Synesthesia: An Intertwining of the Senses

Written by Taylor Le and Edited by Alexander Alva

Sensory interpretation relies on relatively specific regions and neural pathways in the brain. For instance, visual data reaches the retina of each eye, filtered by cells that distinguish colors, and is interpreted by a region of the brain known as the occipital lobe. The corpus callosum, a bundle of nerve fibers that enable communication between the two hemispheres of the brain, ultimately pieces together an overall image using data from each eye [1]. Such interplay between the hemispheres proves necessary for cognitive function, yet it is atypical to observe simultaneous intertwining specifically between distinct sensory pathways, such as hearing and sight. In an estimated two to four percent of humans, a rare neurological condition known as synesthesia involves an intertwining of the senses, such that stimuli that are normally interpreted by one sensory pathway involuntarily stimulate the other senses and their own pathways [2]. In other words, one or more sensory organs and pathways are linked to create an intertwined interpretation.

For instance, grapheme-color synesthesia and chromesthesia are among the various studied types of projection synesthesia, in which colors are instinctively projected and observed with letters, numbers, and even sound. Individuals with grapheme-color synesthesia associate the visual form of numbers and letters (graphemes) with a specific color [3]. In chromesthesia (sound-color synesthesia), hearing sounds with varying tonalities, beats, and rhythms will trigger vivid colors with movement and shape [4]. The extent of synesthesia varies widely among individuals, and there are types that have yet to be thoroughly studied.

The genetic and structural basis of intertwined “brain-wiring” are currently inconclusive, yet there have been two proposed theories that may serve as a foundation for future research. The disinhibited feedback theory is based on a functional abnormality in the brain, in which a pathway that is normally suppressed to prevent crosstalk between the regions of the brain is instead active. In contrast, the cross-activation theory relies on a structural difference, resulting in excess neural connectivity between regions that, in grapheme-color synesthesia for instance, process numbers and letters along with colors [5]. Future research may elaborate on the specific mechanisms and brain composition while using such theories as a basis to examine the complexity of synesthesia.

Two separately conducted studies in 2017 and 2019 used magnetic resonance imaging (MRI) and other visualization methods to map out and determine whether there were any neuroanatomical differences, such as the cerebral structure—grooves, tissue volume, tissue density and fiber organization—and amount of white matter, tissues that interpret sensory information, respectively [6] [7]. Both studies found no significant differences with their respective variables in a relatively small group of people with synesthesia, demonstrating the need for further research with a larger group.

Very few people in the world have been diagnosed with synesthesia and not enough data has been obtained and analyzed to confidently analyze the origins, mechanisms, and differences between the types of synesthesia. Future research requires a much larger group and may involve investigating the genetic and molecular mechanisms behind grapheme-color and sound-color synesthesia, if and how it relates to the cerebral structure, and if there are any connections to novel types of synesthesia. Regardless, synesthesia is a fascinating phenomenon that should be further explored, and such a condition may suggest that sensory interpretation and perception may be more intertwined than scientists have previously understood.

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