Experimental manipulations: Neuroimaging correlational studies cannot determine whether neural activity plays a causal role in the occurrence of mental processes (correlation does not imply causation) and they cannot determine if the neural activity is either necessary or sufficient for such processes to occur. Identification of causation and necessary and sufficient conditions requires explicit experimental manipulation of that activity. If manipulation of brain activity changes consciousness, then a causal role for that brain activity can be inferred. Two of the most common types of manipulation experiments are loss-of-function and gain-of-function experiments. In a loss-of-function (also called "necessity") experiment, a part of the 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 in several ways:
Pharmacological manipulation using various drugs which alter neural activity by interfering with neurotransmission, resulting in alterations in perception, mood, consciousness, cognition, and behavior. Psychoactive drugs are divided into different groups according to their pharmacological effects; euphoriants which tend to induce feelings of euphoria, stimulants that induce temporary improvements in either mental or physical functions, depressants that depress or reduce arousal or stimulation and hallucinogens which can cause hallucinations, perception anomalies, and other substantial subjective changes in thoughts, emotion, and consciousness.
Electrical and magnetical stimulations using various electrical methods and techniques like transcranial magnetic stimulation. In a comprehensive review of electrical brain stimulation (EBS) results obtained from the last 100 years neuroscientist Aslihan Selimbeyoglu and neurologist Josef Parvizi compiled a list of many different subjective experiential phenomena and behavioral changes that can be caused by electrical stimulation of the cerebral cortex or subcortical nuclei in awake and conscious human subjects.
Today, where a definition of the moment of death is required, doctors and coroners usually turn to "brain death" or "biological death" to define a person as being dead; brain death being defined as the complete and irreversible loss of brain function (including involuntary activity necessary to sustain life).