Sound Frequency Predicts the Bodily Location of Auditory-Induced Tactile Sensations in Synesthetic and Ordinary Perception

For individuals who experience sound-touch synesthesia, sounds consistently evoke strong, localized sensations on the body. Interestingly, visual and auditory information can also evoke bodily sensations in ordinary perception, such as the sight or sound of a mosquito producing phantom sensations of touch. We systematically investigated the functional relationship between sound frequency and several characteristics of induced tactile experiences in synesthetes and controls. Sound frequency strongly predicted the location of tactile sensations on the body in both synesthetes and controls. Synesthetes felt sound-induced touch more frequently and in significantly more concentrated locations on their bodies compared to sound-induced touch perceptions in controls. This spatial distribution of touch reflects a behavioral mapping between tonotopy and somatotopy that suggests the involvement of early, tonotopically- and somatotopically-organized brain areas. Sound frequency also predicted the hardness, roughness, and sharpness of tactile sensations in both groups. Together, these findings highlight a strong similarity between auditory-tactile mappings in synesthetic and ordinary perception, suggesting that synesthesia only differs in the strength of the mappings and therefore is on a spectrum with ordinary perception. Furthermore, these findings offer insights into the possible underlying neural mechanisms of sound-touch mappings, suggesting that they rely on cross-modal neural pathways already in use in ordinary perception.