nervous system is diminished or removed in an attempt to determine if it is necessary for a certain process to occur, and in a gain-of-function (also called "sufficiency") experiment, an aspect of the nervous system is increased relative to normal. Manipulations of brain activity can be performed with direct electrical brain stimulation, magnetic brain stimulation using transcranial magnetic stimulation, psychopharmacological manipulation, optogenetic manipulation, and by studying the symptoms of brain damage (case studies) and lesions. In addition, neuroscientists are also investigating how the mind develops with the development of the brain. Physics Physicist Sean M. Carroll has written that the idea of a soul is incompatible with quantum field theory (QFT). He writes that for a soul to exist: "Not only is new physics required, but dramatically new physics. Within QFT, there can't be a new collection of 'spirit particles' and 'spirit forces' that interact with our regular atoms, because we would have detected them in existing experiments." Quantum indeterminism has been invoked as an explanatory mechanism for possible soul/brain interaction, but neuroscientist Peter Clarke found errors with this viewpoint, noting there is no evidence that such processes play a role in brain function; Clarke concluded that a Cartesian soul has no basis from quantum physics. Parapsychology Some parapsychologists have attempted to establish, by scientific experiment, whether a soul separate from the brain exists, as is more commonly defined in religion rather than as a synonym of psyche or mind. Milbourne Christopher (1979) and Mary Roach (2010) have argued that none of the attempts by parapsychologists have yet succeeded. Weight of the soul In 1901 Duncan MacDougall conducted an experiment in which he made weight measurements of patients as they died. He claimed that there was weight loss of varying amounts at the time of death; he concluded the soul weighed 21 grams, based on measurements of a single patient and discarding conflicting results. The physicist Robert L. Park wrote that MacDougall's experiments "are not regarded today as having any scientific merit" and the psychologist Bruce Hood wrote that "because the weight loss was not reliable or replicable, his findings were unscientific." See also Ancient Egyptian concept of the soul Being Chinese room Ekam History of the location of the soul Kami Knowledge argument Metaphysical naturalism Mind–body problem Nafs in Islam Nishimta in Mandaeism The Over-Soul (essay) Paramatman (or oversoul) Philosophical zombie Open individualism Qualia Self Self-awareness Shade (mythology) Soul