Volunteers were told to think about yawning, while they breathed either normal air, pure oxygen, or an air mixture with an above-normal level of carbon dioxide.

当志愿者在正常的空气中、纯氧的环境下，或者混有超过正常的二氧化碳含量的空气中呼吸时，他们被要求要想着打哈欠。