The Opponent Process Theory (OPT) was proposed by Solomon and Corbit (1974). It suggests that any stimulus that produces an immediate hedonic, or emotional effect, also produces an after effect that is opposite in direction to the initial effect.

The magnitude and duration of the a-process is fixed, as it is determined by the onset and offset of the particular stimulus being experienced. The b-process, however, is dynamic. Repeated exposure to the particular stimulus (eg, a drug), causes the b-process to begin earlier, increase in magnitude, and last longer.

These changes in the b-process reverse themselves as time passes without exposure to the given stimulus. Whether the b-process grows in magnitude or not depends on the time interval between stimulations.

If the stimulations are widely spaced, there is no change in the b-process. Accordingly, the emotional state experienced by an organism is determined by the difference in magnitude between the a-process and the b-process at a particular point in time.