In order to understand the brain, neuroscientists use a wide array of methods. The most common ones are (functional) magnetic resonance imaging ((f)MRI), electro-/magnetoencephalography (E/MEG), single-unit recordings, and transcranial direct current/magnetic stimulation (tDCS/TMS). Each of these methods has its own advantages and disadvantages and is suited for answering a particular set of questions.
Magnetic resonance imaging (Figure 1) is a technique which uses strong magnetic fields and field gradients in order to generate pictures of the brain. These offer insight into the anatomy of the brain and can be helpful in detecting, for example, tumors or other brain abnormalities. However, MRI is only capable of producing a static image of the brain, while neuroscientists are more concerned with how the activity in the brain changes over time in response to various task demands.
The observation that oxygenated and deoxygenated blood have different magnetic properties, coupled with the fact that, as regions of the brain become more involved in a task, their demand for oxygen increases, causing more oxygenated blood to flow into these regions, has helped create functional MRI. Consequently, unlike the traditional MRI, fMRI allows researchers to view how the brain responds to various stimuli.
The advantage of fMRI is that it has good spatial resolution (it can be as good as 1 mm). Unfortunately, because changes in blood flow are slow (on the order of seconds) compared to changes in neuronal responses (on the order of milliseconds), this method is said to have very poor temporal resolution.
A second method, which, unlike fMRI, has excellent temporal resolution, is electroencephalography (EEG (Figure 2); or magnetoencephalography (MEG), which will be discussed in the second part of the section).
Neurons in the brain respond to stimuli or communicate with each other by undergoing changes in their membrane potential (i.e., by having electric charges moving from the inside to the outside of the cell or vice-versa). This movement of charge across the membrane represents ionic currents (for a schematic diagram of this process, see Figure 3 below; for more information about this topic, click here). When a lot of neurons with the same orientation have the same ionic currents, these will sum up and give rise to voltage fluctuations large enough to travel through the skull and be measurable by EEG. In other words, EEG monitors changes in the electrical activity of the brain. Because the signal from a single neuron is very weak and therefore it needs to be summed up across a huge number of neurons in order be detectable with EEG, this method has very poor spatial resolution.
Magnetoencephalography (MEG; Figure 4) is similar to EEG in the sense that it is also the result of the coordinated activity of hundreds of thousands of neurons, however, as the name suggests, it measures the magnetic fields induced by the neuronal currents. It suffers from the same drawback as EEG, i.e. poor spatial resolution, but it has the advantage that, unlike the electric signal, the magnetic one is not dampened by the skull.
3. Single-unit Recordings
As the name suggests, single-unit recordings are measures of electrophysiological responses obtained from single neurons. A microelectrode (Figure 5) is either brought in close proximity of the cell (extracellular recording) or inserted into it (intracellular recording) and changes in membrane potential in response to stimulation are recorded. The method is very invasive, however, and generally, it cannot be used in humans (some exceptions exist in the case of patients with treatment-resistant epilepsy, who need to undergo surgery and have electrodes inserted into their brains prior to the operation in order to determine the location of the epileptic foci).
While the previous methods outlined here focus on recording intrinsic brain activity, neuroscientists have also the possibility of inducing changes in this activity and then measuring how this impacts human behaviour. This is done using transcranial direct current stimulation (tDCS) or transcranial magnetic stimulation (TMS; Figure 6), i.e. applying low-intensity direct currents or weak magnetic fields over certain scalp regions. As the intensity of these currents/magnetic fields is very low, they cause absolutely no harm to the individual. The two methods show promising results in the treatment of psychiatric conditions such as major depressive disorder.
This list gives an overview of the most common methods used in neuroscientific research, but this is by no means exhaustive. Other imaging methods include near-infrared spectroscopy (NIRS), positron emission tomography (PET), or different variants of MRI, such as diffusion MRI. Furthermore, these are coupled with behavioural methods, for example, eye-tracking, reaction time measurements, or systematic behaviour observations. For those who are interested in the topic and would like to read more, Wikipedia articles are a great starting point. If you would like to go more in-depth than that, a great resource is the book “Guide to Research Techniques in Neuroscience” (1st edition), by Matt Carter and Jennifer Shieh.