Nonassociative learning in the leech Hirudo medicinalis.

In the present study we examined nonassociative learning of the induction of swimming which was evoked by weak electrical stimulation in the leech Hirudo medicinalis. The behavioural response to stimuli applied repeatedly to the body wall at an inter-trial interval (ITI) of 1 min decreased, eventually ceased, and then recovered spontaneously. More rapid reduction of the behavioural response occurred in repeated training sessions. This decrement of response conformed to the operational definition of habituation. Moreover, a noxious stimulus (i.e. brushing on the skin) facilitated the decremented response (dishabituation). In addition, we compared response decrement in naive animals with decrement in dishabituated and in sensitized animals. The analysis of the best fitting functions of the habituation, the habituation of dishabituation and the habituation of sensitization revealed interesting differences in these processes.