1. Department of Psychology, School of Arts and Sciences (S.E.K., S.M.F., H.J.G.) and Department of Psychiatry, University of Pennsylvania, Philadelphia, Pennsylvania 19104;
2. Physiology and Behavior Group (M.A.), Institute of Animal Sciences, ETH Zurich, 8603 Schwerzenbach, Switzerland
3. School of Medicine (M.R.H.), University of Pennsylvania, Philadelphia, Pennsylvania 19104;