Some of this stuff is hard to measure.<p>Anything that doesn&#x27;t cross the blood brain barrier is never going to get many samples due to the invasive procedures needed.<p>And for more normal blood based measurements, the protocols for measuring change over time requires a number of blood samples taken over time. The more accurate you want to be the more invasive you must become.<p>All the research I have read uses proxy measures, for example using PET scan and MRI techniques to measure brain activity and trying to infer the neurotransmitters based on glandular activity in the brain.