Expression in paradoxical sleep of a conditioned heart rate response

After a session of habituation to a tone, awake rats underwent two conditioning sessions during which the tone was paired with footshock. The tone alone was presented during paradoxical sleep (PS) following each session; it never awakened the animal. Heart rate (HR) was recorded at each tone presentation in wakefulness and PS. During conditioning in wakefulness, tone presentation elicited HR accelerative responses. Tone-evoked HR accelerations were also detected in PS following the two conditioning sessions. Such changes were not observed in rats that received unpaired presentations of tone and footshock. These results demonstrate that an HR conditioned response acquired in wakefulness can be expressed during PS.