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surrounding environment; restricted environmental stimulation interrupts the normal flow of sensory-motor activity; and transcendental meditation may induce a state of alert but content-free "pure consciousness." Claims have been made for the performance-enhancing qualities of each of these states, but a critical review of the available literature indicates that most of these claims are unsupported by scientific data. That is, either the results have been negative, or positive results have been contaminated by the lack of certain critical controls.

To the extent that performance is impaired by subjective feelings of pain and fatigue, hypnosis can enhance performance by reducing a subject's awareness of these potentially demoralizing conditions. This possibility is limited by the role of hypnotizability in moderating the effects of hypnotic suggestion: not everyone is hypnotizable enough to experience this effect. However, even individuals who are not hypnotizable may receive some benefit from the placebo component in hypnotic analgesia or from training in nonhypnotic stress inoculation.

By and large, direct hypnotic suggestions for enhanced performance have no effect on muscular strength and endurance, sensory thresholds, learning, and memory retrieval. Hypnotized subjects may believe that they are doing better, and this belief may have positive motivational properties, but the subjective experience of performance enhancement appears to be illusory.

Transcendental meditation (TM) has been offered as a means of enhancing performance, chiefly by reducing the deleterious effects of stress. Although TM has generated a voluminous body of research, the available studies suffer from a variety of methodological flaws that preclude firm conclusions. For example, it is not clear whether the positive effects observed in TM are due to the specific effects of the unique features of TM or to the frequency and discipline with which TM is practiced.

Restricted environmental stimulation (REST) has been offered as a technique for enhancing human performance, but most of the evidence supporting this proposal is based on the proven therapeutic effects of REST in controlling habit behaviors. There is some anecdotal evidence of the performance-enhancing effects of REST, and a few formal studies, but not enough for firm conclusions about the effects, if any, and their underlying mechanisms.

Although sleep learning is ineffective when measured in terms of an individual's ability to consciously remember material presented during sleep, the committee's last report raised the possibility that sleep learning could be expressed as implicit memory, in the absence of explicit recollection: more recent evidence indicates that this is not the case. Some degree of quasi sleep learning may be possible, but if so it is both likely to be inefficient and to have detrimental effects on a person's subsequent waking performance.