Lesions of periaqueductal gray dissociate-conditioned freezing from conditioned suppression behavior in rats

It is commonly assumed that suppression of an ongoing behavior is an indire ct measure of freezing behavior. We tested whether conditioned suppression and freezing are the same or distinct conditioned responses. Rats were trai ned to press a bar for food and then given fear-conditioning sessions in wh ich a tone was paired with a foot shock (two pairings a day for 2 days). Th ey then received either sham or electrolytic lesions of the periaqueductal gray (PAG). Post-training PAG lesions blocked freezing to the conditioned s timulus (CS), but had no effect on the suppression of operant behavior to t he same CS. Thus, conditioned suppression and freezing, which both cause a cessation in activity, appear to be mediated by separate processes.