Although today differential psychology embraces a large and ever-increasing body of facts, conclusions, and practical recommendations which have wide applicability, a number of critical problems are still unresolved or only partly resolved. One of the most important of these is the relationship between individual variations in human behavior and in- dividual features of a range of correlated physiological functions. Adequate concepts exist for understanding individual variations in some physiological functions underlying specific aspects (speed, tempo, rhythm, amplitude) of the flow of...