PSYC 3620 Cognitive Neuroscience Notes/Study Guides
PSYC 3620 Cognitive Neuroscience Notes/Study Guides PSYC 3620
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Chapter 1 / Introduction to the Nervous System What is cognitive neuroscience? Cognitive neuroscience is the study of how the neurological organization of the brain influences the way people think, feel, and act. It stems from human neuropsychology, which focuses on understanding mental processes in humans with an emphasis on examining the changes in behavior as a result of trauma. Cognitive neuroscientists view the brain as an informationprocessing system whose primary goal is to solve problems. To understand certain phenomena, scientists rely on integrating findings from different approaches. Experimental neuropsychologists work to understand the neural bases of cognition by doing scientific studies comparing individuals who have sustained brain damage with those who haven’t. Clinical neuropsychologists work in health care settings who have sustained brain damage through trauma. They diagnose the cognitive deficits, plan programs of rehabilitation, evaluation the degree to which an individual is regaining function, and determine how environmental factors may moderate the effects. The understanding of the relationship between the brain and the mind is understood from two vantage points: Neurologically oriented approaches emphasize the brain’s anatomy and aim to specify the function of specific circumscribed regions of brain tissue Psychologically oriented approaches emphasize the brain’s mental capacities and aim to understand how different aspects of cognition are supported by the neurological organization of the brain Basic building blocks of the nervous system The human nervous system (brain, spinal cord, nerves, and ganglia) controls the body’s response to internal and external stimuli. It is made up of two main classes of cells: neurons, that carry information by means of a combination of electrical and chemical signals, and glia, which are support cells that outnumber neurons by 10:1. Neurons have three main parts: The dendritic tree receives input from other cells. The cell body contains the nucleus and other apparatus responsible for manufacturing proteins and enzymes that sustain cell functioning The axon is the appendage of the cell along which information is carried Sensory neurons bring information to the CNS Interneurons associate information within the CNS Motor neurons send information from the brain/spinal cord to the muscles Glia cells influence communication by modifying the chemical milieu between them, aid with reorganization after brain damage by removing dead neurons, serve some nutritive needs of neurons, and provide structural support. They are also critical to maintaining the bloodbrain barrier, the mechanism by which many harmful substances are prevented from reaching the brain. It is made up of tightly packed glial cells between blood vessel and neurons. Neuroanatomical terms and brain “geography” The front of the brain is referred to as anterior and the back posterior. Rostral and caudal are also used respectively in animals, where the rostral is the area closer to the head and caudal is in the direction of the tail. The top of the brain is referred to as superior and the bottom is referred to as inferior. In the human brain, dorsal and ventral have meanings synonymous with superior and inferior, respectively. Areas of the middle or in the center of the brain are referred to as medial and areas toward the outside are called lateral. A coronal slice the brain is sliced from ear to ear and separates the front from the back A horizontal slice shows the top and bottom of the brain A sagittal slice separates the left and right side of the brain. More specifically, if the brain is cut in the middle showing the left and right sides, it is called a midsagittal view. The nuclei is a distinct group of neurons whose cell bodies are all situated in the same region. Other terms Contralateral means the opposite side; ipsilateral means the same side Unilateral applies to only one side of the brain; bilateral applies to both sides of the brain Proximal refers to something near an area; distal means far Major subdivisions of the central nervous system The central nervous system includes the brain and spinal cord, whereas the peripheral nervous system compromises all neural tissue beyond the central nervous system, such as neurons. Due to it’s fragile nature, the CNS is encased in bone. Between the neurons and their bony encasements is cerebrospinal fluid (CSF). The spaces that contain CSF are known as ventricles, the most prominent of which are the lateral ventricles. The bloodbrain barrier, which deflects toxins from reaching the brain, also makes it hard for antibodies and white blood cells to reach the brain. Thus, the immune system is in large prevented from protecting the CNS from infection. The following is a closer look at the seven main subdivisions of the CNS: the spinal cord, the medulla, the cerebellum, the pons, the midbrain, the hypothalamus and thalamus, and the cerebral cortex. Spinal cord most sensory neurons relay information to the brain and commands to the muscles via the spinal cord. The bony structure housing the spinal cord, the spinal column, is composed of many sections/vertebrae. Each vertebrae receives sensory information on the dorsal side and convey motor commands to the muscles on the ventral side. Medulla the medulla is the section directly superior to the spinal cord. It is the region of the brain that contains many of the cell bodies of the 12 cranial nerves (nerves responsible for receipt of sensory information and motor control of the head/neural control of the internal organs) Most of the motor fibers cross from one side of the body to the other at the medulla, resulting in the left side of the brain controlling the right side of the body and vice versa. The medulla controls vital functions such as respiration and heart rate The medulla is home to part of a set of neurons known as the reticular activating system, whose neurons receive input from the cranial nerves and project diffusely to many other regions of the brain. o The reticular activating system is important for arousal, attention, and regulation of sleepwake cycles. Cerebellum located posterior to the medulla. This is a region important for the regulation of muscle tone and guidance of motor activity. It helps us to be precise in our movement as well as maintain balance and equilibrium. A specific region called the lateral cerebellum, is linked to motor control and the learning of motor skills. More recent studies suggest the lateral cerebellum is also linked to certain aspects of cognitive processing, allowing for fluidity and precision in mental processes. It is also critical for timing information, acting as the brain’s internal clock. Pons the pons is located superior to the medulla and anterior to the cerebellum. It acts as the main connective bridge from the rest of the brain to the cerebellum. It is: the point of synapse of some cranial nerves acts as an important center for control of eye movements and balance holds the superior olive, one of the points where sound is relayed from the ear to the brain and allows for localization of sound Midbrain superior to the pons lies the midbrain contains two important structures on its dorsal side, the inferior colliculus and the superior colliculus, which play role in orienting to stimuli in the auditory and visual modalities respectively. o The inferior colliculus is a relay point is a relay point for auditory information and helps in localizing sound. It contributes to the reflexive movements of the head and eyes in response to sound. o The superior colliculus allows us to perceive and orient toward large moving objects in the periphery. It also aids in guiding your eyes toward that large object so that it falls in the center of our vision (called foveation) Hypothalamus the general role is to control behaviors that help the body maintain equilibrium and a stable state called homeostasis. Provides signals telling the brain that these behaviors are needed Aids feeding and drinking behavior Regulate body temperature by detecting changes Interacts with the hormonal system giving certain areas of the hypothalamus a role in sexual behavior (sexually dimorphic nucleus), daily rhythms (Suprachiasmatic nucleus), and fight or flight reactions (lateral areas) Thalamus along with the hypothalamus, the thalamus is part of the diencephalon. It is a large relay center for sensory information coming in and motor information leaving it. A relay center is a region in which the neurons from one area synapse onto neurons that then go on to synapse somewhere else in the brain. Ex: the visual system takes information from the retina and synapses onto the lateral geniculate nucleus of the thalamus. The magnocellular layer receives input from cells that are sensitive to low levels of light. The parvocellular layer receives information from cells that are color sensitive and need more light to function. Major subcortical systems get their name because they are located in regions below the cerebral cortex. Two important subcortical systems are: The basal ganglia consists of the caudate nucleus, the putamen, and the globus pallidus and is important for motor control The limbic system consists of the amygdala, the hypothalamus, the cingulate cortex, the anterior thalamus, the mammillary body, and the hippocampus o a series of structures believed to be a circuit for integrating emotional information between various parts of the nervous system Cerebral cortex what most people think of when referring to the brain It is divided into two cerebral hemispheres with gyri (convolutions) and sulci (valleys). If a sulcus is deep it is called a fissure o Three major fissures the central fissure, lateral fissure, and longitudinal fissure The major fissures divide each hemisphere into four large regions, or lobes the frontal lobe, the temporal lobe, the occipital lobe, and the parietal lobe A closer look at the cerebral cortex The system that distinguishes cortical regions on the basis of the pattern of cellular organization is called cytoarchitectonics Cytoarchitectonic divisions All regions of cortex have 5/6 layers, or laminae, of cells, the thickness varies as well as the size and shape of the cells. Neuroanatomists have identified areas of the cortex with similar laminar organization, which established the basis of the Brodmann map. It divides the brain into different areas whose boundaries are not always distinct. Some areas can be defined by their function and some cannot. Each region has an assigned number and is referred to as BA#. Primary sensory and motor cortices The first region in the cortex to receive information about a sensory modality is the primary sensory cortex. The primary motor cortex is the region of the cortex that is the final exit point for neuron’s responsible for the motor control of the body’s muscles. These two share general characteristics of organization: These areas are organized so that specific attributes of the physical world are “mapped” onto brain tissue These maps are distorted relative to the physical world, meaning that they appear to reflect the density of receptors within a system. The mapping of the world into brain tissue occurs in an upside down and backward manner for vision, touch, and motor control. / Motor cortex The primary motor cortex resides directly in front of the central fissure in a long, narrow band called the motor strip. It is also referred to as the motor homunculus. The effects of damage to this area would be muscle weakness to the contralateral side of the body because the cortex controls the amount of force to be applied by muscles. When massive unilateral destruction to the motor strip occurs, paralysis results on the contralateral side of the body and is known as hemiplegia. / Somatosensory cortex The primary somatosensory cortex is the portion of the cortex that receives information about tactile stimulation, proprioception (perception of the position of body parts and their movements), and pressure/pain from internal organs and muscles. The skin contains various nerve endings, or receptors, that respond to pain, pressure, vibration, and temperature. This information reaches the cortex via two routes. One responds to crude, tactile information at the dorsal region of the spinal cord and the other responds to fine touch and proprioceptors at the medulla. The effects of damage to the somatosensory would be fine discriminations of touch on the side of the body contralateral to the damaged cortex. Individuals would be unable to determine how many times they were poked, whether a fabric was velvet or burlap, or that they were being touched in two separate places in proximity (they would think it was the same area). This is called twopoint discrimination. / Visual cortex the primary visual cortex is located in the occipital lobe. The visual system is organized into two visual fields. The right visual field is the information to the right of the point of fixation and is projected to the left half of the retina of both eyes. The left visual field projects to the right half of the retinas of both eyes. How does this occur? Information from the inside half of each retina, known as the nasal hemiretina, crosses the midline of the body at the optic chiasm and projects to the contralateral lateral geniculate nucleus of the thalamus. In contrast, information from the temporal hemiretina projects to the ipsilateral lateral geniculate. Not only is the mapping of the visual world reversed from left to right, it is inverted from top to bottom as well. Destruction of the visual cortex results in an inability to perceive lightdark contrast. If the entire occipital cortex of only one hemisphere is damage, no visual field would be detected in the contralateral visual field, called the homonymous hemianopsia. If the dorsal or ventral portion of the cortex is damaged, in which a quadrant of the visual world is lost, this is called quadranopsia. Scotomas are small portions of lost visual fields. / Auditory cortex The auditory system is organized so that there are both ipsilateral and contralateral projections from the ear to the brain. The primary auditory cortex of the human brain is located in the superior portion of the posterior temporal lobe in an area called Heschl’s gyrus. Unilateral damage to the primary auditory cortex does not impair the ability to perceive all sound. Deficits that occur after damage occurs in one hemisphere are a higher sound threshold and a decrease in ability to localize sound. / Olfactory and gustatory cortex Our sense of smell comes from receptors in the nasal mucosa that send information about odors to the olfactory bulb. When information leaves the olfactory bulb, it can go via a path that mediates our emotional response to smell or a path from the medial dorsal thalamus to the orbitofrontal regions called the primary olfactory cortex. Olfaction is the only system in which information is conveyed ipsilaterally. Our sense of taste comes from our taste bud and either branches off to our limbic system or our primary sensory cortex. The primary sensory cortex for taste I located in the anterior portion of a region called insula, which is tucked inside the lateral fissure. After information has been processed in the primary cortex, it will move to the secondary cortex, and then the association cortex. Association areas Areas of the brain where information from multiple modalities is processed is known as an association area. / Frontal lobe The frontal lobe is described as having three distinct regions: the primary motor region, the premotor region, and the prefrontal region. Frontal regions are often considered the source of some of the most uniquely human abilities and play a role in planning, guidance, and evaluations of behavior. Some results of damage to the frontal lobe: Difficulty organizing behavior to reach a goal Memory loss An increase in psychological inertia, the force that must be overcome to either initiate a process or stop one, which leads to patients to perseverate (perform a behavior repeatedly) Difficulty in modulating behavior Vast change in personality (Phineas Gage) Poor judgment / Parietal lobe The parietal lobe plays a role in integrating information from various sensory modalities, from the sensory world and with information from the memory, and about an individual’s internal state with the external world. Due to it’s integrative function, the deficits observed after parietal lobe damage are often diverse and difficult to conceptualize. In humans, the role of the parietal lobe in multimodal integration is seen in many syndromes: Alexia is the inability to read Agraphia is the inability to write Apraxia is the inability to link skilled motor movement to ideas or representations Spatial cognition is also debilitated leading to the disrupted ability to localize points in space. The syndrome called hemineglect, or hemiattention, leads to the individual ignoring information on one side of space, and act as if that side of the world does not exist. / Temporal lobe Temporal regions are associated with four main functions: memory, visual item recognition, auditory processing, and emotion. They are also important in the formation of long term memories, visual processing, contributing to visual item recognition. Damage can lead to deficits in recognizing common objects and sounds. Also associated with temporal damage are the syndromes of visual agnosia and auditory agnosia. Agnosia is a modalityspecific deficit in recognizing sounds or objects that occurs in the absence of major deficits in basic sensory processing. Chapter 2 / How Neurons Communicate Electrochemical signaling in the nervous system Electrochemical signaling can be broken down into two parts that information is relayed within a neuron by means of an electrical signal, and one neuron influences another via a chemical signal HOW INFORMATION IS TRANSFERRED WITHIN A NEURON When a neuron is in a resting state, the resting potential (difference in electrical charge) is about 70 millivolts (mV). The cell membrane of the neuron essentially creates this charge by separating ions on the inside from the outside. Ions like sodium and potassium can move through a membrane via ion channels. Ion channels are selective; therefore, cell membranes contain several types of channels. Ions are naturally found in different concentrations on either side of the membrane, and when the channels open, the ions will diffuse in the direction that will create equilibrium. Eventually, diffusion would cause the ions to be equally distributed, but active mechanisms like the sodium potassium pump works to prevent equal distribution of ions, maintain the resting potential. When a cell uses its channels and an ions concentration gives enough stimulation, a threshold is passed and the cell “fires”. When firing, the charge reverses from 55mV to 40mV. After reaching the peak, a state called depolarization, the charge retreats back to its resting potential, a state called repolarization. The voltage potential will become slightly more negative than the resting potential during a phase called hyperpolarization. The sequence of events described above, from resting potential and back again, is called action potential (AP). It has three properties: It is selfpropagating, meaning once set in motion nothing else needs to be done The strength will not dissipate with distance traveled It fires in an allornothing response, meaning it either fires to its full extent or it does not The AP is first produced at the axon hillock, then travels the axon to the terminal buttons, which contain synaptic vesicles. These vesicles contain neurotransmitters that are released into the synaptic cleft when the AP reaches them. The region of contact b/t the neuron, the synaptic cleft, and the postsynaptic region is called a synapse. HOW INFORMATION IS TRANSFERRED BETWEEN NEURONS At the synapse, neurotransmitter molecules are released from the presynaptic neuron and received by the postsynaptic neuron, which contains receptors in the dendritic trees. The neurotransmitters fit into a specific region of the receptor, called a binding site, which changes the configuration of the receptor that in turn changes the electrical charge of the neuron. At this point, the chemical signal is transformed back into an electrical one. There are two main classes of receptors that work to produce a local change in the voltage of the dendritic tree of the postsynaptic neuron. The first type are ionotropic receptors that work directly to open/close an ion channel. The second type are metabotropic receptors that work indirectly. Metabotropic receptors are linked to Gproteins (guanyl nucleotidebinding protein). In these circumstances, the neurotransmitter binds to the receptor and a subunit of the protein breaks away and binds directly to an ion channel or activates it by attaching an activating an enzyme situated in the postsynaptic membrane. An enzyme is a molecule that controls a chemical reaction, either by binding together two substances or by cleaving a substance into parts. The enzyme produces another chemical, called the second messenger, that causes a series of steps to occur that open an ion channel. Postsynaptic potentials produced by metabotropic receptors are slower to start but longer lasting than ionotropic receptors. HOW POSTSYNAPTIC POTENTIAL CAN CAUSE AN ACTION POTENTIAL The local changes in the electrical potential can make the electrical charge either more positive or negative than the resting potential. An excitatory postsynaptic potential (EPSP) makes the cell’s charge more positive, reducing the difference of the charge between the inside and outside of the cell, which brings it closer to it’s threshold to fire. An inhibitory postsynaptic potential (IPSP) makes the inside of the cell more negative and moves the cell further away from the threshold of firing. The type of effect, EPSP or IPSP, depends on the receptor type that it binds to. Postsynaptic potentials differ from action potentials in three ways: They are graded, meaning they weaken as they travel They are much smaller in magnitude Whereas APs are always excitatory, postsynaptic potentials can be excitatory or inhibitory Because the potentials are smaller, a single potential is unlikely to cause a cell to fire. It requires a combined effect of the potentials. Think of it as depending not on a single voice from a neighboring neuron, but the chorus of EPSPs and IPSPs produced and whether they occur close together in time and space. The postsynaptic potentials are summated at the axon hillock. Because the potentials are graded, when generated close to the axon hillock they have a larger influence on firing. In general, excitatory synapses are located on the dendritic tree, whereas inhibitory synapses are located on the cell body. Therefore, IPSPs are more likely to be generated closer to the axon hillock, where they can have a greater effect. FACTORS THAT INFLUENCE THE RESPONSIVENESS OF A NEURON Neurons code the intensity of a stimulus via the rate, or pace, of its firing. A strong stimulus will fire many times in succession and a weak stimulus will fire occasionally. The limit of firing rate is about 200 times per second. This limit exists because once an AP has been initiated, it can’t generate another during the depolarization and repolarization phases. This period of inhabitation is called the absolute refractory period. An AP can be produced during the hyperpolarization phase with a stimulation that is substantially higher than the prior AP, called relative refractory period. Several mechanisms exist for clearing neurotransmitter molecules from the synaptic cleft: Reuptake the rapid removal of neurotransmitters back into the terminal buttons by special transporter molecules embedded into the presynaptic membrane Enzymatic deactivation an enzyme separates the transmitter molecules so they become incapable of binding to the receptor. o This process occurs mainly for one neurotransmitter, acetylcholine. The enzyme acetylcholinesterase divides acetylcholine into choline and acetate Astrocytes, a type of glial cell, can take up the neurotransmitter and destroy it by breaking it down. Diffusion can clear neurotransmitters; they simply float out of the range of receptors Autoreceptors located on the presynaptic membrane will bind to the neurotransmitters released by that neuron. They work as a negative feedback mechanism, decreasing the activity of the presynaptic neuron to avoid the cell from being overstimulated Neurotransmitters Many types of neurons can release two or more neurotransmitters. In the following section, we will go over the neurotransmitters that play a major role in the functioning of the CNS. Neurotransmitters have four major characteristics: They are chemicals synthesized within the neuron They are released when the cell is activated by an AP The same response is obtained in the target cell when the transmitter is placed on it artificially When the release of one is blocked, an AP will not result in activity in the postsynaptic neuron There are two main classes of neurotransmitters in the CNS: amino acids, the smallest and most basic building blocks of proteins, and neurotransmitter “systems” that are produced by specific sets of neurons whose cell bodies are located subcortically and whose axons project diffusely throughout the cortex. AMINO ACIDS The two main amino acids in the CNS that act as neurotransmitters are glutamate, which has an excitatory effect, and GABA, which has an inhibitory effect. Two additional amino acids are aspartate, which is excitatory, and glycine, which is inhibitory. / Glutamate There are four main glutamatergic receptors. Three are ionotropic (NMDA, AMPA, and kainate) and the fourth is the metabotropic glutamate receptor. Binding to AMPA and kainate = EPSPs Binding to NMDA= allows the transmitter to regulate the entry of ions and allow those ions to act as second messengers Overactivity of glutamate is thought to play a role in the development of epilepsy. Too much glutamate can produce excitotoxicity, which is excessive activity of receptors that can excite neurons to death. Excitotoxicity is a consequence of ischemia, a form of brain damage that kills neurons due to the lack of oxygen in the brain. / GammaAminobutyric Acid (GABA) GABA is the main inhibitory amino acid in the CNS, used by 40% of receptors. GABAergic input is thought to occur mainly via interneurons. There are two main types of GABA receptors GABA A and GABA B . GABA A is ionotropic and GABA is metabotropic. B Many substances that reduce activity in the CNS bind to GABA receptors. One group is barbiturates, a class of CNS depressants. Another group is benzodiazepines, which are generally used to treat anxiety disorders and seizures. Alcohol produces similar effects. NEUROTRANSMITTER SYSTEMS This class of neurotransmitters differs from amino acids because its members are organized into systems. Acetylcholine is an example that you already known. The other three are known as monoamines, named b/c they derive from an amino acid that has undergone a chemical transformation via enzymatic process. The monoamines are dopamine, noradrenaline (also called norepinephrine), and serotonin. Dopamine and noradrenaline derive from tyrosine and are known as catecholamines. Serotonin derives from tryptophan and is classified as an indolamine. Each of these neurotransmitters is released by a different set of neurons that together form a neurotransmitter system: the cholinergic, dopaminergic, noradrenergic, and serotonergic systems. Agonists are chemicals that mimic the effect of a neurotransmitter an antagonists oppose or diminish the effect on a target neuron. / Cholinergic System The cell bodies of neurons in the cholinergic system are located primarily in the basal forebrain nucleus and the septal nuclei that project to the hippocampus. There are two types of ACh receptors, one ionotropic and one metabotropic. The ionotropic receptor is known as the nicotine receptor and the metabotropic receptor is known as the muscarinic receptor. The cholinergic system plays an important role in maintaining overall cortical excitability. Activity of the system has been linked to paying attention, selective attention, and the ability to orient towards important sensory information. It is also associated with memory processing; acetylcholine depletion is associated with Alzheimer’s disease. ACh may affect both attentional and memory processes b/c it modulates an operation required in both / Dopaminergic System There are three dopaminergic subsystems: the nigrostriatal, mesolimbic, and mesocortical. The two main families of receptors are both metabotropic and classified as D 1 like and D 2 like. D 1 like receptors increase the production of a second messenger, cyclic AMP. o Located exclusively on postsynaptic sites D 2 like receptors decrease the production of cyclic AMP. o Located both postsynaptically and presynaptically, where they serve as autoreceptors The activity of the receptors has been linked to schizophrenia. 2 agonists can often reduce the “florid” symptoms of schizophrenia, which are delusions and hallucinations. The severity of these deficits has been linked to the level of binding of receptors the less binding that 1 occurs, the more likely to be schizophrenic. Other receptors in the D family have specific effects on cognitive and emotional processing. 2 The expression of the receptor is associated with the trait “novelty seeking” which is characterized by exploratory behavior, excitability, and impulsiveness. The nigrostriatal system is made up of cells with bodies located in the substantia niagra projecting to the neostriatum. This portion is important in motor control, specifically regulating the selection, initiation, and cessation of motor behaviors. The mesolimbic system has its cell bodies in the ventral tegmental area and projects to several parts of the limbic system. It is linked to rewardrelated behavior. Dopamine levels in the nucleus accumbens, for example, increase in response to natural reinforcers and drugs of abuse. Dopamine responds to whether a reward exceeds or falls short of what was expected The mesocortical system has its cell bodies in the ventral tegmental area as well. The axons of these cells project to the motor and premotor cortex as well as the prefrontal cortex. This system influences working memory, which allows us to use information to perform tasks, plan, and prepare strategies for problem solving. / Noradrenergic System Neurons that make up this system originate in the locus coeruleus and project to the thalamus, hypothalamus, and the cortex (mainly prefrontal). There are four main types of noradrenergic receptors: α1 , α 2 , β 1 , and β2 . All are metabotropic and coupled to G proteins. The receptors can produce both excitatory and inhibitory effects. Effects of activity in the noradrenergic system: Arousal and attention o Functioning of noradrenaline may be disrupted in ADHD Sleep Working and longterm memory, especially memory that has an emotional component These effects are all very similar to those of the cholinergic system! / Serotonergic System The cell bodies of the serotonergic system are found in the raphe nuclei of the midbrain, pons, and medulla and project to the hypothalamus, hippocampus, and amygdala. Because of its diverse sites of projection, the system influences a large variety of behaviors, including arousal, mood, anxiety and aggression, eating, sleeping and dreaming, pain, sexual behavior, and memory. There are more than 10 types of serotonergic receptors, most of which are metabotropic. Sleep and mood states, specifically depression, have been linked to serotonin levels. Some of the most popular drugs to treat depression are known as serotoninspecific reuptake inhibitors (SSRIs), which increase the amount of serotonin by inhibiting the reuptake of the presynaptic cleft. Serotonin is also linked closely to memory, specifically storing memories into longterm memory. Deficits in learning and memory associated with aging and Alzheimer’s disease coincide with a decline in serotonergic function. INTERACTIONS BETWEEN NEUROTRANSMITTER SYSTEMS These systems are highly interrelated with effects on similar areas of cognitive functioning. New pharmacological interventions combine drugs to treat a disorder. Ex: a treatment approach for Alzheimer’s involves manipulating the cholinergic systems while providing a glutamate antagonist Chemical modulation of neural transmission There are three main ways of modulating neurotransmission: Affecting presynaptic mechanisms can be influenced in a number of ways o One way is to regulate the amount of neurotransmitter that is produced o Another is to modulate the release of the neurotransmitter into the synaptic cleft o The action of autoreceptors can be modulated Modulating the amount of neurotransmitters in the synaptic cleft o One way is to affect reuptake mechanisms o Another is to inhibit action of the enzymes that break them down Affecting postsynaptic mechanisms o Drugs can act as agonists (therefore increasing activity) or antagonists (blocking postsynaptic sites) Myelination The speed at which neurons send electrical signals down their axons depends on the degree to which the axon is insulated by a fatty sheath called myelin. The larger the myelin sheath, the greater the speed. Unmyelinated axons are small and do not carry information over long distances. In contrast, pyramidal cells, which are myelinated, must travel from the brain to the bottom of the spinal cord. It travels very quickly so that we can react and move quickly in response to things. The myelin sheath is produced by a type of glial cells, oligodendrocytes. A portion of the oligodendrocyte wraps itself around the axon; the more turns around the neuron, the greater the insulation. Gaps between myelinated section of an axon are known as nodes of Ranvier. Because myelin is fatty, it is white. Areas of the brain that are made up of myelinated fibers are white matter of the brain and unmyelinated areas are gray matter. When a group of cells send their axons to the same place, the group of axons is called a fiber tract. Chapter 3 / Methods Introduction Cognitive neuroscience requires the integration of information on one topic from multiple perspectives. You might study the brain at a neuroanatomical, neurochemical, or neurophysiological perspective. The method of converging operations refers to a technique that include multiple perspectives that all lead to the same conclusion. Populations of research participants The following sections review the advantages and disadvantages of using three major populations individuals with brain damage, neurologically intact individuals, and nonhuman animals to study the brain. PATIENTS WITH CIRCUMSCRIBED BRAIN DAMAGE If damage to a specific brain region results in an inability to perform a specific mental function, scientists usually assume that the function must have depended on that region. This approach is known as the lesion method. This method has led us to conceptualize the brain as being composed of different subsystems. Now we know realize that these subsystems are located in specific regions of brain tissue a concept called localization of function. In the early 20 century some believed that the brain worked by mass action, meaning that all parts of the brain contributed to all functions. Some that believed this concept also believed that the deficits following brain damage relied on the extent of the damage rather than the area. Today we know this isn’t true, but we do know that no brain region acts in isolation. / Uses of the lesion method The main strength of the lesion method is that it can associate a specific mental process to a specific area of the brain. One downside, however, is that it relies on cases of brain damage that result from unfortunate circumstances. The researcher, therefore, has little control of the area of damage, the extent, and the cause. When using the lesion method, there are two approaches that can be taken: the neural substrate approach and the cognitive function approach. Neural substrate approach will ask what functions are supported by a specific piece of brain tissue? Research will include individuals with damage to a specific area as well as individuals with damage to other areas to determine that a specific region is responsible for that function. Cognitive function approach will select a group of individuals exhibiting the same behavioral symptoms Double dissociation is a method that allows researchers to determine whether two cognitive functions are independent of one another. If they can show that a lesion causes a disruption in one function and not the other, and a different lesion causes a disruption in the opposite functions, the can infer that the functions are independent b/c the viability of one function doesn’t rely on the viability of the other. / Difficulties with the lesion method The major limitations of the lesion method are: 1. Variability in characteristics of the population, location, and extent of the damage a. Compared to research with animals, lesion methods with humans is much less homogenous in terms of its sample. Individuals can vary in age, socioeconomic status, and education. b. Lesions in humans are also less specific, both in extent and origin, those in animal experiments 2. We can’t directly observe the function performed by the damaged portion of the brain. Rather, we can observe how the brain performs without that area. a. Only the region critical to a function can be identified, not the entire set of brain regions that participate b. Deficits may occur because a damaged area is a connecting region that must interact for the function c. A region’s contribution may be “silent”, if the task can be performed in more than one manner 3. The lesion method doesn’t allow us to discern whether damage to an area alters performance because it’s (a) critical or (b) it contains axons, known as fibers of passage, that connect two or more regions. a. A result of the lack of connection between regions is called a disconnection syndrome 4. Finally, we underestimate the role of a specific region in a given cognitive function due to our brain (a) compensating by using a different strategy or (b) reorganizing the brain tissue itself / Singlecase versus group studies Singlecase studies a single individual with brain damage is studied intensively with a variety of psychological tests o Hard to generalize to the public Group studies individuals with brain damage who have similar characteristics are studied as a group o May obscure patterns of behavior because a group average is rarely found in any one individual Multiple casestudy approach researchers validate findings on a series of patients, each of whom is also treated as a singlecase study. Data for each individual is provided, so you can determine the variability across all participants o Can be used to examine whether a relationship exists b/t a cognitive deficit and some other factor of interest NEUROLOGICALLY INTACT INDIVIDUALS Neurologically intact individuals can provide the control group when studying individuals with brain damage. The larger the group, the higher the certainty. Neurologically intact patients must be matched, on a casebycase basis, with an individual with brain damage for demographic variables like age, gender, educational history, and stresses. Because people under stress tend to perform worse on cognitive tasks, it is important to take this into account when examining the relationship between said task performance and brain damage. Stresses like rehabilitation can be matched for a neurologically intact person receiving it due to a bodily injury to someone with a damaged area receiving rehabilitation for their brain injury These individuals can also shed light on individual variations and on how brain structures work together under normal conditions. NONHUMAN ANIMALS Most brain research that isn’t performed on animals is performed on monkeys because they share several basic organization principles. Some mental functions, like language, are more difficult or impossible to study. Researchers have more control over environmental conditions, the size and nature of lesions, and previous life experiences. They can also use certain techniques, like singlecell recordings, that can only be performed on very restricted groups of people. Strict guidelines concerning the ethical treatment of the animals must be adhered to ensure they are experiencing the minimal amount of pain possible. Techniques for assessing brain anatomy Brain imaging techniques can be used in combination with lesion methods to locate the pinpoint of damage being studied. Previously, you had to wait for postmortem examination to localize damage. Older methods were only able to inform us of what part of the skull was missing, however newer methods can allow us to see the internal structures of the brain, which are damaged, and the size and shape of neural structures in neurologically intact people. COMPUTERIZED AXIAL TOMOGRAPHY The density of brain structures can be determined with Xrays in computerized axial tomography (CAT/CT). Dense tissue such as bone appears white, whereas material with less density such as CSF appears black. CAT scans provides a series of “slices” of the brain stacked one above the other. In CAT scans, regions that were damaged long ago appear darker than their surrounding tissue because they are filled with less dense CSF. In contrast, more recent damage occurs to be lighter because the blood is denser than the brain tissue. CAT scans are inexpensive, available in most hospitals, and have no restrictions on who can receive one. MAGNETIC RESONANCE IMAGING Magnetic resonance imaging (MRI) is a technique that relies on the use of magnetic fields to distort the behavior of protons. It relies on three magnetic fields: 1. The static field a constant magnetic field that causes the magnetically sensitive particles to align themselves in the same direction a. MRI machines are classified by the strength of this field, their T (tesla) 2. The pulse sequence an oscillating (moving) magnetic field that causes a disruption in the protons. a. The time it takes for the protons to revert to their original state, the relaxation time, is recorded through a radiofrequency coil that acts as a receiver coil b. The intensity of the signal received by the coil indicates the concentration of the substance in the brain, but not the location 3. The gradient field varies in intensity and provides information of location from which the signals are emanating a. The combination of the location from the gradient field and the intensity of the signal provides a 3D image Two advantages over CAT scans: they don’t require xrays and the spatial resolution is much better. Individuals with pacemakers or metal in their body can’t have an MRI, though. A newer method called diffusion tensor imaging (DTI) can provide information about the structural integrity of brain regions but also the anatomical connectivity between different regions. It identifies the main axis or direction along which water diffuses in nerve fibers. The axis along which water diffusion is greatest indicates the main orientation of whitematter tracts and the degree of diffusion indicates the structural integrity of those tracts. It can be used to investigate the effects of demyelinating disorders like multiple sclerosis, to examine changes in whitematter tracts, and to detect disorders that arise from a partial/complete disconnection of brain regions. A method referred to as diffusion tensor imaging tractography provides information on probable whitematter tracts. Techniques for assessing physiological function FUNCTIONAL BRAIN IMAGING METHODS The function of an area can be determined by measuring how physiologically active an area is related to changes in blood flow or metabolic changes in compounds used. The most common techniques are functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), which uses a radioactive agent to determine the brain’s metabolic activity. These techniques can show how specific regions contribute to task performance under normal circumstances and can observe the entire network of brain structures that participate in performing a function by revealing all brain regions that are active (something that lesion methods can’t do). / Positron emission tomography PET scans can determine the amount of a specific compound that is being used by specific brain regions. Similar to CAT scans, it relies on the use of highenergy ionizing radiation that is introduced to the body. For these molecules to become stable, they release a positron that collides with an electron, producing two photons of light that travel in opposite directions. You can use these directions to work backwards and determine where the molecules are originating from. The time required to obtain a picture is linked to how quickly a given isotope goes from a radioactive state to a nonradioactive state, called its halflife. A related technique is a single photon emission computed tomography (SPECT), which is a scaleddown version of the PET scan. It uses a small set of sensors rather than a ring of them. It has lower spatial resolution and because it takes longer, it is less precise because activity is averaged over a longer time interval. PET scans have two major advantages: they show us how the brain uses specific molecules and they provide information on absolute levels of brain metabolism. Some limitations: due to radiation, the number of scans per year is limited, making it difficult to study something that requires multiple scans. The temporal and spatial resolution is also lower than that of an fMRI. / Functional magnetic resonance imaging Because changes in neuronal activity are accompanied by local changes in other physiological functions like blood flow and blood oxygenation level, these local changes can be used to infer the activity levels of different brain regions. fMRI cannot measure a neuronal response directly, rather it indexes a hemodynamic response, the response of the vascular system to the increased need for oxygen. One method of measuring these changes is known as BOLD (blood oxygen level dependent). It can only provide information about the relative concentration, however. Absolute measurements of the amount of oxygen delivered to the brain are not available. Another method is an arterial spinlabeling technique, which provides information about brain perfusion that can be directly compared across individuals. A magnetic resonance spectroscopy can be used to examine the concentration of other biologically active substances. They provide only very gross information and concentrations must be very high to be detected. Because fMRIs detect a change in the signal from one state to another, it requires that you compare two conditions: the one of interest and a baseline. The baseline is critical for interpretation of results. Some advantages: temporal resolution is better than PET, it is widely available, it’s noninvasive, multiple scans be ran (unlike PET), and it’s more precise. ELECTROMAGNETIC RECORDING METHODS The following methods allows us to record the electrical activity of the brain that results from neuronal firing or the magnetic fields induced by that electrical activity. These methods do a poor job of localizing activity, but provide an accurate measure of activity on a millisecondto millisecond basis. Because of this, these methods provide the best temporal resolution available. / Singlecell recordings these methods will place an electrode into a brain region and record the electrical output of the cell. Once a baseline firing rate is established, researchers will determine what properties of a stimulus make the cell fire maximally above the baseline. Opportunities for these studies in humans are limited. When they are used, though, they can localize tissue that generates seizure activity and use that information to avoid the removal of useful, undamaged tissue. They can also provide information of stimulus properties that make cells fire in a given region. / Electroencephalography Electroencephalography (EEG) techniques record electrical signals produced by the brain through metal electrodes positioned on the scalp. Each electrode acts as its own recording site. One electrode is attached to an electrically inactive site, like the mastoid bone behind the ear, to act as a reference that provides a baseline. The electrical potential recorded is the summed signal of the postsynaptic electrical fields of similarly aligned neuronal dendrites. The frequency and form of the EEG signal can vary according to a person’s state. Clinically, an EEG can detect epilepsy. Experimentally, an EEG can examine certain questions. For example, the suppression of alpha activity, known as alpha suppression, can be used to determine how active the brain is under different conditions. / Eventrelated potentials EEG recordings provide a continuous measure, but eventrelated potentials (ERPs) provide recordings in reference to a specific event, like the presentation of a stimulus. Because of the reaction to a stimulus, ERPs can provide some idea of when processes occur in the brain. The firing after an event results in the creation of a dipole, a small region of electrical current with a positive end and a negative end. Electrodes placed on the scalp detect these dipoles. The waveform recorded will change as time of the onset of the stimulus does. These waveforms can be divided into components, which are characteristic portions of the wave linked to certain psychological processes. Components are given two parts: a letter (P or N depending on positive or negative) and a number (milliseconds, ms, after stimulus presentation the component appears) Components can be exogenous components (linked to physical characteristics and external stimulus, occurring early) or endogenous components (driven by internal cognitive states and occurring later). Major classes of components: Very early components, occurring within 100 ms, are linked to sensory processing Components that appear 100 ms after can be linked to sensory processing and attention Mismatch negativity is a component of N 200 . It occurs when you are presented with an item that is different from previous context (like listening to high pitch tones and then hearing a low pitch). It occurs whether you are paying attention or not. P 300 is a commonly studied component that appears to be related to attention and the updating of memory. It can be elicited by the lack of sensory stimulation, like silence, because even then memory must be updated. The person must also be engaged in N processing for it to occur, unlike 200 . N 400 occurs when individuals detect semantic anomalies. / Magnetoencephalography Magnetoencephalography (MEG) records the magnetic potentials produced by brain activity. The magnetic field can be used to locate the dipole, because it resides between the extreme high points of intensity. The apparatus for collecting MEG data is large, mainly because the sensors (SQUIDS, or superconducting quantum interference devices) only have superconducting properties at low temperatures. They must be encased in large cylinders containing liquid helium. Advantages/disadvantages over recording electric potentials: The strength of magnetic fields isn’t influenced by variations in tissue They provide information about how deep within the brain the source is located It requires a special magnetically shielded room It can’t detect the activity of cells oriented in certain areas b/c the magnetic field will not “emerge” from the brain MEGs are used to localize the source of epileptic activity and to locate primary sensory cortices so avoid them during neurosurgeries. They are also used to understand cognitive processes and the neurophysiology underlying psychiatric disorders. OPTICAL RECORDING METHODS Optical imaging provides information about the source of activity as well as its time course. A laser source of nearinfrared light is positioned on the scalp. Optical sensors will then sense how the path of light is altered. Two types of information are provided: A slow signal measures the absorption of light, but unlike the BOLD method, this can determine the degree by which each substance is absorbed separately rather than a ratio A fast signal measures the scattering of light in relation to the swelling of glia and neurons associated with firing. It occurs simultaneously with neuronal activity. A newer method, eventrelated optical signal or EROS takes advantage of the fast signal to record information locked to an event. It can give information on the source and temporal information. It can’t be used to obtain information about subcortical regions because too much light gets absorbed on the way to and from structures deep in the brain. Techniques for modulating brain activity The best example of modulating or changing brain activity is transcranial magnetic stimulation (TMS). It can be conceptualized as working in the opposite manner from MEG. MEG records magnetic fields produced by the electrical activity of the brain, whereas TMS provides a magnetic field, via a coil, that induces an e
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