By now, we’re all familiar with the colourful spots and blobs associated with brain imaging. The assumption is often that areas which ‘light up’ are increasingly activated – and are, as such, implicitly more ‘important’ – in the task under study. But researchers using brain imaging techniques know this to be untrue. Some of the most interesting blobs that have been identified are known to reflect relative decreases in brain activity, often referred to as deactivations. A particularly interesting example, Dr David Nutt and colleagues found that the effects of psilocybin, the psychedelic found in magic mushrooms, are associated with widespread deactivations – suggesting that its effects are counter-intuitively due to less brain activation and/or altered connectivity between regions. For those interested, the controversy surrounding Dr. Nutt – which began around the time he cheekily argued that taking ecstasy was less harmful than horse-riding (see here for copy of the academic paper) – and his research continue with the funding of a much-needed study looking at the effects of psilocybin on depression.
My colleague Adrianne Huxtable and I briefly weighed in on some of the work related to the interpretation of deactivations in neuroimaging. Because these fMRI studies often use contrasts between two conditions of interest (e.g., brain signals during the experience of pain minus signals during a period of no pain), the identified areas could be reflective of many things of which many ideas abound. For instance, it is unclear how deactivations are related to increases and decreases in neuronal activity and what role specific neurotransmitters and other spatiotemporal dynamics might play.
What this means for the interpretation of neuroimaging signals is still highly debatable. Nonetheless, there are some suggestions which might help make future publications more comparable such as distinguishing between negative BOLD responses and relative deactivations. The take-home message here is that neuroimaging, like all fields of science, is evolving both technically and conceptually – many exciting findings and opportunities will be born from such contentious ideas.
Carhart-Harris RL, Erritzoe D, Williams T, Stone JM, Reed LJ, Colasanti A, Tyacke RJ, Leech R, Malizia AL, Murphy K, Hobden P, Evans J, Feilding A, Wise RG, & Nutt DJ (2012). Neural correlates of the psychedelic state as determined by fMRI studies with psilocybin. Proceedings of the National Academy of Sciences of the United States of America, 109 (6), 2138-43 PMID: 22308440
Hayes DJ, & Huxtable AG (2012). Interpreting deactivations in neuroimaging. Frontiers in psychology, 3 PMID: 22347207
Klingner CM, Huonker R, Flemming S, Hasler C, Brodoehl S, Preul C, Burmeister H, Kastrup A, & Witte OW (2011). Functional deactivations: multiple ipsilateral brain areas engaged in the processing of somatosensory information. Human brain mapping, 32 (1), 127-40 PMID: 21157879