Fig. 1.

Temporal predictions for two types of synesthesia theories. Cross-activation theory (4, 5) predicts that color perception and synesthetic color activation occur at the same time in the visual hierarchy, so neural representations evoked by perception should cross-generalize to synesthesia at the same time. Conceptual mediation theory (6) predicts that synesthetic color is an additional conceptual feature of the stimulus. Conceptual features are activated later in the visual hierarchy than color perception, predicting a delay between color representations evoked by direct perception and synesthetic association. Neural activation pattern evoked by direct and synesthetic color perception can be compared by training a classifier on neural data evoked by colored stimuli and tested on data evoked by (achromatic) synesthesia-inducing symbols. Cross-activation theory predicts on-diagonal generalization at early time points because color information in both direct and synesthetic color experiences should occur simultaneously. Conceptual mediation theory predicts later off-diagonal generalization because for synesthetic color to be elicited, there is an additional stage of accessing the concept of the inducing letter.