Neurologist and author Oliver Sacks brings our attention to Charles Bonnet syndrome — when visually impaired people experience lucid hallucinations. He describes the experiences of his patients in heartwarming detail and walks us through the biology of this under-reported phenomenon.

Color Blindness

Most of us share a common color vision sensory experience. Some people, however, have a color vision deficiency, which means their perception of colors is different from what most of us see. The most severe forms of these deficiencies are referred to as color blindness. People with color blindness aren’t aware of differences among colors that are obvious to the rest of us. People who don’t have the more severe types of color blindness may not even be aware of their condition unless they’re tested in a clinic or laboratory.

Inherited color blindness is caused by abnormal photopigments. These color-detecting molecules are located in cone-shaped cells within the retina, called cone cells. In humans, several genes are needed for the body to make photopigments, and defects in these genes can lead to color blindness.

There are three main kinds of color blindness, based on photopigment defects in the three different kinds of cones that respond to blue, green, and red light. Red-green color blindness is the most common, followed by blue-yellow color blindness. A complete absence of color vision —total color blindness – is rare.

Sometimes color blindness can be caused by physical or chemical damage to the eye, the optic nerve, or parts of the brain that process color information. Color vision can also decline with age, most often because of cataract – a clouding and yellowing of the eye’s lens.

Hallucinations

Hear voices. Your doctor may call this an “auditory hallucination.” You may sense that the sounds are coming from inside or outside your mind. You might hear the voices talking to each other or feel like they’re telling you to do something.

See things. This is also known as a “visual hallucination.” For example, you might see insects crawling on your hand or on the face of someone you know.

Sometimes they look like flashes of light. A rare type of seizure called “occipital” may cause you to see brightly colored spots or shapes.