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Chapter 5

Chapter 5.docx

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Psychology 1000

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Psychology 1 Chapter 5: Sensation and Perception Synesthesia—“mixing of the senses”. Experience sounds as colours or tastes as touch sensations that have different shapes, women are more likely, it is suggested we’re all born with it Binding problem—how do we bind all our perceptions into one complete whole while keeping its sensory elements separate? Sensation—the stimulus-detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brain Perception—making sense of what out senses tell us—is the active process of organizing this stimulus input and giving it meaning SENSORY PROCESSES  Nerve impulses is the only language the nervous system understands Transduction—the process whereby the characteristics of a stimulus are converted into nerve impulses.  Human sensory systems are designed to extract from the environment the information that we need to function and survive Psychophysics—studies relations between the physical characteristics of stimuli and sensory capabilities Studies 2 kinds of sensitivity 1. Absolute limits of sensitivity 2. Differences between stimuli Absolute Threshold—the lowest intensity at which a stimulus can be detected correctly 50% of the time; the lower the absolute threshold the great the sensitivity Decision criterion—in signal detection theory, the potentially changing standard of how certain a person must be that a stimulus is present in order to report its presence. Depends on fatigue, expectation, and the potential significance of the stimulus Signal detection theory—a theory that assumes that stimulus detection is not based on a fixed absolute threshold but rather is affected by rewards, punishments, expectations, and motivational factors Psychology 2 Difference threshold—the smallest difference between two stimuli that people can perceive 50% of the time. Sometimes called the just noticeable difference Weber’s law—states that the difference threshold, or jnd, is directly proportional to the magnitude of the stimulus with which the comparison is being made, and can be expressed as a Weber fraction. The smaller the fraction the greater the sensitivity to differences Sensory adaptation—sensory neurons are engineered to respond to a constant stimulus by decreasing their activity, and the diminishing sensitivity to an unchanging stimulus is called sensory adaptation Adaptation, also called habituation, reduced our sensitivity to certain things that become monotonous (wrist watch) THE SENSORY SYSTEMS Vision  The normal stimulus for vision is electromagnectic energy  Light-- ROY G BIV  Light waves enter the eye through the cornea—a transparent protective structure at the front of the eye  Behind the cornea is the pupil—an adjustable opening that can dilate or constrict to control the amount of light that enters the eye  The pupil’s size is controlled by muscle in the coloured iris that surrounds the pupil  Behind the pupil is the lens, an elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects  Retina—a multilayered tissue at the rear of the fluid-filled eyeball and is actually an extension of the brain. Has 2 types of light-sensitive receptor cells: the rods and cones  The lens reverses the image from right to left and top to bottom when it is projected on the retina Myopia (nearsightedness)—the lens focuses the visual image in front of the retina resulting in blurred image for far away objects. The eyeball is usually longer, front to back Hyperopia (farsightedness)—occurs when the lens does not thicken enough and the image is therefore focused on a point behind the retina Psychology 3 Rods—function best in dim light, are primarily black and white brightness receptors. They are 500x more sensitive to light than the cones but do not give colour sensations Cones—colour receptors; function best in bright illumination  Humans have a mixture of rods in cones since they see in the night and day  Rods are found throughout the retina except in the fovea Fovea—a small area in the centre of the retina that contains only cones  Cones decrease in concentration as you move away from the centre of the retina  Periphery of the retina contains mainly rods  Rods and cones send their messages to the brain via 2 additional layers of cells: bipolar cells and ganglion cells Bipolar cells—have synaptic connections with the rods and cones and in turn, synapse with a layer of about 1 million ganglion cells Ganglion cells—whose axons are collected into a bundle to form the optic nerve  Rods and cones not only form the rear layer of the retina, but their light-sensitive ends actually point away from the direction of the entering light so that they receive only a fraction of the light energy that enters the eye  Rods and cones are connected to the bipolar and ganglion cells  We can more easily detect a faint stimulus if we look slightly to one side so that its image falls not on the fovea but on the peripheral portion of the retina, where rods are packed more densely  In the fovea the densely packed cones each have their own “private line” to a single bipolar cell. As a result our visual acuity, or ability to see fine detail, is greatest when the visual image projects directly onto the fovea  The optic nerve formed by the axons of the ganglion cells exits through the back of the eye not far from the fovea, producing a blind spot, where there are no photoreceptors  Rods and cones translate light waves into nerve impulses through the action of protein molecules called photopigments. The absorption of light by these molecules produces a chemical reaction that changes the rate of neurotransmitter release at the receptor’s synapse with the bipolar cells Psychology 4  The greater thechange in transmitterrelease,thestronger thesignalpassed ontothe bipolar celland,in turn,the ganglion cellswhose axonsformtheopticnerve  If the nerve responses are triggered ateachofthe3levels:rodor cone,bipolar cell,and ganglion cell,the message isonitswaytothevisualrelaystationinthethalamusand the on to the visual cortex  The brightnesssensitivity of boththerodsand theconesdependsinpartonthe wavelength of the light  Rodshave much greaterbrightnesssensitivitythanconesthroughoutthecolour spectrum except at the red end,whererodsare relatively insensitive  Conesare most sensitive to lowillumination inthegreenish-yellowrange Dark adaptation—the progressive improvement inbrightnesssensitivitythatoccursover time under conditionsof lowillumination  After absorbing light,a photoreceptorisdepletedofitspigment moleculesfor aperiod of time  If the eye hasbeen exposed toconditionsofhigh illumination asubstantialamount of photopigment willbe depleted  During the processof dark adaptation,the photopigment moleculesareregenerated, and the receptors sensitivity increasesgreatly  After about 5-10 minutesconesreachtheir max.  Rodsreach their max.in30mins Twotheoriesof colour vision: 1. The trichromatic theory—  It wasdiscovered that anycolour inthevisiblespectrum can beproduced by some combo of the wavelengthsthatcorrespond tothecoloursblue,green,and red in what isknown as additivecolour mixture  Advanced byThomasYoungand Hermann von Helmholtz  There are 3 typesof colour receptorsintheretina.Althoughallconescan be stimulated by most wavelengthstocarrying,degrees,individualconesaremost sensitive to wavelengths thatcorrespond toeither blue, green,or red  Presumably, each of these receptor classes sends messages to the brain, based on the extent to which they are activated by the light energy’s wavelength  The visual system then combines the signals to recreate the original hue  If all 3 cones are equally activated, a pure white colour is perceived Psychology 5  Flaws: says yellow is produced by activity of red and green receptors but people with red-green colour blindness still see yellow and afterimage—an image in a different colour appears after a colour stimulus has been view steadily 2. Opponent-process theory  Formulated by Ewald Hering  Proposed that each of the 3 cone types responds to 2 different wavelengths  One type responds to red or green, another to blue or yellow, and a third to black or white  Flaws: opponent processes do not occur at the level of the cones, but rather certain ganglion cells in the retina and some neurons do and there is no yellow sensitive cone but rather simultaneous input from red and green sensitive cones Dual-process theory—combines trichromatic and opponent-process theories to account for the colour transduction process  Trichromatic was right about the cones containing 1 of 3 protein photopigments that are mostly sensitive to wavelengths roughly corresponding to the colours blue, red, and green  Different ratios of activity in the red-, blue- and green-sensitive cones can produce a pattern of neural activity that corresponds to any hue in the spectrum Trichromats—people with normal colour vision. They are sensitive to all 3 systems: red-green, yellow-blue and black-white Dichromat—a person who is color-blind in only one of the systems (red-green or yellow-blue Monochromat—sensitive only to the black-white system and is totally colour blind Anaylysis and Reconstruction of Visual Scenes  From the retina, the optic nerve sends nerve impulses to a visual relay station in the thalamus  The input is then routed to various parts of the cortex, particularly the primary visual cortex in the occipital love Psychology 6  Studies have shown there is a point-to-point correspondence between tiny regions of the retina and groups of neurons in the visual cortex—the fovea is represented by a disproportionately large area of the visual cortex  Groups of neurons within the primary visual cortex are organized to receive and integrate sensory nerve impulses originating in specific regions of the retina, some of these are known as feature detectors—they fire selectively in response to stimuli that have specific characteristics (David Hubel and Trosten Wiesel)  Other classes of feature detectors respond to colour, depth or movement  Modules within the brain simultaneously analyze colours, shape, distance and movement by engaging in parallel processing of the info and constructing a unified image of its properties  Brief, high-frequency “bursts” of firing in sensory neurons may function as feature detectors and can signal the occurrence of important stimulus in the sensory field  The final stages occur when the info analyzed and recombined by the primary visual cortex is routed to other cortical regions known as the visual association cortex—here, successively more complex features of the visual scene are combined and interpreted in light of our memories and knowledge  If all goes correctly, then a process that began with nerve impulses from the rods and cones now ends with us recognizing a scene and reacting to it  Neurons in the brain respond selectively not only to basic stimulus characteristics such as corners and colours, but also to complex stimuli that have acquired special meaning though experience Audition  Sound is actually pressure waves in air, water, or some other conducting medium. These sound waves have 2 characteristics: frequency and amplitude Frequency—is the number of sound waves, or cycles, per second. The sound waves’ frequency is related to the pitch that we perceive; the higher the frequency (hertz), the higher the perceived pitch  Hertz—is the technical measure of cycles per second; 1hertz=1 cycle/sec  Humans are capable of detecting sound frequencies from 20 hertz up to 20,000 hertz Amplitude—refers to the vertical size of the sound waves—that is, to the amount of compression and expansion of the molecules in the conducting medium. The sound wave’s amplitude is the primary determinant of the sound’s perceived loudness  Decibels—differences in amplitude (db), a measure of the physical pressures that occur at the eardrum Psychology 7  The transduction system of the ear is made up of tiny bones, membranes, and liquid filled tubes designed to translate pressure waves into nerve impulses  Sound waves travel into an auditory canal leading to the eardrum, a moveable membrane that vibrates in response to the sound waves  Beyond the eardrum is the middle ear, a cavity housing 3 tiny bones (smallest in the body, grain of rice)  The vibrating activity of these bones—the hammer (malleus). Anvil (incus) and stirrup (stapes)—amplifies the sound waves more than x30  The hammer is attached firmly to the eardrum  The stirrup is attached to another membrane, the oval window, which forms the boundary between the middle ear and the inner ear  The inner ear contains the cochlea, a coiled, snail-shaped tube about 3.5cm in length that is filled with fluid and contains the basilar membrane, a sheet of tissue that runs its length  Resting on the basilar membrane is the organ of Corti, which contains about 16,000 tiny hair cells that are actual sound receptors  The tips of the hair cells are attached to the tectorial membrane that overhands the basilar membrane along the entire length of the cochlea  The hair cells synapse with the neurons of the auditory nerve which, in turn, sends impulses via an auditory relay station in the thalamus to the auditory cortex, which is located in the temporal lobe MAKE WEB OF THIS  When sound waves strike the eardrum, pressure created at the oval window by the hammer, anvil and stirrup of the middle ear sets the fluid inside the cochlea into motion  The fluid waves that result vibrate the basilar membrane and the membrane above it, causing a bending ot the hair cells in the organ of Corti  This bending of hair cells triggers a release of neurotransmitter substance into the synaptic space between the hair cells and the neurons of the auditory nerve, resulting in impulses that are sent to the brain MAKE WEB OF THIS TOO  Within the auditory cortex, located in the temporal lobe are feature detectors Pitch and Loudness Loudness  High-amp sound waves cause the hair cells to bend more and release more neurotransmitter substance at the point where they synapse with auditory nerve cells, resulting in higher rate of firing within the auditory nerve. Loudness is coded in terms of both the rate of firing in
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