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Last updated: Tue, Jan 14, 2025
The brain contains some 80-90 billion neurons. Some of its features are apparent with the unaided vision—the cerebral cortex, the brain stem, and the cerebellum are all visible from the exterior of the brain. Inside and underneath is another set of structures that make up the remainder of the brain stem and the limbic system.
The brain is highly organized at finer levels, and a large number of smaller structures have been identified through visual analysis, microscopic analysis, and a variety of other techniques. Although it's possible to work out which parts are connected to which anatomically through dissection and microscopic analysis, the short answer is that everything is connected to everything. As with the spinal cord, many synapses are inhibited sometimes but not at others, so this approach, the tracing of connections, has limitations.
Much of our knowledge of how the parts of the brain work together and what the different parts do is based on brain imaging and visualization techniques. Electroencephalography (EEG) is a technology that can detect and analyze electrical activity in the brain. When a neuron fires an action potential, the flow of ions into the cell is a small electrical current. When groups of neurons fire together, the current is larger. This allows electromagnetic sensors placed close to the skull to detect the firing of groups of neurons. With suitable analysis, the point of origin of the current within the brain can be detected approximately, and the timing of the current (when it starts and how long it lasts) can be measured very precisely. MEG (magnetoencephalograpy) is very similar to EEG, but instead detects the magnetic forces that are created by the same small electric currents that EEG measures. EEG and MEG are similar in their overall ability to detect the timing and location of the currents, but because the cortex is so convoluted, one may be more sensitive than the other for activity in different parts of the cortex.
The principal imaging technologies used in research into brain function are functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). MRI works by exciting tissue with magnetic waves and reading the response. Magnetic waves excite tissue because the water molecules that make up most of your tissue have an unevenly distributed electrical charge. They vibrate, and when they vibrate they emit electromagnetic waves. Different tissues contain different amounts of water, which allows the tissue structure to be seen after the electromagnetic waves have been appropriately processed and analyzed. The fMRI used in functional studies of the brain is different from the MRI routinely used by doctors in that it is able to detect the amount of blood flowing through different tissues. The blood flow is a measure of how much energy is being used by an area of tissue.
fMRI can peer into the innermost parts of the brain. It has a time resolution of a small number of seconds and a spatial resolution of tens of cubic millimeters. The time resolution of several seconds means that the technique can reveal the average activation of an area over a period of several seconds, but not a shorter period. This means that short-lived brain processes can't be seen using fMRI. There may be a hundred million synaptic connections in a single cubic millimeter.1 This of course limits the conclusions that can be drawn from the imaging results.
PET studies use a slightly different technology. The positrons that are emitted are the result of radioactive decay of tracer substances injected into the subjects. This technology provides advantages for certain types of studies. For example, if the researchers are interested in a particular molecule, they can make radioactive versions of the molecule, inject them into the subject's bloodstream, and use PET imaging to find out where in the brain these molecules are. Among the disadvantages of PET are that the study of a particular person must be done quickly, and it is not safe to perform the study repetitively with the same subject. PET studies have a temporal resolution of tens of seconds, meaning that it is not able to track changes that happen more quickly than that, or in fact to detect any quick brain processes.
Much of our knowledge of the function of different brain structures is based on creating a situation of interest, for example, submitting subjects to a controlled pain stimulus, then creating and analyzing fMRI or PET images. By building up evidence from different experimental conditions, inferences are made and the function of a structure or region is characterized. If the amygdala is active whenever pain is felt, the amygdala is involved in pain processing. If it is active under condition B but not under condition C, this further refines our knowledge about its role.
---NEW MATERIAL---Scientists have been able in a sense to watch the brain function for the last thirty-odd years. The principal imaging technologies used in research into brain function are functional magnetic resonance imaging (fMRI) and positron emission tomography (PET).
Functional magnetic resonance imaging (fMRI) techniques can observe how much blood is flowing through different areas of the brain while an experimental subject performs a task or reacts to a stimulus. Since more blood flow is needed by areas of the brain that are working harder, these images reveal which areas were working while the subject performed the task. The images have limited time and spatial resolution, and are far from being able to read the subject's mind, but they can tell which brain areas were most involved in performing the task.
MRI works by exciting tissue with magnetic waves and reading the response. Magnetic waves affect the tissue because the water molecules that make up most of our tissue have an unevenly distributed electrical charge. They vibrate under the influence of magnetic waves, and when they vibrate they emit electromagnetic waves. Different tissues contain different amounts of water, which allows the tissue structure to be seen after the electromagnetic emissions have been analyzed.
The fMRI used in functional studies of the brain is different from the MRI routinely used by doctors in that it is able to detect the amount of blood flowing through different tissues. The blood flow is a measure of how much energy is being used by the area of tissue. fMRI can peer into the innermost parts of the brain. It has a time resolution of a small number of seconds, meaning that it can't observe changes that take less than that amount of time. It has a spatial resolution of tens of cubic millimeters. There may be a hundred million synaptic connections in a single cubic millimeter.2 This of course limits the conclusions that can be drawn from the imaging results.
There are many special forms of MRI study. Voxel-based morphometry (VBM) can be used to determine the concentration of gray matter, white matter, and cerebro-spinal fluid, voxel by voxel, throughout the brain. Diffusion MRI is a technique for measuring the diffusibility of water through brain tissues. This provides clues to the structure of brain tissues, since water diffuses directionally in fibrous tissues such as white matter. Fractional anisotropy and tractography are analysis techniques that use diffusion MRI results to make inferences about the tracts, or groups of axons, that connect parts of the brain.
Positron emission tomography or PET studies use a slightly different technology. The positrons that are emitted are the result of radioactive decay of tracer substances injected into the subject. This technology provides advantages for certain types of studies. For example, if the researchers are interested in a particular molecule, they can make a radioactive version of the molecule and use PET imaging to find out where in the brain these molecules end up. Among the disadvantages of PET are that the study of a particular person must be done quickly, and it is not safe to perform the study repetitively with the same subject. PET studies have a temporal resolution of tens of seconds, meaning that it is not possible to track changes that happen more quickly than that.
Much of our knowledge of the function of different brain structures is based on creating a situation of interest, for example, subjecting subjects to a pain stimulus, then creating and analyzing fMRI or PET images. By building up evidence from different experimental conditions, inferences are made and the function of a structure is characterized. If the amygdala is active whenever pain is felt, the amygdala is involved in pain processing. If it is active under condition B but not under condition C, this further refines our knowledge about its role.
---END NEW MATERIAL---The notion of whether fMRI is a modern phrenology is under debate....3