Patient TN was, by his own account, completely blind. Two consecutive strokes had destroyed the visual cortex of his brain, and consequently, his ability to see.
It is not uncommon for stroke patients to suffer brain damage, but the case of TN — referenced by his initials, the general practice in such studies — was peculiar. His first stroke had injured only one hemisphere of his visual cortex. About five weeks later, a second stroke damaged the other hemisphere. An assessment of his brain function revealed that after two strokes, TN, in his 50s, was clinically blind.
Known as selective bilateral occipital damage, TN’s unusual injury made him the subject of much interest while recovering at a hospital in Geneva. Researchers began examining him and discovered that despite his blindness, he had maintained the ability to detect emotion on a person’s face. He responded appropriately — with emotions such as joy, fear, and anger — to a variety of facial expressions. Observed activity in his amygdala — the part of the brain responsible for processing emotions — confirmed the curious results.
To further test the extent of TN’s abilities, researchers from Tilburg University in the Netherlands devised a simple yet decisive experiment: an obstacle course. They arranged boxes, chairs, and various other objects down a long hallway. The team then asked TN to navigate the course without any sort of assistance. TN was skeptical, as he required the aid of a cane and a guide to get around. But eventually, he decided to participate. Researchers recorded the result in their recent paper: “Astonishingly,” the report reads, “he negotiated [the course] perfectly and never once collided with any obstacle, as witnessed by several colleagues who applauded spontaneously when he completed the course.”
TN’s rare condition is known as blindsight. Because his stroke damaged only his visual cortex, his eyes remain functional and as a result can still gather information from his environment. He simply lacks the visual cortex to process and interpret it. Sight has changed for TN from a conscious to a largely subconscious experience. He no longer has a definitive picture of his surroundings, but he has retained an innate awareness of his position in the world. He is, to some degree, able to see without being aware that he is seeing.
The researchers explained that TN’s success indicates that “humans can sustain sophisticated visuo-spacial skills in the absence of perceptual awareness.” Similar abilities have been observed in monkeys, but TN’s is the first study of these abilities in humans. According to Beatrice de Gelder, a neuroscientist from Harvard and Tilburg, who helped conduct the study, “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.”
Intact navigation skills after bilateral loss of striate cortex
Current Biology December 23, 2008
Originally published January 14, 2009