ABSTRACT

Behaviour is generated by a hierarchically arranged system of quasi-autonomous units of action. At the bottom are elementary units of action, which cannot be decomposed into subunits that are themselves units of action. Five kinds of elementary units are described (illustrative examples given in parentheses): oscillators (chewing), error-actuated servomechanisms (the bptokinetic reaction), latching servomechanisms (smooth-tracking), simple reflexes (the vestibulo-ocular coordinate transformations reflex) and directed motions requiring multiple (saccades, reaches). More complex actions are created by neural circuits that coordinate the actions of two or more elementary units. Several of the mechanisms underlying these coordinations involve novel principles, not included in the basic principles of neural integration described by Sherrington. For example, the operation of the complex units known as central pattern generators depends fundamentally on two of these novel principles: (1) oscillator coupling and (2) potentiation and depotentiation.