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Chapter 6

PSYB65 Chapter 6.docx

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Zachariah Campbell

PSYB65 – Chapter 6 Recording the Brain’s Electrical Activity: - Techniques for recording the brain’s electrical activity include: o Single-cell recording o Electroencephalographic recording o Event-related potential recording Singe-Cell Recording: - An electrode is inserted directly into an animal’s brain, adjacent to a single neuron o The neuron’s electrical activity is recorded on a computer, thus supplying information about the activity of that neuron o Most experiments must be done with nonhuman animals since it places the electrodes directly on the brain tissue - Some animal species are preferable to others for studying given behaviours, examples: o Investigators favor nonhuman primates and cats for recording the single-cell activity of visual functions  These species have excellent vision o Barn owl used for studying auditory function  They have excellent hearing and it automatically orients its head to locate the sound of its prey o Rats used for recording single-cell activity associated with spatial behaviour  They are small enough to be physically active in a limited space - Miniaturization, computerization and arrays of as many as 50 thin wires forming an electrode allow the recording of many individual neurons simultaneously o Techniques developed to identify specific neurons so that their activity can be followed for long periods of time, examples:  An electrode that is close to a neuron will provide a large amplitude signal of that neuron’s activity  An electrode a little farther away will provide a smaller amplitude signal produced by the same activity • Ratio of the amplitude of the 2 signals provide a unique signature, that allows researchers to monitor the ongoing activity of that specific neuron - Graphs for single-cell studies are usually drawn with x-axis scaled on the order of seconds o This allows researchers to correlate the serial action potentials produced by a given neuron with the ongoing behaviour of the animal - Action potentials are the currency with which the brain operates o Somatosensory neurons convey action potentials to the spinal cord, and spinal neurons convey them to the cortex o They represent sights, sounds, smells, tastes, sensations of pain and temperature, and our desires and emotions The Neuronal Code: - Neurons exhibit many firing patterns in different animal species o Some discharge at a steady rate that appears unrelated to behaviour o Others fire in bursts in association with an observable behaviour o Others hardly ever discharge at all - Many neurons exhibit a rhythmical discharge that is in some way related to breathing or heart rate. - James Ranck – recorded the activity of single neurons in the limbic region of a rat’s brain o Noticed that the action potentials of a single neuron had a remarkable relation to the rat’s behaviour  When the rat faced in a particular direction, the neuron vigorously fired  When the rat turned away from the direction, the neuron fired more slowly  When the rat faced opposite the neuron’s favoured direction, the neuron did not fire at all - In visual areas of the brain, action potentials in different neurons might be the units of the perceived image o Bright areas of the visual image might be represented by neurons firing more rapidly o Dark areas might be represented by reduced or absent firing (this theory is incorrect) - Neurons encode information in several ways: o A simple way of representing sensory events is with a time code, in which the presence of an event is signaled by neural firing  Ex. as long as a light is present; a neuron discharges; the discharging stops when the light is turned off o Same information could be represented as an event code: a neuron might discharge when the light comes on and then discharge again when the light goes off  The intensity of an event might be represented by a frequency code • Ex. brightness of a light or the intensity of a pain stimulus is represented by the rapidity of a cell’s firing • Pain fibers in the PNS appear to encode pain in this way, a few action potentials signaling mild pain and more frequent action potentials signaling more severe pain  The frequency with which a neuron fires could also represent much more complicated information, examples: • When a neuron in the visual system is very active, it represents the colour red • When it is less active, it represents the colour green o Neurons that encode bimodal information in this way have a resting state characterized by moderate activity Levels of Neural Processing: - The anatomy of the brain suggested to researchers that it must use codes to represent information, examples: o Numbers of retinal ganglion cells and lateral geniculate body cells carrying visual information from the thalamus to area 17 are very low o In higher visual association areas, the number of cells increase  The changing numbers of cells argues that visual information must be transmitted as a code rather than as an image - Single-cell recordings at these different levels show that ganglion cells and LGB cells respond only to dots of lights o 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 o The visual cortex takes information encoded as dots by numerous cells and bars in fewer cells  Translates it into the complex, ongoing visual experience that tells us the look of our world - Neurons that are nearby have different behavioural repertories, suggesting that in association areas of the brain, the networks promote different behaviours o Ex. in Broca’s area, one neuron may be active during word perception and its neighbor may be active during word production - In single-cell recordings, well learned behaviours seem to be encoded by scattered cortical activity o Behaviours that are being newly learned are accompanied by much more widespread excitability in the cortex - These findings suggest that not only is the type of behaviour or stimulus event important for determining whether a neuron changes its rate of firing, but so is context and experience Electroencephalographic Recording: - Hans Berger – Found that voltage fluctuations or brain waves could be recorded by placing the leads from a voltmeter onto the skull o These recordings are called electroencephalograms (EEGs) - EEGs are valuable tool for: o Studying sleep o Monitoring the depth of anesthesia o Diagnosing epilepsy and brain damage o Studying normal brain function - EEG arrangement: (detects the difference in the electrical potentials) o One electrode (active electrode) is attached to the scalp to detect the electrical activity in the underlying brain area o A second electrode (indifferent electrode) is attached to the ear lobe, where there is no electrical activity to detect - The electrical fluctuations in the brain are rather small, much less than a millivolt, when amplified they can be displayed on a polygraph o In the polygraph, the electrical signals powered magnets connects to pens o A motor pulls a long sheet of paper at a constant rate, allowing the patterns of electrical activity to be traced on the paper - The neurons of the neocortex are arranged in horizontal layers, part of the EEG signal comes from the large pyramidal neurons of layers V and VI o Pacemaker cells – ensure that these neurons undergo graded potentials at the same time, so they can synchronize their action potentials - Signal recorded by the EEG consists of the rhythmical graded potentials of neurons o The rhythms of the pyramidal cells are produced in a number of ways:  Cells in the thalamus or brainstem act as pacemakers, driving the graded potentials rhythmically.  The pyramidal cells have intrinsic rhythms and the connections between adjacent neurons can serve to synchronize those patterns  The rhythm of the cells can fluctuate with heart rate or respiration, events that provide oxygen and glucose to the cells and thus influence their activity - The neurons that produce a signal are referred to as the signal’s generator o The electrical activity that is detected comes from generators in the brain that has been demonstrated in a number of ways  During surgery, neurologists have taken EEG recordings both from the skull and from the underlying brain • Found that the rhythms from the 2 locations are similar, but the waves are larger in amplitude when recorded from the brain tissue  In research with animals, microelectrodes placed within neurons have demonstrated that these neurons do generate the waves - The waves recorded from the skull are volume conducted through the brain and through the skull o Conducted in the manner in which waves travel through water o As electrodes are moved farther away from the source, the amplitude of the waves from a given generator grows small  If a number of electrodes are placed on the skull, amplitude differences can be used to estimate the approximate location of the generator - EEG recordings found to be useful in a number of ways: o When a person is aroused, excited or alert, EEG pattern has low amplitude and high frequency  This pattern, called the beta rhythm is typical of an EEG taken from anywhere on the skull of an alert subject (not just human) o When a person is calm and resting quietly, the rhythmical brain waves often emerge  These are called alpha waves that are extremely rhythmical but with waxing and waning amplitude and a frequency of approximately 11 cycles per second • In humans, the largest alpha rhythms are detected coming from the region of the visual cortex at the back of the head • If a relaxed person is disturbed or opens his/her eyes, the alpha rhythm abruptly stops - A voltmeter can be used for monitoring one’s alpha rhythms o It transforms EEG waves into beeps so that the brain-wave rhythm can be heard; once promoted as a tool for learning transcendental meditation - An EEG is a sensitive indicator of conscious states other than arousal and relaxation o As the EEG rhythms become slower in frequency and larger in amplitude, 4 to 7- cycle per second theta waves and finally to 1 to 3-cycle per second delta waves are produced o These brain-wave patterns make the EEG a reliable tool for monitoring waking and consciousness, estimating the depth of anesthesia, evaluating the severity of head injury, and search for other brain abnormalities  If the brain ceases to function (brain death), the EEG becomes a flat line - The EEG finds a useful clinical application in the diagnosis of epilepsy, a condition characterized by changes in consciousness or by convulsions of the body - Some forms of epilepsy: o Petit mal epilepsy – generally associated with brief losses of consciousness, lasting only a few seconds o Grand mal epilepsy – characterized by convulsions of the body, falling down, and loss of consciousness - EEG recordings can provide information both about the cause of epilepsy and about the location of the problem o First, the duration of an epileptic attack correlates closely with the duration of abnormalities in the EEG, can consist of a loss of recording, a slowing of recording or large distinctive spikes  This correspondence indicates that epilepsy is associated with the abnormal activity of neurons o Second, the EEG can identify the region of the brain in which the abnormal rhythm is produced  The focus of the abnormality is usually located in the brain region that first generates the abnormal electrical activity - Computer techniques are used to make comparisons of the onset times and amplitude of EEG waves; reliably indicates the region of the brain in which the abnormal waves originate - EEG imaging is also used to study cognitive functions o Allows recordings to be taken from as many as 125 sites on the skull o The computer makes a 2-dimensional map of the brain surface, with different colours indicating the relative activity of different brain regions  Produces an online representation of the working brain Event-Related Potentials: - Event-related potentials (ERPs) – are brief changes in a slow-wave EEG signal in response to a discrete sensory stimulus o Not easy to detect, because the signal is hidden in the EEG o Consists of a graded potential generated by the sensory stimulus of interest, mixed with many other electrical signals  Therefore 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 o Averaging tends to cancel out any irregular and unrelated electrical activity, leaving only the graded potentials generated by the stimulus event - Event-related potentials represent the location and the time of processing at each generator, yielding a picture of information flow through the brain - ERP consists of a number of negative and positive waves produced in a period of a few hundred milliseconds after a stimulus is presented o Waves depicted as going downward on the ERP graph are called positive o Waves depicted as going upward are called negative  Positive and negative waves are numbered according to the time at which they are produced o Waves produced at longer latencies, from 100 – 300 ms after a stimulus is presented are likely to be related to the meaning of a stimulus  Ex. the words “cat” and “rat” contain distinctive peaks and patterns that allow researchers to differentiate one response from the other - Maps of cortical function can be produced using ERPs o ERP is made up of positive and negative waves and each wave is produced by a different neural generator  By a different group of neurons responding successively to the signal with a change in their electrical activity • Ex. P3 produced 300 ms after stimulus presentation, represents the process of decoding the meaning of the sounds - Computerized averaging techniques reduce the masses of information obtained to simpler comparisons between electrode sites o Ex. if the focus of interest is P3, a computer record displays an image of the skull in which only the amplitude of P3 is shown o Record is then converted into a colour code, creating a graphic representation showing which brain regions are most responsive to the signal - ERP is used to study the normal function of the pathway through: o Which the signal passes o The normal function of the nuclei taking part in processing the signal o The cognitive processes in the neocortex that are employed in discriminating or learning about the signal - Measures are taken from both hemispheres, so studies of ERPs recorded during cognitive tasks compare the different responses of the hemispheres with the stimulus signal - ERPs also reveal electrical changes associated with the planning and execution of movement o Ex. researchers identified certain potentials produced in the motor cortex later than 300 ms after the presentation of a given stimulus  They call it a readiness potential because it signals an impending movement Magnetoencephalography: - When a magnetic field passes across a wire, it induces a current in the wire o When a current flows along a wire, it induces a magnetic field around the wire o This reciprocal relation between electricity and magnetism is also seen in neurons - Neural activity also produces a magnetic field o 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 provide a description of the electrical activity of neurons and a 3-dimensional localization of the ell 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 o 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  Disadvantage of the MEG is its cost; equipment for producing it is expensive - The heart of a MEG probe is a sensing device containing the special superconducting coils needed to detect the brain’s very weak magnetic fields o This is called SQUID (superconducting quantum interference device)  It is immersed in liquid helium to keep it at the low temperature necessary for superconductivity o Each probe produces an “isocontour map,” a chart with concentric circles representing different intensities of the magnetic field  Such gradient maps allow the calculation of the 3-dimensional location of the neurons generating the field Brain Stimulation: - Early studies of brain stimulation indicated that movements are elicited by o Stimulating the motor cortex o Sensations are elicited by stimulating the sensory cortex o Complex cognitive functions such as speech are disrupted by stimulating speech areas of the cortex - Electrical stimulation of the brainstem can also activate many complex actions o Such as mating, aggression, nest building or food carrying in animals Intracranial Brain Stimulation: - Electrical stimulation of the brain is used for applications other than just mappings its functional regions o Ex. stimulation of certain regions of the brainstem produce pleasurable sensations o Early studies experimented with this form of brain stimulation to treat depression o Can also be used to treat the abnormal discharges of the brain in epilepsy because it can arrest neural activity - Electrical stimulation of the brain can potentially aid in movement production after brain injury or in conditions where brain disease has produced a movement abnormality o Ex. stimulation of cortex adjacent to tissue damaged by stroke can aid in recovery - One used application of electrical brain stimulation is in the treatment of Parkinson’s disease and some other disorders of movement o Parkinson’s disease – characterized both by tremors and by akinesia, which is an absence or poverty of movement o 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 - This neurosurgery facilitates normal movements by fixing an electrode in place in the globus pallidus or subthalamic nucleus and connecting it to an external electrical stimulator controlled by the patient o When a patient turns the switch on, appropriate current is delivered to the electrode, both lessening abnormal movement and improving desired movement - Although DBS treatment can give a Parkinson patient from months to years of relief from disease symptoms, the treatment can have adverse effects o Cognitive changes including changes in mood and anxiety ensue o Electrode implants can produce infection and irritation and frequently DBS is ineffective - What limits both experimentation with intracranial electrical stimulation and its use in treating brain disorders is its invasiveness o The skull must be opened to introduce the electrode  This procedure can damage the brain or introduce infection Transcranial Magnetic Stimulation: - The relation between magnetism and electricity forms the basis of transcranial magnetic stimulation (TMS) o It is a noninvasive method that allows the brain to be stimulated through the skull o A small wire coil is placed adjacent to the skull and a high voltage is passed through the coil in pulses as fast as 50 times per second o Each pulse of electricity produces a rapid increase and then decrease in the magnetic field around the coil  The magnetic field easily penetrates the skull and changes the electrical activity of adjacent neurons - TMS was originally used by neurosurgeons to stimulate brain tissue to monitor its functional condition during brain surgery; to identify the function of the tissue o It was clear that TMS did not harm tissue, so it could be used to stimulate the normal brain through the skull - One use of TMS was to map the brain in regard to functional areas o By moving the TMS stimulation, researchers can identify and map the functions of the cortex in just the same way as with intracranial electrical stimulation - These initial experiments suggested that TMS might be used therapeutically for many of the same purposes for which electrical stimulation by intracranial electrodes is used o Can TMS be used as a therapy for patients with movement disorders? Etc. X-Ray Imaging Techniques: - X-rays are important for medical diagnosis, especially when looking for evidence of a brain tumour, stroke or abnormality in brain vasculature o Most obvious limitation of X-ray techniques is that they produce a static, 2- dimensional image of what should be a 3-dimensional structure o However, now there are new methods that allow brain structure and function to be imaged together Conventional Radiography: - Conventional radiography – consists of passing X-rays through the skull onto an X-ray sensitive film. o As the X-rays travel though 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  Less still by fluid such as that in t
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