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PSYB65H3 (479)
Chapter 6

Chapter 6

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Zachariah Campbell

Chapter 6: Imaging the Brain’s Activity • Angelo Mosso (1846– 1910) was the first to experiment with the idea that changes in the flow of blood in the brain might provide a way of assessing brain function during mental activity. Mosso knew that, in newborn children, the fontanelles— the soft areas on a baby’s head where the bones of the skull are not yet fused — can be seen to pulsate with the rhythm of the heartbeat. • Electrical recording methods detect changes in the electrical activity of neurons. • Brain stimulation methods induce changes in the electrical activity of the brain. • X- ray imaging methods are sensitive to the density of different parts of the brain, the ventricles, nuclei, and pathways. • Dynamic imaging methods record and manipulate ongoing changes in brain activity, including the electrical activity of cells, biochemical events, differences in glucose consumption, and the flow of blood to various regions. Recording The Brain’s Electrical Activity • The techniques for recording the brain’s electrical activity include ( 1) single- cell recording; ( 2) electroencephalographic recording; and ( 3) event-related potential recording. • Action potentials are the currency with which the brain operates. The sensation of a mosquito landing on your arm is conveyed from one neuron to the next in the form of action potentials: somatosensory neurons convey action potentials to the spinal cord, and spinal neurons convey them to the cortex. In the cortex, action potentials record the perception that a mosquito is on your arm. When the cortex instructs the hand to swat at the mosquito, it sends the message in the form of action potentials. • Single- cell recordings at different levels show that ganglion cells and LGB cells respond only to dots of light, whereas the cells in the primary visual cortex respond to bars of light of specific orientation. Cells in higher visual areas respond to more- complex stimuli, including the position and movement of objects, and perhaps even to the specific features of the face such as “ Halle Berry” or “ Grandmother.” In some way, the visual cortex takes information encoded as dots by numerous cells and bars in fewer cells and translates it into the complex, ongoing visual experience that tells us the “ look” of our world. Electroencephalographic Recording • A simple technique for recording the electrical activity of large regions of the human brain was developed in the early 1930s by German physiologist Hans Berger. He found that voltage fluctuations, or “ brain waves,” could be recorded by placing the leads from a voltmeter onto the skull. These recordings, called electroencephalograms (electro, for “ electrical,” encephala, for “ brain,” and grams, for “ graphs”) or EEGs, are a valuable tool for (1) studying sleep, (2) monitoring the depth of anaesthesia, (3) diagnosing epilepsy and brain damage, and (4) studying normal brain function. • Polygraph: Apparatus for simultaneously recording blood pressure, pulse, and respiration, as well as variations in electrical resistance of the skin; popularly known as a lie detector. • The neurons of the neocortex are arranged in horizontal layers, and a substantial part of the EEG signal comes from the large pyramidal neurons of layers V and VI. Pacemaker cells ensure that these neurons undergo graded potentials at the same time, presumably so that they can synchronize their action potentials. The signal recorded by the EEG consists of the rhythmical graded potentials on many thousands of neurons. • Volume conducted: Refers to electrical potential re-corded in tissue at some distance away from its source. • Beta rhythm: Irregular electroencephalographic activity ranging from 13 to 30 Hz and generally associated with an alert state. • When a person is calm and resting quietly, especially with eyes closed, the rhythmical brain waves often emerge. These so- called alpha ( a) waves are extremely rhythmical but with waxing and waning amplitude and a frequency of approximately 11 cycles per second. • Some forms of epilepsy, called petit mal (from the French words mean-ing “ little bad”) epilepsy, are generally associated with brief losses of consciousness, perhaps lasting only a few seconds. Other forms of epilepsy may be associated with a loss of memory lasting for many minutes. Still other forms, called grand mal (meaning “ big bad”) epilepsy, are characterized by convulsions of the body, falling down, and loss of consciousness. Event –Related Potentials • Event- related potentials, or ERPs, are brief changes in a slow- wave EEG signal in response to a discrete sensory stimulus. An ERP is not easy to detect, be-cause the signal is “ hidden” in the EEG. The ERP, which consists of a graded potential generated by the sensory stimulus of interest, is mixed with many other electrical signals and so is impossible to spot just by visually inspecting an EEG. One way to detect an ERP is to produce the stimulus repeatedly and average the recorded responses. Averaging tends to cancel out any irregular and unrelated electrical activity, leaving only the graded potentials generated by the stimulus event. • Readiness potential: Evoked potential that occurs just before a movement. Magnetoencephalography • Neural activity, by generating an electrical field, also produces a magnetic field. Although the magnetic field produced by a single neuron is extremely small, the field produced by many neurons is sufficiently strong to be recorded on the surface of the skull. Such a record is called a magnetoencephalogram (MEG), and it is the magnetic counterpart of the EEG or ERP. • Calculations based on MEG measurements not only provide a description of the electrical activity of neurons but also permit a three- dimensional localization of the cell groups generating the measured field. Magnetic waves being conducted through living tissue undergo less distortion than electrical signals do, and so an MEG can have a higher resolution than an ERP. Thus, a major advantage of the MEG over the EEG and ERP is its ability to more precisely identify the source of the activity being recorded. • The heart of a magnetoencephalogram probe is a sensing device containing the special superconducting coils needed to detect the brain’s very weak magnetic fields. This so- called SQUID (superconducting quantum interference device) is immersed in liquid helium to keep it at the low temperature necessary for superconductivity. One or more probes are moved across the surface of the skull, sending signals to the SQUID. • Parkinson’s disease is characterized both by tremors and by akinesia, an absence or poverty of movement. When electrodes are implanted in the brain so that deep brain stimulation (DBS) can be applied to a number of brainstem regions, both tremors and akinesia are lessened. Transcranial Magnetic Stimulation • The relation between magnetism and electricity forms the basis of transcranial magnetic stimulation (TMS), a noninvasive method that allows the brain to be stimulated through the skull. In the TMS technique, a small wire coil is placed adjacent to the skull. A high voltage current is passed through the coil in pulses as rapid as 50 times per second. • Transcranial magnetic stimulation was originally used by neurosurgeons to stimulate brain tissue to monitor its functional condition during brain surgery and to identify the function of the tissue. From this initial use, it became clear that TMS did not harm tissue, even after thousands of pulses of stimulation, and so could be used to stimulate the normal brain through the skull. X-Ray Imaging Techniques Conventional Radiograph • The first method for producing a visual image of the brain, conventional radiography, consists of passing X- rays through the skull onto an X- ray- sensitive film. As the X- rays travel through the head, they are absorbed to different degrees by different tissues: to a great degree by dense tissue such as bone, to a lesser degree by neural tissue, and less still by fluid such as that in the blood vessels and ventricles. Pneumoencephalography • Pneumoencephalography (literally, air– brain graph) is a method for enhancing conventional X- ray ra
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