A PROBLEM faced by animals as they move about is to distinguish between sensory input which is generated as a direct consequence of the animals' own movements (called reafference1), and that generated by independent external events. One way an animal might handle reafference would be to use an approximate copy of motor output (corollary discharge2) to repress or otherwise compensate for it. Cockroaches usually react to gentle puffs of air by rapidly running away, the so-called "escape response". Since they do not respond similarly to air currents set up by their own walking (even though the receptors involved are very sensitive to slight air movements from any direction3), they must have some mechanism for recognising and ignoring reafferent input. The escape response had originally been thought to be initiated by the activity of the large interneurones known as giant fibres 4. This view has been disputed5,6, however, and the function of the giants is at present unclear. I report here evidence that some of these giants are directly excited during walking by neural input which may represent corollary discharge from motor centres in the thorax, and which may aid the insect in handling reafferent input.
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