The "psychic secretions" that interfered with Pavlov's experiments on digestion were:

a. primary reinforcers.

b. conditioned responses.

c. conditioned stimuli.

d. secondary reinforcers.

e. unconditioned responses.

The answer is b. conditioned responses.

The starting Pavlov's idea was that whenever dog sees food, it will salivate. Here food is unconditioned stimulus and salivation is unconditioned response. This is because dog does not need to learn to salivate when see the food. But, in his experiment, Pavlov used a bell as a neutral stimulus. Any time he served the food to the dog, he would rang a bell. After a number of repeats, the dog salivated whenever it heard a bell no matter the food was not there. Now, the bell is not any more the neutral stimulus, but the conditioned stimulus. The response to the conditioned stimulus (the bell) is called a conditioned response.