Behavioral and brain mechanisms of grapheme-color synesthesia and their relationships with perceptual binding and visual imagery

Synesthesia is an unusual blending of the senses that occurs in about four percent or more of the human population. Much effort has been devoted to establishing criteria to define what synesthesia is ever since the phenomenon reemerged as a fascination within the scientific community in the late 1970s. To date, the most common criteria for synesthesia are that synesthetic experiences be automatic, consistent, rely on an external stimulus that triggers the phenomenological experience, and that this experience is fully conscious to the mind. This framework allows for some differentiation of synesthetes compared to non-synesthetes within the human population, and yet it also creates a self-selecting bias in the synesthetic population; if the scientific community defines criteria for synesthesia, and then only studies people whom fit those criteria, the resulting data will likely validate the definitions if only because they have been defined that way. What is left unknown are ways that synesthetes, as a community of otherwise normal human beings, vary in subtle ways, both in their psychophysical behavior and in their neurobiological form and function in relation to other human beings who do not experience any form of conscious, unusual sensory blendings yet defined as synesthesia.The studies described in this thesis explore whether perception in the population of individuals currently defined as synesthetes is in fact uniquely different from perception in the rest of the human population. These unique differences in perception are also used here to better inform our understanding of the functions of the human brain. Chapter 2 introduces the concept of perceptual binding and its relation to synesthesia. Some synesthetes experience colors that are associated with letters and numbers, and these so-called grapheme-color synesthetes may rely on similar brain mechanisms to bind their synesthetic colors to space as the ones they (and most humans) use to bind color to space normally. Chapter 3 addresses the question of binding with regard to an unusual phenomenon specific to grapheme-color synesthetes: that it is possible for some of these synesthetes to experience two colors that are spatially co-localized without blending. The results of this behavioral study will be shown to correlate with the vividness of visual imagery, a measure that extends beyond synesthetic phenomenology. Finally, Chapter 4 demonstrates how synesthetes differ from well-matched non-synesthetes in relation to behavior and the anatomy of the brain. Specifically, synesthetes have more vivid visual imagery as a population, more arborized white matter, and show a positive correlation between vivid imagery and increased axonal branching that is absent in non-synesthete controls. Together, these studies suggest that the brains of synesthetes rely on attention-specific mechanisms used by most humans to bind color to space. However, synesthesia as a whole may not simply be one end of a continuum of brain differences. Rather, synesthetes may be unique both in their phenomenological experiences of the world, and in some ways, the organization of the brain that creates them.