Cherish Madu
This wiki briefly explores the methods that those interested in neuroscience most frequently employ when investigating and explaining cognitive processes in terms of brain structure and function. The advantages and disadvantages of each method will also be discussed.
Transcranial Magnetic Stimulation
Overview, Advantages, Disadvantages
Optogenetics
Overview
Optogenetics combines techniques from genetics and optics to deepen our understanding of neural circuitry and how it relates to behavior. Optogenetics is a tool that makes use of microbial opsins that can be activated by light. For the most part, viruses are engineered with specific promoters that drive expression of ospin genes and are injected into mice or rats.
Advantages
Disadvantages
Electroencephalography
Overview
Electroencephalograpy (EEG) recordings measure electric activity in the brain through surface electrodes that are placed along the scalp. These surface electrodes measure voltage, and the difference in voltage between the surface electrodes and a reference electrode (placed elsewhere on the head) are amplified, digitized, and then recorded (CITE 8). EEG does not measure the elecric potential of a single neuron (CITE 9). Instead it measures the dendritic field potentials of groups of neurons. In this way, the signal becomes large enough for EEG to detect. Event-related potentials (ERPs) are extracted from the EEG signal to relate electrical activity to neural function. ERPs can do this because they have the advantage of linking electrical activity with time.
EEG has many functional uses in assessing the global state of the brain. Clinically, EEG is used to differentiate epileptic seizures from fainting and movement disorders (CITE 2). EEG is also used to diagnose coma and brain death.
Advantages
EEG is a non-invasive technique as it does not require surgery or the injection of hazardous materials. ERPs also have excellent temporal specificity, within milliseconds (CITE 33).
Disadvantages
EEG has poor spatial resolution because it records mainly from the most superficial layers of the cortex. Neurons within sulci, as opposed to within gyri, or other sub-cortical layers are not detected using EEG.
Positron Emission Tomography
Overview
Positron emission tomography (PET) is a three-dimensional brain imaging technique that takes advantage of changes in metabolism to localize brain activity and functional processes in the body. The most active parts of the brain use more metabolic processes than do relatively inactive parts of the brain and PET is able to track this metabolic flow. Flourine-18 or oxygen-15, the most commonly used radioactive tracer isotopes, is injected into the bloodstream where it disperses to more active parts of the brain. Inside the brain, the radioactive tracer decays into a positron and an electron. When the positron collides with an electron, two gammas rays are produced. The accumulation of these pairs of gamma rays is measured by gamma-ray detectors which are placed all around the subject's head. Computer analyses construct the radioactive tracer concentration into three-dimensional images. PET is often combined with CT or MRI scans for superimposition of the images of concentration onto anatomic images of the subject's brain.
PET is not only used in neuroimaging. It also has applications in oncology and pharmacology. Flourodeoxyglucose (FDG) is primarily used in oncology because of its ability to track glucose metabolism in cancer tissues. Flourodeoxyglucose flourine-18 (FDG-PET) is also used to detect patients at risk for stroke in cardiology.
Advantages
PET has the advantage of having high spatial resolution (approximately a centimeter). PET is also used in humans and can be used to study higher-order functioning that is relevant to our population.
Disadvantages
Radioactive isotropes need to be created by cyclotrons either on-site or near the hospitals using PET. This process is very costly. Moreover, because the positron has to travel before it collides with an electron to produce gamma-rays, temporal resolution is very low. It takes close to a minute for the concentration of gamma-rays to accumulate into a reliable signal. Also, PET is a somewhat invasive procedure because it requires the injection of radioactive molecules into the bloodstream. However, the dose of radiation is reduced in isotopes with short half-lives.
Functional Magnetic Resonance Imaging
Overview
Like PET, Functional magnetic resonance imaging (fMRI) results in the imaging of active brain locations. FMRI, however measures blood flow using the blood-oxygen-level-dependent (BOLD) contrast (CITE). BOLD contrasting takes advantage of the fact that oxyhemoglobin (oxygen-rich) and deoxyhemoglobin (oxygen-poor) have differing magnetic resonance signals, hence the contrast. Functioning areas of the brain require more oxygen and thus more blood flow that non-active parts of the brain.
FMRI has spurred the creation of companies that market lie-detectors based upon the imaging technique. Such companies claim that the pre-frontal cortex (PFC) is more active when individuals are planning to or contemplating a lie (CITE 64).
Advantages
FMRI is a fairly safe technique as it does not require the injection of radioactive materials, unlike PET. Also, the spatial resolution of fMRI is higher than PET, at a few millimeters. FMRI also delivers relatively high temporal resolution (a few seconds), although it does not reach the standards set by optogenetics.
Disadvantages
FMRI is an extremely costly technique because magnetic resonance scanners alone can cost upwards of $3,000,000 depending on tesla level.
References
Huettel, S. A.; Song, A. W.; McCarthy, G. (2009), Functional Magnetic Resonance Imaging (2 ed.), Massachusetts: Sinauer, ISBN 978-0-87893-286-3