The authors also designed tiny vests for the animals that emitted red light that could pass through the rodents bodies all the way to their hearts. When a mouse's vest emitted the pulse of light, the animals engineered heart muscles fired, causing the heart to beat faster. The team trained the animals to expect a shock when they pressed a lever for water as a reward. Using the optogenetic system, the team raised the animal's heart rates from the normal 660 beats per minute to 900