1. BOLLES, R. C. Theory of motivation. New York: Harper & Row, 1967.
2. PANKSEPP, J., & TROWILL, J. A. Positive and negative contrast effects with electrical stimulation of the brain. Physiological Behavior, in press.
3. SIEGEL, S. Nonparametric statistus. New York: McGraw-Hill. 1956.
4. TROWILL, J. A., PANKSEPP, J., & GANDELMAN, R. An incentive model of rewarding brain stimulation. Psychological Review, in press.