When is it possible to see without looking? When a person suffers from blindsight.

Blindsight is a bit of an oxymoron, I mean, if you’re blind, how is it possible to see at the same time? In order to understand the condition of blindsight, we have to explore how the brain actually processes visual information.

How the brain processes visual information

Unless you’re an ophthalmologist with intimate knowledge of vision, I bet you thought the same as me when it comes to vision. Visual information enters the eyes and is processed in the visual cortex area of the brain. We see once this processing has taken place.

The truth is far more complicated. We use half of our brain to process visual information. A research team at the Massachusetts Institute of Technology have been exploring how the brain processes visual information.

“We’ve found that even supposedly simple parts of the brain are doing complex, sophisticated processing of such things as visual illusions.” MIT team

We don’t just use our eyes to see

The team describe vision as an orchestra, where groups of cells within the brain interact in order to process all of the various parts of our vision. For example, colour, depth, size, shape, light and dark, movement, shadows, perception, everything.

In fact, the moment we look at something, this information travels to at least 10 different areas of the brain. Each one has their own processing function. The main area is the visual cortex. This is a large area responsible for our conscious vision. It is also the first to receive visual information from the eyes.

We need the visual cortex

The visual cortex processes the majority of what we see and performs complex functions such as ‘filling in’ details.

For example, take a piece of paper with horizontal lines and place it next to an identical sheet. However, move one sheet slightly lower so that the lines are just slightly askew and don’t run in a continuous straight line. Your brain will try to adjust what it sees by ‘filling in’ the small gap between the lines.

The brain is able to fill in by using information from a different area of the brain. This is called a ‘subjective contour’ and is an optical illusion. It is where the brain compensates for a lack of information. It is a complex cognitive process that occurs in the visual cortex and could explain blindsight.

“Our work is the first and most important step in showing that right in the earliest stages of the visual cortex, where visual input first enters the brain, there are groups of cells that break down these stimuli and respond to them. That leaves open the question of how higher-order visual cortex areas further process these kinds of stimuli.” MIT Team

What if there is damage to the visual cortex?

How does damage to the visual cortex affect our sight? Conscious vision is lost when the visual cortex is damaged. People with this damage suffer from ‘cortical blindness’. This means that although visual information is still getting through, it is not being processed in the visual cortex.

But notice, I said ‘conscious’ vision. If you lose your eyes, there is no information to send, whether the visual cortex is up to processing it or not. When you are cortically blind, your eyes are still working, but the normal way of processing the information is gone.

However, there are still areas of the brain able to use this information. Blindsight relates to these phenomena.

What is blindsight?

A few studies have documented this phenomenon. Researcher Larry Weiskrantz coined the term blindsight in 1974. He described blindsight as ‘the ability in people who are cortically blind to respond to visual stimuli’.

It’s important to understand what a cortically blind person is not able to see before we go on to explore what they can. Someone who is cortically blind cannot read a book, watch a film, walk to the bus stop or recognise friends or family.

However, they are able to guess, with more accuracy than if by chance, where a light is flashing, or movements. Remarkably, they can also see emotions and whether a person is looking at them or not.

How people with blindsight can see

One study with a blindsight patient called TN explored whether a person with this type of blindness can tell if someone is looking at them.

Pictures of faces were presented to TD.  There was a mixture of pictures; some showed faces looking directly at TD, in others their gaze was averted. TN correctly identified the faces staring at him. Brain activity in the amygdala, an area thought to be responsible for processing emotions and threats, spiked when TN looked at the direct gazes.

Not only can cortically blind patients tell whether a person is looking at them, they can also correctly identify emotions.

A study in the Netherlands used pictures of a smiling person and a frowning one. Researchers showed these pictures to the participants. Researchers then recorded any changes in facial muscles. The results showed that the participants responded by mimicking the emotions they had ‘seen’.

The great news is that the brain is able to compensate when a part of it is damaged. Moreover, it can be ultra-sensitive when our conscious processes let us down.

Not only that, but all of the compensating areas seem to reside in the oldest and most basic processing parts of the brain. These are the areas that would have been essential for basic human survival thousands of years ago. However, to survive in today’s world we need all our facilities functioning.

As Marco Tamietto, lead author on the Netherlands study reminds us:

“To really understand the intentions of others, we have to be conscious.”

And that means we need the visual cortex.


  1. http://www.bbc.com/

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