A jumble of letters. Image credit: Matthias Heyde (CC0)
Perception feels like a window onto the world, but it never is. What we see is built from what reaches the eye and from what the brain adds. In synesthesia, that second part is easy to spot. People with synesthesia involuntarily experience additional sensations triggered by ordinary stimuli, for example, seeing the number 5 as always appearing red, or hearing a trumpet and perceiving it as bright yellow. This is not imagination or a choice; the experience simply happens. The most common form is grapheme-color synesthesia, where letters and numbers reliably evoke specific colors. Synesthesia is not a 'disease', most synesthetes enjoy the additional sensations and find them useful (e.g., the additional colors help to remember a telephone number). Although synesthesia has been studied for decades, measuring it objectively has proven difficult because researchers have had to rely largely on self-report.
Strauch, Leenaars and Rouw studied what happens to the eyes when people experience the colours. They tested 16 grapheme-color synesthetes and two matched control groups of 16 participants each, measuring pupil sizes while the participants viewed gray digits.
Synesthetes' pupils became smaller for digits evoking brighter synesthetic colors and became larger for darker ones despite identical physical stimulation of the digits on the screen. Pupil responses were also stronger for digits for which synesthetes had reported a strong color association. Synesthetes did not seem to use visual imagery to evoke their colors, as their pupil responses were too fast for deliberate imagery.
The control group, who had to report a random color with digits, did not show effects in their pupil size related to color brightness. Furthermore, controls needed more effort to perform the task. I.e., their pupils widened more than those of synesthetes doing the same task, and more than in controls with no color task.
Pupillometry offers researchers a low-cost, objective tool for identifying synesthesia. Objective measurement helps demonstrate that synesthetic experiences are 'real' and offers a method to objectively map out (variations in) synesthetic experiences. This is particularly important for synesthetic children, who are often misunderstood when they report their experiences during schoolwork involving letters or digits.
Pupillometry makes synesthesia usable as a model for a larger question. Because the sensory extra (the color) has no physical counterpart on the screen, the method opens a way to study how the brain builds conscious perception.
