Pavlovian phenomena in conditioned acceleration—Stimulus summation

Abstract

Four naive male rats were separately fear-conditioned to two stimuli (light and sound) by pairing these stimuli with shocks. During Sidman avoidance these unreinforced stimuli elicited increases in response rates (conditioned acceleration). In a multiple schedule with light-shock and sound-shock reinforcements in the first component, the rats were tested for summation effects in the second (avoidance) component by presenting the light twice, the sound twice and the compound twice. They were then extinguished by eliminating the shocks in the first component. The effects of compounding were evidenced by: (1) a sizeable and reliable amplitude increase to the compound stimulus during conditioning; (2) a smaller and less consistent decrease in latencies during conditioning in all four subjects; and (3) a greater resistance to extinction for the compound stimulus as measured by mean amplitudes for all four subjects and on six of seven extinction sessions. Latency data during extinction were inconclusive. An additional unexpected finding was what appeared to be a two-factor secondary extinction of the avoidance itself as a result of Pavlovian extinction. The responding in the first four minutes of avoidance was inhibited to such an extent that the number of shocks received during this period increased 700 per cent from the last three conditioning sessions to the first three extinction sessions.