'Blindsight' phenomenon

Tuesday December 23 2008

“A man who was left completely blind by multiple strokes has been able to navigate an obstacle course using only his "sense" of where hazards lie,” The Daily Telegraph reported. It said that researchers had already found that the man used ‘blindsight’ to react to facial expressions in other people such as fear, anger and joy. They tested the extent of this ability by constructing an obstacle course for him to navigate, which he did without striking any of the objects in his path.

This story is based on a case report of a man demonstrating ‘blindsight’, an ability that has previously only been described in monkeys. The man’s blindness was caused by strokes and was a result of damage to the brain rather than his eyes. The researchers say that this shows there are pathways in the brain other than those known to be involved in vision that give humans navigational skills in the absence of sight. This finding is intriguing and may lead to future study.

When evaluating a piece of research, it is important to consider how strong the evidence for it is. In this case, this type of study is often thought of as the lowest level of evidence. Further research that demonstrates this ability in others is needed.

Where did the story come from?

The research was carried out by Professor Beatrice de Gelder from the University of Tilburg in the Netherlands, and colleagues from the Netherlands, USA, Italy, Switzerland and Scotland. It was partly funded by grants from several sources including the European Union. The study was published in the peer-reviewed science journal Current Biology .

What kind of scientific study was this?

In this case report, the researchers described a single patient, referred to as patient TN, who had had at least two strokes which had damaged both sides of his brain. After the two main strokes, he was left with clinical blindness over his whole visual field. The researchers say this was caused by the loss of nerve cells in the areas of brain where the nerve signals from the eyes eventually end up, known as the primary visual (striated) cortex, and in the pathways that lead to these, known as the geniculostriate pathways. This loss of function in the visual cortex was confirmed by brain imaging assessments using advanced functional MRI scans.

The researchers say that the first indication that patient TN had ‘affective blindsight’ was when they noticed that he reacted to facial expressions that he could not see. To confirm this, they tested him with brain scans to show that parts of his brain reacted to the emotional expressions he was exposed to in other people including fear, anger and joy.

The researchers also tested TN with an electroencephalograph (EEG), which detects the electrical currents within the brain from electrodes placed on the surface of the scalp. This identified which parts of the brain activated when objects or flashing lights were placed in various parts of the man’s visual fields.

Patient TN’s ability to navigate was then tested as he was asked to walk down a long corridor in which various barriers such as waste paper baskets, tripods and small boxes had been placed.

What were the results of the study?

The MRI and EEG tests showed that the man completely lacked any functional visual cortex. The abilities he retained did allow him to successfully navigate down the corridor. A video shows him avoiding six or seven blockages.

What interpretations did the researchers draw from these results?

The researchers say this demonstrates that pathways other than the usual geniculostriate pathways are being used and means that humans can retain navigational skills in the absence of sight. This is similar to what has been previously reported in monkeys.

They conclude that “it remains to be determined which other pathways account for the retained navigational skill”. They say that the scans showed that, when TN’s visual field was stimulated, there was a different type of activation pattern in the left hemisphere compared to the right one. This suggests that part of the explanation might lie in how the nerve signals are transferred from one side of the brain.

What does the NHS Knowledge Service make of this study?

Case reports are often the first study type in humans. As an observational study, they provide preliminary information and can be the starting point for future studies. If more people develop the condition, a case series can be formed or alternatively other observational studies with control groups can be designed.

Case reports are the lowest level of evidence commonly referred to in a hierarchy of study types. This is because without a comparison group or even the ability to make recordings in similar patients it's not possible to draw general conclusions. For example, it's not possible to know what aspects of these findings would be the same or different for anyone else with this type of blindness or pattern of brain damage.

In this type of study, it is also important for repeat tests to be conducted, preferably using a new group of researchers, independent of the original observers. This is so that the lack of function in the visual cortex can be confirmed as this is critical to the idea that the man has no sight. The researchers have done this using several different techniques themselves, but say that it was difficult as the patient was unable to keep his eyes still for testing. They say that in one test “one could not be certain that absolutely all visual cortex had been destroyed or inactivated.”

The exciting thing about this observation, is that ‘blindsight’ in humans is now known to be possible. Using currently available imaging techniques, such as the advanced MRI-scanner, more of the specific visual pathways in the brain can be mapped.

Analysis by Bazian
Edited by NHS Choices