Abstract
Grapheme–color synesthesia is a condition in which objectively achromatic graphemes induce concurrent color experiences. While it was long thought that the colors emerge during perception, there is growing support for the view that colors are integral to synesthetes’ cognitive representations of graphemes. In this work, we review evidence for two opposing theories positing either a perceptual or cognitive origin of concurrent colors: the cross-activation theory and the conceptual-mediation model. The review covers results on inducer and concurrent color processing as well as findings concerning the brain structure and grapheme–color mappings in synesthetes and trained mappings in nonsynesthetes. The results support different aspects of both theories. Finally, we discuss how research on memory colors could provide a new perspective in the debate about the level of processing at which the synesthetic colors occur.
Highlights
Synesthesia is a phenomenon in which a stimulus elicits an additional, unstimulated experience
Case reports show that the font type and capitalization of the inducer can affect the hue or the vividness of the concurrent color (Ramachandran & Hubbard, 2001) and that synesthetic colors can transfer to perceptually similar graphemes of a previously unknown alphabet in synesthetes (Mroczko, Metzinger, Singer, & Nikolic, 2009)
While it is often argued that this color popout effect in visual search is found in synesthetes (Palmeri et al, 2002; Ramachandran & Hubbard, 2001), a closer look at the data shows a difference between the benefit of synesthesia in these search tasks and pop out
Summary
Synesthesia is a phenomenon in which a stimulus (inducer) elicits an additional, unstimulated experience (concurrent; Grossenbacher & Lovelace, 2001). We will focus on two models that could be considered as two ends of the continuum from low- to high-level explanations of the origin of synesthetic colors: the crossactivation theory (Hubbard, Brang, & Ramachandran, 2011) and the conceptual-mediation model (Chiou & Rich, 2014) The former model posits that synesthesia arises during sensory processing due to an irregular activation of neurons encoding the concurrent feature. Because children acquire literacy in a common environment within a given culture, with similar toys and educational materials, it is fair to assume that some grapheme–color mappings should be highly prevalent across the population of synesthetes Another implication is that, after excessive training with precolored graphemes, nonsynesthetes should experience concurrent colors with achromatic letters and numbers
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