Visual illusions can be caused by imagination

A fantastic study just published in Cognition reports that the motion aftereffect illusion, where staring at something constantly moving in one direction causes illusory movement in the opposite direction when you look away, can be caused just by imagining that the movement is happening.

The effect is occasionally called the ‘waterfall illusion’ because it can be triggered by staring at a waterfall for a few minutes and then looking at the nearby bank, which will seem as if it is moving upward, in the opposite direction to the falling water.

It was traditionally explained by the fact that direction-specific motion-detecting neurons in the brain’s visual areas ‘habituate’ or adapt to constant movement by slowly becoming less active, as if they barely need to keep reporting with the same vigour because they’re just detecting more of the same.

According to this explanation, when you look away, these ‘habituated’ neurons are caught off guard and the neurons that look out for motion in the opposite direction are relatively stronger and so, until the balance is readdressed, give the impression that the world is moving contrary to your past experience.

As with most of these things, it turns out not to be quite so simple, but the effect is so easily invoked that it is used widely in vision and motion research.

One of the key findings in this area is that visual imagery activates some of the same areas as actually seeing what you’re thinking of. In other words, the brain seems to simulate the visual experience actually in the visual system.

Or at least, that’s what it looks like from the brain scans, but just because the same areas are active during both tasks, it doesn’t mean the same neurons are being used. It could be completely different processes at work that just happen to share the same neural office space.

So here’s the cool bit. This new study, led by psychologist Jonathan Winawer, asked participants to briefly view a moving pattern. It only appeared briefly, not long enough to cause the effect, and then disappeared.

Then were then shown the same pattern, without any movement, and were asked to imagine that it was moving in the same way. After a short while, the pattern was replaced by a picture of motionless dots, and they were asked to indicate if they saw the dots moving in a particular direction.

If the effect appeared, participants should see the dots moving in the opposite direction.

The participants were asked to imagine different directions and types of motion and then were given the same task but where they didn’t need to imagine anything, as the pattern moved by itself.

As expected, the moving pattern caused a clear motion aftereffect, but rather wonderfully, the effect appeared after participants had simply imagined the movement. It wasn’t as strong but it was clearly there.

They researchers also asked the participants after which direction would they expect the dots to go in, to check they hadn’t heard about the effect or were just doing what they thought was expected of them, and they couldn’t reliably give the correct direction that the effect would cause.

This provides good evidence that when imagine visual experiences we’re actually running a simulation in the same parts of the brain that are used to actually see the world.

Link to PubMed entry for study.
pdf of full text paper.