The experiment consisted of four sections: Pretest and Tasks 1, 2, and 3. In the web-based Pretest, we identified for each synesthete (1) synesthetic color categories elicited by each of a set of familiar Hiragana characters, and (2) novel Thai characters for which the synesthete did not experience synesthetic color at this time. Based on the results of the Pretest, we selected six familiar-novel grapheme pairs as stimuli for each synesthete. In the laboratory, each synesthete first selected synesthetic colors for the six familiar graphemes by using an RGB color palette (Task 1). The participant then learned the six novel-familiar grapheme pairs (Task 2). After the learning session, the participant performed the synesthetic color selection task for both familiar and novel graphemes (Task 3). The distances between the colors selected for the learned novel graphemes in Task 3 and those for the corresponding familiar graphemes in Task 1 were calculated as indicators of the degree of synesthetic color transfer.
Twenty-seven Japanese grapheme-color synesthetes (21 females, six males, mean age = 29.5 years, SD = 10.0 years) whose first language was Japanese participated in this study. All reported experiencing synesthetic colors when viewing Hiragana characters, as well as when viewing digits, letters of the English alphabet, and other types of Japanese characters (i.e., Katakana and Kanji characters). Participants were randomly assigned to either the heterogeneous (n = 13) or the homogeneous (n = 14) condition.
Among the entire set of 71 Hiragana characters, we used all the 46 basic characters as familiar graphemes. Hiragana is a Japanese script that represents vowels or syllables (i.e., combinations of a consonant and a vowel) and is typically mastered by Japanese people in childhood. We used the 42 consonant characters in current use in the Thai language as novel graphemes. These are orthographically dissimilar to Japanese and Latin graphemes and to Arabic digits, and Japanese people typically have low exposure to them in daily life.
Each participant completed a web-based Pretest at a self-chosen location. Subsequently, the participant engaged in two computer-based tasks in a darkened room and one writing task under room light.
At some time before the day of the laboratory experiment, each participant completed a web-based Pretest (created on Google Forms) concerning their synesthetic experiences. The Pretest consisted of two sections. In the first section, participants selected synesthetic colors for each of the familiar Hiragana graphemes. On each trial, one of the 46 characters was presented at the top of the computer screen, and the participant chose one color term corresponding to the character from among 13 color terms frequently used by Japanese (Kuriki et al., 2017) meaning red, yellow, green, blue, pink, orange, brown, purple, light blue, yellow-green, white, black, and gray, respectively. They also rated how accurately each of the color terms they selected represented the synesthetic color they experienced, using four options: Accurate, Somewhat accurate, Not very accurate, The grapheme elicits only a weak sensation of a color in the first place. Each of the 46 characters was presented only once.
In the second section of the Pretest, participants were asked to indicate the degree of their sensation of synesthetic color when viewing the novel Thai graphemes. On each trial, one of the 42 characters was presented at the top of the screen, and the participant reported the color intensity it elicited using one of four options: Strong sensation of color, Slightly strong sensation of color, Little sensation of color, No color.
Task 1: Synesthetic color measurement of familiar graphemes
On each trial of Task 1, each of the familiar graphemes (46 Hiragana basic characters) was presented on the top of a computer screen (Mitsubishi Diamondtron M2 RDF223G), and participants used a computer mouse to select a corresponding color from an RGB 2563 color palette displayed below it. If the presented character elicited no synesthetic color, they were asked to select a “no color” option. Stimulus characters were presented in random order, and no character was repeated.
Task 2: Learning session
In the second task, participants learned a group of six novel Thai characters, each of which was arbitrarily associated with a different familiar Hiragana character. These associations were completely unrelated to the actual sounds of Thai characters. A different set of stimulus characters was selected for each participant based on the results of the Pretest. In the heterogeneous condition, the synesthetic colors for the six stimulus Hiragana characters were selected from six different color categories (i.e., color terms). In the homogeneous condition, on the other hand, all six stimulus Hiragana characters elicited synesthetic colors of the same category. In both conditions, Hiragana characters were selected from those for which the color terms were rated as “accurate” or “somewhat accurate” representations of the synesthetic colors, and Thai characters were selected from those eliciting ratings of “little sensation of color” or “no color.” Further, we avoided including in the stimulus set Thai characters that were highly visually similar (see Fig. S1 in Supplementary Material 1).
The learning session consisted of the repetition of a sequence of events that included a handwriting section and a test section. This sequence is shown in Fig. 1. In the handwriting section, participants were first given a sheet of paper on which six pairs of familiar graphemes and novel graphemes were printed, and they wrote each pair of graphemes five times with a pencil. They then proceeded to the test section, which consisted of the following steps: They were first asked to write the novel grapheme that corresponded to each familiar grapheme, and were then asked to write the familiar grapheme corresponding to each novel grapheme. After the participant repeated this three times and correctly reproduced all the graphemes on the last repeat, he or she was asked to recall the six pairs of novel and familiar graphemes. The learning session ended if the participant succeeded in recalling all pairs in succession; therefore each participant completed at least four sets of handwriting and test sections.
Task 3: Synesthetic color measurement for novel and familiar graphemes
Participants selected a color corresponding to each novel grapheme (Thai character) and familiar grapheme (Hiragana basic character). All 42 Thai characters (including the six graphemes used in the learning session) and 46 Hiragana basic characters (including the six graphemes used in the learning session) were used as stimuli. The apparatus and procedure were the same as in Task 1.
Following the completion of Task 3, participants were asked whether there were Thai characters that elicited a synesthetic color at the beginning of Task 2 (i.e., the first time they saw Thai characters in the laboratory experiment).