Neuroelectical and Imaging Techniques PDF

# Neuroanatomy and Biological Process ## Neuroelectrical Techniques * Nerve cells, like all living cells, maintain an *electrical charge* across their outer membrane. * Since the electrical signals produced by nerve cells are comparatively *small*, they must be amplified before they can be accurat...

# Neuroanatomy and Biological Process ## Neuroelectrical Techniques * Nerve cells, like all living cells, maintain an *electrical charge* across their outer membrane. * Since the electrical signals produced by nerve cells are comparatively *small*, they must be amplified before they can be accurately measured. Today, this is accompanied by using electronic amplifiers, much like those employed in *home audio equipment*. * The *size* and *placement of the electrodes* determine what aspects of neural activity will be recorded. * Very large electrodes reflect the activity of larger populations of nerve cells: smaller ones can record more *localized neuroelectric events*. ### The Electroencephalogram: A technology for studying brain activity through recordings from electrodes placed on the scalp. When such a recording is obtained from electrodes placed directly on the brain surface, usually during neurosurgery, the measure is called the *electrocorticogram* (*ECoG*). EEG recordings measure the activity of large numbers of cells, known as a *field potential*. * Hans Berger made the first EEG recording, shown here. * Berger observed that his recordings varied with *wakefulness*, *sleep*, *anesthesia*, and *epilepsy*. * Several patterns of EEG activity are termed *alpha*, *beta*, *theta*, and *delta* - they differ in their frequency and amplitude. * The waking human EEG is characterized by an alteration between two patterns: * *alpha activity* - a rhythmic, high-amplitude, 8 to 12 Hz pattern, * *beta activity*: a low-voltage tracing at more than 13 Hz. * *Theta activity* is between 5 and 7 Hz and typically is of medium amplitude. * The EEG is generated primarily by the activity of large numbers of nerve cells within the brain. Because the skull, which encloses the brain beneath the scalp, is an *electrical insulator*, it is impossible, under most circumstances, to conclude which portion of the brain is generating any particular part of the EEG signal. * *Computerized analysis* of EEG recordings can be used to generate *animations* of activity over time and for the *construction of three-dimensional maps* of brain activity. The EEG of a sleeping human displays distinct stages: | Stage | Description | | :------ | :----------------------------------------------------------------------------------------------- | | Awake | Normal waking EEG. | | Stage 1 | Often called *drowsy*, but it isn't true sleep. | | Stage 2 | The majority of sleep time is spent in this stage. | | Stage 3,4 | This is *slow-wave sleep* and is characterized by very slow waves called *delta waves*. | | REM | *Rapid eye movement sleep* is active, like waking. | ## Magnetic Recording: * *Magnetoencephalography (MEG)* - magnetic recording of brain activity from the scalp is strictly an experimental procedure where, magnetic rather than electrical recording, is the object of interest. * The most important difference between MEG and EEG is that the skull is *electrically resistant* but *magnetically transparent*. This means that the skull gravely distorts the localization information that would otherwise be present in the scalp-recorded EEG, whereas much *localization information* is preserved in the MEG record. * *Magnetic recordings* may therefore be of significant value in localizing the source of signals produced by populations of nerve cells within the brain if its formidable technical problems can be resolved. * Recordings of the magnetic fields produced by the brain can be taken much *faster* than either fMRI or PET scans - providing a *moment-by-moment picture* of brain activity. MEG has the added advantage of being *silent*. * To record the tiny magnetic fields generated by the brain, a series of *super-cooled sensors* known as *superconducting quantum interference devices* (*SQUIDs*) are arrayed around the participant's head. ## Event-Related Potentials: * An *event-related potential* (*ERP*) is a component of the EEG that is triggered in association with *sensory*, *motor*, or *mental events*. ERPs are used extensively to study the *time course* of higher-level processes in the human brain, such as *perception* and *attention*. * ERPs are typically *small fluctuations* produced by the processing of a *sensory stimulus* or *motor events*. * For example, *young children with autism* often behave as though their hearing were impaired. When spoken to by parents or others, a child with autism often shows no reaction. By observing *evoked potentials to sound*, we can determine whether the child can actually hear. ## Microelectrode Recording: * *Microelectrodes* are very small electrodes with very small tips - they can be used to record the electrical activity of *single nerve cells*. The micropipette is filled with a conductive solution such as *potassium chloride*. * Microelectrodes may be used for either *extracellular* or *intracellular recording*. For extracellular recording, the electrode is placed near the nerve cell. In this position, it can measure the current flowing from the nerve cell into the extracellular fluid that surrounds it. For intracellular recording, the microelectrode is inserted into the interior of the nerve cell itself. ## Patch Clamps: * Nerve cells regulate their *electrical activity* by controlling small pores, or *channels*, in their outer membrane. * A patch clamp is an adaption of the glass micropipette method in which a small amount of suction is applied to the fluid-filled recording electrode. * If the tip of the electrode is placed on the outer surface of the cell membrane, a tight *mechanical* and *electrical seal* results. The result is that the electrode measures *electrical current* only from the portion of the membrane that is clamped to the electrode. This way, the *activity of individual membrane channels* can be measured. ## Imaging Techniques ### Computerized Tomography (CT) Computerized tomography (*CT)* was invented in 1972 by *Godfrey Hounsfield* and *Allan Cormack*. CT technology provided the *first high-resolution* look at a living brain. More modern CT technology allows for the construction of highly detailed three-dimensional images. CT scans provide *no information* regarding activity levels in the brain. ### Positron Emission Tomography (PET) - PET allowed researchers to observe *brain activity* for the first time. *Michael Phelps* and *Edward Hoffman* began to apply this basic technique of gamma invention to the study of brain function. - PET brain studies combine *radioactive tracers* with a wide variety of *molecules*, including oxygen, water, and drugs. - Each gamma ray resulting from the breakdown of the tracer is recorded by detectors and fed to a computer, which reconstructs the data into images. - Typically, programmers have assigned *red and yellow* to areas of high activity and *green, blue, and black* to areas of low activity. ### Magnetic Resonance Imaging (MRI) Magnetic resonance imaging, or *MRI*, is an imaging technique that provides very high-resolution structural images. It has become a standard medical diagnostic tool and a valuable research asset. *Raymond Damadian*, *Larry Minkoff*, and *Michael Goldsmith* produced the first MRI image in 1977. This imaging technology uses *powerful magnets* to align *hydrogen atoms* within a magnetic field. * *Radio frequency* (*RF*) pulses are directed at the part of the body to be imaged, producing “*resonance*", or spinning, of the hydrogen atoms. * When the RF pulses cease, the hydrogen atoms return to their natural alignment within the magnetic field. As the atoms “*relax*", each becomes a *miniature radio transmitter*, emitting a characteristic pulse that is detected by the scanner. #### Functional MRI (fMRI) A technology using a series of MRI images taken 1 to 4 seconds apart to assess brain activity. The first fMRI of the brain was conducted by *Belliveau, et. al* (1991). - The use of fMRI to track *blood flow* in the brain was previewed in the 19th century by *William James*. - MRI technology has significant advantages over both CT and PET. - It can provide images taken at any angle without any movement of the individual. - In tracking brain activity, fMRI is considered superior in both *spatial* and *temporal resolution* to PET scan.