ASSOCIATIVE LEARNING MODIFIES THE SHORTENING REFLEX IN THE SEMIINTACTLEECH HIRUDO-MEDICINALIS - EFFECTS OF FAIRING, PREDICTABILITY, AND CSPREEXPOSURE

Three experiments addressed the importance of the inter-event relation ships of contiguity and contingency for associative learning in the se mi-intact leech. It was found that both of these relationships are imp ortant for the leech to acquire a learned association between a touch (conditional stimulus, CS) and shock (unconditional stimulus, US). The learning can be extinguished if training is followed by explicitly un paired presentations of the CS and US, which removes the contiguity be tween the stimuli. Learning is degraded by the introduction of unpredi cted USs, as well as by unreinforced presentations of the CS (CS preex posure), both manipulations reduce the contingency between the CS and US. These results suggest that the associative process in both vertebr ates and invertebrates share considerable functional similarity in the inter-event relationships important to learning.