In the 1930s, it was possible it was possible to gain some basic idea of brain activity by placing electrodes on the scalp. EEG is still useful today, but can not tell us much about the interior workings of the brain. The 1970s saw the advent of the CAT scanner.
CAT scanners can 'see' the difference between bone, flesh and cavities.
They don't show us nervous activity. A positron is a small particle given off by a radioactive substance. It is perhaps a bit like an electron. In PET, the patient is injected with a harmless very mildly radioactive marker. Various markers are used.