Blindsight occurs in some people who have suffered damage to their visual cortex. The visual cortex is the primary place to which the brain routs information from the eyes. It is responsible for decoding visual information and can detect patterns and motion. Damage to this area, therefore, can lead to complete blindness - even in a person with healthy eyes.
Often, however, whatever damages the visual cortex (usually a stroke) damages only a part. This condition, called a scotoma, creates a large "hole" in the vision of the affected person where they cannot consciously detect any visual stimuli. In addition, they are often unable to accurately reproduce a drawing, leaving out the section the lies in the affected region of their vision.
Missing Information: Any sensory data that is directed to the damaged part of the visual cortex is ignored by people with a scotoma. If the person has blindsight, however, he or she may be able to identify stimuli presented to the blind spod using unconscious brain pathways. Photo Credit: Stanford University
When shown a stimulus in the affected region of their vision, patients will deny that they see anything. However, when pressed just to make a guess as to where the stimulus came from, they can often identify the direction of the source with an accuracy far better than chance. Some people can even identify whether the stimulus was an "x" or an "o", despite not consciously knowing either way. Psychologists were quick to recognize the huge implications of this - apparently, visual processing occurred in places other than the visual cortex.
In addition, a research team led by Prof. Tadashi Isa and Dr. Masatoshi Yoshida of the National Institute for Physiological Sciences, Japan, demonstrated that monkeys with damage to their visual cortex can be trained to recognize stimuli in their blind spots. The team trained monkeys to look at a visual stimulus at all points in their visual field. At first, the monkeys were mostly unable to track any stimulus in the affected region of their vision, but after 2-3 months they were tracking visual stimuli in all areas of their vision with the same accuracy. This result suggests that visual information can take pathways around the visual cortex and still provide useful information - something that could perhaps be used for therapeutic purposes in humans.
More research, to be published in Current Biology, recently demonstrated this ability in humans. An unusual patient with selective damage to the visual cortex in both hemispheres, which led to complete blindness, successfully navigated an obstacle course of boxes and chairs without the use of a cane or a guide. For all other purposes, the patient is blind - he usually doesn't walk without a cane and requires a someone to guide him around buildings. However, brain imaging shows that he emotionally responds to facial expressions and can track visual stimuli unconsciously, thanks to a separate brain pathway for visual information that doesn't involve the visual cortex.
Beatrice de Gelder of Tilburg University, The Netherlands and of the Martinos Center for Biomedical Imaging and Harvard Medical School, cited an evolutionary reason for the alternative pathway - it's an outdated mechanism that helps us determine our spatial situation unconsciously: "It's a part of our vision that's for orienting and doing in the world rather than for understanding," she said. "All the time, we are using hidden resources of our brain and doing things we think we are unable to do."
The research sheds light on an area of psychology that receives a huge amount of attention these days: the interpretation of sensory data. Studying conditions like blindsight allows psychologists to identify all the ways visual information is processed and could eventually help engineers develop ways of helping blind people see, regardless of the reason.
For patients with brain damage, de Gelder wants to help them realize what they are capable of: "There is much that patients can do outside the grip of their being too aware of what they cannot do."