For Merleau-Ponty, synesthesia is a phenomenon which requires us to reconsider the paradigm of sensation and perception. He writes;
From the perspective of the objective world with its opaque qualities, or from the objective body with its isolated organs, the phenomenon of synesthesia is paradoxical. The attempt is thus made to explain it without touching the concept of sensation: it will be necessary, for example, to assume that stimulations ordinarily circumscribed within a region of the brain (the optical zone or auditory zone, for instance), become capable of intervening beyond these limits, and that in this way the specific quality is associated with a non-specific quality. Whether or not there are arguments in cerebral physiology for this explanation, it does not account for synesthetic experience, which thus becomes a new opportunity to put the concept of sensation and objective thought into question. For the subject does not tell us merely that he has a sound and a color at the same time: it is the sound itself that he sees, at the place where colors form. This formula is literally rendered meaningless if vision is defined by the visual quale, or sound by the sonorous quale. But it falls us to construct our definitions in such a way as to find a sense for this experience, since the vision of sounds or the hearing of colors exist as phenomena. And they are hardly exceptional phenomena. Synesthetic perception is the rule and, if we do not notice it, this is because scientific knowledge displaces experience and we have unlearned seeing, hearing, and sensing in general in order to deduce what we ought to see, hear, or sense from our bodily organization and from the world as it is conceived by the physicist.
[Merleau-Ponty, M. (1945/2012) Phenomenology of Perception. New York: Routledge. p.237-8]
According to Merleau-Ponty, we have to learn to see, hear, and sense once again, by thinking of the meaning of synesthesia.