On the energy cost of mental effort.

The discovery of the Law of Conservation of Energy in the 1840s had consequences for psychological theory. Does the process of thinking involve a novel form of energy that is not recognized by physical science? E. L. Youmans (1821-1887) argued that "mental operations are dependent upon material changes in the nervous system." Kurd Lasswitz (1848-1910) introduced the term "psychophysical energy," based upon the electrical activity of the brain. At the beginning of the twentieth century Alfred Lehmann (1858-1921) claimed that intense mental effort leads to a net increase in oxygen utilization and regarded this as evidence of a specific psychic energy. His views were adopted and extended by Hans Berger (1873-1941). F. G. Benedict (1870-1957), drawing upon extensive experience with balance experiments conducted on humans in large-scale respiration calorimeters, concluded that mental effort probably had no effect upon the brain's metabolism. Modern approaches to the problem make use of PET imaging, which detects local changes in glucose utilization by the brain during cognitive activity.