This article is from the Piano Playing from Memory FAQ, by Isako Hoshino rmmpfaq@yahoo.com with numerous contributions by others.
Visual memories of scores are retained in varying degrees by
different people. I do not know how prevalent the so-called
"photographic" memory is, nor how people who allegedly have this
ability use it while playing from memory. (Do they play from the
score in the "mind's eye" while playing without it physically?)
For most of us visual memory tends to be an ability to recollect
where things are in the score when we refer to it after the piece
has been partially or fully memorized. For people who recall more
vivid images of the score, conceivably these images could serve as
cues while playing from memory. Markings made on the score which
relate to playing from memory may be recalled in this manner also.
 
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