THE uncanny ability of blind people to "sense" unseen objects has been demonstrated for the first time in sighted volunteers whose vision was blanked out by scientists.
The findings suggest "blindsight", which has been observed in blind people whose eyes function normally but who have suffered damage to the brain's visual centre, is a real and not imagined phenomenon.
In tests, the blind have been able to distinguish basic shapes of objects they cannot see, as well as their orientation and direction of motion. On other occasions a blind person has reported experiencing a "feeling" that an object is present, while not being able to see it.
A number of theories have been proposed to explain "blindsight". Generally, it is suggested that other parts of the brain besides the primary visual cortex respond to nerve messages from the eyes at an unconscious level.
Scientists from the University of Houston in Texas, temporarily blinded a group of 12 volunteers by using an
electromagnetic field to shut down the primary visual cortex. Images were then flashed in front of them on a screen.
In one experiment, volunteers were shown either a horizontal or vertical bar. In another, a red or green dot appeared.
Most of the time, the volunteers were unaware of the images with which they were presented. But they guessed either the orientation of the bar or the colour of the dot correctly more often than would have been expected by the law of averages.
The researchers wrote in the journal Proceedings of the National Academy of Sciences: "Despite unawareness of these 'targets', performance on forced-choice discrimination tasks for orientation and colour were both significantly above chance."
They said the findings suggested that a visual pathway bypassing the primary visual cortex must be responsible for "blindsight".