Behavior, Hierarchical Organization of

Elementary units of behavior are assemblages of neural, endocrine, and muscular components sufficient to explain naturally occurring actions. All such assemblages require an initiator component, where the neural signals that ultimately drive the action originate, conductor components that relay the signals from the initiators to the effectors, and effector components that translate the signals into movements and secretions. Some elementary units also contain computational elements, which perform basic computational operations like coordinate transformations, and some units contain memory elements, which hold the values of control variables, such as the angular velocity of a target. Neural circuits carrying timing signals and specifying the values of operating parameters, such as pacemaker periods, co-ordinate the operation of elementary units to form complex units, which are themselves coordinated to form still more complex units, yielding a hierarchical control structure. The behavioral expression of the units active high in the hierarchy exhibits many different forms, because different values for control parameters lead to different patterns from the same unit, and because much of the coordination is achieved by the selective potentiation and depotentiation of the lower units, so that the spectrum of potentiated lower units actived on a given occasion depends on occasion-specific trigger stimuli.