With a study of a blind man who successfully nagivated an obstacle maze unaided, scientists have shown for the first time that it is possible for people who have been left blind after severe brain damage to the visual (striate) cortex to be able to use an ability called “blindsight”, where they can detect things in their vicinity without being aware of seeing them.
The case study of a male patient who is known as TN, appears in the December 23rd issue of Current Biology and was the work of Dr Beatrice de Gelder of Tilburg University, The Netherlands and of the Martinos Center for Biomedical Imaging and Harvard Medical School in the US, together with colleagues from research centres in Italy, the US, the UK and Switzerland.
The researchers said their findings show the importance of other routes in the brain, which are active not only in people who suffer severe damage to the part of the brain that processes sight, but also in all of us as we go about our everyday lives.
Other studies have shown that monkeys with similar brain damage also have this ability.
TN became blind after he suffered two strokes which selectively damaged the visual cortex in both hemispheres of his brain. The patient has no awareness of being able to see and no activity in the visual cortex, the part of the brain that processes visual signals.
De Gelder said:
“This is absolutely the first study of this ability in humans.”
“We see what humans can do, even with no awareness of seeing or any intentional avoidance of obstacles. It shows us the importance of these evolutionarily ancient visual paths. They contribute more than we think they do for us to function in the real world,” she added.
The researchers had already heard that TN had what is called “blindsight”, the ability to detect things in the environment without being aware of seeing them. He is able to sense and respond to a variety of facial expressions in other people. When they watched his brain patterns responding to these expressions the researchers found they were consistent with known patterns for emotions like anger, fear and joy.
TN is blind: he walks like a blind person, using a stick and with the help of others to find his way around inside buildings.
For the study, de Gelder and colleagues made an obstacle course of randomly arranged boxes, chairs and other objects and invited TN to cross it without help and without using his cane.
They had already established using functional and anatomical brain imaging assessments that TN completely lacked any functional visual cortex and the strokes, which lesioned each visual cortex in turn, have left him with “clinical blindness over his whole visual field”.
The scientists were astonished when TN succeeded in navigating the obstacle course perfectly without help and without his stick. He didn’t collide with any of the obstacles and received a resounding and spontaneous round of applause from onlookers when he reached the finish.
The researchers suggest this case study shows there are alternative pathways in the brain that behave like visual pathways and help people keep a spatial sense of where they are relative to things around them, without having to pay conscious attention to noticing them.
They wrote that the case:
“Demonstrates that extra-striate pathways in humans can sustain sophisticated visuo-spatial skills in the absence of perceptual awareness, akin to what has been previously reported in monkeys. It remains to be determined which of the several extra-striate pathways account for TN’s intact navigation skills.”
De Gelder suggested that the alternative pathways are:
” Part of our vision that’s for orienting and doing in the world rather than for understanding.”
“All the time, we are using hidden resources of our brain and doing things we think we are unable to do,” she said, explaining that this is an important message for patients with brain damage:
“There is much that patients can do outside the grip of their being too aware of what they cannot do,” de Gelder added.
The researchers themselves and other experts have said that the news is exciting but more research is needed to verify the findings.
“Intact navigation skills after bilateral loss of striate cortex.”
Beatrice de Gelder, Marco Tamietto, Geert van Boxtel, Rainer Goebel, Arash Sahraie, Jan van den Stock, Bernard M.C. Stienen, Lawrence Weiskrantz, Alan Pegna.
Current Biology Vol. 18, Issue 24, pp. R1128-R1129, 23 December 2008.
Sources: Journal abstract, Cell Press.
Written by: Catharine Paddock, PhD