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The concept of “releasing system” suggests a neuronal sensorimotor interface translating perception into action (Figure 2.2). At its afferent (input) side this interface has stimulus recognition and localization properties; its efferent (output) side runs (“commands”) the corresponding motor pattern generating system (Table 2.4). Theoretically, the simplest structure of a ballistic releasing system would entail a “command neuron,” CN, operating in a processing stream that—once triggered—proceeds to completion.

Table 2.4 Command neuron hypothesis.

At first glance the giant axon of giant lateral interneuron in crayfish, controlling the escape tail-flip in response to mechanosensory stimulation, serves as a CN (Wiersma & Ikeda 1964). The idea that a neuron triggers a behavior was challenging and sparked intense debate among neuroethologists. Kupfermann and Weiss (1978) pointed out that a CN must fulfill two conditions: its excitation is not only necessary but also sufficient to activate the behavior. This strong definition can be examined: electrically exciting that neuron should be sufficient to elicit the corresponding behavior; removing it should abolish the behavioral response to the peripheral stimulus. For experimental studies, the best candidate of a CN is the reticulospinal Mauthner cell in teleost fish that, in response to vibratory stimulation, triggers the fast-body-bend escape reaction. However, quantitative investigations showed that a Mauthner cell did not fulfill the “double-condition” (Eaton 2001); a command neuron that does it, convincingly, remains to be discovered. If several neurons are involved in a command function, we use the term “command system.”

In fact, a releasing system—relying on attention and motivation—may take advantage of adequate receptor cells and assemblies of feature-sensitive/selective interneurons. This system translates the information of a sign-stimulus into a command, which is appropriate to activate the corresponding motor system (Figure 2.2).