figure 7.7 The biology of habituation Research with a simple organism, Aplysia, reveals the precise biological mechanism that underlies the behavioral change known as habituation. When an external stimulus first makes contact with a part of the Aplysia (its siphon), the Aplysia strongly withdraws its gill. After repeated presentations of the stimulus, it withdraws its gill only slightly. A reduction in the amount of neurotransmitter released by a motor neuron explains this change in behavior.