In this context papilla refers to the projection created by the tumor, not a tumor on an already existing papilla (such as the nipple).

Maybe a coming of age in the American sensual darkness, never seeing an unsmudged nipple, an uncensored vagina, has left me forever infected with an unquenchable lust of the eye.

Researchers take advantage of this fact by rigging up two rubber nipples, so that if a baby sucks on one, it hears a recording of its mother's voice on a pair of headphones, and if it sucks on the other nipple, it hears a recording of a female stranger's voice.