A man who is clinically blind was able to navigate an obstacle course successfully using "ancient visual paths," European researchers say in a study to be released Tuesday.

The Swiss patient, a doctor known as TN, was left blind after consecutive strokes. Tests showed he had no activity in the visual cortex, the main region of the brain that processes what people see. The man had selective damage to the visual cortex in both sides of the brain.

"This is absolutely the first study of this ability in humans," said Beatrice de Gelder of Tilburg University, the Netherlands, and of the Martinos Center for Biomedical Imaging at Harvard in Massachusetts.

"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."

Preliminary tests showed TN had blindsight — the ability to detect things in the environment without being aware of seeing them. Tests monitoring activity in his brain showed that he responded to facial expressions of fear, anger and joy in others.

In Tuesday's issue of the journal Current Biology, de Gelder and her colleagues described tests of TN's navigational ability.

Applause for collision-free result

TN usually uses a stick to track obstacles and someone helps to guide him when he walks around buildings. The researchers built an obstacle course of randomly arranged boxes and chairs, and asked him to cross it without using his cane or the help of someone else.

"An experimenter always followed behind him during his traversing the course in case of a fall or collision, which seemed a real possibility given his clinical blindness," the study's authors wrote.

"Astonishingly, he negotiated it perfectly and never once collided with any obstacle, as witnessed by several colleagues who applauded spontaneously when he completed the course."

It is theoretically possible that TN used sound waves instead of sight as a guide, the researchers said. But they added it is unlikely since no sounds were detected from the people or objects, and he navigated the course more accurately than humans can do using sounds.

The experiment suggested that the brain has alternative visual paths that allow people to orient themselves to obstacles.

"All the time, we are using hidden resources of our brain and doing things we think we are unable to do," de Gelder said.

"There is much that patients can do outside the grip of their being too aware of what they cannot do."

The findings are important because they help us to understand how vision might have evolved, said Colin Ellard, associate chair of the department of psychology at the University of Waterloo in Ontario.

Visual ability without consciousness

People tend to think vision works like a camera, with light coming in through the eye, forming an image on the retina, and the brain reconstructing a mental image.

But parts of our visual system do not need consciousness, such as how the size of the pupil changes in response to different levels of illumination, Ellard said.

Our brains are organized with relatively independent systems solving problems such as reaching out and grabbing an object, understanding a facial expression or judging if a fruit is ripe.

"So a brain lesion that makes it impossible for a patient to 'see' in the everyday sense of that word might leave intact all kinds of interesting visual abilities that don't involve consciousness at all," Ellard said in an e-mail. "That's what we're seeing in this patient."

This study highlights how these different abilities depend on different parts of the brain, he said.

Similar findings were reported in Helen, a monkey with damage in both sides of the brain. In the monkey's case, the lesion was not absolutely complete in one hemisphere, so researchers surmised a small region of intact vision in the far periphery of the right visual field added some ambiguity to the animal's obstacle course performance.