Complex sentence: The animal is then tested by being played an intermediate stimulus C, e.g. a 15 Hz tone, and observing whether the animal presses the lever associated with the positive or negative reward, thereby indicating whether the animal is in a positive or negative mood.
Split into simpler sentences: The animal is then tested by being given an intermediate stimulus C, e.g. a 15 Hz tone, and observing whether the animal presses the lever associated with the positive or negative reward. Theoretically, this indicates whether the animal is in a positive or negative mood.