Given that synesthesia, similarly to the color rotation gear, involves systematic distortions of color perception that are consciously experienced by the subjects, analyzing synesthetic experiences appears relevant in the context of the present discussion. This is why proponents of the sensorimotor theory of color perception might be interested in examining whether their postulates also apply in cases employing such synesthetic color-addition gear (cf. Hurley & Noë 2003; Fingerhut 2011; Mroczko-Wąsowicz & Werning 2012; Seth 2014; Ward 2012). The relevant propositions of enactivism may be stated as follows:
determining the modality of perceptual experience by specific sensorimotor signature (i.e., dependency between sensory stimulations and the activity of the perceiver, including their motor actions, bodily changes, or behavioral skills), as well as a necessary possession of such sensorimotor knowledge of contingencies enabling any perception;
flexibility of perceptual experience manifested in the ease of its modification and adaptation based on learning a new set of sensorimotor contingencies (Noë 2005);
and finally epistemic reliability of conscious perceptual experiences and their counterfactual richness (Metzinger 2014; Seth 2014; see also Seth this collection).
The above basic assumptions underlying sensorimotor enactivism of perception may be challenged by synesthesia in the following way:
Synesthetic concurrent percepts (e.g., visual experiences) are generated internally, not via a direct relation of a synesthete with the surrounding environment. They are triggered without employing the regular sensorimotor signature related to these concurrents, like eye saccades in normal vision. For such permanent inducer-concurrent couplings, the concurrent modality and its experiences are never related to their normal sensorimotor signature.
Synesthetic associations cannot be learned or adapted to, in contrast to various manipulations of sensory input such as, for example, spatial displacement, color inversion, or auditory-visual sensory substitution (sometimes called an artificial synesthesia), which are used by sensorimotor enactivists as examples of the perceptual system’s adaptation involving an appropriate adjustment of sensorimotor contingencies. Unlike the majority of learned pairings, synesthetic associations are rigid and not flexible enough to adapt, irrespective of the amount of exposure to contradictory experiences or training (Baron-Cohen et al. 1993; Deroy & Spence 2013).
As a final point, although synesthetic colors are reported to be as vivid as non-synesthetic colors, synesthetes immediately detect the difference between them, which confirms the absence of perceptual presence or phenomenal transparency in synesthesia, meaning its opacity or experiential unrealness, which is the availability of earlier processing stages to attention (Metzinger 2003a, 2003b, 2014; Seth 2014).
As a kind of reply to these challenges, sensorimotor enactivits could claim that enactivism focuses on standard perceptual mechanisms and therefore has difficulties explaining perception-like experiences in synesthesia, as well as that synesthetic concurrents (often colors) lack some important features of typical perceptual experiences and properties of sensorimotor engagement, e.g., corporeality. However, this would not really be explanatory. One possible way of vindicating how the enactive theory could accommodate such atypical non-adaptive color conditions is to claim that there is actually no need for synesthetic colors to adapt, because they do not carry any information about the colors of objects in the synesthetes’ environment—whereas adaptation is a retrieval of how things are colored. To put it in enactive terms, synesthetic colors do not figure in patterns of appearance reflecting dynamic relations between perceiver, object, and light (Ward 2012). Unlike the rotation gear, synesthesia does not determine the way things appear to the perceiver; i.e., the way worldly objects and surfaces modify the light is not affected.