[i] is Lighter and More Greenish Than [o]: Intrinsic Association Between Vowel Sounds and Colors

in Multisensory Research
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It has recently been reported in the synesthesia literature that graphemes sharing the same phonetic feature tend to induce similar synesthetic colors. In the present study, we investigated whether phonetic properties are associated with colors in a specific manner among the general population, even when other visual and linguistic features of graphemes are removed. To test this hypothesis, we presented vowel sounds synthesized by systematically manipulating the position of the tongue body’s center. Participants were asked to choose a color after hearing each sound. Results from the main experiment showed that lightness and chromaticity of matched colors exhibited systematic variations along the two axes of the position of the tongue body’s center. Some non-random associations between vowel sounds and colors remained effective with pitch and intensity of the sounds equalized in the control experiment, which suggests that other acoustic factors such as inherent pitch of vowels cannot solely account for the current results. Taken together, these results imply that the association between phonetic features and colors is not random, and this synesthesia-like association is shared by people in the general population.

Multisensory Research

A Journal of Scientific Research on All Aspects of Multisensory Processing



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  • Vocal tract representation of CASY with its articulator variables for generating vowel sounds. F: mandibular condyle, C: tongue body center, B: tongue blade, T: tongue tip, J: jaw. The jaw position is given by the angle (JA) from a horizontal line at the joint with the constant distance from F. The tongue body is represented as an imaginary circle with a fixed radius. The position of the tongue body is given by the position of the circle’s center, which is determined by the angle (CA) from the line F–J and the length (CL) of the line F–C. The tongue blade and tip are attached to the tongue body’s circle. The tongue body’s articulator variables (CA and CL) were modulated parametrically to manipulate vowel sounds. All the other articulatory variables were fixed, including the jaw position that affects mouth openness. The positions of the tongue body’s center used for generating the 42 auditory stimuli are superimposed on the outline of the vocal tract.

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  • Acoustic variations of the auditory stimuli in the main experiment generated by articulatory synthesis. Acoustic properties of the vowel stimuli were displayed as brightness of each circle. (A) pitch, (B) intensity, (C) the first (left) and the second (right) formants. Stimuli with higher intensities and frequencies are represented with lighter shades.

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  • Luminance (L) results. (A) The relationship between the tongue body’s position (height and frontness) and L values of the matched colors. The dark green and blue lines indicate results from the main and the control experiment, respectively. The bold lines indicate statistical significance (p<0.05, F-test). The shades indicate ±1 standard error of the mean (SEM). (B) The group mean L values for each of the 42 auditory stimuli based on the tongue body’s position are shown as the level of lightness. Stimuli matched with lighter colors (larger L) were shown in lighter shades.

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  • Chromaticity results along the green–red continuum (a). (A) The relationship between the tongue body’s position (height and frontness) and a values of the matched colors. Dark green and blue lines indicate results from the main and the control experiment, respectively. The bold lines indicate statistical significance (p<0.05, F-test). The shades denote ±1 SEM. (B) The group mean a values for each of the 42 auditory stimuli based on the tongue body’s position are represented as colors with reference to the green–red color axis. More reddish colors denote larger a values whereas more greenish colors denote smaller a values.

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  • Chromaticity results along the blue–yellow continuum (b). (A) The relationship between the tongue body’s position (height and frontness) and b values of the matched colors. The dark green and blue lines indicate results from the main and the control experiment, respectively. The bold lines indicate statistical significance (p<0.05, F-test). The shades denote ±1 SEM. (B) The group mean b values for each of the 42 auditory stimuli based on the tongue body’s position are represented as colors with reference to the blue–yellow color axis. More yellowish colors denote larger b values whereas more bluish colors denote smaller b values.

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