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A new paper details the first-ever reported case of synesthesia in a congenitally blind person. Jonathan Knowles/Getty Images

  • Researchers examined the first-ever reported case of a congenitally blind person with synesthesia.
  • They say their findings shed light on the role of vision in the condition.
  • They say further research is needed to understand the underlying mechanisms of synesthesia.

Synesthesia is a neurological condition in which stimulation of one sense leads to an experience in another, such as seeing colors when listening to music.

Some cases of synaesthesia occur after brain damageTrusted Sourcedrug use, or sensory loss. However, many report symptoms for as long as they can remember.

Between 80 and 97% of people with synesthesia report color-related synesthesia.

The majority of synesthesia cases have reported visual experiences. Reports of non-visual synesthesia are rare, and until now, there have been no reports of synesthesia in people with congenital blindness.

In a recent case study, researchers reported a case of a 40-year-old Italian male with a Ph.D. in Computer Science who was born blind, has no history of neurological or psychiatric disorders, and has synesthesia.

“This paper calls upon us to discard the dogma that the visual sense must exist for synesthesia to occur simply by demonstrating the synesthetic experiences of a 40-year-old ‘congenitally blind’ person who had such experiences since age 4,” Dr. Howard R. Krauss, director of Neuro-Ophthalmology for the Pacific Neuroscience Institute, CA, not involved in the study, told MNT.

The study was published in Neuropsychologia.


In their paper, the researchers noted that congenital blindness is a rare condition that affects around 0.03% of the population. As synesthesia affects around 3% of people, the joint probability of having both conditions is 0.0009%.

For the study, the researchers examined “CB,” an Italian man. In interviews, he reported that whenever he hears, writes, reads, or thinks of whole numbers, he experiences them having a shape and specific textural properties.

For example, he experienced numbers one and two as having a cardboard-like texture and the number three as “soft velvet.”

CB further said the numbers, represented as cubes, are aligned in a 3D mental space comprising of mountains and a vacuum in the upper and spatial borders, which activates every time he thinks, writes, or hears a number.

Whenever he retrieves numerical knowledge, he finds himself in a mental space where he moves through numbers similarly to how one would through their own home.

He reported that the numbers move independently of him and that he can explore them from “different perspectives” i.e. from different sides: above or from below.

The researchers noted that the ability to explore the synesthetic spatial environment in such a way matches previous reports of synesthesia.

CB also reported similar associations with letters, days, and months.


After the interview, CB underwent an experiment alongside 10 non-synesthetic individuals: eight sighted people who were matched for education and two people who were blind from birth.

In the experiment, the participants were presented with a board containing 40 rectangular cards, each covered by a different texture mentioned by CB during the interviews. The experiment sought to assess participants’ consistency in retrieving and matching concepts with textures between two sessions conducted 30 days apart.

The researchers found that CB matched the same numbers with the same textures 75% of the time between the two sessions.

The same was true for:

  • 42% of letters
  • 42% of days of the week
  • 8% of months

Meanwhile, those in the control group showed a test-retest consistency of:

  • 7% for numbers
  • 8% for letters
  • 9% for days of the week
  • 18% for months

The researchers noted that in the domains of numbers, letters, and days of the work, CB outperformed all of the controls.

Underlying mechanisms

When asked how synesthesia may work in blind people, Roberto Bottini, associate professor at the Center for Mind/Brain Sciences at the University of Trento in Italy, and study author told Medical News Today:

“Until now, there have been no reports of congenitally blind people with synesthesia, despite synesthesia being one of the most studied phenomena in psychology. With evidence of congenital blindness disrupting other forms of sensory crosstalk (e.g., multisensory integration, crossmodal correspondences), we thought that blindness might prevent the development of synesthesia.”

– Prof. Bottini

“[We thought this could be the case as] in some cases of multisensory integration, vision has a pivotal role in creating a common reference frame onto which other senses converge.”

“So, long story short, the case of CB allowed us to test the hypothesis that congenital lack of vision may prevent radical forms of sensory crosstalk in synesthesia,” he explained.

Dr. Bottini went on to say that the underlying mechanisms for both sighted and congenitally blind people may be the same and that the exact mechanisms behind synesthesia remain unknown.

Professor Yanchao Bi, at the IDG/McGovern Institute for Brain Research in Beijing Normal University, China, told MNT that she agreed that the same mechanisms might underlie cases of synesthesia in those who are sighted and congenitally blind.

When asked to explain a possible theory for the emergence of synesthesia, she said:

“One general hypothesis is that synesthesia emerges due to excessive neural connections between cortical regions. For instance, in a sighted brain, the neural systems processing visual color and graphemes — the smallest unit in a writing system i.e. letters and numbers — are closely wired together and may cause seeing letters as colors.”

“In this case, the brain regions processing letterforms and number forms may be strongly connected with those processing textures and spatial patterns,” she explained.

“These new findings push this general hypothesis further, [making us consider how] nonvisual shape, nonvisual textures, and nonvisual spatial properties are processed in the brain, [and how excessive connections could lead to specific patterns],” she added.

The researchers say their findings shed light on the mechanisms behind sensory crosstalk in synesthesia.

When asked about limitations to the research, Dr. Bottini noted that they could not learn much about the underlying mechanisms of synesthesia. To address this, they are now running a functional magnetic resonance imaging (fMRI) study with CB.

He added that congenital blindness may still reduce the likelihood of developing synesthesia and that this could only be known through epidemiological studies.

Dr. Krauss added that in the absence of medical records, it might be difficult to establish whether CB was “completely blind” at birth or if he may have had some level of visual experience.

“The results are exciting,” said Dr. Krauss, “I often say to medical students and those in postgraduate studies, ‘50% of what you will learn in your training will be proven false in your lifetime.’”

He continued to say that these findings demonstrate how science is not just a ‘collection’ of facts that can be used to further specific agendas, but rather “a methodology for observation, questioning, and the development and testing of hypotheses.”

Dr. Bi added that these findings invite many new questions, such as whether the spatial patterns between numbers CB described are how numbers are typically processed in the brain.

“I also wonder about the neural basis of these specific patterns of synesthesia, where these unusual connections are made yet the representations for each modality remain intact (i.e., not messing up the knowledge about numbers and letters),” she added.

She highlighted that these findings also “showcase the power of studying special populations.” Such special cases, she says, provide unique opportunities to “dissect different channels to understand the principles in which the brain and mind process information.”

Source: Synesthesia: First-ever reported case of congenitally blind person