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> He had synesthesia that made him unable to forget things for most of the life.

Some counterpoints from the Wikipedia article as well:

> According to autobiographical diary of Shereshevsky, found by Reed Johnson, he "did not, in fact, have perfect recall". Details of Shereshevsky's biography are different in his own writing from Luria's account. [...] Reynberg recalls that Shereshevsky "could be forgetful", and that he "trained hours a day for his evening performances", because he needed "consciously try to commit something to memory".