Neuroethological approaches to the study of motor development in chicks: achievements and challenges.

Chicks and chick embryos provide a useful model system for the study of issues related to the development of motor behaviors. EMG and kinematic analyses of leg movements have been used to provide new data on the organization of embryonic motility. These data suggest that the circuitry needed to produce a basic, coordinated motor pattern is available early in development. This circuitry then appears to be retained throughout life. Evidence from analysis of EMG patterns and leg deafferentation studies suggest that the output of this basic circuit can be modulated by sensory input to produce the motor patterns of later behaviors, such as hatching and walking. If the same circuitry is present throughout life, then mechanisms for initiation and termination of particular behaviors must be available to ensure that specific behaviors are turned on and off at appropriate times. For example, hatching can be turned on by a specific sensory signal: proprioceptive signals from the bent neck. In addition to reviewing current research on the development of chick motor behaviors, methodological considerations and suggestions for future research are presented.