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Chapter 5

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Western University
Psychology 2220A/B
Scott Mac Dougall- Shackleton

Chapter 5: The Research Methods of Biopsychology METHODS OF VISUALIZING AND STIMULATING THE LIVING HUMAN BRAIN • Prior to the early 1970s, biopsychological research was impeded by the inability to obtain images of the organ of primary interest: the living brain • Conventional X-ray photography is next to useless for this purpose • When an X-ray photograph is taken, an X-ray beam is passed through an object and then onto a photographic plate • Each of the molecules through which the beam passes absorbs some of the radiation; thus, only the unabsorbed portions of the beam reach the photographic plate • X-ray photography is therefor effective in characterizing internal structures that differ substantially from their surroundings in the degree to which they absorb X-rays • However, by the time an X-ray beam has passed through the numerous overlapping structures of the brain • It carries little information about the structures through which it has passed • Contrast X-Rays: • Contrast X-ray techniques involve injecting into one compartment of the body a substance that absorbs X-rays either less than or more than surrounding tissue • The injected substance then heightens the contrast between the compartment and the surrounding tissue during X-ray photography • One contrast X-ray technique, cerebral angiography, uses the infusion of a radio- opaque dye into a cerebral artery to visualize the cerebral circulatory system during X-ray photography • Cerebral angiograms are most useful for localizing vascular damage, but the displacement of blood vessels from their normal position also can indicate the location of a tumor • X-Ray Computed Tomography: • Computed tomography (CT) is a computer assisted X-ray procedure that can be used to visualize the brain and other internal structures of the living body • On one side of the cylinder is an X-ray tube that projects an X-ray beam through the head to an X-ray detector mounted on the other side • The X-ray tube and detector automatically rotate around the head of the patient at one level of the brain, taking many individual X-ray photographs as they rotate • The meager information in each X-ray photograph is combined by a computer to generate a CT scan of one horizontal section of the brain • Magnetic Resonance Imaging: •MRI is a procedure in which high-resolution images are constructed from the measurement of waves that hydrogen atoms emit when they are activated by radio-frequency waves in a magnetic field • Provides clearer images of the brain than does CT • In addition to providing relatively high spatial resolution (the ability to detect and represent differences in spatial location), MRI can produce images in 3D • Positron Emission Tomography: • PET provide images of brain activity (functional brain images) rather than images of brain structure (structural brain images) • In one common version of PET, radioactive 2-deoxyglucose (2-DG) is injected into the patients carotid artery • 2-DG is rapidly taken up by active cells Chapter 5: The Research Methods of Biopsychology • However, cannot be metabolized, therefore it accumulates in active neurons - or in associated astrocytes - until it is gradually broken down • Each PET scan is an image of the levels of radioactivity in various parts of one horizontal level of the brain • Functional MRI • fMRI produce images representing the increase in oxygen flow in the blood to active areas of the brain • Possible because of two attributes of oxygenated blood •First, active areas of the brain take up more oxygenated blood than they need for their energy requirements, and thus oxygenated blood accumulates in active areas of the brain •Second, oxygenated blood has magnetic properties • The signal recorded by fMRI is called the BOLD signal (the blood-oxygen-level- dependent signal) • fMRI has four advantages over PET: 1. Nothing has to be injected into the subject 2. It provides both structural and functional information in the same image 3. Its spatial resolution is better 4. It can be used to produce 3D images of activity over the entire brain • fMRI technology is too slow to capture many neural responses - it takes 2 or 3 seconds to create an fMRI image, and many neural responses, such as action potentials, occur in milliseconds • Magnetoencephalography: • MEG measures changes in magnetic fields on the surface of the scalp that are produce by changes in underlying patterns of neural activity • Its major advantage over fMRI is its temporal resolution; it can record fast changes in neural activity • Transcranial Magnetic Stimulation: • PET, fMRI, and MEG all have the same weakness: They can be used to show a correlation between brain activity and cognitive activity, but they can’t prove that the brain activity caused the cognitive activity • TMS is a techniques for affecting the activity in an area of the cortex by creating a magnetic girls under a coil positioned next to the skull • In effect, the magnetic stimulation temporarily turns off part of the brain while the effects of the disruption on cognition and behavior are assessed • TMS is often employed to circumvent the difficulty that brain imaging studies have in determining causation RECORDING HUMAN PSYCHOPHYSIOLOGICALACTIVITY • Scalp Electroencephalography: • The EEG is a measure of the gross electrical activity of the brain • It is recorded through large electrodes by a device called and electroencephalograph (EEG machine), and the technique is called electroencephalography • The scalp EEG signal reflects the sum of electrical events throughout the head Chapter 5: The Research Methods of Biopsychology • These events include action potential and postsynaptic potentials, as well as electrical signals from the skin, muscles, blood, and eyes • Its value as a research and diagnostic tool rests on the fact that some EEG wave forms are associated with particular states of consciousness or particular types of cerebral pathology • Because EEG signals decrease in amplitude as they spread from their source, a comparison of signals recorded form various sites on the scalp can sometimes indicate the origin of particular waves • Psychophysiologists are often more interested in the EEG waves that accompany certain psychological events than they are in the background EEG signal • These accompanying EEG waves are generally referred to as event-related potentials (ERPs) • Sensory evoked potential - the change in the cortical EEG signal that is elicited by the momentary presentation of a sensory stimulus • The cortical EEG that follows a sensory stimulus has two components: the response to the stimulus (the signal) and the ongoing background EEG activity (the noise) • The signal is the part of any recording that is of interest; the noise is the part that isn’t • The problem in recording sensory evoked background EEG is often so great that the sensory evoked potential is masked • A method used to reduce the noise of the background EEG is signal averaging • First, a subjects response to a stimulus is recorded many times • Then a computer identifies the millivolt values of each of the traces at its starting point and calculates the mean of the scores • Next, it considers the value of each of the traces 1-millisecond from its start and calculates the mean of these values • It repeats this process at the 2-msec mark, the 3-sec mark and so on • When these averages are plotted, the average response evoked by the click is more apparent, because the random background EEG is canceled out by the averaging • The analysis of average evoked potentials (AEPs) focuses on the various waves in the averaged signal • Each wave is characterized by its direction, positive, or negative, and by its latency • The P300 wave is the positive wave that occurs about 300 milliseconds after a momentary stimulus that has meaning for the subject • The proportions of an evoked potential recored in the first few milliseconds after a stimulus are not influenced by the meaning of the stimulus for the subject called farfield potentials • The spatial resolution of these techniques is sufficiet to enable the amplitude of evoked EEG signals recored on the cortex to be color coded and plotted on the surface of a 3D MRI • Muscle Tension: • Muscle fiber contracts in an all-or-none fashion when activated by the motor neuron that innervates it Chapter 5: The Research Methods of Biopsychology • At any given time, a few fibers in each resting muscle are likely to be contracting, thus maintaining the overall tone (tension) of the muscle • Movement results when a large number of fibers contract at the same time • Electromyography is the usual procedure for measuring muscle tension • The resulting record is called an electromyogram (EMG) • EMG activity is usually recorded between two electrodes taped to the surface of the skin over the muscle of interest • Eye Movement: • The electrophysiological technique for recording eye movements is called electroculography, and the resulting record is called an eletroculogram (EOG) • Electroculography is based on the fact that there is a steady potential difference between the front (positive) and back (negative) of the eyeball • Skin Conductance: • Emotional thoughts and experiences are associated with increases in the ability of the skin to conduct electricity • The the most commonly employed indexes of electrodermal activity are: •Skin conductance level (SCL) - the measure of the background level of skin conductance that is associated with a particular situation •Skin conductance response (SCR) - a measure of the transient changes in skin conductance that are associated with discrete experiences • Although the main function of sweat glands is to cool the body, these glands tend to become active in emotional situations • Cardiovascular Activity: • The cardiovascular system has two parts: the blood vessels and the heart • Three different measures of cardiovascular activity are frequently employed in psychophysiological research: •Heart Rate • The electrical signal that is associated with each heartbeat can be recorded through electrodes placed on the chest • The recording is called an electrocardiogram (ECG) • The average resting heart rate of a healthy adult is about 70 beats per minute, but it increases abruptly at the sound, or thought of a dental drill •Blood Pressure • Measuring arterial blood pressure involves two independent measurements: a measurement of the peak pressure during the periods of heart contraction, the systoles. and a measurement of the minimum pressure during the periods of relaxation, the diastoles • Normal resting blood pressure for an adult is about 130/70 mmHG • A chronic blood pressure of more than 140/90 mmHG called hypertension •Blood Volume • Plethysmography refers to the various techniques for measuring changes in the volume of blood in particular part of the body • One method of measuring these changes is to record the volume of he target tissue by wrapping a strain gauge around it Chapter 5: The Research Methods of Biopsychology • Another plethysmographic method is to shine a light through the tissue under investigation and to measure the amount of the light that is absorbed by it • The more blood there is in a structure, the more light it will absorb INVASIVE PHYSIOLOGICAL RESEARCH METHODS • Most physiological techniques used in biopsychological research on laboratory animals fall into one of three categories: lesion methods, electrical stimulation methods, and invasive recording methods • Stereotaxic Surgery • Stereotaxic surgery is the means by which experimental devices are precisely positioned in the depths of the brain • Two things are required in stereotaxic surgery: an atlas to provide directions to the target site and an instrument for getting there • The stereotaxic atlas is used to locate brain structures in much the same way that a geographic atlas is used to locate geographic landmarks • In contrast to the surface of the earth, which has only two dimensions, the brain has three • Bregma - the point on the top of the skull whee two of the major sutures intersect • Stereotaxic instrument has two parts: a head holder, which firmly holds each subjects brain in the prescribed position and orientation; and an eletrode holder, which holds the device to be inserted • A system of precision gears allows the electrode holder to be moved in 3D: anterior-posterior, dorsal-ventral, and lateral-medial • Lesion Methods • In those methods, a part of the brain is removed, damaged, or destroyed; then, the behaviour of the subject is carefully assed in an effort to determine the functions of the lesioned structure • Four types of lesions: •Aspiration Lesions • When a lesion is to be made in an area of cortical tissue that is accessible to the eyes and instruments of the surgeon, aspiration is frequently the method of choice • Cortical tissue is drawn off by suction a fine-tipped handheld glass pipette •Radio-Frequency Lesions • Small subcortical lesions are commonly made by passing radio-frequency current through the target tissue from the current destroys the tissue • The size and shape of the lesion are determined by the duration and intensity of the current and the configuration of the electrode tip •Knife Cuts • Sectioning is used to eliminate conduction in a nerve or tract • A tiny, well placed cut can unambiguously accomplish this task without producing extensive damage to surrounding tissue •Cyrogenic Blockade • An alternative to destructive lesions is cyrogenic blockade Chapter 5: The Research Methods of Biopsychology •When coolant is pumped through an implanted cyroprobe neurons near the tip are cooled until they stop firing •The temperature is maintained above the freezing level, so there is no structural damage •Then, when the tissue is allowed to warm up, normal neural activity returns •A cyrogenic blockade is functionally similar to a lesion in that it eliminates the contribution of a particular area of the brain to the ongoing behavior of the subject •This is why cyrogenic blockades are sometimes referred t as reversible lesions • Interpreting Lesion Effects •Lesion effects are deceptively difficult to interpret •Because the structures of the brain are small, convoluted, and tightly packed together, even a highly skilled surgeon cannot completely destroy a structure without producing significant damage to adjacent structures • Bilateral and Unilateral Lesions •As a general principle the behavioral effects of unilateral lesions are much milder than those of symmetrical bilateral lesions, particularly in nonhuman species •Most experimental studies of lesion effects are studies of bilateral, rather than unilateral lesions • Electrical Stimulation • Clues about the function of neural structure can be obtained by stimulating it electrically • Electrical brain stimulation is usually delivered across the two tips of a bipolar electrode • Electrical stimulation of the brain is n important biopsychological tool because it often has behavioral effects • The particular behavioral response that is elicited depends on the location of the electrode tip, the parameters of the current, and the test environment in which the stimulation is administered • Invasive Electrophysiological Recording Methods • Intracellular Unit Recording • Intracellular unit recordings, provides a moment by moment record of the graded fluctuations in one neurons membrane potential • Most experiments are performed on chemically immobilized animals • Extracellular Unit Recording • Extracellular unit recording provides a record of the firing of a neuron but no information about the neurons membrane potential • Difficult to record extracellularly from a single neuron in a freely moving animal without the electrode tip shifting away from the neuron, but it can be accomplished with special flexible microelectrodes • It is now possible to simultaneously record extracellular signals from up to about 100 neurons by analyzing the correlations among the signals picked up through several different electrodes implanted in the same general area • Multiple-Unit Recording Chapter 5: The Research Methods of Biopsychology •In multiple unit recording, the electrode tip is much larger than that of a microelectrode; thus, it picks up signals for many neurons •A multiple unit recording is a graph of the total number of recorded action potentials per unit of time • Invasive EEG Recording •Cortical EEG signals are frequently recorded through stainless tell skull screws, whereas subcortical EEG signals are typically recorded through stereotaxically implanted wire electrodes PHARMACOLOGICAL RESEARCH METHODS • Routes of Drug Administration • In most psychpharmacological experiments, drugs are administered i one of the following ways: 1. They are fed to the subject 2. They are injected through a tube into the stomach 3. They are injected hypodermically into the peritoneal cavity of the abdomen into a large muscle into the fatty tissue beneath the skin • A problem with these peripheral routes of administration is that many drugs do not readily pass through the blood-brain barrier • To overcome this problem, drugs can be administered in small amounts through a fine, hollow tube, called a cannula, that has been stereotaxically implanted in the brain • Selective Chemical Lesions • The effects of surgical, electrolytic, and cyrogenic lesions are frequently difficult to interpret because they affect all neurons in the target ares • In some cases it is possible to make more selective lesions by injecting neurotoxins that have an affinity for certain components of the nervous system • Measuring Chemical Activity of the Brain • Two techniques that have proved particularly useful in biopsychological research are the 2-deoxyglucose technique and cerebral dialysis • The 2-Deoxyglucose Technique •Entails placing an animal that has been injected with radioactive 2-DG in a test situation in which it engages in the activity of interest •The subject is killed, and its brain is removed and sliced •The slices are then subjected to autoradiography; they are coated with a photographic emulsion, stored in the dark for a few days, and then developed much like film •Areas of the brain that absorbed high levels of the radioactive 2-DG during the test appear as black spots on the slides • Cerebral Dialysis •Cerebral dialysis is a method of measuring the extracellular concentration of specific neurochemicals in behaving animals •Cerebral dialysis
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