The Collaborative International Dictionary of English v.0.48:

perceptivity \per`cep*tiv"i*ty\
   (p[~e]r`s[e^]p*t[i^]v"[i^]*t[y^]), n.
   The quality or state of being perceptive; power of
   perception. --Locke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

perceptivity
    n 1: a feeling of understanding [syn: insight,
         perceptiveness, perceptivity]