It is not uncommon for stroke patients to suffer brain damage, but the case of one patient was peculiar. This patient was, by his own account, completely blind. Two consecutive strokes had destroyed the visual cortex of his brain, and consequently, his ability to see. 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, the patient, who was in his 50s, was clinically blind.
Known as selective bilateral occipital damage, this patients’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.
His 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 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 news that there are “colours” that you cannot see should not be new news to you. The idea that mixtures of colours based off the visible spectrum that you still cannot see may be however. Certain colours, such as pink, are mixtures of different wavelengths of light, but other colours that are mixtures simply cannot be perceived and sound a bit like a real life octarine. These are colours such as red-green and blue-yellow, which are not actually what you get when you mix the two, but really a reddish variety of green or a bluish yellow colour. So why can’t we see these colours? The answer lies in what are known as “opponent neurons” in the eye’s retina. When red is seen one type of these neurons will fire, which the brain sees as red, when green is seen the neuron is silent and this lack of signaling is perceived as green. Interestingly this is also the basis of Red-Green colour blindness. So although these colours actually exist, we cannot see them because we cannot have a neuron firing and not firing at the same time.