A recent article in The Psychologist by Carhart, Kaelen and Nutt (here) reviews what is known about the action of chemicals that cause hallucinations: LSD from ergot fungi, dimethyltryptamine from ayahuasca and psilocybin from magic mushrooms.
The molecular action of the hallucinogens is to excite particular pyramidal neurons by mimicking the action of the transmitter serotonin. These layer 5 pyramidal neurons are important for projecting to lower centers outside the cortex and within it. They mostly project to neurons that are inhibitory. The net effect is that the exciting of the pyramid cells creates an inhibitory signal from other neurons. It tends to generally shut things down. The oscillations in the cortex, so important to the workings of the brain, are decreased in strength and also desynchronized. The disruption of brain waves seems to stem from the interference with the pyramidal cells – inhibitory cell chains.
This decrease in activity is especially evident in some very important hubs in the brain: the thalamus, posterior cingulate cortex and the medial prefrontal cortex, all integrating and executive control hubs. This may be the source of the lack of integration and constraint seen in hallucinations. There is a lack of distinctness in the network structure of the brain and networks seem to melt into one another. One of the effects of this is increased cognitive flexibility and ability to learn. This may be very useful to therapists in controlled doses. The inhibition of some centers allows areas they control to escape that control. When the cat is away, the mice with play.
It is interesting that the hallucinogens “profoundly alter the quality of consciousness whilst leaving arousal or wakefulness intact.” How does the hallucinogen create the complex vivid visual hallucinations or the loss of ego? During periods of hallucination there are ‘phasic discharges’ from the hippocampus, amygdala and septal nuclei (medial temporal lobe sites) in contrast to the general disorder of brain waves. This resembles the activity of the medial temporal lobe in REM sleep and dreaming. Stimulation of the MTL experimentally can produce hallucinations and distortions of vision.
“One of the most common yet abstract experiences described in relation to the hallucinogenic drug state is a disintegration or dissolution of the self or ego. Such an experience is difficult to fathom from the vantage of normal waking consciousness, where an integrated sense of self is felt as pervasive and permanent. It is perhaps not surprising therefore that the experience of ego-disintegration is described as profoundly disconcerting and unusual …Classic accounts of so-called ‘mystical’ or ‘spiritual’ experiences have placed emphasis on the necessity for self or ego disintegration for their occurrence. Thus, in order to investigate the neurobiological basis of ego-disintegration and mystical-type experiences, it is useful to first examine the neural correlates of self-awareness.” The strength of alpha waves in the posterior cingulate cortex, a major hub in the default mode network is correlated with the strength of the self. In hallucinogen sessions, the activity of the PCC decreases in correlation with ego-disintegration. The self is also weak during dreaming.