Reference: Nair, A., & Brang, D. (2019). Inducing synesthesia in non-synesthetes: Short-term visual deprivation facilitates auditory-evoked visual percepts. Consciousness and Cognition, 70, 70-79.

Can you see sounds or taste colors? Does thunder makes you see red swirls or your doorbell evoke a flash of blue light? If so, you probably have synesthesia – a neural condition that merges different senses in the brain and creates unique sensory experiences.

Some people with synesthesia, called synesthetes, experience smells when listening to music, or else associate colors with letters and numbers. In rarer mirror-touch synesthesia, synesthetes see other people touching and feel touched themselves.

If you have no idea what it’s like to have these sensory experiences, you are not alone – synesthesia is a rather uncommon condition, only affecting 1-20% of people (1).

(1) In auditory-visual synesthesia, sounds evoke visual sensations. A person with this type of synesthesia may strongly associate the color orange with the first few notes of their favorite song.

Non-synesthetes are in the majority, but for those who still want to know what it’s like to see sounds, here’s some good news! Psychologists at the University of Michigan have managed to create artificial synesthetic experiences in non-synesthetes. The recipe? Sit in the dark, close your eyes, and listen to some beeping sounds.

How to see sounds

In the study, participants who did not have synesthesia went through a session of visual deprivation by sitting in a dark room with their eyes closed. During that time, they heard short beeping noises. They were told that those noises can make some people see flashes of light or other visual sensations, which are called auditory-evoked visual percepts. The participants pressed a button if they experienced such a visual percept.

In any psychological study, researchers aim to avoid biasing their participants towards any experimental outcome, as that may produce unreliable data. A good strategy is to not tell the participants about the study’s hypothesis and, to prevent people from figuring it out themselves, to introduce a measure that is not actually used in the experiment.

To avoid bias, the research team did not tell their participants the true purpose of the study, but instead designed a “decoy” experimental task. Between the beeping noises, the participants also heard letters of the alphabet and were instructed to imagine them and describe their shapes. This data was not used in the actual analysis of synesthetic experiences.

This “decoy” task also served to ensure that the participants remained awake and alert throughout the experience, because sitting in a dark room for half an hour sounds like a perfect opportunity for a nap.

Seeing the light

Across three different experiments, up to of participants experienced auditory-evoked visual percepts. These visual percepts differed quite a bit: most people reported flashes of light, but others saw blue colors or movement out of the corner of their eyes.

The researchers found that within half an hour of visual deprivation, a subset of non-synesthetes could see images when they heard sounds. The evoked images ranged from bursts of light to dark blotches to blue clumps in the corner of the eye.

There was also a spatial correlation: if sounds came from one of the corners of the dark room, the participants would be more likely to see visual percepts in that corner, too. In addition, louder beeps created more visual percepts, but sometimes beeps weren’t even necessary – the participants had visual sensations simply in response to spoken words of the “decoy” task.

 When the study participants were seated in a dark room with their eyes closed and heard short beeping noises, they often saw flashes of light.

(2) When the study participants were seated in a dark room with their eyes closed and heard short beeping noises, they often saw flashes of light.

What causes synesthesia?

What can this study tell us about the cause of synesthesia?

Scientists have come up with two possible mechanisms underlying synesthesia. In one of them, called the cross-activation model, synesthetes are proposed to have abnormally strong nerve connections between sensory regions in the brain, either due to high usage or to decreased pruning of neurons during childhood. As nerve tissue takes time to grow, synesthesia would take several years to develop under this model.

By contrast, the disinhibited feedback model budgets for only a few minutes to produce a synesthetic experience. This model holds that synesthete brains have normal anatomical connections between sensory brain regions, but they are simply less suppressed than in other people. Information from the auditory processing region may travel more freely to the visual cortex, and vice versa.

The study by the University of Michigan team lends strong support to the disinhibited feedback model, as it only took half an hour to create synesthesia-like experiences in non-synesthetes.

Another phenomenon supporting this model is drug-induced synesthetic experiences. Psychedelic substances, like LSD, can have weak synesthetic effects by creating temporary associations between letters or colors, or sounds and colors (2).

Needless to say, we do not advise taking any drugs to experience synesthesia.  Turning the lights off, closing your eyes, and listening to some sounds is a much better option – and a more relaxing one, too!

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Additional References:

(1) Simner, J., & Carmichael, D. A. (2015). Is synaesthesia a dominantly female trait?. Cognitive Neuroscience, 6(2-3), 68-76.

(2) Terhune, D. B., Luke, D. P., Kaelen, M., Bolstridge, M., Feilding, A., Nutt, D., … & Ward, J. (2016). A placebo-controlled investigation of synaesthesia-like experiences under LSD. Neuropsychologia, 88, 28-34.

Images:
(1) Photo by bruce mars from Pexels
(2) Photo by Francis Seura from Pexels