For many people, experiencing literal synesthesia may be hard to imagine. How can somebody possibly hear the color green or taste sadness, one might ask. Still, the evidence for synesthesia as a real phenomenon is compelling, and grasping the technical nature of literal synesthesia may help the reader better understand the state of mind of the narrator of “The Yellow Wallpaper.”
In a study conducted just over a decade ago, a group of researchers used functional magnetic resonance imaging (fMRI) technology to track synesthetic activity in the brains of people with “colored-hearing” synesthesia, where auditory stimuli result in color perception (Nunn 371).
In a typical human brain, color stimuli trigger the primary visual cortex known as V1/V2—which generally processes all visual stimuli–as well as the more specialized “color center” known as V4/V8 in the left hemisphere of the brain. These scientists hypothesized that, assuming synesthetic perception of color occurs in the same part of the brain as typical perception of color, V4/V8 would respond in colored-hearing synesthetes to certain sound stimuli as it normally does when presented with corresponding color stimuli.
Three experiments were conducted:
- In the first, the response of the V4/V8 areas in synesthetes to heard words was compared to responses in non-synesthetes to heard words and visual stimuli. The result was that both synesthetes and non-synesthetes showed activation of the perisylvian regions of the brain, known for processing language, when presented with heard words, but only synesthetes showed activity in the V4/V8 areas (371).
- In the second, the response of the V4/V8 areas in synesthetes to heard words was compared to the response of V4/V8 in synesthetes to color. The result was that both groups of synesthetes showed activity in the V4/V8 areas (though only the group that was presented with color stimuli showed activity in V1/V2).
- And in the third, the response of the V4/V8 areas in synesthetes to heard words was compared to the response (if any) of V4/V8 in the brains of non-synesthetes imagining color (Nunn 371). The result was that, no matter the instruction or the amount of practice, non-synesthetes who were told and prepared to imagine color did not exhibit any increase in activity in the V4/V8 areas (373-74).
In the words of the authors of this study, “The data reported here, demonstrating differences between synesthetes and controls in activation of a color-selective region by spoken words, lend such phenomena an authenticity beyond reasonable doubt” (373).