1. Annau, Z., and Kamin, L. J.: The conditioned emotional response as a function of the intensity of the US. J. Comp. Physiol. Psychol., 54, 428–432, 1961.
2. Anohkin, P. K.: The role of the orienting-exploratory reflex in the formation of the conditioned reflex. In, L. G. Voronin, A. N. Leontiev, A. R. Luria, E. N. Sokolov, and O. S. Vinogradova (eds.): Orienting Reflex and Exploratory Behavior. Moscow, Academy of Pedagogical Science, 1958.
3. Azrin, N. H., and Hake, D. F.: Positive conditioned suppression: conditioned suppression using positive reinforcers as the unconditioned stimuli. J. Exp. Anal. Behav., 12, 167–173, 1969.
4. Boakes, R. A., Halliday, M. S., and Poli, M.: Response additivity: effects of superimposed free reinforcement on a variable-interval baseline. J. Exp. Anal. Behav., 23, 177–191, 1975.
5. Brady, J. V.: Does tetraethylammonium reduce fear. J. Comp. Physiol. Psychol., 46, 307–310, 1953.