As the patterns in the bars change, they seem to move out of alignment; similarly, the circles seem to grow and shrink as the concentric patterns move inwards or outwards. But as you’ve probably guessed, the bars aren’t really moving and the circles don’t really change size: it’s an optical illusion.
You may well have seen illusions like this before. Researchers have been using them to test our perception for decades. But a new study might explain how they work, and suggests that our brains are constantly trying to predict the future when it comes to moving objects.
Neil Roach from the University of Nottingham, UK, and colleagues conducted an experiment to try and find out why these shapes appear to shift.
They asked volunteers to stare at a cross in the centre of a screen, which contained two bars of drifting patterns similar to those in the video above. The volunteers then had to spot a target – a fainter, smaller version of the pattern, as it popped up on the screen at the end of one of the bars.
The volunteers were better at spotting the target when it appeared at the leading edge of the bar – that is, the end that the internal lines were drifting towards, compared to the tail end. This advantage only occurred when the patterns inside the target were consistent with a continued motion of the bar.
Roach suggests this might be a result of the brain predicting what a moving object is going to do next. Volunteers were therefore more skilled at detecting the target when it matched with their internal predictions.
When we watch the video, we predict the motion that will happen next at the leading edge. Because we are better at detecting the motion at the leading edge than the trailing edge this creates an asymmetry that leads to the feeling that the bar is moving. There are still some discrepancies between the experiment and the illusion, says Roach, which will need further investigation.