INSTRUMENTAL LEARNING IN TOADS (BUFO-ARENARUM) - REINFORCER MAGNITUDEAND THE MEDIAL PALLIUM

Two experiments studied the effects of shifts in the amount of water r einforcement on the instrumental performance of toads (Bufo arenarum) using one trial per day. In the first experiment, a shift from a large to a small reward magnitude led to a gradual change in performance wi thout evidence of negative contrast. A shift from large or small rewar d magnitude to extinction led to similar extinction rates and provided no indication of the magnitude of reinforcement extinction effect. In the second experiment, the gradual change in performance (with no con trast) after a shift from a large to a small reward magnitude was comp letely eliminated by ablation of the medial pallium. These results pro vide support for the hypothesis that the medial pallium in amphibians is homologous to the hippocampal formation in mammals and also indicat e that species differences in these learning phenomena may be related to the differential development and differentiation of the hippocampal formation.