Long interested in the origins of animal behavior, Hamburger undertook a series of experiments in the 1950s and 1960s to investigate the neurobiological basis for ontogeny of behavior in the chick. When he entered this field, Hamburger was only dimly aware of the intense debate occurring among psychologists over the innate versus learned quality of even the simplest behavior. This was the period of the 1950s and 1960s, when American behaviorism (stimulated by the work of psychologist B. F. Skinner) was in its ascendancy, creating an atmosphere that left little room for ideas about innate factors in the origin of animal behavior. Recognizing that much of embryogenesis might also be guided by internal factors, Hamburger recognized that the behaviorist view would require that the sensory and motor systems in the embryo would develop simultaneously, so that motor activity could occur in response to sensory input. Hamburger’s approach was to test these alternative hypotheses by determining how neuronal development correlated with the development of observable behavior in the chick embryo in vivo. In a series of ingenious experiments with several graduate students and post-docs at Washington University, Hamburger noted (1) That the motor neurons begin to grow out from the developing spinal cord before the development of sensory neurons, and (2) The process begins at the anterior portion of the embryo and proceeds steadily toward the posterior.
Through a series of experiments in which portions of the spinal cord were removed, it was possible to determine clearly that early movements are endogenous, and require no input from sensory nerves to take place. Initially he noted that the movements were random but as development proceeded they become more coordinated, and the frequency of limb and head movements increased. The next step was to correlate electrical activity (using neurophysiological recording techniques) of the developing neurons with observed movements of the limbs and head. This work demonstrated clearly that early activity in limbs arises from innervation by developing motor neurons, and that coordination of these movements occurs at the level of the spinal cord. These results refuted the orthodox behaviorist claims that all early behavior is a result of conditioned responses that depend on input from sensory neurons. It was this work and many ramifications that laid the groundwork for studying the origins of later and more complex behavior, and its origins in neuronal organization.