Synesthesia is often described as a crossing of the senses, hearing colors, seeing sound, and so on. To me it appears that this is touching on something more fundamental and broad than just sensory crossing. A specific kind of multi-sensory associative processing. Synesthetic connections could be the building blocks of our ordinary perception.
I suspect it goes beyond multi-sensory and could be described as multi-cognitive. The term multi-sensory might be appropriate if we redefine how we view the concept of “senses”. Rather than imagining senses to be exclusively describing reception of the external environment, we can also describe senses as reception of internal cognitive processes. I feel my stomach, I can feel my bones, but are these senses “touch”? I can also observe my thoughts and emotions. These should be classified as certain kinds of senses as well, at least in my opinion. Anything that can be felt could be a kind of “sense”. Within this conceptualization of the senses, this broader synesthesia process that I daydream about can be defined as a “crossing of the senses”.
To give some examples, one could associate an emotion with a visual cue. One could associate an emotion with a smell. One could associate a smell with the sound of a bell. One could associate mental imagery with a sound. This last one in particular seems to occur in some cannabis users. This occurs to me under the effects of cannabis. The sounds in songs seem to bring up mental perceptions of moving visual mechanisms, like springs, pistons, cannons, and other often strange and mechanical, physical, and visual representations. This is not a hallucination, but something more internal, like an imagination. It is as strong as the effect that saying “dog” has on my mind. This sound “dog” brings me intrusive thoughts about the concept, appearance, and meaning of “dog”. Somehow the sounds of instruments being played brings up similar mental representations that correlate with how I imagine the object creating the sound would look and behave. It is a prediction I make based on what I’ve already experienced by listening to the sounds of the universe in my past.
The more ordinary and non-lingual kind of synesthesia that we evolved to have can be demonstrated by imagining a quarter falling to hit a wooden floor. The visual observation of this quarter touching the wood becomes associated with the sound emitted from the collision. Your brain connects these two as if they were a unified experience.
I view language to be a synesthetic phenomena. Humans have learned that they can utilize synesthesia to communicate. We synchronize our auditory-concept synesthesia among entire cultures so that we can emit sounds that evoke a sense of meaning that is often tied to memories of multisensory experiences. Consider the case of the dog again. This is experienced as a concept, a visual memory, the sound of the dog (bark), and even far deeper thoughts about how a dog behaves. This response is so strongly conditioned in us that we find it very difficult to suppress this reaction. I’d urge you to attempt to do so.
Synesthesia seems to be something that is innate in children, hypothetically until around 8 months old, where the brain connections that manifest synesthetic experiences seem to be pruned (Ward & Simner 2020; The Synesthetic World Of Childhood). It is thought that those who retain synesthesia as adults have also retained these connections in the brain and have increased structural connectivity and potentially diminished pruning (Rouw & Scholte 2007; Sinke et al 2012; Zamm et al 2013). It is likely to be the case that reinforced perceptions remain, while neglected or failing perceptions fall apart and become pruned away. As we build our perception during infanthood, we may calibrate many aspects of our senses in order to create senses that allow us to reach our goals without error. This may be a case of classical conditioning. Failing to reach our goals is a form of punishment while reaching them is rewarding. I believe that these punishments and rewards condition us to correctly perceive the world. Though, that isn’t to suggest that our perception as adults is always correct. A calibrated perception simply becomes useful enough for reaching our goals and then becomes reinforced by rewards so that we can continue reaching our goals in the future.
There is another important consideration about synesthesia being innate. It isn’t that we are necessarily born with synesthetic experiences preprogrammed, but we may develop it quickly and also lose it quickly during some perceptual critical period. Perceptions may become very easily associated together, but also dissociated again too. Eventually, with enough experience, we begin to narrow down which associations are “correct”. It may be as if we go from making very broad conditioned responses to more fine-tuned associations, which tend to not include the kind of perceptions we label synesthetic.
As a note, I suspect psychedelics (which are popular synesthetogens) revert us back to a similar critical period during the main effects. I’ve talked about the evidence for this hypothesis in The Phoenix Effect.
One commonly studied form of synesthesia is known as grapheme-color synesthesia, where individuals will associate letters of the alphabet with colors. One study found that many people’s grapheme-color synesthesia maps onto the colors assigned to letters of popular refrigerator magnet sets (Witthoft & Winawer, 2006), suggesting that exposure to these fridge magnets during childhood. This may be a case of residual infanthood synesthesia being reinforced by exposure to these fridge magnets.
This brings us to synesthesia training. A group of researchers has managed to train individuals to become synesthetes (Bor et al., 2014). They essentially conditioned people to associate colors to letters using this fridge magnet idea. They used software that presented colored letters and startling noises to individuals to get them to develop conditioned responses. They also had the participants read books with color-coded letters.
Something similar was explored in another study that trained people to hallucinate the sound of a tone (Powers, Mat & Corlett, 2017). This one paired a visual cue with a tone, and exposed the participant to this in repetition. Eventually, the subjects reported hearing a nonexistent tone when presented the visual cue alone. I’d wonder if this is similar to the fridge magnet synesthesia that was observed in the other study. I’d wonder if the sense of a unified perception is based on similar mechanisms.
I don’t yet fully understand the methods of the synesthesia training study, but here’s the exciting part: my current goal is to train a select few individuals to acquire synesthetic experiences themselves. Check out this page for more information on that. If you’d like to somehow help me or take part as a participant, email me at email@example.com. I am particularly interested in finding someone willing to develop a simple software. This will help reduce human error that could stem from training people manually.
Bor, D., Rothen, N., Schwartzman, D. J., Clayton, S., & Seth, A. K. (2014). Adults can be trained to acquire synesthetic experiences. Scientific reports, 4(1), 1-8.
Powers, A. R., Mathys, C., & Corlett, P. R. (2017). Pavlovian conditioning–induced hallucinations result from overweighting of perceptual priors. Science, 357(6351), 596-600.
Rouw, R., & Scholte, H. S. (2007). Increased structural connectivity in grapheme-color synesthesia. Nature neuroscience, 10(6), 792-797.
Sinke, C., Neufeld, J., Emrich, H. M., Dillo, W., Bleich, S., Zedler, M., & Szycik, G. R. (2012). Inside a synesthete’s head: a functional connectivity analysis with grapheme-color synesthetes. Neuropsychologia, 50(14), 3363-3369.
Ward, J., & Simner, J. (2020). Synesthesia: The current state of the field. In Multisensory Perception (pp. 283-300). Academic Press.
Witthoft, N., & Winawer, J. (2006). Synesthetic colors determined by having colored refrigerator magnets in childhood. Cortex, 42(2), 175-183.
Zamm, A., Schlaug, G., Eagleman, D. M., & Loui, P. (2013). Pathways to seeing music: enhanced structural connectivity in colored-music synesthesia. Neuroimage, 74, 359-366.