The Collaborative International Dictionary of English v.0.48:

mentation \men*ta"tion\ (m[e^]n*t[=a]"sh[u^]n), n.
   the process of thinking (especially thinking carefully);
   mental activity.

   Syn: thinking, thought, cerebration, intellection.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mentation
    n 1: the process of using your mind to consider something
         carefully; "thinking always made him frown"; "she paused
         for thought" [syn: thinking, thought, thought
         process, cerebration, intellection, mentation]