Increased firing of neurons in the posterior hypothalamus which precede classically conditioned pupillary dilations

Paralyzed cats were used as subjects in a classical conditioning experiment where each subject was exposed to 40 explicitly unpaired 1-s bursts of white noise and 0.5-s paw shocks. This training was followed by 60 trials of the two stimuli paired, where the white noise immediately preceded the paw shock. Following this training, the subjects were re-exposed to 40 trials of the explicitly unpaired procedure. The pupil was monitored as the behavior and electrodes implanted in the thalamus, the dorsal hypothalamus and the posterior hypothalamus recorded the activity of clusters of cells. Only the cells in the posterior hypothalamus showed robust changes in firing rates that preceded the pupillary behavior, both (a) on any particular trial and (b) as the learned association was being demonstrated behaviorally across trials.