Colored photisms prevent object-substitution masking in digit-color synesthesia

For C, a digit-color synesthete, viewing a digit elicits a photism (an experience of a highly specific color), which is perceived as externally projected onto the digit. We used an object substitution paradigm to demonstrate the influence of C's photisms on her perception of digits. Object substitution refers to a form of masking that depends upon two critical factors: attention must be taxed (e.g., distributed among numerous distracters), and the mask must remain on-screen for a sufficient duration after the target has been removed. Results showed that for C, even under conditions that produced maximal object substitution in control participants, her endogenous coloring of target digits prevented object substitution. Thus, the present study clearly showed that C's photisms influence her ability to detect digits.