An animal need not send out electrical pulses to encounter this problem. As the Swedish neuroscientist Björn Merker notes, it results just from being able to move. An earthworm withdraws when something touches it – the touch might be a threat. But every time the worm crawls forward, it causes part of its body to be touched in just the same way. If it withdrew at every touch, it could never move at all. The worm succeeds in moving forward by canceling the effects of those self-produced touches.
