- Researchers looked at the first-ever reported case of a person born blind with synesthesia.
- They say their findings shed light on the role of vision in disease.
- They say more research is needed to understand the mechanisms underlying synesthesia.
Synesthesia is a neurological condition in which stimulation of one sense leads to an experience in another, such as seeing colors while listening to music.
Some cases of synesthesia occur after
Between 80 and 97% of people with synesthesia report color-related synesthesia.
The majority of synesthesia cases have reported visual experiences. Reports of nonvisual synesthesia are rare, and so far there have been no reports of synesthesia in people with congenital blindness.
In a recent case study, researchers reported the case of a 40-year-old Italian man with a doctorate. in computer science who was born blind, has no history of neurological or psychiatric disorders and suffers from synesthesia.
“This article invites us to reject the dogma that the visual sense must exist for synesthesia to occur simply by demonstrating the synesthetic experiences of a 40-year-old ‘birth-blind’ person who has had such experiences since age 4 years,” Dr. Howard said. R. Krauss, director of neuro-ophthalmology for the Pacific Neuroscience Institute, CA, not involved in the study, said MNT.
The study was published in Neuropsychology.
In their article, the researchers noted that congenital blindness is a rare disease that affects approximately 0.03% of the population. Since synesthesia affects about 3% of people, the joint probability of having both conditions is 0.0009%.
For the study, the researchers looked at “CB”, an Italian. In interviews, he reported that whenever he hears, writes, reads, or thinks about whole numbers, he experiences them with specific shape and textural properties.
For example, he experienced numbers one and two as having a cardboard-like texture and number three as “soft velvet”.
CB further stated that the numbers, represented as cubes, are aligned in a 3D mental space including mountains and a void in the upper and spatial boundaries, which activates whenever he thinks, writes or hears a number.
Each time he retrieves numerical knowledge, he finds himself in a mental space where he moves through the numbers in the same way as one would in his own house.
He reports that numbers move independently of him and that he can explore them from “different perspectives”, that is, from different sides: from above or from below.
The researchers noted that the ability to explore the synesthetic spatial environment in this way is consistent with previous reports of synesthesia.
CB also reported similar associations with letters, days, and months.
After the interview, CB underwent an experiment alongside 10 non-synaesthetic people: eight sighted people who were matched for education and two people who were born blind.
In the experiment, participants were presented with an array containing 40 rectangular cards, each covered with a different texture mentioned by CB during the interviews. The experiment aimed 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 weekdays
- 8% of months
Meanwhile, those in the control group showed test-retest consistency of:
- 7% for numbers
- 8% for letters
- 9% for weekdays
- 18% for months
The researchers noted that in the areas of numbers, letters, and working days, CB outperformed all controls.
When asked how synesthesia might 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 said Medical News Today:
“So far, there have been no reports of congenitally blind people with synesthesia, although synesthesia is one of the most studied phenomena in psychology. With evidence of congenital blindness interfering with other forms of sensory crosstalk (eg, multisensory integration, cross-modal correspondences), we thought that blindness might prevent the development of synesthesia.
– Professor Bottini
“[We thought this could be the case as] in some cases of multisensory integration, vision plays a central role in creating a common frame of reference towards which other senses converge.
“So in short, the CB case allowed us to test the hypothesis that congenital lack of vision can prevent the radical forms of sensory crosstalk in synesthesia,” he explained.
Dr Bottini went on to say that the underlying mechanisms for sighted and congenitally blind people may be the same and the exact mechanisms behind synesthesia remain unknown.
Professor Yanchao Bi, from the IDG/McGovern Institute for Brain Research at Peking Normal University, China, said DTM that she agreed that the same mechanisms might underlie cases of synesthesia in sighted and congenitally blind people.
When asked to explain a possible theory for the emergence of synesthesia, she said:
“A general hypothesis is that synesthesia emerges due to excessive neural connections between cortical regions. For example, in a sighted brain, the neural systems processing visual color and graphemes – the smallest unit of a writing system, i.e. letters and numbers – are closely linked and can make see the letters as colors.
“In this case, the regions of the brain that process letters and numbers may be strongly linked to those that process textures and spatial patterns,” she explained.
“These new findings push this general hypothesis further, [making us consider how] non-visual form, non-visual textures and non-visual spatial properties are processed in the brain, [and how excessive connections could lead to specific patterns],” she added.
The researchers say their findings have shed light on the mechanisms behind sensory crosstalk in synesthesia.
Asked about the limitations of the research, Dr. Bottini noted that they could not learn much about the mechanisms underlying synesthesia. To address this issue, they are currently conducting a functional magnetic resonance imaging (fMRI) study with CB.
He added that congenital blindness can further reduce the likelihood of developing synesthesia and this can only be known through epidemiological studies.
Dr Krauss added that in the absence of medical records, it could be difficult to establish whether CB was “completely blind” at birth or whether he may have had some level of visual experience.
“The results are exciting,” Dr. Krauss said, “I often tell medical students and graduate students, ‘50% of what you learn in your training will turn out to be wrong in your lifetime.'”
He went on to say that these findings demonstrate how science is not just a “collection” of facts that can be used to advance specific agendas, but rather “a methodology for observing, questioning and developing and hypothesis testing”.
Dr. Bi added that these findings raise many new questions, such as whether the spatial patterns between numbers described by CB match the way 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, but the representations of each modality remain intact (i.e., don’t spoil knowledge about numbers and the letters),” she added.
She stressed that these findings “also show the power of studying particular populations.” These special cases, she says, offer unique opportunities to “dissect different channels to understand the principles by which the brain and mind process information.”