"In hypnosis, we know you can alter things like gastric acid secretion, heart rate, blood pressure and skin conductance," Spiegel said.

On the non-dominant hand, the volunteers wore a sensor that measured skin conductance, which is itself an indicator of physiological, and perhaps emotional, arousal.

But machine learning technologies will also make it possible to discover "unknown signals," such as the significance of skin conductance for a variety of conditions.