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PSYA01H3 (1,184)
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Chapter 5

Chapter 5 Sensation

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University of Toronto Scarborough
Steve Joordens

Chapter 5 Sensation ----------Sensory processing Sensation: the detection of the elementary properties of a stimulus Perception: the detection of the more complex properties of a stimulus, including its location and nature; involves learning Sensory mechanisms: visual, auditory, gustatory, olfactory, somatosensory ***transduction Transduction: the conversion of physical stimuli into changes in the activity of receptor cells of sensory organs (the process by which the sense organs convert energy from environmental events into neural activity) Receptor cell: a neuron that directly responds to a physical stimulus, such as light, vibrations, or aromatic molecules ***sensory coding A code is a system of symbols or signals representing information. Two general forms: anatomical coding & temporal coding Anatomical coding: a means by which the nervous system represents information; different features are coded by the activity of different neurons Sensory coding for the body surface is anatomical. The primary somatosensory cortex contains a neural “map” of the skin. Temporal coding: a means by which the nervous system represents information; different features are coded by the pattern of activity of neurons (It is the coding of information in terms of time. The firing of a particular set of neurons (an anatomical code) tells where the body is being touched; the rate at which these neurons fire (a temporal code) tells how intense that touch is.) ***psychophysics: a branch of psychology that measures the quantitative relation between physical stimuli and perceptual experience *the principle of the just-noticeable difference JND: the smallest difference between two similar stimuli that can be distinguished; also called difference threshold Weber fractions: the ratio between a just-noticeable difference and the magnitude of a stimulus; reasonably constant over the middle range of most stimulus intensities The curve experiments: the amount of physical energy necessary to produce a jnd increases with the magnitude of the stimulus b S=kI K= mathematical constant that adjusts for the way physical intensity is measured I= intensity; b is a number between 0 and 1 (If bigger than 1, like figure 5.2) *signal detection theory Threshold: the point at which a stimulus, or a change in the value of a stimulus, can just be detected Difference threshold: an alternative name for just-noticeable difference Absolute threshold: the minimum value of a stimulus that can be detected Signal detection theory: a mathematical theory of the detection of stimuli, which involves discriminating a signal from the noise in which it is embedded and which takes into account participants’ willingness to report detecting the signal Receiver operating characteristic curve (ROC curve): a graph of hits and false alarms of participants under different motivational conditions; indicates people’s ability to detect a particular stimulus The signal detection method is the best way to determine a person’s sensitivity to occurrence of a particular stimulus. Signal detection theory emphasizes that sensory experience involves factors other than the activity of the sensory systems, such as motivation and prior experience --------Vision ***light Wavelength: the distance between adjacent waves of radiant energy; in vision, most closely associated with the perceptual dimension of hue(ranging from 380 through 760 nanometres) All other radiant energy is invisible to eyes Entire range of wavelength: electromagnetic spectrum The part our eyes can detect: visible spectrum ***the eye and its functions Cornea: the transparent tissue covering the front of the eye Sclera: the tough outer layer of the eye; the “white” of the eye Iris: the pigmented muscle of the eye that controls the size of the pupil The aqueous humour nourishes the cornea and other portions of the front of the eye (A disorder known as glaucoma) Lens: the transparent organ situated behind the iris of the eye; helps focus an image on the retina Accommodation: changes in the thickness of the lens of the eye that focus images of near or distant objects on the retina Farsighted: eyes too short; nearsighted: eyes too long Retina: the tissue at the back inside surface of the eye that contains the photoreceptors and associated neurons Photoreceptor: a receptive cell for vision in the retina; a rod or a cone Optic disc: a circular structure located at the exit point from the retina of the axons of the ganglion cells that form the optic nerve (blind point) The information from the photoreceptors is transmitted to neurons that send axons toward one point at the back of the eye-the optic disc. All axons leave the eye at this point and join the optic nerve, which travels to the brain. Bipolar cell: a neuron in the retina that receives information from photoreceptors and passes it on to the ganglion cells, from which axons proceed through the optic nerves to the brain Ganglion cell: a neuron in the retina that receives information from photoreceptors by means of bipolar cells and from which axons proceed through the optic nerves to the brain Retina: ganglion cell layer(front), bipolar cell layer(middle), photoreceptor layer(back) Light passes from front to back, visual information passes from back to front then to brain The human retina contains two general types of photoreceptors: 125 million rods and 6 million cones Rod: a photoreceptor that is very sensitive to light but cannot detect changes in hue Cone: a photoreceptor that is responsible for acute daytime vision and for color perception Fovea: a small pit near the centre of the retina containing densely packed cones; responsible for the most acute and detailed vision ***transduction of light by photoreceptors Photopigment: a complex molecule found in photoreceptors; when struck by light, it splits and stimulates the membrane of the photoreceptor in which it resides Rhodopsin: the phtopigment contained by rods The photoreceptors of the human eye contain four kinds of photopignments (one for rods and three for cones) The detection of light requires that photons split molecules of rhodopsin or one of the other photopigments ***adaptation to light and dark Dark adaption: the process by which the eye becomes capable of distinguishing dimly illuminated objects after going from a bright area to a dark one ***eye movements Elements of the visual system are not responsive to an unchanging stimulus The eyes also make three types of “purposive” movements: vergence movements, saccadic movements, and pursuit movements Vergence movement: the co-operative movement of the eyes, which ensures that the image of an object falls on identical portions of both retinas Saccadic movement: the rapid movement of the eyes that is used to scanning a visual scene, as opposed to the smooth pursuit movements used to follow a moving object Pursuit movement: the movement that the eyes make to maintain an image of a moving image upon the fovea ***color vision There are three types of cones in the human eye, each containing a different type of photopigment. Each type of photopigment is most sensitive to light of a particular wavelength. The differences in wavelength alone do not account for the differences in the colors we can perceive *the dimensions of color (three physical dimensions: wavelength, intensity, purity) Three perceptual dimensions: Hue: a perceptual dimension of color, most closely related to the wavelength of a pure light Brightness: a perceptual dimension of color, most closely related to the intensity or degree of radiant energy emitted by a visual stimulus Saturation: a perceptual dimension of color, most closely associated with purity of a color *color mixing: the perception of two or more lights of different wavelengths seen together as light of an intermediate wavelength Color mixing is adding, pigment (paint) mixing is subtracting *color coding in the retina Trichromatic theory: the theory that color vision is accomplished by three types of photoreceptors, each of which is maximally sensitive to a different wavelength of light These receptors are sensitive to three of the colors: blue, green, red (Thomas Young) Three types of photopigments: 420, 530, and 560nm (blue, green, red cones) Red and green cones are present in about equal proportions; there are far fewer blue cones Two types of ganglion cells encode color vision: red/green cells and yellow/blue cells. Opponent process: the representation of colors by the rate of firing of two types of neurons: red/green and yellow/blue. (Red and yellow high, green and blue low) (An axon that signals red or green and yellow (or yellow or blue) can either increase or decrease its rate of firing. It cannot do both at the same time) *negative afterimages: the image seen after a portion of the retina is exposed to an intense visual stimulus; a negative afterimage consists of colors complementary to those of the physical stimulus The most important cause of negative afterimage is adaptation to the rate of firing of retinal ganglion cells (rebound effect) *defects in color vision Protanopia: a form of hereditary anomalous color vision; caused by defective “red” cones in the retina (lack of the photopigment for red cones, or red cones filled with green photopigment) (x chromosome) Deuteranopia: a form of hereditary anomalous color vision; caused by defective “green” cones in
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