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The brain blurs fast-moving images

Fast vision The brain sees fast-moving objects by using blurs or streaks, as seen in photographs, researchers have found.

The research, by an international team of researchers, is published today in the Proceedings of the Royal Society B.

"The brain doesn't see instantaneously. It takes about 100 milliseconds for the neurones in the brain to fully encode information," says a co-author on the paper, Professor David Alais of the University of Sydney's school of psychology.

"If you move things really fast it will blur - exactly like a blurred camera image."

Traditionally, scientists would have considered this "smeared image" to have been a problem for the brain to contend with.

But in 1999, a researcher by the name of Wilson Geisler proposed that this blurring could be useful in the brain's processing of fast-moving images.

His idea was that since the smearing would always be in the direction the object is moving, it might help you see accurately which way it's going, says Alais.

Scientists have traditionally believed the brain perceives form (static objects) and motion using two separate neuronal pathways. But Geisler argued that form-sensitive neurones, not usually involved in detecting motion, could pick up the orientation of a static motion streak.

So, for example, the brain perceives an object moving rapidly horizontally to the left, by combining the output of form-sensitive cells that pick up the horizontal motion streak, and motion-sensitive cells that pick up the leftward movement.

Geisler said this process kicks into action when an object moves above a critical speed - defined as twice its width over 100 milliseconds, says co-author Dr Deborah Apthorp of the University of Wollongong's school of psychology.

Apthorp used functional MRI scans to look for evidence to support Geisler's ideas, as part of her PhD, under Alais' supervision.

MRI study

Apthorp says evidence from previous behavioural experiments in humans have supported Geisler's theory, but there has never been any direct evidence from the human brain.

"We wanted to put people in the scanner and see if we got a similar result," she says.

Apthorp placed people in a fMRI scanner and measured their brain activity as they were shown a number of stimuli.

The study participants were shown fast moving images moving upwards to the left or the right, and slow moving images moving up to the left or right. They were also shown static images tilted to the left or the right.

The researchers found that when the images were moving fast enough to produce a motion streak, the pattern of brain activity across the early visual cortex (the part of the brain involved in basic visual processing) was similar to that when people were viewing static images with the same orientation.

When the motion was too slow to form streaks, the brain showed a different pattern of activity.

"This suggests motion streaks are measurable in the brain," says Apthorp. "It is direct evidence from the human brain to support Geisler's model and shows motion and form pathways are not as separate as people think."

Apthorp carried out the MRI scans in London with support from the Australian Federation of Graduate Women, the Wellcome Trust and Sydney University's School of Psychology.

She says the findings have implications for computational models and the design of experiments used in the study of visual deficits and robotic vision.

Unsettling movies explained?

Apthorp says the new findings could explain why some people are reporting that the new "Hobbit" movie - shot at 48 frames per second rather than the usual 24 frames per second - looks "strange and unsettling".

"This could be that, because the new high frame rate reduces motion blur in the movie, the motion actually looks less realistic to the human visual brain," she says.