There are multiple techniques available in research and medicine to study the brain for various purposes including diagnosis and psychology research.
Functional magnetic resonance imaging (fMRI) is used moreso in research than medicine, and gives a picture of brain activity based on increased blood flow correlated with neural activity.
This is based on the knowledge that wherever in the brain higher activity takes place, an increase in blood will take place. This creates a gradient between the deoxygenated haemoglobin in the blood and the oxygenated haemoglobin in the blood. Because oxygen in haemoglobin is bound to the iron present, deoxygenated haemoglobin will have unbound iron ions. These respond magnetically to the magnetic field applied during the procedure.
By processing the data obtained through the procedure (which involves picking up the blood flow differences via magnetic fields and radio frequency waves), a map of brain activity emerges. Within an experiment, this is cross-referenced with a control state, whether for a patient or group of participants in a study, in order to interpret the data.
Limitations of fMRI include noise which interferes with the signals e.g. from heat in the vicinity of the scanner, movement of the participant or system noise coming from the hardware, and statistics needed to make use of the raw data. Sometimes the statistics used was not robust enough to distinguish between real differences in brain activity and noise. Famously, the fMRI data of a dead salmon was used to show real differences in brain activity obtained by using flawed statistics.
Computerised tomography (CT) is another brain imaging technique used for other parts of the body too, which uses X-rays processed by a computer in multiple angles to create a multiple slice picture of the brain. It is used a lot in detecting brain infarction, tumours and haemorrhages.
The issue raised by CT is exposure to radiation which itself can cause cancer.
Positron emission tomography (PET) is an imaging technique that relies on a radioactively labelled molecule (a tracer) injected into the patient to pick up metabolic signals from the body. It is used in the diagnosis of cancer metastasis and Alzheimer’s disease.
The tracer used is a common molecule in the body such as glucose, water or urea, and following its injection into the patient, a waiting period is undertaken to allow for the molecule to disperse via the blood stream to the target location. After waiting for the required period of time, the patient is ready for scanning. The scanner picks up the signal given when the positron (like an electron but positive instead of negative) from the radioactively labelled glucose or other tracer is annihilated by the contact with an electron in its environment. It only travels up to 1 mm in the body before this happens.
Since the use of glucose by the brain is correlated with increase blood flow to an active area, the data can be likened to that obtained via fMRI, which also relies on the link between brain activity and blood flow.
Limitations of PET include exposure to ionising radiation from the tracer injected into the patient, a well as the procedure being expensive.
The concept of electroencephalography (EEG) is based on the ionic current produced from brain activity. This can be recorded using electrodes placed on the scalp. Unlike the other techniques, this gives a trace of different brain waves as opposed to a picture of the brain.
EEG can be used to diagnose and track the progression of epilepsy and coma. The waves are characterised by being alpha, beta, delta or theta waves based on their frequency. For example, alpha waves are of 8-12 Hz.
Limitations of EEG include low spatial resolution and a poor signal-to-noise ratio. This means that it’s very difficult to speculate which area of the brain signals might be coming from, even with using many electrodes spread out to cover all areas; and that in order to obtain good quality data for studies, a large sample of participants is required.