Friday Illusion: How a cheetah can hide its spots

Sandrine Ceurstemont, editor, New Scientist TV

Be careful if you're walking in the jungle: what may seem like moving spots could actually be a cheetah. Now new animations by Stuart Anstis and his team from the University of California in San Diego illustrate the effect by showing how our brain can interpret a moving scene in different ways.

The video starts with couples gazing into each others' eyes. As they rotate, you are likely to keep seeing four distinct couples as we are primed to recognise male-female pairs. However, in the next clip, where faces are replaced with dots in a similar arrangement, what initially appear to be groups of rotating dots will probably soon turn into two floating squares. A third animation demonstrates that by adding more pairs of dots, the motion of the whole takes over as most people will see two pulsing octagons. A final clip shows that linking the pairs of dots makes the smaller groupings stick out from the overall formation.

These animations demonstrate that our brain can favour either the overall shape or its components depending on the arrangement. In many cases, we can perceive a scene in different ways and alternate between the configurations. However, Anstis and his team showed that over time, our brain usually favours one arrangement over the other by remembering how it has previously processed information in the scene.

Typically, we see the motion of the smaller groupings first before perceiving an overarching shape. "In our view, the grouping of moving spots into local motion clusters is an early, fast, pre-attentive event, while grouping them into global motion clusters is a slower, high-level process," write Anstis and colleagues.? "Reverting to the cheetah example, it is a modest visual achievement to group some of the moving spots locally into legs or a tail, but a prey's actions and survival will ultimately depend on organizing them globally into a whole cheetah."

If you enjoyed this post, see how an artificial shadow can trick your brain or watch how an animated Pac-man can create phantom shapes.?

Journal reference, Journal of Vision, DOI: 10.1167/11.3.13

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