1. Bartoshuk, A. K. Electromyographic gradients in goal directed activity. Canad. J. Psychol., 1955, 9, 21–28.
2. Hilgard, E. R. Methods and procedures in the study of learning. In S. S. Stevens (Ed.), Handbook of experimental psychology. New York, Wiley, 1951, 517–567.
3. Malmo, R. B. Physiological gradients and behavior. Psychol. Bull., 1965, 64, 225–234.
4. Obrist, W. D. Skin resistance and electroencephalographic changes associated with learning. Unpublished doctoral dissertation, Northwestern University, 1950.
5. Stennett, R. G. The relationship of performance level to level of arousal. J. exp. Psychol., 1957, 54, 54–61.