Network architectures and circuit function: Testing alternative hypothesesin multifunctional networks

Understanding how species-typical movement patterns are organized in the ne rvous system is a central question in neurobiology. The current explanation s involve 'alphabet' models in which an individual neuron may participate i n the circuit for several behaviors but each behavior is specified by a spe cific neural circuit. However, not all of the well-studied model systems fi t the 'alphabet' model. The 'equation' model provides an alternative possib ility, whereby a system of parallel motor neurons, each with a unique (but overlapping) field of innervation, can account for the production of stereo typed behavior patterns by variable circuits. That is, it is possible for s uch patterns to arise as emergent properties of a generalized neural networ k in the absence of feedback, a simple version of a 'self-organizing' behav ioral system. Comparison of systems of identified neurons suggest that the 'alphabet' model may account for most observations where CPGs act to organi ze motor patterns. Other well-known model systems, involving architectures corresponding to feed-forward neural networks with a hidden layer, may orga nize patterned behavior in a manner consistent with the 'equation' model. S uch architectures are found in the Mauthner and reticulospinal circuits, 'e scape' locomotion in cockroachs, CNS control of Aplysia gill, and may also be important in the coordination of sensory information and motor systems i n insect mushroom bodies and the vertebrate hippocampus. The hidden layer o f such networks may serve as an 'internal representation' of the behavioral state and/or body position of the animal, allowing the animal to fine-tune oriented, or particularly context-sensitive, movements to the prevalent co nditions. Experiments designed to distinguish between the two models in cas es where they make mutually exclusive predictions provide an opportunity to elucidate the neural mechanisms by which behavior is organized in vivo and in vitro. Copyright (C) 2000 S. Karger AG, Basel.