Early and Modern Developments
Neuroanatomy involves taking slices of brain tissue and inspecting them under a microscope. Using staining techniques, it is possible to identify patterns of connectivity among the cells as well as differences in cell morphology (the form, shape, size, and structure of cells). Staining creates contrast in the tissue, so details that might not otherwise be observable can be analyzed.
Using electrical stimulation of cortical tissue, the brain of a conscious individual is stimulated with a mild electric current. Because the brain has no pain pathways itself, the electrical stimulation is not painful. Instead the sensations and the actions triggered by stimulation allow identification of which areas of the brain are involved in specific cognitive, motor, and perceptual functions. For example, stimulation of the somatosensory cortex corresponding to the fingers may cause a tingling sensation in the fingers, while stimulating the motor cortex may cause the fingers to twitch.
In single-cell recording, a thin electrode is inserted into the brain in close contact with a single cortical cell (a cell in the cortex of the brain). The electrical activity of the cell is recorded while the subject is exposed to various stimuli, such as different types of light. The lesion technique involves severing or otherwise damaging specific areas of the brain and then observing which functions are disrupted.
Modern developments in studying the brain have changed assumptions about brain structure and function. For example, neurogenesis is the process by which new neurons are formed in the brain. Scientists once believed that neurogenesis occurred only during embryonic development. Modern research has revealed that it continues in some brain regions throughout a person's life span. The organization of brain circuitry also changes throughout life as a function of experience, a property called brain plasticity. Brain plasticity can be influenced by a number of factors, including maturation, aging, diet, life events, drugs, hormones, disease, and stress. Better scientific understanding of brain plasticity may lead to new techniques that may help people recover from brain injuries and diseases.
Brain Imaging Techniques
An electroencephalogram (EEG) is a recording of the brain's electrical activity made by placing electrodes on the scalp. When large populations of neurons are active together, they produce electric potentials that are large enough to be recorded from outside the body. The patterns of electrical activity (called an EEG signature) differ depending on whether the person is awake, drowsy, relaxed, excited, or in various stages of sleep.
In event-related potential (ERP), EEG traces from a series of trials are averaged together by aligning the records in reference to an external event, such as the onset of a signal or response. The result is called an ERP. Two ERP types that have been studied are P300, a valley that occurs about 300 ms after the presentation of an unexpected or unusual stimulus, and N400, a peak that occurs about 400 ms after such a stimulus.
Transcranial magnetic stimulation is a powerful, noninvasive method of brain stimulation that allows the researcher to interfere with normal neural activity. Repetitive TMS (rTMS) refers to the application of a brief train of pulses. Effects on cognitive tasks are usually disruptive, which is why TMS is sometimes called a "virtual lesion" method. This mechanism effectively prevents the continuation of ongoing neural activity that might be relevant for task performance, allowing the precise location of the brain regions involved in the task to be identified.