By JOHN NEWELL
LONDON - People who are not blind can still fail to see, as a result of unattended visual information escaping awareness.
By studying such loss of awareness after brain damage, combined with brain scanning, scientists are beginning to unravel the neural basis of conscious visual experience.
Their work at the Institute of Cognitive Neuroscience at the University College London is also helping to lead to new treatments for conditions which affect many thousands of people, usually as a consequence of stroke.
Light reaches our eyes from many sources and many visible objects, but at any one time we are consciously aware of only a few of these stimuli. .
We can to some extent choose where we direct our attention, and this can alter our visual awareness of the world even when the image entering our eye remains unchanged.
Experiments have shown that we can become completely blind to information that we are directly looking at, if our attention is directed to something else. This is called "inattentional blindness."
Its existence means that the generally held belief that we see and attend to everything in our field of vision simultaneously is an illusion.
Inattentional blindness can be brought about by some forms of brain damage in which patients become permanently unaware of events on one side of their field of view, usually the left side.
When such patients are asked to draw objects such as an animal, a bicycle, a human face or a clock face, parts of which are situated in the area they are unaware of, then they do not draw those parts at all.
They believe their drawings are complete.
Patients with this condition, known as spatial neglect, show their unawareness of one side of their field of view in other ways. They may ignore people approaching from the neglected side, eat food from only one side of their plate and shave or make up only one side of their face.
More research has shown that many people affected by spatial neglect are not completely blind on the affected side. They sometimes see isolated events there.
For such people, loss of awareness of the left-hand side may occur only when competing events are taking place in the right-hand side.
Magnetic resonance imaging (MRI) has shown that one part - the right parietal lobe - is usually damaged in patients suffering from spatial neglect.
Because visual perceptions are processed in the brain, a number of other areas of the brain become involved in order to acquire properties such as colour, movement and identification.
MRI scanning has shown that these other areas may remain undamaged in a patient affected by damage to the right parietal lobe.
It has also shown that objects in the neglected left-hand side of the field of vision may still produce a response in these undamaged areas of the brain, even though the patient remains unaware of that response.
The fact that neglected events are still being processed in the patient's brain suggests it might be possible to bring them back into consciousness.
One function of the parts of the visual cortex that remain undamaged in these patients is the ability to to organise visual images into distinct objects.
By using computer-generated displays, Professor Jon Driver and his colleagues at the Institute for Cognitive Neuroscience have shown that some patients affected by visual neglect can have some conscious vision restored in the neglected part of their visual field.
This provides evidence that some neglected events can be brought back into the awareness of a patient affected by visual neglect, by harnessing the residual function of unaffected areas of the brain.
Wearing spectacles with the right half of the lens for each eye covered has also been shown to help some patients affected by spatial neglect to regain some awareness of events in the left-hand side of their field of vision.
So has the use of prisms which redirect light from the left to the right-hand side of the visual field.
There is hope that more effective treatments may be devised.
Researchers are discovering that the brain is more plastic and adaptable and has more capacity for self repair than used to be thought, even in middle and old age.
Herald Online Health
Scientists begin to unravel the neural basis of sight
AdvertisementAdvertise with NZME.