Early and Modern Developments
Phrenology was one of the earliest attempts to map cognitive function onto brain function. The technique involved feeling the conformation of the skull and then attributing bumps and valleys in specific areas to mental faculties and character traits. Phrenology was practiced well into the 20th century but has been discredited by modern scientific research. In its place there are four main techniques for studying the brain: neuroanatomy, electrical stimulation, single-cell recording, and the lesion technique.
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
Brain imaging techniques are used to assess different brain areas and functions. Different brain imaging techniques have different advantages. Some produce detailed three-dimensional images, whereas others reveal the brain regions active during various tasks. A computed tomography (CT) scan produces a 3D visual image of the brain constructed by using X-rays to detect differences in tissue density. High-density tissue absorbs more radiation than low-density tissue, allowing for the construction of a highly detailed image from X-rays taken from multiple angles.Positron emission tomography (PET) maps brain activity by measuring blood flow to brain regions experiencing heightened neural activity. Brain regions with the highest activity during specific cognitive tasks are the regions important to completing those tasks. A PET scan involves injecting an unstable form of oxygen (which has a half-life of 123 seconds) in the form of water into the bloodstream of a person engaged in a cognitive task. The PET scanner measures the positrons emitted as the isotope decays. A computer is used to average this activity over a number of individuals, and these averages are superimposed on a standardized brain map showing the degree of neural activity in various areas of the brain. Researchers can use PET scans to understand how brain activity differs across groups of people or changes for individuals across tasks. For example, a researcher may compare brain activity of musicians versus nonmusicians while listening to recordings of jazz improvisation to explore whether experienced musicians process the music in different parts of their brain. Or a researcher may compare brain activity for sober versus intoxicated individuals to explore how alcohol impacts brain activity.
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.