In experiments, rats were conditioned to fear the chime via electric shock until they would consistently freeze up each time the chime sounded. The next day, rather than undergoing extinction learning, the variable group was injected with BDNF. The control rats were left untouched. The next day, researchers began sounding the chime. The control rats seized up as expected, fearful of the shock. The variable rats did not. Further testing showed that the BDNF experiment very closely mirrored genuine extinction training.