Nonassociative learning in the leech Hirudo medicinalis

In the present study we examined nonassociative learning of the induction o f swimming which was evoked by weak electrical stimulation in the leech Hir udo medicinalis. The behavioural response to stimuli applied repeatedly to the body wall at an inter-trial interval (ITI) of 1 min decreased, eventual ly ceased, and then recovered spontaneously. More rapid reduction of the be havioural response occurred in repeated training sessions. This decrement o f response conformed to the operational definition of habituation. Moreover , a noxious stimulus (i.e. brushing on the skin) facilitated the decremente d response (dishabituation). In addition, we compared response decrement in naive animals with decrement in dishabituated and in sensitized animals. T he analysis of the best fitting functions of the habituation, the habituati on of dishabituation and the habituation of sensitization revealed interest ing differences in these processes. (C) 2001 Elsevier Science BV. All right s reserved.