In a remarkable experiment, researchers found that a man who was rendered completely blind by several strokes could deftly navigate an obstacle course unaided, easily avoiding boxes and sidling around pieces of office equipment. The patient, known only as TN, was left blind after damage to the visual (striate) cortex in both hemispheres of the brain following consecutive strokes. His eyes are normal but his brain cannot process the information they send in, rendering him totally blind [BBC News]. Researchers say TN’s successful performance was an example of the phenomenon “blindsight,” and say it suggests that some small amount of information is being transmitted from his undamaged eyes to a more primitive part of his brain, which operates beneath the level of consciousness.
TN usually walks with a cane, but researcher Beatrice de Gelder convinced him to put it aside and to try to navigate the obstacle course without its help. He was able to do so flawlessly, despite being unable to consciously see any of the obstacles. Head down and hands loose by his side, he twisted his body to slalom slowly but surely between a camera tripod and a swingbin, and neatly stepped around a random series of smaller items. “At first he was nervous,” says de Gelder. “He said he wouldn’t be able to do it because he was blind.” The scientists broke into spontaneous cheers when he succeeded [Nature News].
The experiment, published in Current Biology [subscription required], is the most advanced example of blindsight yet recorded. Brain scans had previously revealed that TN could recognise facial expressions, since the amygdala region showed increased activity when he was faced with fearful, angry or joyous faces. This suggested his mind could process some visual information, albeit subconsciously [New Scientist]. Researchers have also tested patients who have lesions on one side of their visual cortex, leaving them blind in one direction of sight. But when objects were placed in those patients’ blind spots, they were able to tell researchers whether they were oriented horizontally or vertically.
Some experts are skeptical of the latest findings from TN’s stroll down the corridor, and say that his hearing should have been blocked to rule out the possibility that he was getting an auditory assist: Some blind people can use their hearing to assist in navigation, sensing the reflection of sound waves to help them locate obstacles. “We don’t think this is likely to be the answer [in TN] because echolocation is not so efficient for small objects, and he could avoid them with precision,” says de Gelder [Nature News].
The latest findings will have limited applications in medicine and therapy, as most cases of blindness are not caused by brain damage. But de Gelder says the study illuminates the sophistication of our vision system: “It shows us the importance of these evolutionary ancient visual paths. They contribute more than we think they do for us to function in the real world.”