Answer:
They discovered "how to control the heart"
Explanation:
In concrete, they were able to reinforce heart-rate increase and they were able to reinforce heart-rate decreases, which means they found a way to make a heart start pumping or even to stop the heart.
They also were able to control heart rate with a discriminative stimulus, this means that they could make a heart, for example, go from 100 bpm (beats per minute) to 70 bpm.