Recently I've been using new internets social thingy 2.0, Quora, which, according to Wikipedia, is "a social networking website that aggregates questions and answers to many topics and allows users to collaborate on them." So let's go with that.
Anyway, recently someone asked the question, "What can we measure using neuroimaging techniques?"
This is a good question. So I started to write an answer. An hour later, it had become a monster, and I thought it might be worth sharing here. Here it is:
Because this is a very broad topic, I will do my best to address the most common aspects of neuroimaging without going into too much detail. Please feel free to add more to this list, and I recognize I am being neither complete nor fully extensive.
Broadly speaking, I would say there are three categories of neuroimaging: structural, functional, and chemical. These can then be subdivided into non-invasive, semi-invasive, and invasive, which delineate the degree of physical invasiveness involved in the imaging method. That is, cutting open the skull and implanting electrodes would be considered invasive, whereas putting the electrodes on the head (such as in scalp EEG) is non-invasive. Because I'm not proficient in animal imaging methods I will focus on human studies, most of which are non- or semi-invasive, with a few exceptions.
Any technique that images structures of the brain. This would include CT (Computed Tomography), MRI (Magnetic Resonance Imaging), and DTI (Diffusion Tensor Imaging).
CT scanning is non-invasive uses x-rays to image tissue density. It is very rapid and can detect cerebral hemorrhaging in the early (acute) stage. It is most often, therefore, used for medical purposes.
Structural MRI is non-invasive and often provides better contrast resolution than CT with similar (and again, often better) spatial resolution. Unlike CT, structural MRI provides excellent tissue delineation, allowing users to visualize boundaries between grey and white matter in the brain, for example. Structural MRI is often used in neuroimaging to calculate volume of different brain regions or to define regions of brain damage or tumor.
DTI is non-invasive and can be done on most research MRI scanners. It involves using a special scanning and reconstruction sequence to image the flow (or, more specifically, constraints in the flow) of water through the brain. Because water flow is constrained by the axons (white matter) in the brain, it can be used to image large axonal connections between brain regions.
Any technique that quantifies some metric of brain activity. This would include EEG (ElectroEncephaloGraphy), MEG (MagnetoEncephaloGraphy), fMRI (functional MRI), PET/SPECT (Positron Emission Tomography/Single Positron Emission Computed Tomography), NIRS (Near-InfraRed Spectroscopy), and, to a certain extent, TMS (Transcranial Magnetic Stimulation) and TDCS (Transcranial Direct Current Stimulation), along with several others.
For all functional imaging methods, researchers often make use of the cognitive subtraction method originally established in reaction time studies by Franciscus Donders. The underlying assumption in these studies is that activity in brain networks alters in a task-dependent manner that becomes evident after averaging many event-related responses and comparing those against a baseline condition. Deviations from this baseline reflect a change in the neuronal processing demands required to perform the task of interest.
For example, if you want to study the effects of thumb movement on brain activity, you would have a person move their thumb many dozens or hundreds of time while interrogating brain activity. You would then call the time of thumb movement time zero, and take a set time window around that thumb movement and average the brain activity across those many trials. Because brain activity is usually very "noisy", it's hard to detect specific, thumb-movement activity at any give time. But averaging across all of these times highlights thumb-specfic brain activity.
EEG is most commonly non-invasive, although it can be invasive. Invasive EEG can be referred to as ECoG (ElectroCorticoGraphy) when electrodes are placed specifically on the cortex of the brain or, more generally, as iEEG (intracranial EEG) or ICE (IntraCranialElectrophysiology). Because neurons communicate via electrical action potentials generated by ionic current flow differentials, each neuron acts has a little source and sink. The integrated electrical activity of millions of neurons is what is picked up in EEG. Because the power of the electrical signal drops of as a function of distance from the source, the EEG signal is (generally) dominated by surface cortical signals. Scalp EEG is characterized by its generally low spatial resolution but excellent temporal resolution. To unpack that, when the electrodes are placed non-invasively on the scalp the signal being recorded is far away from the brain source. Also, because the skull is not transparent to the brain electrical signals being measures, the signals get spatial smoothed and smeared (I've published on this before with an interesting patient group with their skulls surgically removed; see Voytek et al., 2009 for details). This makes signal source localization a mathematically intractable problem, though there are very good methods of constraining the search space to improve methods. However, because the electrical signal amplitudes can be queried at the rate limited only by the signal amplifier, brain activity can be measured on a sub-millisecond timescale.
iEEG is an invasive method that involves implanting recording electrodes directly onto or in the brain. This is done for medical and surgical reasons (usually to pinpoint the source of epileptic activity). Researchers collaborate with the patients who have had the procedure done to request to obtain the recordings from the patients' brains since the opportunity is so unique and rare. This method is so valuable because, like EEG, the temporal resolution is excellent, but because the researcher knows exactly where the electrodes are placed, the spatial resolution is vastly superior to scalp EEG.
MEG is similar to EEG, however it measures changes in the associated magnetic fields of the neurons generating electrical potentials. Because the skull is transparent to magnetic fields, however, the spatial resolution of MEG is said to rival that of fMRI while maintaining the superior temporal resolution of EEG. However, like EEG, the signal is biased toward cortical sources.
fMRI is different from EEG and MEG in that it does not measure neuronal activity directly. Rather, what is measured is the hemodynamic response, or activity-related changed in blood flow. This is usually measured as blood-oxygen level dependent (BOLD) activity. When neurons are active they utilize metabolic resources such as glucose and oxygen, so the assumption in fMRI is that task-related changes in BOLD reflect changes in the brain's oxygen use in any given region. Because I'm not an fMRI researcher, I will rather defer to an excellent post on neuroskeptic, "fMRI in 1000 words".
PET and SPECT are similar to fMRI in that, depending on the radiotracer used, it indirectly measures neuronal activity through blood flow or cerebral metabolism. As with fMRI, the assumption is that changes in metabolites reflect task-related changes in brain activity in a specific region. PET is semi-invasive in that it requires an injection or inhalation of a radioactive substance. To measure brain activity, either radioactive water, or glucose, etc. are injected into a person. As these radioactive molecules diffuse through the bloodstream they are deposited in certain regions. As the radioactive portions decay they give off gamma rays. Through coincidence detection, these gamma ray sources can be localized and activity maps reconstructed. Regions with a higher density of emitted gamma rays are assumed to represent regions of greater neuronal activity.
NIRS is another method of functional neuroimaging, unfortunately I'm not very familiar with it, so I defer to wikipedia though I cannot vouch for its accuracy.
TMS and TDCS can be utilized for neuroimaging in a sense. For example, repetitive TMS (rTMS) can be used to alter cortical activity in a small patch of cortex. rTMS is referred to as applying a "virtual lesion" in that the small stimulated region of cortex stops working as efficiently for a few minutes. If there are behavioral changes associated with the application of rTMS then researchers can infer function associated with that region.
Any technique that can measure the specific concentration, usage, or flow of a specific neurochemical would full under the auspice of chemical neuroimaging.
PET can also be used in chemical neuroimaging. Rather than injecting radioactive versions of neuronal metabolites researchers can inject radioactive versions of neurotransmitters or other substances. For example, if one wanted to specifically examine the functional changes in dopamine activity, then radioactive dopamine can be injected.
Various methods of MRI are also capable of chemical neuroimaging, however I am less familiar with those, as well.