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Jean-Michel Hupe

How different are synesthetes ? From neural substrates to personality

A few percent of the population, « synesthetes », experience additional but systematic associations, like grapheme-color synesthetes who arbitrarily associate a specific color to each letter or number. Synesthesia is often described as a neurological condition, meaning that synesthesia would be caused by a structural or functional anomaly in the brain of synesthetes. At present, the two prominent theories of synesthesia suppose neurological differences indeed : the cross-activation theory suggests extra neuronal connections between neighboring areas of synesthetes ; the disinhibited feedback theory supposes a difference of neuronal transmission in synesthetes. However, most results in favor of one or the other, obtained mostly with functional and structural MRI, were not reproducible, casting some doubts on whether any neurological marker of synesthesia has been discovered yet. I propose a new theory for grapheme-color synesthesia, the “palimpsest hypothesis”, which predicts neither extra neural connections nor stronger neuronal transmission in synesthetes. This theory is a variant of the recycling hypothesis for reading, in which the neural development of written language expertise requires the recycling of brain regions predisposed to expertise acquisition into reading regions. The palimpsest hypothesis supposes that for grapheme-color synesthetes recycling involves neuronal networks that were already specialized for color perception. Synesthetic colors would be the remains of this former expertise. In this theory, the cause of synesthesia could be searched in the creative imagination of children, especially when they learn to read and write. Synesthetic associations could then be considered as a special kind of childhood memories, rather than a neurological condition. Then we may expect that synesthetes differ from the rest of the population by other characteristics, and indeed synesthesia has been purportedly associated with other personal characteristics, like better memory, mental imagery, personality and creativity. Global measures of functional and structural brain organization have also suggested differences in synesthetes. We compared synesthetes and matched controls in a series of tests of global cognition and creative thinking, as well as questionnaires of personality and mental imagery. We did find a few, but modest, differences. If as a group, synesthetes differed from non-synesthetes, these differences were less pronounced than those observed between men and women in our sample (that is within French culture), suggesting that on an individual level, synesthetes may not show exceptional characteristics beyond their phenomenal synesthetic experiences.