Habituation of the shortening reflex in the medicinal leech

The shortening reflex was elicited in food-sated Hirudo medicinalis by light flashes delivered at 20-s intervals over a 40-trial session. The reflex was enhanced if animals were stored at 5-7 degrees C rather than 20 degrees C. Short-term, that is, within-sessions, habituation was readily observed. Dishabituation could be produced by a single electric shock at Trial 30. However, the shock also enhanced responding when delivered before Trial 1 (sensitization). No long-term, that is, between-sessions, habituation occurred over 6 consecutive days of testing; on the contrary, responding gradually increased.