For instance, in psilocybin research with human volunteers, Dr. Carhart-Harris obtained results with an fMRI which shows a global decrease in brain activity, especially frontal areas, while Dr. Vollenweider using PET found global increases, especially in the same frontal areas. The difference has been interpreted as a result of the low temporal resolution of PET. But I still don't understand why a decrease in brain activity would appear as an increase simply due to a low temporal resolution.

The other confounds are mostly related to method of administration. Vollenweider administered it orally while Carhart-Harris administered it intravenously. I have a feeling this is the culprit, having to do with metabolizing the drug and it's distribution to other organs which intravenous administration doesn't produce and certainly the onset has an effect as well.

The fMRI machine also makes a lot of noise that could have caused decreased frontal activity which is characteristic of anxiety.

But generally, is completely opposite results due to PET vs fMRI something we find often in the neuro community? And if so, why didn't they teach me that in university when comparing all the neuroimaging methods?

Overall, how are we supposed to interpret the PET research given the new fMRI results? Does this shed light on the inaccuracy of our understanding of brain activity, or can anything at all be said about the psychedelic state using the PET data? Is it to be ignored? For over 40 years now psychedelic science has been based on the increased neural activity showed by these PET results, and now an fMRI study is telling us that it's all backwards... what are we to make of the previous data?