Blind patient does not perceive objects or colors, but does perceive movements
Seeing despite blindness – a mysterious phenomenon © Freeimages
But not completely blind: Neuroscientists have studied the brain of a woman who is blind – but can still see. In the patient a large part of the visual center is destroyed. Nevertheless, she perceives certain things quite consciously, she sees moving objects. The explanation: The brain of the blind has found ways to bypass the defective areas and to transmit visual motion stimuli via detours – an impressive example of the plasticity of the human thought organ.
Actually, the matter seems clear: When we see, the eye and the brain create images of the environment in our head. If the eye does not work or if the visual center in the brain is destroyed, this does not work anymore – we are blind. Or not after all? Mysteriously, there are people who are clearly blind, but still see. These "blind men" instinctively avoid obstacles or inexplicably reach for objects with unerring accuracy. In short: They seem to react very well to certain optical stimuli, even if often unconsciously.
A bobbing ponytail
Neuroscientists led by Michael Arcaro of Harvard Medical School in Boston have now reported a particularly unusual case of blindsight: 48-year-old Scottish woman Milena Canning woke up blind from a coma 18 years ago after a severe respiratory infection and several strokes. After a short time, however, she discovered that her sight was apparently not completely lost.
This is how the patient suddenly perceived the glitter of a gift bag. She saw the bobbing ponytail of her daughter, but not her face, the raindrop running down the window pane, but not what was behind the pane. To this day, the blind woman can see things if they meet one condition: They must move. How can this phenomenon be explained?
To find out, Arcaro and his colleagues ran some tests with Canning: How would she react to moving objects, and what would happen in her brain in the process? It turned out: In the experiment, Canning was able, for example, to perceive the movement, size and speed of a ball rolling toward her – and to grab it unerringly at the right moment. When asked about the color of the object, she could not give a clear answer.
A look at the brain using functional magnetic resonance imaging finally revealed the patient’s secret: "Canning is missing tissue the size of an apple in the rearmost part of the cerebrum – almost her entire occipital lobe is no longer functioning," reports co-author Jody Culham from the University of Western Ontario in London, Canada. This area of the brain is responsible for processing visual stimuli. Despite the defect in this visual center, however, Canning’s brain has found ways to create visual images: it simply bypasses the damaged areas.
Impressive feat of adaptation
"Figuratively speaking, the highway of the visual system leads to a dead end in Milena’s case. But it has found byways to bypass this highway and still transport some visual stimuli – especially motion – to other areas of the brain," Culham explains. As the researchers report, this phenomenon of seeing movement blindly is also known as Riddoch syndrome and is extremely rare.
"Our work provides possibly the most complete characterization of a single person’s visual system to date," says Culham. In their opinion, this analysis impressively demonstrates one thing in particular: the amazing plasticity and adaptability of the human brain.