Altered Perception: An Introduction to Synesthesia
By Ajay Chatha (Editor)
Imagine being able to “hear” colors, or “taste” shapes; this phenomenon is known as synesthesia. Synesthesia is a neurological condition in which the stimulation of one sensory modality (the inducer) triggers the involuntary perception of another modality (the concurrent). While synesthesia can take many forms, the phenomenon is always involuntary. For example, a synesthete might perceive a taste likened to vanilla as they hear a specific note being on the piano. The synesthete in the this example has no way of turning the simultaneous perception of the taste off. Another synesthete could see a nebulous yellow tint in their periphery whenever they see the letter ‘A’. This second example demonstrates grapheme-color synesthesia (the inducer, which is a grapheme in this case, and the concurrent, which is a color, are combined when naming a synesthesia type), which is the most common form of synesthesia. It’s important to note that even among synesthetes who have the same form of synesthesia, they will most likely have different associations (another grapheme-color synesthete could associate blue with the grapheme A).
 Example of a synesthete’s grapheme perception.
There is little concrete evidence to explain the mechanisms behind synesthesia. Until the mid-2000s, research suggested that only 1 in 25,000-100,000 people had synesthesia. Combined with the fact that the condition is harmless, there was little incentive for research to be done in the field. But as the condition started gaining attention through media and the internet, more and more people started sharing their experiences with the condition. So more recent research shows that the actual number is closer to 1 in 2,000 (some estimates suggest that the number is as high as 1 in 100). This has prompted an influx of new studies being done on the condition, which so far have revealed that the condition is biological, automatic, and most likely hereditary.
 fMRI scan comparing a synesthete and a control’s brain while viewing a grapheme. The color-selective hVR region is shown in pink, and the yellow/orange/red shows a varying range of brain activation.
There are currently only a few theories explaining the mechanism behind the condition. One mechanism is termed cross-activation; this mechanism suggests that the lack of pruning between certain adjacent regions of the brain can connectivity between those regions and their respective modalities. Another mechanism is called disinhibited feedback, which suggests that information is not inhibited in feedback pathways in synesthetes. This means that as information travels back from cortical regions to early sensory regions, the later stages of sensory processing could bleed into and influence earlier stages of processing. While the disinhibited feedback theory isn’t as flushed out as the cross-activation theory, it can be used to explain why non-synesthetes taking certain drugs can experience synesthesia-esque effects. Unlike cross-activation, which proposes that synesthetes fail to undergo specific neural pruning, the inhibited feedback theory suggests that the neural connections in synesthetes are all normal. This means that it is possible for non-synesthetes to experience synesthesia-esque perception, especially when under the influence of mind-altering drugs.
Some scientists have completely circumvented these two theories, and have posed a less-technical theoretical explanation for synesthesia, called Ideasthesia. This theory refuted the idea that modalities trigger other modalities, by suggesting that concepts trigger a modality. This theory is less of an explanation of synesthesia and more of a replacement—it’s fundamentally different in that it is a semantic model. While this phenomenon is so radically different than synesthesia that it might soon become its own neurological condition, it has helped make the semantic approach to analyzing modalities mainstream in synesthesia research.