"Why are we sensitive to optic flow?", Simon Rushton (University of Cardiff)

Simon Rushton (University of Cardiff)

Why are we sensitive to optic flow?

In science fiction films the movement of stars seen from inside the spaceship gives the viewer a vivid sensation of forward movement.  The movement of the stars creates “optic flow”, a similar pattern of image motion is generated whenever we move.  The human brain is exquisitely sensitive to optic flow.  Why?  I will outline some potential answers.

The standard answer is that we use it to guide locomotion, an intuitive idea that originated with Grindley in the 1940s and was popularised by Gibson.  In early work, I challenged this idea and suggested that humans use the egocentric direction of the target to guide locomotion.  More recently, with colleagues, I have been looking at the role of “allocentric location” cues in guiding locomotion in enclosed spaces.  This work leaves the standard answer looking rather shaky.

With Paul Warren I put forward an alternative suggestion for why we are sensitive to optic flow – to help in the identification of object movement during self-movement (“flow-parsing”).  We, and others, have now put together a fairly substantial body of work that addresses this hypothesis.  I’ll give a brief overview of this work.

I’ll finish with a tentative idea.  If we were to directly see the images on our retinas we would see a world in continual motion.  This is obviously not what we perceive, we see a stable scene.  I suggest that the brain’s sensitivity to optic flow could provide a mechanism for predicting, and hence, perceptually stabilising, the retinal image.