Scalar Expectancy Theory is grounded in the decision theory framework within which most contemporary psychophysical models are elaborated. In this framework, the values of stimulus variables determine the values of one or more decision variables in the nervous system. By the value of a variable, we mean the strength or magnitude of a signal that represents an objectively specifiable aspect of the subject’s experience, such as the duration of the CS-US interval or the ratio of two rates of reinforcement. Because the signal specifies information about the world, the processing of that signal is called information processing. The observer’s response depends on whether the strength of the decision variable is above or below a threshold. The threshold is called the decision criterion. It is assumed to be to some extent under the subject’s control, because it may vary with the payoff matrix, which is the set of (implicit or explicit) rewards and punishments contingent on whether the subject correctly or incorrectly judges whether the stimulus does or does not have a given property.