Whether hypnosis is an altered state of consciousness has been debated for nearly 200 years. Today, brain imaging studies seem to confirm that hypnosis is an altered state. Numerous studies using electroencephalography (EEG), positron emission tomography (PET), and functional magnetic resonance imaging (fMRI) show distinct changes in the brain during hypnosis and in response to suggestion.
Brain imaging studies observing the general effect of the hypnotic state (as opposed to the effects of specific suggestions) have found that hypnosis causes observable changes in the brain areas and systems involved in:
• Consciousness and sense of self.
• Attentional absorption and spontaneous conceptual thought.
• Concentration, attentional control and executive function (reasoning, problem solving, planning, self-control, and cognitive flexibility).
• Higher cortical functions.
• Awareness and control of internal bodily processes and emotions.
• Emotional evaluation and worrying. 
Studies observing the effects of hypnotic suggestions have most frequently centered on the neurophysiological response to suggestions for pain, motor function and limb paralysis, mental imagery, and memory. These studies demonstrate not only that changes occur in brain physiology, but also that these changes explain the phenomena and behaviors observed in hypnosis. Hypnotic suggestion has been shown to affect the brain areas and systems involved in:
• Consciousness, sleep, and alertness.
• Motor control.
• Autonomic functions, emotion, motivation, impulse control, reward anticipation, and decision-making.
• Color perception.
• Reasoning and decision making.
• Imagery, self-awareness, and motor control.
Neural activity in hypnosis
Hypnosis causes changes in brain activity and connectivity consistent with decreased self-consciousness, increased control of internal sensations and emotion, and less worry.
A study by researchers at Stanford University in July 2016 considered what takes place in the brain in general during hypnosis. Functional magnetic resonance imaging (fMRI) was used to observe brain activity in 57 subjects in hypnosis. Changes were observed in three specific areas of the brain:
1. Reduced connectivity between the dorsolateral prefrontal cortex (part of the executive control network involved in planning and decision making) and the posterior cingulate cortex (the part of the default mode network active during self-related thinking). This reduced connection between planning of actions and self-awareness may account for the immediacy of action that takes place during hypnosis. In hypnosis, as in states of deep absorption in a task or performance, one acts spontaneously, without reflecting upon actions.
2. Increased connectivity between the dorsolateral prefrontal cortex (part of the executive control network involved in planning and decision making) and the insula (the part of the salience network involved in sensing and regulating internal bodily processes). This may be responsible for the increased control over bodily and emotional processes in hypnosis.
3. Reduced activity in the dorsal anterior cingulate cortex (part of the salience network involved in the emotional evaluation of errors and worrying). The dorsal ACC is also active during effortful performance. This reduced activity may explain why, in hypnosis and states of deep absorption, actions and performance take place effortlessly and with less worry.