The lessons for enactive theories of color perception, pointed to by Grush et al. in their target article, may also be expanded by including challenges constituted by other atypical color conditions, namely synesthetic color experiences.
Synesthesia is traditionally considered to be a phenomenon in which the stimulation of one sensory or cognitive pathway (the inducer) elicits involuntary and consistent sensory experiences (the concurrent) in the same or another modality (Baron-Cohen et al. 1987; Baron-Cohen & Harrison 1997; Ramachandran & Hubbard 2001a, 2001b). As a result, the stimuli corresponding to the inducer and the experiences associated with the concurrent form a highly integrated percept—a phenomenally-unified experience which may cover not only sensory modalities, but also various mental domains including conceptual, emotional, bodily, and motor aspects (Mroczko-Wąsowicz & Werning 2012; Mroczko-Wąsowicz 2013). Such unification incorporates the central system and early stages of processing. Some synesthetes see colors when dealing with letters or numerals. Individuals with another kind of synesthesia perceive colored patterns in space when hearing sounds. The prevalence of the phenomenon depends on the particular type of synesthetic association, with grapheme-color synesthesia being the most common (Cytowic & Wood 1982; Mroczko-Wąsowicz & Nikolić 2013).
Color sensations are the most frequent synesthetic concurrents (Marks & Odgaard 2005), demonstrating color opponent properties and neural representations more or less similar to veridical color experiences (Nikolić et al. 2007; Hubbard et al. 2005; cf. Hupé et al. 2012; Van Leeuwen et al. 2010). For some forms of synesthesia color concurrents may also originate from information processing in regions of the cortex other than the visual. Recent neuroimaging studies demonstrate that synesthetic colors for numbers or mathematical formulas may also be produced when the visual cortex is not involved, i.e., by the activation of temporal, parietal, and frontal brain areas (Bor et al. 2007; Hupé et al. 2012; Brogaard et al. 2013). This suggests that information processing in non-visual brain regions may be a source of concurrent colors and therefore some forms of synesthesia can be seen as high-level perception proceeding via non-standard mechanisms. Such high-level synesthetic color perception for mathematical skills, though quite unusual, may provide supportive evidence for the conception of phenomenal liberalism and cognitive phenomenology.