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20 Behavioral Choice

William Kristan, Rhanor Gillette


All animals with central nervous systems are faced with a bewildering array of choices to make. We are bombarded with stimuli constantly, and we need to decide which of them to respond to, as well as how and when to respond. In fact, different animals and different individuals are characterized by the behavioral choices they make as much as by their physical appearance. There has been a rash of recent interest in finding the neural mechanisms used in making such choices. This chapter champions the comparative approach, using animals best suited to approach the question of behavioral choice. We have used leeches and sea slugs (Phyla Annelida and Mollusca, the major members of the Superphylum Lophotrochozoa), but we assume, based on recurring examples of the success of the comparative approach in neurobiology, that what is found in these animals will be found again and again across many animal phyla (Bullock 1993). In this chapter, we present a series of examples showing that the strategy for decision making in leech and slug nervous systems is not competition among dedicated neuronal circuits, but rather a reconfiguration of shared neuronal networks. We argue that circuitry sharing is likely to be a common mechanism in all nervous systems more complex than those of leeches and slugs.

A Comparative Approach
The experiments discussed in this chapter depend heavily on a variety of favorable properties of invertebrate nervous systems. First, many invertebrate neurons are large, making electrophysiological experiments relatively easy. Second, invertebrate neurons are...

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