Carhart-Harris RL, Feilding A, Nutt DJ et al. – Proceedings of the National Academy of Sciences Jan 2012.
This study measured the neural correlates of the transition from normal waking consciousness to the psychedelic state, via administration of psilocybin or placebo while undergoing fMRI imaging. Two separate sets of scans (ASL and BOLD – see methods) were performed, with both running as task free functional scans. 
See METHOD below for more details
FINDINGS AND IMPLICATIONS
The primary finding was that the psychedelic experience is correlated with reductions in cerebral blood flow and metabolism, with the magnitude of this decrease predicting the intensity of the subjective effects.These reductions were most pronounced in brain areas that are part of the ‘default mode network’ (DMN) – a network of higher-order brain regions, actived during self-referencing and other high-level functions linked to the self . These regions include the posterior cingulate cortex, the anterior cingulate cortex and the medial prefrontal cortex. For example, metabolism in the PCC in resting state is usually around 20% higher than most other brain regions. Psilocybin decreased blood flow in the PCC by up to 20% in some subjects. Furthermore, the strong link between these regions and self-related processing might allow a possible explanation for the phenomenon of ego dissolution or disturbed ego boundaries in psychedelic states via the decrease in activity of these self-related regions. This topic has since been explored within our collaboration by re-analysing this fMRI data .
These DMN regions also host the highest number of connections to and from other areas of the brain, making them important connector hubs for efficient information transfer. This crucial communicative and integrative functions of the DMN might explain why deactivating these regions causes such profound changes to consciousness and sensory experience. Many functions of the brain heavily rely on top-down communication from cognitive to sensory regions, hence disruption in this communication could be a general mechanism to explain the many diverse aspects of the psychedelic experience.
- Fifteen healthy volunteers were scanned with Arterial spin labelling (ASL) perfusion and a separate fifteen healthy volunteers with blood-oxygen level dependent (BOLD) fMRI.
- ASL perfusion is a measure of blood flow within the brain, gained by magnetically labelling blood water and then imaging its movement. Blood flow will follow increases in neural activity or decrease in cases of lower activity.
- BOLD is a a measure of changes in blood oxygenation in the brain, (relying on the different magnetic properties of oxygenated and deoxygenated blood) which provides an indirect measure of metabolic (and therefore neural) activity.
- In the ASL scanning group, the subjects received placebo in the first scan and psilocybin in the second. In the BOLD scanning group, the placebo and psilocybin sessions were carried out in a balanced order (i.e. a randomised half the group had their psilocybin scan first and placebo second, the rest placebo first and psilocybin second). In the BOLD group the scans were separated by at least 7 days.
- The IV infusion occurred 6 minutes into each functional scan, with the participant giving intensity ratings at the start of the scan, immediately before infusion, 5 minutes post-infusion, and 12-min post infusion (each scan lasted 18 minutes, and participants underwent two scans in each session). The onset of psilocybin’s effects occur around 5 seconds post infusion
- The intensity of the subjective effects was rated via button press on a 0–10 visual scale (10 = extremely intense effects) at the start of each functional scan, just before infusion, 5-min postinfusion, and 12-min postinfusion.
 Carhart-Harris et al (2012) Neural correlates of the psychedelic state as determined by fMRI studies with psilocybin. Proceedings of the National Academy of the Sciences USA. 2012 Feb 7;109(6):2138-43. doi: 10.1073/pnas.1119598109. Epub 2012 Jan 23.
 http://en.wikipedia.org/wiki/Default_network / Raichle (2010) Two views of Brain Function Trends Cog Sci 2010 Apr; 14(4): 180-190
 Carhart-Harris et al (2012) Functional Connectivity Measures After Psilocybin Inform a Novel Hypothesis of Early Psychosis.Schizophr Bull. 2012 Oct 27. [Epub ahead of print] http://www.ncbi.nlm.nih.gov/pubmed/23044373 /
Hagmann et al (2008) Mapping the structural core of human cerebral cortex. PLoS Biol 6:e159