Information about the structure and function of the human brain comes from a variety of sources. Most information about the cellular components of the brain and how they work comes from studies of animal subjects, using techniques described in the brain article. Some techniques, however, are used mainly in humans, and therefore are described here.
By placing electrodes on the scalp it is possible to record the summed electrical activity of the cortex, in a technique known as electroencephalography (EEG). EEG measures mass changes in population synaptic activity from the cerebral cortex, but can only detect changes over large areas of the brain, with very little sensitivity for sub-cortical activity. EEG recordings can detect events lasting only a few thousandths of a second. EEG recordings have good temporal resolution, but poor spatial resolution.
Apart from measuring the electric field around the skull it is possible to measure the magnetic field directly in a technique known as magnetoencephalography (MEG). This technique has the same temporal resolution as EEG but much better spatial resolution, although not as good as fMRI. The greatest disadvantage of MEG is that, because the magnetic fields generated by neural activity are very weak, the method is only capable of picking up signals from near the surface of the cortex, and even then, only neurons located in the depths of cortical folds (''sulci'') have dendrites oriented in a way that gives rise to detectable magnetic fields outside the skull.
Structural and functional imaging
There are several methods for detecting brain activity changes by three-dimensional imaging of local changes in blood flow. The older methods are SPECT and PET, which depend on injection of radioactive tracers into the bloodstream. The newest method, functional magnetic resonance imaging (fMRI), has considerably better spatial resolution and involves no radioactivity. Using the most powerful magnets currently available, fMRI can localize brain activity changes to regions as small as one cubic millimeter. The downside is that the temporal resolution is poor: when brain activity increases, the blood flow response is delayed by 1–5 seconds and lasts for at least 10 seconds. Thus, fMRI is a very useful tool for learning which brain regions are involved in a given behavior, but gives little information about the temporal dynamics of their responses. A major advantage for fMRI is that, because it is non-invasive, it can readily be used on human subjects.
Effects of brain damage
A key source of information about the function of brain regions is the effects of damage to them. In humans, strokes have long provided a "natural laboratory" for studying the effects of brain damage. Most strokes result from a blood clot lodging in the brain and blocking the local blood supply, causing damage or destruction of nearby brain tissue: the range of possible blockages is very wide, leading to a great diversity of stroke symptoms. Analysis of strokes is limited by the fact that damage often crosses into multiple regions of the brain, not along clear-cut borders, making it difficult to draw firm conclusions.
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