Synesthesia: the blending of two or more sensory experiences, or the automatic activation of a secondary (indirect) sensory experience due to certain aspects of the primary (direct) sensory stimulation.

Vision is not an encapsulated system. It interacts with and depends on other sensory modalities. For example, when you move your head in one direction, your eyes reflexively move in the opposite direction to compensate, allowing you to maintain your gaze on the object that you are looking at. This reflex is called the vestibulo-ocular reflex. It is achieved by integrating information from both the visual and the vestibular system (which knows about body motion and position). You can experience this compensation quite simply. First, while you keep your head still and your gaze looking straight ahead, wave your finger in front of you from side to side. Notice how the image of the finger appears blurry. Now, keep your finger steady and look at it while you move your head from side to side. Notice how your eyes reflexively move to compensate the movement of your head and how the image of the finger stays sharp and stable. Vision also interacts with your proprioceptive system, to help you find where all your body parts are, and with your auditory system, to help you understand the sounds people make when they speak. You can learn more about this in the multimodal module.

Finally, vision is also often implicated in a blending-of-sensations phenomenon known as synesthesia. Synesthesia occurs when one sensory signal gives rise to two or more sensations. The most common type is grapheme-color synesthesia. About 1 in 200 individuals experience a sensation of color associated with specific letters, numbers, or words: the number 1 might always be seen as red, the number 2 as orange, etc. But the more fascinating forms of synesthesia blend sensations from entirely different sensory modalities, like taste and color or music and color: the taste of chicken might elicit a sensation of green, for example, and the timbre of violin a deep purple.

The experience of involuntary sensory cross activation where the presentation of a particular stimulus elicits a secondary sensory-perceptual experience. It most commonly occurs in the association of color with linguistic stimuli such as letters, numbers, words, or music, but can also occur between other senses. Although synesthesia can be acquired or transient due to trauma or drug use, there is also a strong genetic component, with a prevalence of about 1 in 2,000 individuals and a female to male ratio of 6:1.

What does the color blue taste like? Does guitar music smell sweet or spicy? These questions might sound like nonsense. But for people with a condition called synesthesia, they describe real experiences.

We have five senses: touch, sight, taste, sound, and smell. Most people experience their senses one at a time. In synesthesia, one sense can be experienced at the same time as another. For example, a person with synesthesia might feel something rough brush against their hand every time they smell a flower.

They can also experience the same sense in two ways. For example, seeing letters or numbers in a specific color, like the letter “A” always being red no matter what color it’s written in. This is the most common type of synesthesia.

Synesthesia isn’t very common. It sometimes runs in families. This makes researchers think that genes may play a role.

Synesthesia isn’t dangerous, and rarely causes problems for people. Most people with synesthesia say they enjoy their special senses.

For researchers, “studying synesthesia allows us to think about what the brain does,” explains Dr. Krish Sathian, a neurologist at Pennsylvania State University.

Scientists don’t know what happens in the brain to cause synesthesia. “There are a few explanations that have been put forward,” Sathian says. These involve the way neurons talk to each other. Neurons are the brain cells that process information from the senses.

Some researchers think that people with synesthesia have extra connections between neurons in some areas of the brain. Others think that the direction that information can flow between brain cells might be different.

Sathian’s lab uses imaging tests to watch how the brain works. They’ve found that, even in people without synesthesia, the senses often work together in ways previously unrealized.

For example, when you touch an object and try to identify its shape, the part of your brain that processes sight becomes active along with the area that processes touch.

Sathian’s team showed that people had more difficulty sensing an object’s shape by touch when the brain area that processes sight was blocked using magnets.

His team has wondered if synesthesia is an extreme version of these brain collaborations. They tested whether people with synesthesia are better at tasks that use more than one area of the brain. People with synesthesia performed faster at a task that linked language with a specific shape than people without the condition. But they weren’t any better at other tasks requiring more than one sense.

Synesthesia remains a mystery for now. But Sathian and other NIH-funded researchers plan to keep studying it. The condition may hold keys to understanding things that are fundamental to being human, such as language and memory.

“It’s been shown that people with synesthesia have better memories than people who don’t,” Sathian says. “Maybe understanding synesthesia could give us insight into some of the mechanisms that underlie memory. And then perhaps eventually to some better treatments for people who have memory disorders.”

Synesthesia is different for everyone who has it. People with the condition may experience:

• Seeing certain letters or numbers in specific colors
• Having a certain taste in your mouth triggered by hearing a word
• Feeling a sensation of touch when you smell something
• Seeing a color or shape in response to hearing a sound