Synesthesia is loosely defined as a merging of the senses, the notion that one sensation can trigger a separate perception. This is most commonly manifested as individuals who perceive color with letters or numbers (known as grapheme-color synesthesia), however other variants include experiencing taste with words, texture with taste, or color with music. It is important to note that the secondary, or concurrent, sensation does not replace the primary, or inducer, perception, but is instead experienced in addition to it. There are currently 61 known variants of synesthesia, spanning the perceptual spectrum of color, taste, sound, touch, and even higher level cognitive conceptions, such as language. At first glance, these phenomena seem quite unique and binary (after all, you either taste rectangle or you don't), and the prevalence of synesthesia was originally thought to be roughly 1 in 1000. However, new research on sensory integration has revealed that varieties of synesthesia may be much more common than originally thought, and that there may be a sliding scale in the amount of sensory cross-talk among individuals. Synesthesia is thought to derive from the periphery activation of neurons in unrelated sensory regions from the primary stimulus. For example, regions involved in processing linguistic or textual information may "bleed over" and begin to activate neurons in the area of the visual cortex pertaining to color. Functional imaging analyses confirm these suspicions, with both primary (inducer) and secondary (concurrent) neural regions experiencing activation upon an encounter with a trigger stimulus, such as the activation of both the auditory and visual cortices while listening to music. This phenomenon is also associated with a crucial increase in connectivity between the relevant regions, with an increase in white matter tracts (myelinated axons that carry signals throughout the brain) demonstrated using diffusion tensor imaging analysis. However, how these connections are created or maintained is not as well established.
One hypothesis is that as children our brains are much more malleable, containing many more synapses between neurons and allowing the potential for any number of neural connections to be made. However, as we age these connections are pruned out, leaving only those that are most highly utilized. This process is extremely important and beneficial to our cognitive development, allowing us to become much more efficient at common processes and behaviors. However, this does mean that we lose some of the more unusual connections that are not typically used, perhaps including some of these inter-sensory interactions. If you indulge in the type of thought processes that would activate these connections as a child though, such as pondering about the precise burnt shade of the number 5 (does it have more of an orange or red tint?), then perhaps these connections may remain.
Adults who report experiencing strong sensations of synesthesia are acutely aware of these extra perceptions, the blues from Beethoven's 9th Symphony being quite pronounced and demanding attention. However other individuals may have much more subtle experiences, feeling merely a suggestive halo of violet in accompaniment. Furthermore, interactions between the senses are not limited to just those few who experience synesthesia. A recent article by Courtney Humphries from the Boston Globe highlighted how some restaurant, manufacturing and marketing companies are trying to tap into the common cross-sensory phenomena that we all have to make the consummatory experience more enjoyable. For example, research out of UCLA and Oxford University on the perceptual effects of the interactions between senses have shown that the crisp, both in regards to texture and sound, of a potato chip can alter the way we taste it, and that the color of a strawberry mouse influences our perception of its sweetness.
Finally, on a less scientifically established note, is the idea of reading people's "auras". These claims are often disregarded as quack extrasensory phenomena, but is it possible that some individuals may experience the cross-talk between color and identity that others have with numbers or days of the week? The fusiform face area (FFA) is a highly established region of the visual cortex that is crucial in processing facial information. Could the perception of auras arise from an interaction between color modalities and the FFA? In her article in the British Journal of Psychology, psychologist Julia Simner touches on this greater conceptual notion of synesthesia, citing individuals who experience an anthropomorphism of letters or numbers, associating them with personality traits. Although not a merging of the senses per se, this ordinal linguistic personification does share similar tendencies to the more traditional sensory grapheme-color associations, as well as the potential merging of color and person-hood.
Regardless of its form, synesthesia provides us with yet another reason to explore the fascinating world of neuroscience in a less conventional way, and helps us to remember just how intricate and magical our brains can be.
(Thanks to Sam Greenbury, Josh Keeler, and Lindsey Heck for the inspirational discussions for this post. For those who are interested in more information on synesthesia, I highly recommend the book "Wednesday is Indigo Blue: Discovering the Brain of Synesthesia" by Dr. Richard Cytowic and Dr. David Eagleman.)