KRISTAN, W.B.: How the leech nervous system chooses among behaviors.

When presented with conflicting mechanical stimuli, leeches typically produce distinct responses, not combinations of responses. For instance, touching the back end of a leech normally elicits swimming, whereas touching the front end reliably elicits shortening. Presented together, these stimuli always elicit shortening. In fact, shortening always overrides swimming, even if the shortening stimulus is given in the midst of a swimming episode. These individual behaviors can be initiated by artificially activating command-like neurons, whose somata are located either in the anterior brain or in segmental ganglia. Surprisingly, such neurons are themselves activated during several behaviors. For instance, stimulating swim trigger neurons reliably turns on the swim motor program, yet these neurons also are strongly activated during shortening. Based upon these kinds of results, we now consider that each behavior is triggered and maintained by overlapping sets of these command-like neurons rather than by “dedicated” neurons that are active exclusively in a single behavior. To enable us to test this possibility, we have been using voltage-sensitive dyes to determine the connectivity among these neurons and show the activity patterns of many neurons during the decision-making time, just before a motor pattern is expressed.