An implicit premise in neuropsychopharmacology with regard to the psychological aspects is that all states of mind, including both normal and drug-induced altered states, and diseases involving mental or cognitive dysfunction, have a neurochemical basis at the fundamental level, and certain circuit pathways in the central nervous system at a higher level. (See also: Neuron doctrine) Thus the understanding of nerve cells or neurons in the brain is central to understanding the mind. It is reasoned that the mechanisms involved can be elucidated through modern clinical and research methods such as genetic manipulation in animal subjects, imaging techniques such as "functional magnetic resonance imaging" (fMRI), and "in vitro" studies using selective binding agents on live tissue cultures. These allow neural activity to be monitored and measured in response to a variety of test conditions. Other important observational tools include radiological imaging such as "positron emission tomography" (PET) and "single-photon emission computed tomography" (SPECT). These imaging techniques are extremely sensitive and can image tiny molecular concentrations on the order of 10−10 M such as found with extrastriatal D1 receptor for dopamine.