There's the limit for our eyes!

The discovery by an international team has important implications for road design and safety, driver and pilot training, fast-moving sports and other areas of human visual activity affecting safety and performance.

"We have found that there is a limit to the maximum number of directions, and hence distinct objects, that the human brain can see at any given instant in time. That limit is either two or three, depending upon how the directions are defined.

"An example of this is when we're at a road roundabout -- although we can see cars coming and going in different directions, we can't actually keep close tabs on more than three simultaneously.

"Rather, what the brain does is process them in series, like cars heading from the right, then in the roundabout, followed by cars on the left," said Mark Edwards of The Vision Centre and The Australian National University, who led the team.

The same theory also applies to multi-tasking. Dr Edwards said: "We may assume that people who are good at multitasking can process lots of different things at once, but what may be closer to the reality is that they are actually able to switch their attention faster to the next item."

For the study, the scientists tested the brain's ability to detect signal directions using random-dot stimuli which consisted of a field of scattered dots, with groups of intermingled dots moving in different directions.

They found that people could not detect more than two signal directions at once if those signals moved in different directions. The people could detect three signals when they differed in speed or depth.

However, once these limits were exceeded, instead of seeing distinct signals, what they saw was only randomly moving noise.

"In order to see motion, the signal intensity -- proportion of dots moving in a given direction -- needs to be of at least a certain value. For the transparent displays used here, that intensity level has to be over 40 per cent.

"What this means is there has to be at least 40 per cent of the dots moving in a given direction to stimulate the brain cells that are processing that motion," Dr Edwards said.