[techdragon]

Some of this stuff is hard to measure.

Anything that doesn't cross the blood brain barrier is never going to get many samples due to the invasive procedures needed.

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.

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.