Brewer and his team found two notable trends in the results of the study. First, experienced meditators showed deactivation of the part of the brain known as the default mode network (DMN), a region involved in self-referential processing, including daydreaming. All three forms of meditation showed similar results. This discovery is interesting because one of the goals of meditation is to remain focused, and deactivation of the DMN seems to show that meditation is functionally doing just that in the brain. As meditators self-reported significantly less mind-wandering, these results support the hypothesis that deactivation of the DMN is related to a reduction in mind-wandering.
Gary Weber's site covers a bit of recent research in "Persistent Meditative States - How?" On why the pleasurable states of meditation do not seem to habituate, he writes:
The big question is why the great pleasures of "thought-free" meditation, presumably operating by the same dopamine/opiod neurochemistry system, persist, and do not saturate and operate in the same way to produce less pleasure with more craving. Patricia's paper, and Buddhism, develops the idea that our repetitive thought patterns can be viewed as a form of addiction. If we dramatically reduce the internal narrative, the dopamine down-regulation/reduction apparently does not occur. Instead of an endlessly repetitive stream of thoughts, the brain is engaged in a dance of open awareness with its continually changing show, and all of its concomitant neurotransmitter-induced pleasure.And another video of Robin Carhart-Harris talking about his research. I had not realized that his research with psilocybin was based around the idea that there may be a neurobiological basis for some of Freud's theories. He was particularly influenced by Grof's book, Realms of the Human Unconscious: Observations from LSD Research.
LSD, for example, promoted REM during sleep, and dreams, as theorized by Freud, were the key to the unconscious. With psilocybin, decreased blood flow was seen in the primary motor cortex, thalamus, brainstem, and subgenual cingulate. The latter was mentioned as being active in repression. Increased blood flow was seen in areas of visual association, which is similar to REM.
Although there was a decrease in the blood flow to the thalamus, there aren't a lot of 5-HT2A receptors there, so that implies that there are indirect or feedback effects at work.