needs to know where the gaps are that it can fly through. Since the background behind a gap is further away than the things around it the optical flow shown by that gap will be slower than that of its surroundings. Hence the bird knows  that that's where it can go. These different rates of flow offer clues about the opportunities afforded by the environment. Is it a gap I could fly through is it a perch I could land on? The more general idea is that the value of perception is in showing you what possibilities, affordances, are available.

Most recent references to optical flow are in the field of computing and image generation but there has been recent academic research that supports the idea that optical flow is important for balance when we're walking. Supply people with images where the optical flow doesn't quite match their motion,as perceived in other ways, e.g. through their sense of balance, and they feel very uneasy and may even fall over.

My own , and so far untested, hypothesis is that optical flow is something that we've evolved to enjoy. Even though we're not as fast as many other animals we are capable of keeping going for much longer and were able to pursue our prey until they got too tired to continue. Hunters who enjoyed being on the move were more likely to be successful and hence more likely to pass on their genes. 

Perhaps this is what explains our love of speed and why the motor car, in particular, has been so successful at invading our culture. What it presents is a hyper real version of what we've already evolved to enjoy. Unfortunately, by disconnecting the experience from any significant physical activity we're now at increasing at risk from diseases of physical inactivity.

Luckily, hyper motion can be achieved whilst still being physically active. All it takes is a bicycle.