Would not the act of memorization an infringing copy? Copyright itself does not allow a provision where a non-ephemeral copy may be stored, regardless of the medium or duration. You would, of course, have the positive defense of fair use - if you were sued for your infringing copy, you could mount a defense that the storage falls under the fair use provisions, but you would still be required to defend yourself at your own expense. Would it make a difference if we, one day, developed a method of reading memories. Someone with a photographic memory could then be used to recreate the work from their copy - clearly a violation, and hence the storage is a violation (excepting backup/fairuse - which is still an infringement, but a special case of permitted infringement)