Behavioral and neural characteristics of short-latency and long-latency conditioned responses in cats

Head movements to the conditioned stimulus (a tone CS to the left ear) were studied in 6 cats. An attempt was made to differentiate an orienting, short-latency (alpha) response from the long-latency conditioned (delayed) response. The unconditioned stimulus (UCS) was a brain stimulation to the lateral hypothalamus eliciting, in addition to orienting and approach behavior, a specific, stereotypical head movement. These behavioral characteristics of the unconditioned head movement were used for differentiating it from the conditioned short-latency head movement. Paired conditioning and randomly unpaired control sessions (5 daily sessions each) were given in balanced order to each animal. Evoked neural responses in the hippocampus and cingulate cortex were recorded simultaneously to compare the time-amplitude characteristics of evoked responses to earlier findings in multiple-unit recordings. The results supported the differentiation of the behavioral responses. The time-amplitude course of the evoked neural responses showed complex changes, appearing as an increase in the negativity during the alpha-response period (150- to 450-ms interstimulus interval [ISI]) and as an increase in the positivity during the long-latency period (700- to 1,000-ms ISI + 150- to 450-ms UCS period) on omitted-UCS (CS-alone test) trials.