The effects of hyperstriatal lesions on one-trial passive-avoidance learning in the chick.

The roles of different forebrain structures in memory formation in the chick were investigated using restricted radio-frequency lesions. Young chicks will spontaneously peck at a small bright bead. If the bead has been coated with a distasteful substance, the chicks learn not to peck a similar bead on subsequent presentation. Thus, in a single trial chicks learn not to peck at an aversive stimulus. Bilateral lesions of the intermediate part of the medial hyperstriatum ventrale (IMHV) prevented the acquisition of this 1-trial passive-avoidance learning task. However, neither bilateral lesions of the lateral cerebral area (LCA) nor sham operation affected learning. IMHV has previously been shown to be critically involved in the learning process of imprinting in the chick. The present experiment demonstrates that the role of IMHV in learning is not restricted to imprinting. The function of IMHV in imprinting has been suggested to be concerned with the recognition of objects. In the present study sham-operated and LCA-lesioned chicks recognized the visual characteristics of the training bead and did not avoid a bead of a different color in the test. We propose that IMHV-lesioned chicks fail to learn the task because they are unable to recognize the visual characteristics of the training bead and consequently cannot make the association between the bead and the unpleasant taste.