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Chapter 5

Chapter 5 Notes.docx

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PSYC 271
Richard Beninger

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Cindy Zhu, PSYC271 Fall 2011 Page 1 of11 Chapter 5: Research Methods of Biopsychology 5.1 Methods of Visualizing and Stimulating the Living Human Brain • X-ray photography: X-ray beam is passed through an object and then onto a photographic plate, and can see structures that differ substantially from their surroundings in the degree to which they absorb X-rays. However, for the numerous overlapping structures of the brain which are similar in their ability to absorb X-rays, quite useless. Contrast X-Rays • Contrast X-ray techniques: inject a substance that absorbs X-rays Cerebral less or more than surrounding tissue. Injected substance increases angiography contrast during X-ray photography. • Cerebral angiography uses a radio-opaque dye in a cerebral artery to visualize the cerebral circulatory system, and look for vascular damage. X-Ray Computed Tomography • Computed tomography (CT): computer-assisted X-ray procedure. Patient lies with head positioned in the centre of a large cylinder, with an X-ray source and an X-ray detector opposite each other, which rotates around the head and takes multiple X-ray photographs around each level of the brain. The X-ray photographs are combined by a computer to generate a horizontal section of the brain. • Process is repeated for all levels of the brain, combined to form a 3D representation. Magnetic Resonance Imaging • Magnetic resonance imaging (MRI): Magnetic field aligns hydrogen atoms which are then excited by radio waves, causing them to spin; when they return to the alignment imposed by the magnetic field, they emit radio frequencies which are recorded, and used to reconstruct intensity and origin of signals depending on differential densities of brain parts. These provide clearer images than CT with high spatial resolution, and can also produce 3D images (structural Midsaggital MRI MRI). scan Positron Emission Tomography • PET was the first brain-imaging technique to provide images of brain activity (functional) rather than images of brain structure. • Radioactive 2-deoxyglucose (2-DG) is injected into the carotid artery, and is taken up by brain cells but cannot be metabolized; it accumulates in active neurons that mistake it for a glucose energy source. The PET scan looks at the level of radioactivity (gamma rays) in each horizontal level PET scan showing of the brain, and with superimposing on a brain image can activity for subject pinpoint the areas activated. Other isotopes, with short doing a certain task half-lives. Cindy Zhu, PSYC271 Fall 2011 Page 2 of11 Functional MRI • Functional MRI (fMRI): produces images representing functional increase in oxygen flow in the blood to active areas of the brain. Uses two attributes of oxygenated blood: 1) active areas of brain take up more oxygenated blood than they need, and so it accumulates; 2) oxygenated blood has magnetic properties by influencing magnetic fields on iron in blood. • BOLD signal: the blood-oxygen-level-dependent signal recorded by fMRI. • Advantages over PET: 1) no injection required; 2) same structural and functional information; 3) better spatial resolution; 4) can produce 3D images of activity over entire brain • However, still has flaws: relation between BOLD signal and neural activity not that straightforward, and fMRI can’t capture rapid neural responses. Magnetoencephalography • Magnetoencephalography (MEG): measures changes in magnetic fields on surface of the scalp, produced by changes in underlying patterns of neural activity. • Advantage: temporal resolution, ability to record fast changes in neural activity Transcranial Magnetic Stimulation • PET, fMRI and MEG can all show correlation between brain activity and cognitive activity, but they cannot prove that brain activity caused the cognitive activity. • Transcranial Magnetic Stimulation (TMS): affects activity in an area of the cortex by creating a magnetic field next to the skull, so that the targeted part of the brain temporarily turns off, and the disruptive effects on cognition and behaviour can be assessed. 5.2 Recording Human Psychophysiological Activity • Recording physiological activity from the surface of the human body Scalp Electroencephalography • Electroencephalography: electroencephalogram (EEG) measures gross electrical activity of the brain by using electrodes taped to the scalp. Can study specific EEG wave form associations to particular states of consciousness or particular types of cerebral pathology. • Alpha waves: 8-12 per second high- amplitude waves associated with relaxed wakefulness. • Since EEG signals decrease over distance, we can infer the origin of particular waves by recording from various sites. • Event-related potentials (ERPs): EEG waves that accompany certain psychological events, such as the sensory evoked potential – change in cortical EEG signal elicited by momentary presentation of a sensory stimulus. • We must distinguish the response to the stimulus (signal) from the ongoing background EEG activity (noise). A way to reduce background EEG noise is signal averaging: Cindy Zhu, PSYC271 Fall 2011 Page 3 of11 taking many data points of a response signal and then averaging the EEG millivolts value at each data point; the random background noise is cancelled out, leaving the average evoked potentials (AEPs). • Each wave in the averaged signal is characterized by its direction (positive or negative) and its latency. E.g. P300 wave is a positive wave 300 milliseconds after a meaningful stimulus is presented. • Far-field potentials: evoked potentials in the first few milliseconds after a stimulus is presented, which are not influenced by the meaning of the stimulus, instead originating far away in the sensory nuclei of the brain stem. Muscle Tension • Electromyography (EMG) measures muscle tension (the number of muscle fibers contracting) as an indicator of psychological arousal. EMG activity is recorded between two electrodes on the skin surface over the muscle of interest. • Raw EMG signal is calculated to give the total amount of EMG spikes per time unit (e.g spikes/0.1s), and the integrated signal is then plotted to give a simple, continuous measure of muscle tension. Eye Movement • Electrooculography (EOG): based on fact that there is a steady potential difference between the front (positive) and back (negative) of the eyeball. • When the eye moves, a change in electrical potential between electrodes placed around the eye can be recorded – two electrodes on either side of the eye for horizontal movements, and two above and below for vertical movements. Skin Conductance • Emotional thoughts and experiences linked to increases in ability of skin to conduct electricity. This is probably due to sweat glands, which become active in emotional situations. • Skin conductance level (SCL): measure of background level of skin conductance associated with a particular situation. • Skin conductance response (SCR): measure of transient changes in skin conductance associated with discrete experiences. Cardiovascular Activity • Heart rate: electrical signal associated with heartbeat is recorded through electrodes on the chest for an electrocardiogram (ECG). Average is about 70 beats per minute. • Blood pressure: Systoles are peak pressures during heart contraction, while diastoles are minimum pressures during heart relaxation. Blood pressure is expressed as systolic/diastolic, in mmHg; normal blood pressure is about 130/70 mmHg, while over 140/90 is hypertension. Measured with a sphygmomanometer. • Blood volume: Plethysmography are techniques for measuring changes in volume of blood in a particular part of the body, in association with psychological events. Methods include recording volume by wrapping a strain gauge around target (e.g. finger), and shining light through the tissue and measuring its absorbance (more blood absorbs more light). 5.3 Invasive Physiological Research Methods • Invasive, direct techniques employed in studies of laboratory animals. Stereotaxic Surgery • Stereotaxic surgery: method by which experimental devices are precisely positioned in the depths of the brain. Cindy Zhu, PSYC271 Fall 2011 Page 4 of 11 • Stereotaxic atlas: used to locate brain structures, with a series of individual maps of 2D frontal sections. Distances are given in millimetres from a designated reference point, such as the bregma, the point on the top of the skull where two of the major sutures (seams in the skull) intersect. • Stereotaxic instrument: has a head holder, that firmly holds the subject’s brain in the right position and orientation, and an electrode holder that holds the device to be inserted. Precision gears allow the electrode holder to be moved along the anterior-posterior, dorsal-ventral, and lateral-medial axes. Lesion Methods • A part of the brain is removed, damaged, or destroyed, and the behaviour of the subject is assessed post-surgery to determine the functions of the lesioned structure. • Aspiration lesions: cortical tissue drawn off by suction through a pipette (white matter and major blood vessels undamaged), made when target area is accessible to surgeon’s eyes and instruments • Radio-frequency lesions: Subcortical lesions made by passing radio-frequency current through target tissue from the tip of an electrode, where the heat from the current destroys the tissue. Size and shape of lesion are determined by duration and intensity of the current. • Knife cuts: Sectioning to eliminate conduction in a nerve or tract. • Cryogenic blockade: Coolant is pumped through an implanted cryoprobe, and neurons near the tip are cooled until they stop firing; temperature is above freezing so there is no structural damage, and normal neural activity returns when the tissue warms up. Sometimes known as reversible lesions. • Interpreting lesion effects: Lesions often causes significant damage to adjacent structures of the intended target, since brain structures are so small, convoluted and tightly packed together. This can lead to misleading interpretations – idea that e.g. “amygdala lesions” effects only attributed to amygdala damage (and none to other structures), or idea that “amygdala lesions” include the entire amygdala and therefore behaviours uninfluenced by the lesion are not affected by the amygdala. • Bilateral and unilateral lesions: Unilateral lesions are restricted to one half of the brain, and tend to be milder than those of bilateral lesions where lesions involve both sides of the brain. Electrical Stimulation • Delivered across the tips of a bipolar electrode, which sends weak pulses of current that immediately produce an increase the firing of neurons at the tip of the electrode. • Often behavioural effects are opposite to that of a lesion, and can elicit sequences like eating, drinking, sleeping, depending on location of stimulation. Invasive Electrophysiological Recording Methods • • Moment-by-moment record of graded • fluctuations in a neuron’s membrane • potential. • • • Cindy Zhu, PSYC271 Fall 2011 Page 5 of11 • • Electrode in ECF outside neuron, can’t Cortical: stainless steel skull screws • Subcortical: stereotaxically implanted wire record its membrane potential. electrodes • • • • 5.4 Pharmacological Research Methods • How psychopharmacologists manipulate and record from the brain using chemical methods: main strategy is to administer drugs that increase or decrease the effects of particular NTs and to observe the behavioural consequences. Routes of Drug Administration • 1) Fed to subject; 2) injected through a tube into the stomach (intragastrically); 3) injected hypodermically into the peritoneal cavity of the abdomen (intraperitoneally, IP), into a muscle (intramuscularly, IM), into the fatty tissue beneath the skin (subcutaneously, SC) or into a large surface vein (intravenously, IV). • Problem that drugs do not pass the blood-brain barrier: administered in a cannula, a fine hollow tube stereotaxically implanted in the brain. Selective Chemical Lesions • More selective lesions may be possible by injecting neurotoxins with affinity for certain parts of the nervous system. For example, kainic acid and ibotenic acid are taken up by cell bodies at the site of canula injection, and destroys them, but not other neurons with axons in the area. • 6-hydroxy-dopamine (6-OHDA) is taken up only by neurons that release norepinephrine or DA. Measuring Chemical Activity of the Brain • 2-deoxyglucose technique: an animal injected with radioactive 2-DG is put in a test situation that engages it in the activity of interest, so neurons active during the test absorb the 2-DG. • The subject’s brain is then removed and sliced, and subjected to autoradiography, where they are coated with a photographic emulsion and then developed. Areas of the brain that absorbed high levels of radioactive 2-DG appear as black spots, and the density of spots in various brain regions is coded. • Cerebral dialysis: Measures extracellular concentrations of specific neurochemicals in behaving animals. Involves implantation of a fine tube with a semipermeable section, into which extracellular chemicals from the structure of interest can diffuse, and be collected for chemical analysis. Locating Neurotransmitters and Receptors in the Brain • Immunocytochemistry: Locating particular neuroproteins in the brain by labelling their specific antibodies with a dye or radioactive element, exposing brain tissue to the labelled Cindy Zhu, PSYC271 Fall 2011 Page 6 of 11 antibodies, and then looking at regions of antibody accumulation that mark the locations of the target neurprotein. • Can be used to locate NTs by binding to their enzymes, which are proteins that synthesize that particular NT for the neuron. • In situ hybridization: Insert hybrid strands of mRNA with sequence complementary to that of the mRNA that directs synthesis of the target neuroprotein, labelled with a dye or radioactive element; expose brain tissue to labelled hybrid RNA; after they bind to the complementary mRNA strands, mark location of neurons that release the target neuroprotein. 5.5 Genetic Engineering Gene Knockout Techniques • Gene knockout techniques create organisms that lack the particular gene under investigation, to research neural mechanisms of behaviour. • Melapnopsin knockout mice (gene for synthesis of melanopsin deleted, a protein in retinal neurons) used to study role of melanopsin in regulating light-dark cycles that control circadian rhythms. Found knockout impaired ability to adjust circadian rhythms in response to changes in light-dark cycle. Gene Replacement Techniques • Gene replacement techniques: transgenic mice with genetic material of another species. • Transgenic mice with defective gene associated with schizophrenia displayed cerebral abnormalities and abnormal behaviours reminiscent of human schizophrenia – confirmed this gene was a causal factor in schizophrenia in this family. • Another variation: gene is replaced with an identical one except for addition of a few bases, which act as a switch turning that gene on or off in response to certain chemicals. The chemicals can then be used to activate or suppress the gene at points of development. Fantastic Fluorescence and the Brainbow • Green fluorescence protein (GFP): a jellyfish protein that gives bright green fluorescence under blue light. GFP is activated in cells under investigation so they can be easily visualized – by inserting gene only into target cells, or inserting it into all cells but only expressing it in the targets. • GFP gene can be altered to synthesize proteins with different colour fluorescence – with each colour marking a separate protein, can see different neurons with distinct colours due to h
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