PSYC 3510 Behavioral Neuroscience Exam 3 Materials
PSYC 3510 Behavioral Neuroscience Exam 3 Materials PSYC 3510-001
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CHAPTER 7 MECHANISMS OF PERCEPTION: HEARING, TOUCH, SMELL, TASTE, AND ATTENTION This chapter focuses on the remaining four, aside from vision, of the exteroceptive sensory systems: auditory (hearing), somatosensory (touch), olfactory (smell), and gustatory (taste). 7.1 PRINCIPLES OF SENSORY SYSTEM ORGANIZATION The sensory areas of the cortex are considered to be of three different types: primary, secondary, and association. 1. The primary sensory cortex receives most of its input directly from the thalamic relay nuclei of that system. 2. The secondary sensory cortex receives most of their input from the primary sensory cortex or from other areas of the secondary sensory cortex of that system. 3. Association cortex receives input from more than one sensory system. The interactions among these three types of sensory cortex are characterized by three major principles: hierarchical organization, functional segregation, and parallel processing. Sensory systems are characterized by hierarchical organization, specifically on the hierarchy of the specificity and complexity of their function. Each level of a sensory hierarchy receives most of its input from lower levels and adds another layer of analysis before passing it up on the hierarchy. The higher the level of damage, the more specific and complex the deficit (ex: destruction of receptors produces complete loss of ability to perceive while destruction of an area of association produces complex deficits). We divide the general process of perceiving into two general phases: sensation, detecting the presence of stimuli, and perception, the higherorder process of integrating, recognizing, and interpreting complete patterns of sensations. It was once thought that the areas of a sensory system were functionally homogeneous, however research shows that each of the three levels of cortex (primary, secondary, association) are functionally segregated, meaning they have distinct areas that specialize in different kinds of analysis. Our sensory systems are parallel systems, in which information flows through components over multiple pathways. Parallel systems feature parallel processing the simultaneous analysis of a signal in different ways by the multiple parallel pathways of a neural network. There appear to be two kinds of parallel streams in our sensory systems: one that influences behavior without conscious awareness and one that influences behavior by engaging our conscious. In summary, multiple specialized areas, at multiple levels, are interconnected by multiple parallel pathways. Each area, for example, of our visual system is responsible for perceiving specific aspects of visual scenes (shape, color), yet complex stimuli are perceived as integrated wholes, not as combinations of independent attributes. Through something we call the binding problem, we question how the brain combines individual sensory attributes to produce integrated perceptions? One possible solution is that there is a single area of the cortex at the top of the hierarchy that receives signals from all areas and combines them, however there is no area of the cortex that all sensory systems report. 7.2 AUDITORY SYSTEM The amplitude, frequency, and complexity of molecular vibrations are most closely linked with loudness, pitch, and timbre, respectively. Pure tones (sine wave vibrations) exist only in lab settings; in real life sound is associated with complex patterns of vibrations. Complex sound waves can be broken down into sine waves, thus when all sine waves are added together you produce the original sound. Fourier analysis is the mathematical procedure for breaking down complex waves into their sine waves. For any pure tone, there is a close relationship between frequency and pitch, but the sounds we perceive are very complex. The pitch of such complex sounds are related to their fundamental frequency (the highest frequency of which the various component frequencies of a sound are multiples). The pitch of a complex sound may not be directly related to the frequency of any of the sound’s components; referred to as the missing fundamental. The Ear 1. Sound waves travel from the outer ear down the auditory canal and cause the tympanic membrane (the eardrum) to vibrate. 2. These vibrations are then transferred to the three ossicles the small bones of the middle ear: the malleus (hammer), the incus (anvil), and the stapes (stirrup). 3. The vibrations of the stapes (stirrup) trigger vibrations of the membrane called the oval window, which in turn transfers the vibrations to the fluid of the snailshaped cochlea. 4. The cochlea is a long, coiled tube with an internal membrane running almost to its tip. This membrane is the auditory receptor organ, the organ of Corti. 5. Each pressure change at the oval window travels along the organ of Corti as a wave. The organ of Corti is composed of two membranes: the basilar membrane, where the auditory receptors (hair cells), are mounted, and the tectorial membrane. 6. A deflection of the organ of Corti produces a force that stimulates the hair cells, which in turn increases firing in axons of the auditory nerve a branch of cranial nerve VIII. 7. The vibrations of the cochlear fluid are scattered by the round window, an elastic membrane in the cochlea wall. The cochlea is remarkably sensitive. Different frequencies produce maximal stimulation of hair cells at different points along the basilar membrane with higher frequencies producing greater activation closer to the windows and lower frequencies producing greater activation at the tip of the basilar membrane. Like the cochlea, other structures of the auditory system are arranged according to frequency. Thus, the organization of the auditory system is primarily tonotopic. Somehow our auditory system sorts individual frequency messages into separate categories and combines them so that we hear each source of complex sound independently the mechanism underlying this ability is still a mystery. The semicircular canals are the receptive organs of the vestibular system, which carries information about direction and intensity of head movements, which helps us with balance. From the Ear to the Primary Auditory Cortex Instead of a major auditory pathway to the cortex, there is a network of auditory pathways. 1. The axons of each auditory nerve synapse in the cochlear nuclei, which project to the superior olives on both sides of the brain stem. (hindbrain) 2. The axons of the olivary neurons project via the lateral lemniscus to the inferior colliculi, where they synapse. (midbrain) 3. The neurons, from the inferior colliculi, project to the medial geniculate nuclei of the thalamus, which in turn project to the primary auditory cortex. (forebrain) Subcortical Mechanisms of Sound Localization Analysis of subcortical auditory pathways is difficult due to its complexity, but the localization of sounds in space is a very well understand function. Localization of sounds in space is mediated by the lateral and medial superior olives. Medial superior olives respond to differences in the time of arrival from the two ears, while neurons in the lateral superior olives respond to differences in the amplitude of sounds from the two ears. The olives project to the superior colliculus (not shown in 7.6 or explained above), which receive auditory input. It appears that the general function of the superior colliculi is locating sources of sensory input in space. Research in barn owls, whose sound localization is better than any other tested animal, shows that their superior colliculus region is very finely tuned. Auditory Cortex Recent progress in the study of the human auditory cortex has resulted from fMRI studies in humans and invasive studies in monkeys. The primary auditory area, and two other areas, are called the core region found in the temporal lobe. Surrounding the core region is a band, often called the belt of areas of secondary cortex. Areas of secondary cortex outside the belt are called parabelt areas. Two important principles of organization of primary auditory cortex: it’s organized in functional columns (neurons respond optimally to sounds in the same frequency range) and it’s organized tonotopically, on the basis of frequency. Because we lack a clear understanding of dimensions along which the auditory cortex evaluates sound, we haven’t made a lot of progress in studying it. Researchers used to use pure tones to study how neurons respond to stimuli, but their responses to them were weak. Recently it was discovered the neurons in monkeys respond much stronger to monkey calls, which is changing the practice of using pure tones. There are to main cortical streams of auditory analysis: the prefrontal cortex and the posterior parietal cortex. It has been hypothesized that the anterior auditory pathway (prefrontal) is involved in the what of sounds, while the posterior auditory pathway (parietal) is involved in the where of sounds. fMRIs help us to investigate sensory system interactions, with an advantage of showing activity throughout the brain rather than one area. Evidence has been repeatedly found showing sensory interactions at the lowest level of the hierarchy in areas of primary sensory cortex, which suggests that sensory system interaction is not merely tagged on after unimodal analyses are complete, but that the interactions are an early and integral part of sensory processing. Most auditory neurons respond to changes in frequency rather than pitch (using the missing fundamental technique). Research shows that a small area anterior to primary auditory cortex contained neurons that responded to pitch rather than frequency, suggesting this area is where frequencies of sound are converted to the perception of pitch. Research has involved monkeys, but humans have a similar “pitch area”. Effects of Damage to the Auditory System Most of the human auditory cortex is in the lateral fissure, so it is rarely destroyed in its entirety, and if it is, there is almost always extensive damage to surrounding tissue. We rely on surgically placed lesions in nonhumans to study the effects of auditory cortex damage. Because of the parallel organization, severe hearing problems typically result from damage to the inner ear or the middle ear or to the nerves leading from them (rather than from central damage). There are two common classes of hearing impairment: damage to the ossicles (conductive deafness) and damage to the cochlea or auditory nerve (nerve deafness). The major common cause of nerve deafness is the loss of hair cell receptors. The first sign of agerelated hearing loss is the deficit in perceiving high frequency sounds. Hearing loss is sometimes associated with ringing of the ears, tinnitus. When one ear is damaged, the ringing is perceived as coming from that ear, however cutting the nerve has no effect on the ringing. This suggests that changes to the central auditory system, caused by deafness, are the cause of tinnitus. Cochlear implants can convert sounds picked up by a microphone to electrical signals, which are then carried into the cochlea by a bundle of electrodes. 7.3 SOMATOSENSORY SYSTEM: TOUCH AND PAIN Sensations from your body are referred to as somatosensations. The system that mediates these sensations is split into three systems: (1) an exteroceptive system, which senses external stimuli applied to the skin, (2) a proprioceptive system, which monitors information about the position of the body that comes from receptors in the muscles, joints, and organs of balance, and (3) an interoceptive system, which provides general information about conditions within the body like temperature and blood pressure. This discussion deals mostly with the exteroceptive system, which is also split into three divisions: a division for perceiving mechanical stimuli (touch), one for thermal stimuli (temperature), and one for nociceptive stimuli (pain). Four types of cutaneous receptors: Free nerve endings simplest / neuron endings with no specialized structures on them / sensitive to temperature change and pain Pacinian corpuscles largest and deepest / adapt rapidly and respond to sudden displacements of the skin but not to constant pressure Merkel’s disks adapt slowly and respond to gradual skin indentation Ruffini endings adapt slowly and respond to gradual skin stretch Stereognosis the identification of objects by touch. Having some receptors that adapt quickly and some that adapt slowly provides information about both the dynamic and static qualities of tactual stimuli. Although sensory receptors are specialized in sensitivity, they tend to function in the same way: stimuli applied changes the chemistry of the receptors which changes the permeability, resulting in a neural signal. Neural fibers that carry information from cutaneous receptors gather together in nerves and enter the spinal cord via the dorsal roots. The area of the body that supplies organs with the left and right dorsal roots is called a dermatome. Two Major Somatosensory Pathways Somatosensory information ascends from each side of the body over two major pathways: the dorsalcolumn mediallemniscus system and the anterolateral system. Both systems **tend to carry specialized information, however separation of function in the two pathways is NOT complete. The dorsalcolumn medial lemniscus system tends to carry information about touch and proprioception. The sensory neurons enter the spinal cord via a dorsal roots, ascend ipsilaterally (same side of body) in the dorsal columns, and synapse in the dorsal column nuclei of the medulla The axons of dorsal column nuclei decussate (cross over to the other side of the brain) and then Ascend in the medial lemniscus to the contralateral (opp. side) ventral posterior nucleus o Most neurons of the ventral posterior nucleus project to the primary somatosensory cortex, others to the secondary somatosensory cortex, or the posterior parietal cortex. The anterolateral system tends to carry information about pain and temperature. Most dorsal root neurons synapse as soon as they enter the spinal cord The axons of most of the secondorder neurons decussate (cross over to the other side of the brain) but then ascend to the brain in the contralateral anterolateral portion of the spinal cord The anterolateral system comprises three different tracts: the spinothalamic tract (which projects to the ventral posterior nucleus of the thalamus, as does the dorsalcolumn system) the spinoreticular tract (which projects to the reticular formation) the spinotectal tract (which projects to the tectum) If both somatosensory paths are completely transected by a spinal injury, the patient can feel no body sensation from below the level of the cut. Clearly, when it comes to spinal injuries, lower is better!! Mark, Ervin, and Yakolev (1962) studied the effects of lesions to the thalamus on the chronic pain of cancer patients. o Lesions to the ventral posterior nuclei, which receive input from the spinothalamic tract and the dorsalcolumn mediallemniscus system, produced some loss of sensitivity to touch, temp. change, and to sharp pain, but had no effect on deep and chronic pain o Lesions to the parafascicular and intralaminar nuclei, both receive input from the spinoreticular tract, reduced deep chronic pain without disrupting cutaneous sensitivity. Cortical Areas of Somatosensation Penfield and his colleagues mapped the primary somatosensory cortex of patients. When stimulation was applied to the postcentral gyrus, the patients reported somatosensory sensations in various parts of their bodies. They discovered that the human primary somatosensory cortex (SI) is somatotopic organized according to a map of the body surface. This map is referred to as the somatosensory homunculus. A second somatotopically organized area (SII) lies ventral to the SI in the postcentral gyrus. SII receives most of its input from SI and is this regarded as secondary somatosensory cortex. In contrast to SI, whose input is largely contralateral, SII receives input from both sides of the body. Most output of SI and SII goes to the association cortex of the posterior parietal lobe. The receptive fields of many neurons in the primary somatosensory cortex, like the visual system, can be divided into antagonistic excitatory and inhibitory areas. Studies that explored single neurons in SI found the same columnar organization characteristic of other areas of primary sensory cortex. Each cell of a particular column of SI had a receptive field on the same part of the body and responded most to the same type of tactile stimuli. Unit recordings suggested that primary somatosensory cortex is composed of four functional strips, each with a similar, but separate, somatotopic organization. One would find that as one moved from anterior to posterior, the preferences of the neurons would tend to become more complex, suggesting an anteriortoposterior hierarchical organization Effects of Damage to the Primary Somatosensory Cortex Like the effects of damage to the primary auditory cortex, effects of damage to the primary somatosensory cortex are often very mild. Presumably because, like the auditory system, our somatosensory system features numerous parallel pathways. Following a surgery in which a unilateral excision that included the SI, patients displayed two minor contralateral deficits: a reduced ability to detect light touch and a reduced ability to identify objects by touch. Somatosensory System and Association Cortex Somatosensory signals are ultimately conducted to the highest level of the sensory hierarchy, to areas of association cortex in the prefrontal and posterior parietal cortex. Posterior parietal cortex contains bimodal neurons, neurons that respond to activation of two different sensory systems. In this case, the bimodal neurons respond to somatosensory and visual stimuli. The visual and somatosensory receptive fields are spatially related; for example, if a neuron has a somatosensory receptive field centered in the left hand, its visual field is adjacent to the left hand. The Case of W.M., Who Reduced His Scotoma with His Hand: Man lost vision on left side. With his left hand in his lap, he could detect almost all stimuli presented in his right visual field, but only 14% of stimuli in his left visual field. However, when he extended his left hand into his left visual field, his ability to detect stimuli improved significantly. Somatosensory Agnosias There are two major types of somatosensory agnosia: Astereognosia the inability to recognize objects by touch. These are rare. Asomatognosia the failure to recognize parts of one’s own body. o Usually unilateral, affecting only the left side of the body, associated with extensive damage to the right posterior parietal lobe. o Often accompanied by anosognosia the failure of neuropsychological patients to recognize their own symptoms o Commonly a component of contralateral neglect the tendency not to respond to stimuli that are contralateral to a righthemisphere injury Perception of Pain The perception of pain is paradoxical in three important respects: Adaptiveness of pain pain is important for our survival; it warns us to stop engaging in potentially harmful activities or to seek treatment Lack of clear cortical representation of pain it has no obvious cortical representation. Painful stimuli activate many areas of cortex, but particular areas vary from person to person. o Painful stimuli elicit responses in SI and SII, however removal of SI and SII is not associated with any change in the threshold for pain. o The cortical area most frequently linked to the experience of pain is the anterior cingulate cortex, which is involved with the emotional reaction to pain rather than the perception itself Descending pain control our pain can be effectively suppressed by cognitive and emotional factors. o The gatecontrol theory accounts for this ability to block pain. They theorized that signals descending from the brain can activate neural gating circuits in the spinal cord to block incoming pain signals. Three discoveries led to the identification of a descending paincontrol circuit: The discovery that electrical stimulation of the periaqueductal gray has analgesic (painblocking) effects. The discovery that PAG and other areas had specialized receptors for opiate and analgesic drugs like morphine. The discovery of the isolation of several internally produced opiate analgesics (endorphins) Neuropathic pain is severe chronic pain in the absence of a recognizable pain stimulus. It typically develops after an injury. Although the exact mechanisms of neuropathic pain are unknown, it is somehow caused by pathological changes in the nervous system induced by the original injury. It is caused by abnormal activity in the CNS, this cutting nerves from the perceived location of pain often brings little or no comfort. Medications are also ineffective against neuropathic pain. 7.4 CHEMICAL SENSES: SMELL AND TASTE Olfaction (smell) and gustation (taste) are our chemical senses because their function is to monitor the chemical content of the environment. When we eat, smell and taste act together and create flavor. This flavor recognition is the main adaptive role of the chemical senses in humans. The contribution of olfaction for flavor is often underestimated. In many species aside from humans, pheromones (chemicals that influence the physiology and behavior of same species) are released. For example, male hamsters will kill unfamiliar males and mount unfamiliar females, however when male hamsters are unable to smell they will engage in neither aggressive nor sexual behavior. Similarly, when male intruders are swabbed with vaginal secretions of females, they are not seen as a threat but as something to lust by other males. Findings that suggest humans release sexual pheromones: Olfactory of women is greatest when ovulating/pregnant Menstrual cycles synchronize Ability to identify gender through breath/underarm odor Ability of men to judge stage of menstrual cycle by odor Remember, there is no direct evidence that human odors serve as sex attractants… we find most odors above unattractive. Olfactory System The olfactory receptor cells are located in the upper part of the nose, embedded in a layer of mucuscovered tissue called the olfactory mucosa. Their dendrites are located in the nasal passages and their axons pass through the cribriform plate (portion of the skull) and enter the olfactory bulbs, where they synapse via the olfactory tracts to the brain. For decades, it was thought that humans had very few receptors, however recent studies suggest we have almost 1,000 (mice have 1,500). In mammals, each olfactory receptor cell contains only one type of receptor protein molecule. All receptors appear to be scattered throughout the mucosa. Each receptor responds in varying degrees to a wide variety of odors, seeming to be encoded by component processing (pattern of activity across receptor types). Despite the receptors being scattered throughout the mucosa, all receptors project to the same location in the olfactory bulb. The axons terminate in discrete clusters (olfactory glomeruli) of neurons near the surface of the olfactory bulbs. There are two glomeruli in each bulb for each protein. The olfactory receptor cells differ from the receptor cells in other sensory systems in one way: new cells are created throughout each individual’s life to to replace those who have deteriorated (only takes about 2 weeks to die). The receptor cells grow until they reach appropriate sites in the bulb. Each olfactory tract projects to several structures of the medial temporal lobes, including the amygdala and the piriform cortex (considered the primary olfactory cortex, adjacent to the amygdala). The olfactory system is the only sensory system whose major sensory pathway reaches the cerebral cortex without first passing through the thalamus. Two major pathways leave the amygdalapiriform area. One projects to the limbic system, while mediates emotional responses to odors. The other projects via the medial dorsal nuclei (of the thalamus) to the orbitofrontal cortex the area of the cortex on the inferior surface of the frontal lobes, next to the eye sockets (orbits). The orbitofrontal cortex is thought to process the conscious perception of odors. Gustatory System Taste receptors are found on the tongue and oral cavity in clusters of about 50 (called taste buds). Taste buds on the tongue are often located around papillae. Unlike olfactory receptors, taste receptors don’t have their own axons. Each neuron that carries signals away from a taste bud receives input from many receptors. Our five primary tastes are sweet, sour, bitter, salty, and unami (meaty). Many taste can’t be created by combinations of the primary tastes, making it apparent that our taste perception is not componentprocessed. We have 33 gustatory receptor proteins: 1 unami, 2 sweet, and 30 bitter. Sour and salty don’t seem to have receptors, but influence taste receptor cells by acting directly on their ion channels. Only one type of receptor protein appears in each receptor cell. Gustatory afferent neurons leave the mouth as part of the: Facial cranial nerve (VII) from the front of the tongue Glossopharyngeal cranial nerve (IX) from the back of the tongue Vagus cranial nerve (X) from the back of the oral cavity These fibers all terminate in the solitary nucleus of the medulla, where they synapse on neurons that project to the ventral posterior nucleus of the thalamus. The gustatory axons of the ventral posterior nucleus project to the primary gustatory cortex, close to the somatosensory homunculus, and then to the secondary gustatory cortex. Unlike projection of other sensory system, projections of the gustatory sensory system are ipsilateral (same side of body), thus tastes seeming to be encoded in the brain by profile of activity in groups of neurons. Brain Damage and the Chemical Senses The inability to smell is called anosmia. The most common cause is a blow to the head that displaces the brain within the skull and shears the olfactory nerves where they pass through the cribriform plate. Other neurological disorders associated with lack of smell: Alzheimer’s, Down syndrome, epilepsy, multiple sclerosis, Korsakoff’s syndrome, and Parkinson’s disease. The inability to taste is called ageusia. It’s very rare because sensory signals are carried via three pathways. Limited taste to 2/3 of the tongue on one side is observed sometimes after damage to the ear on the same side of the body. This is because the branch of the facial nerve (VII) that carries gustatory information from the anterior 2/3 of the tongue passes through the middle ear. 7.5 SELECTIVE ATTENTION Selective attention is the process by which we perceive only a small subset of the many stimuli we receive and ignore the rest. Within the definition, selective attention has two characteristics: (1) it improves perception of the stimuli of focus and (2) it interferes with the perception not of focus. We can also focus our attention in two ways: (1) internal cognitive processes/endogenous attention like looking at a table for keys or (2) external events/exogenous attention like looking at your lamp because your cat knocked it over. The cocktailparty phenomenon is the fact that even while focusing on one conversation, the mention of your name in a loud room will always catch your attention. This suggests that we can block most things, while still unconsciously monitoring the blockedout stimuli just in case something comes up. Change blindness is the inability to detect change because when looking at a scene, we have no memory for the parts of the scene (most of it) that aren’t the focus of attention. Moran and Desimone (1985) demonstrated the effects of attention on neural activity in monkeys. They trained them to stare at a fixation point and recorded the activity of neurons in the ventral stream, which is particularly sensitive to color. When the monkey was trained to perform a task that required attention to the red cue, the neural response to the red cue was increase and the response to the green cue was reduce (and vice versa with attending to green). In general, anticipation of a stimulus increases neural activity in the same circuits affected by the stimuli itself. This neural mechanism is surprising because it involves a surprisingly high degree of neural plasticity. Simultanagnosia is difficulty in attending to visually to more than object at a time due to damage in the dorsal stream of the posterior parietal cortex. CHAPTER 6 THE VISUAL SYSTEM: HOW WE SEE 6.1 LIGHT ENTERS THE EYE AND REACHES THE RETINA The light reflected into your eyes from the objects around you is the basis for your ability to see them therefore if there is no light, there is no vision. Light can be thought of in two ways: discrete particles moving through space (photons) or as waves of energy (measured between 380 and 760 nanometers for our purposes). Two properties of light: Wavelength plays a role in perception of color Intensity plays a role in the perception of brightness The Pupil and the Lens The amount of light reaching the retinas is regulated by the irises, which give our eyes their color. Light enters the eye through the pupil, the hole in the iris. The adjustment of pupil size in response to illumination represents a compromise between sensitivity (detecting dimly lit objects) and acuity (details). High illumination gives high sensitivity, so pupils take advantage and constrict, letting us focus on detail. Low illumination lack of sensitivity, so we sacrifice acuity by dilating our pupils to let in more light. Behind each pupil is a lens, which focuses incoming light on the retina. When we direct our gaze at something, the tension on the ligaments holding each lens in place is adjusted by the ciliary muscles, and the lens takes its natural cylindrical shape. This allows us to bend light and bring objects into focus. When we focus on a distant object, the lens is flattened. This process of adjusting the configuration of the lenses to bring images into focus on the retina is called accommodation. Eye Position and Binocular Disparity One reason vertebrates have two eyes is that we have two sides: left and right; by having one eye on each side they can see in almost every direction without moving their heads. Why do we have our eyes side by side then, and not to the side? This arrangement sacrifices our ability to see behind us, but we are able to see what is in front of us, with both eyes, simultaneously. The placement of our eyes gives our visual system the basis for creating threedimensional perceptions. The movements of eyes are coordinated so that each point in the visual world is projected to corresponding points on your two retinas. To accomplish this, your eyes converge (turn slightly inward). The two retinas never exactly correspond because your two eyes do not view the world from the same position. Binocular disparity is the difference in the position of the same image on the two retinas. It is greater for close objects than for distant objects; therefore your visual system can use the degree of binocular disparity to construct one 3D perception from two 2D retinal images. 6.2 THE RETINA AND TRANSLATION OF LIGHT INTO NEURAL SIGNALS After light passes through the pupil and the lens, it reaches the retina. The retina then converts light to neural signals, conducts them toward the CNS, and participates in the processing of the signals. The retina is composed of five layers of different types of neurons: receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells. Amacrine and horizontal cells are specialized for lateral communication (communication across the major channels of sensory input). Retinal neurons communicate chemically via synapses and electrically via gap junctions. Light reaches the retina in an insideout manner. It reaches the receptor layer after passing through the other four layers. Then, once the receptors have been activated, the neural message is transmitted back out through the retinal layers to the retinal ganglion cells, whose axons project across inside of the retina before gathering together in a bundle and exiting the eye ball. This insideout arrangement creates two visual problems: Incoming light is distorted o Incoming light being distorted is minimized by the fovea, an indentation at the center of the retina specialized for highacuity. The thinning of the retinal ganglion cell layer at the fovea reduces the distortion. For the bundle of retinal ganglion cell axons to leave the eye, there must be a gap in the receptor layer (called the blind spot). o The blind spot is made less problematic through completion, using information provided by the receptors around the blind spot to fill in the gaps in your retinal images. This shows that our visual system does much more than create a faithful copy of the external world. Completion is NOT just a response to blind spots, but a fundamental visual system function. It extracts key information (edges and their location) and creates the perception of the entire object from that partial information. Surface interpolation is the process by which we perceive surfaces; extracting information about edges and making inferences about the appearance of large surfaces. Cone and Rod Vision There are two different types of receptors: coneshaped receptors called cones and rodshaped receptors called rods. The observation of different species having different types of receptors led to the duplexity theory of vision the theory that cones and rods mediate different kinds of vision. Cones Species only active in the day tend to have coneonly retinas Photopic vision, conemediated vision, predominates in good lighting and provides high acuity colored perceptions of the world. Present in the fovea More in the temporal hemiretina, the half of the retina next to the temples. Rods Species active only at night tend to have rodonly retinas Scotopic vision, rodmediated vision, predominates when there is dim illumination and not enough light to reliably excite the cones. It lacks both detail and color. Not present in the fovea, but on the boundaries of the foveal indentation More present in the nasal hemiretina, the half of each retina next to the nose The difference between photopic and scotopic vision results in part from a difference in the way the two systems are “wired.” Scotopic vision involves several hundred rods converge on a single retinal ganglion cell Photopic vision involves just a few cones converging on each retinal ganglion cell Spectral Sensitivity Wavelength has a substantial effect on the perception of brightness. Because our visual systems are not equally sensitive to all wavelengths in the visible spectrum, lights of the same intensity but of different wavelengths can differ markedly in brightness. A graph of relative brightness of lights of the same intensity presented at different wavelengths is called a spectral sensitivity curve. The most important thing to remember about these curves is that humans and other animals with both rods and cones have two of them: Photopic spectral sensitivity curve determined by having subjects judge the brightness of different wavelengths of light shone on the fovea Scotopic spectral sensitivity curve determined by asking subjects to judge the brightness of different wavelengths of life shone on the periphery of the retina at an intensity too low to activate the few peripheral cones that are located there Because of the difference between the two sensitivities, an interesting effect occurs during the transition from photopic to scotopic vision: the Purkinje effect (some colors/objects seeming bright in daylight and then seeming dim in low light while other previously dim colors/objects seem bright). Eye Movement We can ask ourselves how we can see such a richly colored and detailed perception of the world when our cones are restricted to a few degrees in the center of your visual field. To answer, what we see it determined by what is projected on the retina, but also a summation of recent visual information. This temporal integration allows us to blink while still keeping a vivid perception of the world around us. Even when we fix our gaze on an object, our eyes continuously move. These involuntary fixational eye movements are of three kinds: tremor, drifts, and saccades (small jerky movements or flicks). Ironically, if we fixate perfectly and didn’t have these involuntary movements, our world would fade and disappear because our visual neurons respond to change. If there is no change, there is nothing for the neurons to respond to, thus creating an empty field of vision. Visual Transduction: The Conversion of Light to Neural Signals Transduction is the conversion of one form of energy to another. Visual transduction is the conversion of light to neural signals by the visual receptors. A breakthrough for visual transduction came when a red pigment, now called rhodopsin, was extracted from rods. When the pigment was exposed to continuous intense light, it was bleached and lost its color, and its ability to absorb light. When it returned to the dark it regained its redness and its lightabsorbing capacity. It is clear that rhodopsin’s absorption of light (and the accompanying bleaching) is the first step in rodmediated vision. The degree to which rhodopsin absorbs light under various situations predicts how humans see under the same conditions. The absorption spectrum of rhodopsin and the human scotopic spectral sensitivity curve illustrate this. Our sensitivity to various wavelengths is a direct consequence of rhodopsin’s ability to absorb them. It is a Gprotein coupled receptor that responds to light rather than neurotransmitter molecules. When rods are in darkness, their sodium channels are partially open, thus keeping rods slightly depolarized and allowing a steady flow of excitatory glutamate neurotransmitters molecules to emanate from them. When they’re bleached by light, the sodium channels close and hyperpolarizes the rods, and reduces the release of glutamate. 6.3 FROM RETINA TO PRIMARY VISUAL CORTEX The retinageniculatestriate pathways conduct signals from each retina to the primary visual cortex, or the striate cortex, via the lateral geniculate nuclei of the thalamus. About 90% of axons of retinal ganglion cells become part of the retinageniculatestriate pathways. The main take away from the below figure is that all signals from the left visual field reach the right primary cortex: Either ipsilaterally from the temporal hemiretina of the right eye Or contralaterally, via the optic chiasm, from the nasal hemiretina of the left eye The opposite is true of all signals from the right visual field. Each geniculate nucleus has six layers and each layer receives input from all parts of the contralateral visual field of one eye. In other words, each lateral geniculate nucleus receives visual input only from the contralateral visual field; three layers from one eye and three from the other. Most neurons that project to the primary visual cortex terminate in the lower part of cortical layer IV, producing a characteristic striation, hence the name striate cortex. The retinageniculatestriate system is retinotopic; each level of the system is organized like a map of the retina. This means that two stimuli presented to adjacent areas of the retina excite adjacent neurons at all levels of the system. The retinotopic layout of the primary visual cortex has a disproportionate representation of the fovea; considering it is a small part of the retina but about 25% of the cortex is dedicated to analysis of its input. Two parallel channels of communication flow through each lateral geniculate nucleus. Parvocellular layers (P layers) o Runs through top four layers o Composed of neurons with small cell bodies o Responsive to color, fine pattern details, stationary/slowly moving objects o Cones are the majority of input Magnocellular layers (M layers) o Runs through bottom two layers o Composed of neurons with large cell bodies o Responsive to movement o Rods are the majority of input 6.4 SEEING EDGES Edges are the most informative features of any visual display because they define the extent and position of the various objects in it. In a sense, a visual edge is nothing: it is merely a place where two different areas of a visual image meet. The perception of an edge, though, is the perception of a contrast between two adjacent areas of the visual field. Lateral Inhibition and Contrast Enhancement Mach bands are nonexistent stripes of brightness and darkness running adjacent to the edges of objects that enhance the contrast at each edge and make them easier to see. This contrast enhancement is something that we are normally unaware of every edge we look at is highlighted for us by the contrasting mechanisms of our nervous systems. Classic studies performed on horseshoe crabs because, unlike other mammals, their eyes are composed of very large receptors called ommatidia, each with its own large axon. Lateral inhibition occurs when a receptor fires and it inhibits its neighbors laterally via the lateral neural network. The neural basis of contrast enhancement can be understood in terms of the firing rates of the receptors on each side of an edge. A receptor in intense light neighboring dim light will fire more because it’s receiving the same stimulation as other intensely lit receptors, but it’s receiving less inhibition from its dimly lit neighbors. A receptor in dim light neighboring intense light will fire less, because it’s receiving the same stimulation as other dimly lit receptors, but it’s receiving more inhibition from its intensely lit neighbors. Our visual system and these mechanisms are what create Mach bands. Receptive Fields of Visual Neurons David Hubel and Torsten Wiesel won the Nobel Prize for research on the neural mechanisms of vision by studying single neurons in cats and monkeys. The steps are below: The tip of a microelectrode is positioned near a single neuron in the part of the visual system that is under investigation During testing, eye movements are blocked by paralyzing the eye muscles and the images on a screen are focused sharply on the retina by an adjustable lens They then identify the receptive field of the neuron. A receptive field is the area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron. The final step in the method is to record the responses of the neuron to various stimuli within its receptive field to characterize the types of stimuli that most influence its activity. Then this entire process is repeated for another neuron, and then another, etc. Receptive Fields: Neurons of the RetinaGeniculateStriate System Hubel and Wiesel studied three levels of the retinageniculatestriate system: retinal ganglion cells, lateral geniculate neurons, and striate neurons of lower layer IV. They found little change in the receptive fields, but commonalties: Receptive fields in the foveal area of the retina were smaller; consistent with the fact that the fovea mediate highacuity vision All receptor fields were circular All neurons were monocular; having a receptive field in one eye but not the other Many neurons had receptive fields that comprised an excitatory area and an inhibitory area separated by a circular boundary o An excitatory area would display a burst of firing (“on” firing) o An inhibitory area would display an inhibition of firing (“off” firing) and then a burst of firing when the stimulus (light) was taken away. This “on” or “off” firing was dependent on whether the neurons were oncenter cells or off center cells. Oncenter cells respond to lights shone in the central region of their receptive fields with “on” firing and to lights shone in the periphery with “off” firing Offcenter cells respond to lights shone in the central region with “off” firing and to lights shone in the periphery with “on” firing Oncenter and offcenter cells respond best to contrast, so the most effective way to influence the firing rate is to maximize the contrast between the center and the periphery of its receptive field by entire illuminating one area and leaving the other completely dark. This helped Hubel and Wiesel to conclude that one main function of neurons in the retinal geniculatestriate system is to respond to the degree of brightness contrast between the two areas of their receptive fields. Most visual systems are continually active, even when there is no visual input. Receptive Fields: Simple and Complex Cortical Cells The striate cortex neurons, neurons of the lower layer IV, are exceptions to the majority of striate neurons. Most neurons of the primary visual cortex fall into two classes: simple or complex. Simple cells (like lower IV neurons): have receptive fields that can be divided into “on” and “off” regions and responsive to diffuse light are monocular the main difference between simple and lower IV: the borders of “on” and “off” regions are straight lines rather than circles Complex cells are more numerous and: have rectangular receptive fields, like simple respond best to straightline stimuli, like simple and are unresponsive to diffuse light they have larger receptive fields no division between “on” and “off” regions are binocular, responding to both eyes o Binocular complex cells respond best when the same stimulus is presented to both eyes at once but in slightly different positions, making it likely they play a role in depth perception Columnar Organization of Primary Visual Cortex Two important conclusions from studying the receptive fields of primary visual cortex neurons: 1. The characteristics of the receptive fields of visual cortex neurons are attributable to the flow of signals from neurons with simpler receptive fields to those with more complex fields a. Signals flow from on/off center cells to simple cells and then to complex cells 2. Neurons are grouped in functional vertical columns (vertical in this case means at right angles to the cortical layers) a. Vertically introduced electrodes show that each cell in the column has a receptive field in the same area of the visual field, respond best to straight lines in the same orientation, and those neurons are either monocular or binocular with ocular dominance are all most sensitive to light in the same eye b. Horizontally introduced electrodes show each successive cell encountered is likely to have a receptive field in a different location, respond best to straight lines of a slightly different orientation, and the electrode introduced moves through areas of lefteye dominance and righteye dominance (ocular dominance columns) Plasticity of Receptive Fields of Neurons in the Visual Cortex Most neuroscientific research is based on two implicit assumptions: The mechanisms of visual processing can be identified by studies using simplified, artificial stimuli The receptive field properties of each neuron are static, unchanging properties of that neuron Research of real scenes involving movement, rather than simple stimuli like bars and lights, suggests that neither of these assumptions is correct. These natural scenes indicate the response of a visual cortex neuron depends on not only the stimuli but on the larger scene. This plasticity, or the ability to adapt to change, appears to be a fundamental property of visual cortex function. These simple stimuli studies cannot provide a complete explanation of how our visual system works. 6.5 SEEING COLOR The correct term for colors is hues. The perception of an object’s color depends on the wavelengths of light that it reflects into the eye. Most objects absorb the different wavelengths of light that strike them to varying degrees and reflect the rest. The mixture of wavelength that objects reflect influences our perception of their color. The component theory (trichromatic theory) was proposed by Thomas Young and refined by Hermann von Helmholtz. This theory suggests that there are three different kinds of color receptors (cones), each with a different spectral sensitivity, and the color presumed is due to a ratio of activity in the three kinds of receptors. The opponentprocess theory, proposed by Ewald Hering, suggests that there are two class of cells for encoding color and another for brightness. One class signals red by changing its activity in one direction / green for the other direction Another class signals blue / and its complement yellow Brightnesscoding cells to signal black and white Complementary colors are pairs of colors (red and green light) that produce white or gray when combined in equal measure. Hering based his opponentprocess theory on several observations: Complementary colors cannot exist together (there is no reddish green) The afterimage produced by staring at red is green and vice versa Both colorcoding mechanisms coexist in our visual systems. When a technique for measuring the absorption spectrum of the photopigment contained in a single cone was developed, researchers confirmed that there are three different kinds of cones in the retina and each has a different photopigment with its own characteristic absorption spectrum. They also found that at all levels, there are cells firing in one direction to one color and in the opposite for its complementary color. Primates are trichomats, most mammals are dichromats (difficulty seeing light at the red end of the spectrum), and some have four (allowing them to see ultraviolet light that we don’t). Color Constancy and the Retinex Theory Color constancy is the fact that the perceived color of an object is not a simple function of the wavelengths reflected by it / the tendency for an object to stay the same color despite major changes in the wavelengths of light it reflects Color constancy is counterintuitive but it also has a great advantage that it improves our ability to tell objects apart in a memorable way. We are normally unaware of this feature of our vision. Edwin Land developed lab demonstrations of color constancy. He used three adjustable projectors, each permitting only one wavelength of light. Land found that adjusting the amount of light emitted from each projector had no effect at all on the perception of its colors. Ex: he measured reflected wavelengths of a rectangular judged to be pure blue and another red. When he changed each wavelength emitted to be the same, judges still perceived the original red rectangle as red. This color constancy occurs as long as the object is illuminated with light that contains some short, medium, and long wavelengths. Land’s Retinex theory suggested the color of an object is determined by its reflectance the proportion of light of different wavelengths that a surface reflects. It also suggests the visual system calculates the reflectance of surfaces, and thus perceives their colors, by comparing the light reflected by adjacent surfaces in at least three different wavelength bands. His research is important to neuroscientists because it suggests one type of cortical neuron that is likely to be involved in color vision. If our perception depends on the analysis of contrast between adjacent areas of the visual field, then the critical neurons should be responsive to color contrast and they are! These specialized cells are called dualopponent color cells. o Dualopponent color cells are not evenly distributed throughout the primary visual cortex, but concentrated in peglike columns. They are rich in cytochrome oxidase; thus their distribution in the primary visual cortex can be visualized. These columns are seen as blobs, luckily that’s the scientific label for them. 6.6 CORTICAL MECHANISMS OF VISION AND CONSCIOUS AWARENESS The entire occipital cortex as well as large areas of temporal and parietal cortex are involved in vision. The primary visual cortex receives most of its input from the visual relay nuclei of the thalamus via the lateral geniculate nuclei. Located in the posterior region of the occipital lobes. The secondary visual cortex areas are located in two regions: o The prestriate cortex is the band of tissue in the occipital lobe that surrounds the primary visual cortex o The inferotemporal cortex is the cortex of the inferior temporal lobe The association cortex is located in several parts of the cerebral cortex, but the largest single area is in the posterior parietal cortex. Damage to Primary Visual Cortex: Scotomas and Completion Damage to an area of the primary visual cortex produces scotoma an area of blindness in the corresponding area of the contralateral visual field of both eyes. Patients suspected of damage to this area are given a perimetry test that eventually provides a map of the visual field of each eye, which indicates any areas of blindness. Many patients with extensive
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