Image caption All in the mind: blocking out background distractions is a feature of people with higher IQs, the Rochester test showed

People with higher IQs are slow to detect large background movements because their brains filter out non-essential information, say US researchers.

Instead, they are good at detecting small moving objects.

The findings come in a study of 53 people given a simple, visual test in Current Biology.

The results could help scientists understand what makes a brain more efficient and more intelligent.

We expected that all participants would be worse at detecting the movement of large images, but high IQ individuals were much, much worse... Michael Melnick, University of Rochester

In the study, individuals watched short video clips of black and white bars moving across a computer screen. Some clips were small and filled only the centre of the screen, while others filled the whole screen.

The participants' sole task was to identify in which direction the bars were drifting - to the right or to the left.

Participants also took a standardised intelligence test.

The results showed that people with higher IQ scores were faster at noticing the movement of the bars when observing the smallest image - but they were slower at detecting movement in the larger images.

Michael Melnick of the University of Rochester, who was part of the research team said the results were very clear.

"From previous research, we expected that all participants would be worse at detecting the movement of large images, but high IQ individuals were much, much worse.

The authors explain that in most scenarios, background movement is less important than small moving objects in the foreground, for example driving a car, walking down a hall or moving your eyes across the room.

Image caption People with higher IQs appear to be able to concentrate better

As a person's IQ increases, so too does his or her ability to filter out distracting background motion and concentrate on the foreground.

In an initial study on 12 people, there was a 64% correlation between motion suppression and IQ scores. In this larger study on 53 people, a 71% correlation was found.

In contrast, previous research on the link between intelligence and reaction times, colour discrimination and sensitivity to pitch found only a 20-40% correlation.

But the ability to ignore background movements is not the only indicator of intelligence.

"Because intelligence is such a broad construct, you can't really track it back to one part of the brain," says Duje Tadin, who also worked on the study.

"But since this task is so simple and so closely linked to IQ, it may give us clues about what makes a brain more efficient, and, consequently, more intelligent.

"We know from prior research which parts of the brain are involved in visual suppression of background motion.

"This new link to intelligence provides a good target for looking at what is different about the neural processing, what's different about the neurochemistry, what's different about the neurotransmitters of people with different IQs."