Synesthesia, a rare neurological condition, describes an involuntary, automatic union of the senses or secondary experience caused by an external stimulus. The word “synesthesia” comes from the Greek meaning “to perceive together.” These secondary experiences often occur through separate modes such as an internal auditory association with a visual stimulus. Whether the original information is tangible, audible, or visual, the key identifier of synesthesia is simultaneous, intermixed sensory experiences.
There is not a singular type of synesthesia. Both the trigger and concurrent association can be a variety of pairs, but the most common are grapheme-color and sound-color synesthesia. In grapheme-color synesthesia, individual letters and numbers are associated with a color. The visible shape triggers a secondary visual cue and appears tinged or shaded with a specific hue. In sound to color associations, or chromesthesia, a typical experience is comparable to Disney’s Fantasia, where individual sounds trigger a visualization of color. Lexical-gustatory synesthesia is a particularly rare form of the condition where individual spoken words elicit taste sensations in the individual’s mouth. Additional types of synesthesia include spatial sequence and number form synesthesia.
Individuals within the same category of modality interaction reported differences in their specific associations, although these responses occured from the same stimuli and through the same modality. Yet, consistency within different types of synesthesia provides little insight into the neurological, psychological, cognitive, or perceptual basis of the condition. The patterns or shared experiences between synesthetes merely provide a rudimentary diagnostic tool to distinguish between those with the condition and those without.
This neurological condition is not in the forefront of psychiatric or medical investigation, but has been the subject of research for decades (1, 3). Recently, a study was conducted that examined gray matter structure and functioning in grapheme-color synesthetes through magnetic resonance imaging (3). The resulting images revealed patterns that indicate “partly shared mechanisms for all grapheme–color synesthetes, particularly in posterior superior parietal lobe, which is involved in the integration of sensory information” (3). Despite this persistent research, the neurological mechanism of synesthesia is still largely unknown.
The genetic mechanisms for synesthesia remain equally elusive. In large case studies, samplings indicate little about the exact loci or means through which the condition occurs. Early researchers proposed that the condition may be an X linked trait; however, current research indicates that synesthesia appears to hold no gender bias (1).Further, according to a study conducted by Edward M. Hubbard, even a pair of monozygotic twins was discordant for synesthesia (1).
This condition is entirely distinct from adopted practices such as mnemonic devices or metaphor and from experiences such as hallucination because the neurological phenomena experienced by synesthetes are completely involuntary. Often, synesthetes do not recognize that their behavior is abnormal, because the relation occurs simultaneously between modalities and is often undisruptive if not helpful to their daily cognition. In other words, the experience is unlearned and thus an inherent part of cognition, so the experiences are unquestioned. Nonetheless, there have been reports of some synesthetes experiencing sensory overload, becoming exhausted from so much stimulation (2).
- Edward M. Hubbard, Neurocognitive Mechanisms of Synesthesia. (Elsevier Inc., 2005)
- Baaron-Cohen S, Harrison J, Goldstein LH, Wyke M, Coloured speech perception: Is synaesthesia what happens when modularity breaks down?”. (1993)
- Rouw, Romke, and H. Steven Scholte. Neural basis of individual differences in synesthetic experiences. (The Journal of Neuroscience, 2010): 6205-6213.