Draulio de Araujo of the Brain Institute at the Federal University of Rio Grande do Norte recruited ten frequent ayahuasca users, and scanned their brains while they looked at natural photographs, imagined them with their eyes closed, and viewed scrambled versions of the photos. Afterwards, the participants drank ayahuasca-infused tea and, once the effects of the drug had kicked in, repeated the same tasks while their brains were scanned again.

The researchers then compared the brain activity patterns from the scans performed before and after ayahuasca intake. As expected, viewing the natural and scrambled photos activated the primary visual cortex in the occipital lobe at the back of the brain, and several other visual areas.

But these same areas were even more active when the participants closed their eyes and imagined the photos after taking ayahuasca.

...

The scans also showed that the imagery task performed after ayahuasca intake was associated with increased activity in the parahippocampal gyrus, which is involved in retrieval of autobiographical memories and encoding of contextual knowledge, and the frontopolar prefrontal cortex, which is implicated in working memory and imaging future events. The researchers also observed altered connectivity between all these areas, and particularly in the relationship between the frontal and occipital cortices.

The study suggests that the visions induced by ayahuasca engage the brain's memory circuits, and that this may "feed" activity in the primary visual cortex, which in turn drives activity in the other visual areas. The observation that the primary visual cortex is more active during imagery with eyes closed than during viewing of actual images is in keeping with numerous reports of the vividness of ayahuasca visions. All of these effects are thought to be mediated by increased activation of serotonin receptors throughout the brain regions involved.

http://www.guardian.co.uk/science/neurophilosophy/2011/sep/23/1