1. I.P. Pavlov, Conditioned Reflexes (Oxford University Press, Oxford, 1927).
2. P.C. Holland, "Occasion-setting" in Pavlovian feature positive discriminations, in Quantitative Analyses of Behavior; Vol. IV Discrimination Processes, M.L. Commons. R.J. Hermstein, and A.R. Wagner. Eds. (Ballinger. Cambridge, MA. 1983)
3. R.A. Rescorla, Inhibition and facilitation, in information Processing in Animals: Conditioned Inhibition. R.R. Miller and N.E. Spear. Eds. (Edbaum, Hillsdale, Nl, 1985).
4. Combinations of modulators trained with the same and different target stimuli