Take the 2-minute tour ×
Cognitive Sciences Stack Exchange is a question and answer site for practitioners, researchers, and students in cognitive science, psychology, neuroscience, and psychiatry. It's 100% free, no registration required.

I am trying to understand a neuroscience article by Karl Friston. In it he gives three equations that are, as I understand him, equivalent or inter-convertible and refer to both physical and Shannon entropy. They appear as equation (5) in this article (PDF; DOI 10.1007/s00422-010-0364-z). Here they are:

  • Energy minus entropy: $F = −{\ln p(\tilde s,Ψ|m)}q + {\ln q(Ψ|μ)}q$
  • Divergence plus surprise: = $D(q(Ψ|μ)||p(Ψ|\tilde s,m)) − \ln p (\tilde s|m)$
  • Complexity minus accuracy: = $D(q(Ψ|μ)||p(Ψ|m)) − {\ln p(\tilde s|Ψ,m)}q$

The things I am struggling with at this point are:

  1. the meaning of the || in the 2nd and 3rd versions of the equations
  2. the negative logs

Any help in understanding how these equations are actually what Fristen claims them to be would be greatly appreciated. For example, in the 1st equation, in what sense is the first term energy, etc.?

share|improve this question
Cross-posted on Cross Validated. –  Nick Stauner Jul 11 '14 at 23:48
+1 for also being confused by much of Fristen's writing. –  zergylord Jul 15 '14 at 17:02

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.