Negative and positive visual hypnotic hallucinations: attending inside and out.

Hypnotic perceptual alteration affects brain function. Those hypnotic instructions that reduce perception by creating an illusory obstruction to it reduce brain response to perception in the cognate sensory cortex, as measured by event-related potential (ERP) amplitude and regional blood flow (PET). Those hypnotic instructions that affect the subject's reaction to perception activate the anterior attentional system, especially the anterior cingulate cortex in PET studies. Hypnosis involves activation without arousal and may be particularly mediated via dopaminergic pathways. Hypnotic alteration of perception is accompanied by measurable changes in both perceptual and attentional function of those specific regions of the brain that process these activities, modulated by the nature of the specific hypnotic instruction. Positive obstructive hallucinations seem to allow for a hypnotic focus inward, activating the functioning of attentional neural systems and reducing perceptual ones.