The description of rat drug-induced behavior: kinematics versus response categories

The study of rat drug-induced locomotor behavior is largely based on the assumption that behavior consists of a sequence of response categories performed by the whole animal one at a time. By analysing this behavior under (+)-amphetamine (5 mg/kg), we illustrate how even a precise definition of such categories may not be sufficient for the establishment of behavioral variables that have a "physiological reality." We describe the changes of relation between the parts of the rat's body in reference to selected coordinate systems, and show that a great variety of locomotor patterns observed under amphetamine can be reduced to as few as 3 descriptive component-variables. These continuous and relatively independent variables, which behave predictably in the course of drug action, operate simultaneously. Variations in their relative timing of onset and termination account for the apparent variability in observed behavior. The economy and generality of a description based on these variables suggest the existence of corresponding central mechanisms of control.