Conditioned inhibition of the nictitating membrane response in rabbits following hypothalamic and mesencephalic lesions.

Rabbits were trained on a Pavlovian conditioned inhibition (CI) task using light as the reinforced conditioned stimulus (CS+) and the same light compounded with a tone as the nonreinforced CS-. The conditioned response was the nictitating membrane response. After attaining a criterion of CI performance, animals received radio-frequency lesions of the hypothalamus (n = 11) or midbrain (n = 14). For the hypothalamic lesion cases, primary damage extended from the optic chiasm to the pretectal region. For the mesencephalic lesion cases, primary damage ranged from the most rostral portions of the periaqueductal grey (PAG) caudally to the tegmental reticular formation at the level of the third nerve. Prior research suggested that the hypothalamic lesions would disrupt retention of CI by increasing responding to the CS-. Except where a lesion impinged upon the zona incerta, no CI disruption was observed. In accordance with previous studies (Berthier, N.E. and Moore, J.W., Physiol. Behav., 25 (1980) 667-673; Mis, F.W., J. Comp. Physiol. Psychol., 91 (1977) 975-988), post-lesioning CI disruption was observed in some of the mesencephalic lesion cases involving the posterior commissure, PAG and/or accessory oculomotor nuclei. However, CI performance recovered over the course of retraining.