To say that a reinforcement is contingent upon a response may mean

nothing more than that it follows the response. It may follow because

of some mechanical connection or because of the mediation of another

organism; but conditioning takes place presumably because of the

temporal relation only, expressed in terms of the order and proximity

of response and reinforcement. Whenever we present a state of affairs

which is known to be reinforcing at a given drive, we must suppose

that conditioning takes place, even though we have paid no attention

to the behavior of the organism in making the presentation. A simple

experiment demonstrates this to be the case.

A pigeon is brought to a stable state of hunger by reducing it to 75

percent of its weight when well fed. It is put into an experimental cage

for a few minutes each day. A food hopper attached to the cage may

be swung into place so that the pigeon can eat from it. A solenoid and

a timing relay hold the hopper in place for five sec. at each

reinforcement.

If a clock is now arranged to present the food hopper at regular

intervals with no reference whatsoever to the bird's behavior, operant

conditioning usually takes place. In six out of eight cases the resulting

responses were so clearly defined that two observers could agree

perfectly in counting instances. One bird was conditioned to turn

counter-clockwise about the cage, making two or three turns between

reinforcements. Another repeatedly thrust its head into one of the

upper corners of the cage. A third developed a 'tossing' response, as if

placing its head beneath an invisible bar and lifting it repeatedly. Two

birds developed a pendulum motion of the head and body, in which the

head was extended forward and swung from right to left with a sharp