Involvement of the avian hypothalamus in defensively conditioned heart rate change.

Hypothalamic involvement in visually conditioned heart rate change (established by pairing light and foot-shock) was studied in 128 pigeons by evaluating conditioning performance following lesions of various hypothalamic areas. Extensive destruction of the posterior hypothalamus severely impaired development of conditioned heart rate change. Anterior hypothalamic lesions also produced serious deficits, though not as severe as following posterior hypothalamic damage. Partial posterior hypothalamic, unilateral and tuberal lesions produced only minor to moderate deficits. The critical locus for profound impairment of conditioned response development appeared to be the medial hypothalamus, and it is suggested that more specifically it is the terminal field of the archistriatal projection upon the medial hypothalamus. It is concluded that the medial hypothalamus is essential for the development of defensively conditioned heart rate change, and based on previous findings it is suggested that the critical descending pathway for expression of this conditioned response involves the archistriatal projection upon the medial hypothalamus and subsequently a polysynaptic pathway through the ventral brainstem.