Rick Cosslett, of Takaka, asks :-

My favourite tramping party trick gets an airing if ever we come upon a substantial waterfall in the bush. First, my companions are asked to choose a spot in the landscape near the waterfall. Next, they stare at a fixed spot in the face of the waterfall. When, after 30 seconds they are asked to switch their gaze back to their chosen spot the landscape there appears to travel upwards.

What is happening?

Gordon Sanderson, an opthalmologist at the University of Otago's School of Medicine, responded.

Well done for observing this effect, many people never manage to see it even after their attention has been drawn to it. It actually has a name: it is called the waterfall illusion.

Fly fishermen who have been standing in a river for some time, occasionally report a similar illusion when they step out onto the bank and notice it seeming to move in the opposite direction. It may also be an explanation for why people feel off balance when they step off a moving boat. It was originally described by Aristotle and has been observed and described in different forms by many scientists ever since.

What you are noticing is referred to as the 'motion after effect'. It seems to be due to a form of spatial adaptation in the visual cortex (the part of the brain we use to see). That may be the mechanism we use to find our ‘sea legs’ in a moving environment, such as a boat. It also forms the basis of many visual illusions including the famous spiral after effect.

The reason we can be fairly confident that it is cortical in origin is due to the fact that you can observe the moving object with one eye covered, then still experience the motion after effect with the original eye shut and the other eye open. You might like to try that the next time you are watching a waterfall.

There is a similar after effect called the McCullough effect; that effect although it involves colours, is also cortical in origin, and like the motion effect it persists for some considerable time afterwards. However the motion after effect seems to disappear after only a few minutes, whereas the McCullough effect can last for some hours.