In this era, behavioral pharmacology became a central tool in psychiatry, transforming drug therapy from an empirical practice to a systematic, evidence-based discipline. Researchers integrated chemistry, physiology, and clinical observation to study how drugs shape mood, behavior, and brain activity, using controlled human studies and early electrophysiological endpoints to guide interpretation. The approach emphasized quantitative, mechanistic investigations of drug action, including the role of physico-chemical factors in determining efficacy and safety.