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Chapter 5

Chapter 5 - Sensation & Perception.docx

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Psychology 1000

Sensation & Perception Synesthesia  Mixing of senses  May experience sounds as colours or tastes as touch sensations that have different shapes  Women are more likely to be synaesthetes than men  Cross-wiring  One activity in one part of the brain evokes responses in another part of the brain dedicated to another sensory modality  Word-colour linkages – hearing certain words is associated with neural activity in parts of the visual cortex – doesn’t occur  Theory o One  Pruning of neural connections that occurs in infancy has not occurred  Brain regions retain connections that are absent in most people o Two  Deficit in neural inhibitory processes in the brain that ordinarily keep input from one sensory modality from ―overflowing‖ into other sensory areas and stimulating them Introduction to the Process  Transduction o Sensory receptors must translate this information into the language of nerve impulses  Feature Detectors o Specialized neurons called feature detectors break down and analyze the specific features of the stimuli  Numerous stimulus ―pieces‖ are reconstructed into a neural representation that is then compared with previously stored information, such as our knowledge of what particular objects look, smell, or feel like  This matching of a new stimulus with our internal storehouse of knowledge allows us to recognize the stimulus and give it meaning. We then consciously experience a perception Sensation vs. Perception  Sensation o Stimulus-detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brain  Perception o Making ―sense‖ of what our senses tell us—is the active process of organizing this stimulus input and giving it meaning – decision making 1 Sensory Processes  Contact with outer world only possible through neurons that are specialized sensory receptors – transform energy forms into language of nerve impulses o Energy Forms – Senses  Taste – gustation  Sight – vision  Hearing – audition  Touch - pain, pressure, temperature  Smell – olfaction  Sense magnetic field  Electric currents  Infrared radiation  Balance and body position  Chemical composition of blood  Immune system– detection of foreign invaders  Stimuli to which different animals are sensitive vary  Sensory equipment adaptation to the environment  Transduction o Process whereby the characteristics of a stimulus are converted into nerve impulses o Range of stimuli and the manner in which sense organs carry out  Sensory systems designed to extract information form the environment to survive  Psychophysics o Studies relations between the physical characteristics of stimuli and sensory capabilities o Concerned with two kinds of sensitivity  Absolute limits of sensitivity  Weakest salt solution  Softest sound  Differences between stimuli  Smallest difference in brightness  Weight differences Stimulus Detection: The Absolute Threshold  Absolute Threshold o Lowest intensity at which a stimulus can be detected correctly 50 percent of the time o Lower absolute threshold, the greater the sensitivity 2 Signal Detection Theory  Individuals apparent sensitivity can fluctuate  Concept of fixed absolute threshold is inaccurate – no single point on the intensity scale that separates nondetection from detection  Range of uncertainty – people set their own decision criterion o Standard of how certain they must be that a stimulus is present before they will say they detect it o Change from time to time – depends on fatigue, expectation and potential significance of stimuli  Signal Detection Theory o Concerned with the factors that influence sensory judgments o Experiment example  Participants are told that after a warning light appears, a barely perceptible tone may or may not be heard  Tell the experimenter whether they heard the tone  Four possible outcomes  Tone - ―Yes‖ (a hit) or ―No‖ (a miss)  No tone - ―Yes‖ (a false alarm) or ―No‖ (a correct rejection)  At low stimulus intensities, both the participant's and the situation's characteristics influence the decision criterion  Can be influenced to become bolder or more conservative by manipulating the rewards and costs for giving correct or incorrect responses.  Increasing the rewards for hits or the costs for misses results in lower detection thresholds o Ex.  Navy radar operator more likely to notice a faint blip during wartime than during peacetime (disastrous consequence for miss)  Doctors who will not perform a risky medical procedure without strong evidence to support their diagnosis – patients careful about ―Yes‖ responses as costs are higher - higher detection thresholds Difference Threshold  Subtle differences important  Difference Threshold (jnd – just noticeable difference) o Smallest difference between stimuli that can be perceived50% of the time  Weber’s Law o Difference threshold is directly proportional to the magnitude of the stimulus with which the comparison is being made, and can be expressed as a Weber fraction o Breaks down at incredibly high and low intensities o Smaller the fraction – greater sensitivity to differences o Ex.  jnd value for weights is a Weber fraction of approximately 1/50  If you lift a weight of 50 grams, a comparison weight must weigh at least 51 grams in order for you to be able to judge it as heavier 3  If the weight were 500 grams, a second weight would have to weigh at least 510 grams (i.e., 1/50 = 10 grams/500 grams) for you to discriminate between them Sensory Adaptation  Sensory systems are finely attuned to changes in stimulation  Sensory Adaptation o Sensory neurons are engineered to respond to a constant stimulus by decreasing their activity and diminishing sensitivity to unchanging stimulus o Part of everyday experience o Reduce overall sensitivity – frees senses from constant and mundane informative changes in environment Sensory Systems Vision  Normal stimulus: electromagnetic energy, or light waves  Measured in nanometres (or one billionth of a metre)  Electromagnetic Spectrum o X-rays o TV & Radio signals o Infrared o Ultraviolet o Visual spectrum  Wavelengths from 700 (red) to 400 (blue-violet) nanometers Human Eye  Cornea o Light waves enter the eye through o Transparent protective structure at the front of the eye  Pupil o Behind cornea o Adjustable opening that can dilate or constrict to control the amount of light that enters the eye o Size is controlled by muscles in the coloured iris that surrounds the pupil o Low levels of illumination cause the pupil to dilate, letting more light into the eye to improve optical clarity o Bright light triggers constriction of the pupil  Lens o Elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects o Lens of the eye focuses on the visual image on the light-sensitive retina  Retina o Multi-layered tissue at the rear of the fluid-filled eyeball o Lens reverses the image from right to left and top to bottom when projected on the retina o Brain reconstructs the visual input into the image that we perceive o Ability to see clearly depends on the len’s ability to focus the image 4 directly onto the retina  Myopia o Good vision for nearby objects but difficultly seeing faraway objects (nearsightedness) o Lens focuses the visual image in front of the retina (too near the lens), resulting in a blurred image for faraway objects o Eyeball is longer (front to back) than normal  Hyperopia o Excellent distance vision but have difficulty seeing close-up objects clearly o Farsightedness o Lens does not thicken enough and the image is therefore focused on a point behind the retina (too far from the lens) o Eyeball becomes shorter overtime o Middle-aged people acquire reading glasses  Astigmatism o Refractive errors due to a curvature of the cornea Photoreceptors: Rods & Cones  Retina o Multi-layered screen that lines the back surface of the eyeball and contains specialized sensory neurons – an extension of the brain o Contains two types of light-sensitive receptor cells, called rods and cones because of their shapes o 120 million rods, 6 million cones in human eye  Rods o Function best in dim light o Primarily black-and-white brightness receptors o 500 times more sensitive to light than are the cones – no not give rise to colour sensations o Retinas of night creatures – contain only rods o Humans – rods are found throughout retina except in the fovea (small area in the centre of the retina) o Periphery of the retina contains mainly rods  Cones o Colour receptors o Function best in bright illumination o Active during the day – pigeon and chipmunk o Humans – decrease in concentration as one moves away from the centre of the retina  Send messages to the brain via two additional layers of cells  Bipolar Cells o Synaptic connections with the rods and cones o Synapse with a layer of about one million ganglion cells, whose axons are collected into a bundle to form the optic nerve  Process o Input from more than 126 million rods and cones is eventually funneled into only one million traffic lanes leading out of the retina toward higher visual centres 5 o Rods and cones not only form the rear layer of the retina – light-sensitive end actually point away from the direction of the entering light so that they receive only a fraction of the light energy entering o Many rods are connected to the same bipolar cell  Combine or ―funnel‖ their individual electrical messages to the bipolar cell – additive effect of many signals may be enough to fire it  We can more easily detect a faint stimulus  Ex. Dim star - If we look slightly to one side so that its image falls not on the fovea but on the peripheral portion of the retina – rods are packed most densely o Cones that lie in the periphery of the retina  Share bipolar cells  In fovea, the densely packed cones each have their own private lane to a single bipolar cell  Visual acuity (fine detail) is greatest when the visual image projects directly onto fovea  Focusing results in the firing of large number of cones and their private-line bipolar cells o 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 (no photoreceptors)  Unaware of blind spot because our perceptual system ―fills in‖ missing part of field Visual Transduction: From Light to Nerve Impulses  Transduction o Process whereby the characteristics of a stimulus are converted into nerve impulses  Photopigments o Rods and cones translate light waves into nerve impulses through the action of protein molecules o Absorption of light by molecules produces a chemical reaction – changes the rate of neurotransmitter release at the receptor’s synapse with the bipolar cells o Greater the change in transmitter release – stronger the signal passed to bipolar cells and in turn to the ganglion cells whose axon forms the optic nerve  Nerve Responses 6 o Triggered at three levels  Rod or cone  Bipolar cell  Ganglion cell o Message is instantaneously on its way to the visual relay station in the thalamus – then onto visual cortex of brain Brightness Vision & Dark Adaptation  Rods are far more sensitive than cones under conditions of low illumination  Brightness sensitivity of both the rods and the cones depends in part on the wavelength of the light  Rods have much greater brightness sensitivity than cones throughout the colour spectrum except at the red end, where rods are relatively insensitive  Cones are most sensitive to low illumination in the greenish-yellow range of the spectrum  Dark adaptation is the progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination  After absorbing light, a photoreceptor is depleted of its pigment molecules for a period of time  If the eye has been exposed to conditions of high illumination, such as bright sunlight, a substantial amount of photopigment will be depleted  During the process of dark adaptation, the photopigment molecules are regenerated, and the receptor's sensitivity increases greatly  Cones gradually become sensitive to fainter lights as time passes, but after about 5 to 10 minutes in the dark, their sensitivity has reached its maximum  Rods - whose photopigments regenerate more slowly, do not reach their maximum sensitivity for about half an hour Colour Vision  Two theories of colour vision Trichromatic Theory  Additive Colour Mixture o Three types of colour receptors in the retina o A beam of light of a specific wavelength directed onto a white surface is perceived as the colour that corresponds to that wavelength on the visible spectrum o Beams of light that fall at certain points within the red, green, or blue colour range are directed together onto the surface in the correct proportions o Combined or additive mixture of wavelengths will result and any colour in the visible spectrum can be produced o White at the point where all three colours intersect o Assumes that colour perception results from the additive mixture of impulses from cones that are sensitive to red, blue, and green o Problems  Theory: yellow is produced by activity of red and green receptors  People with red-green colour blindness are able to experience yellow  Colour Afterimage 7  An image in a different colour appears after a colour stimulus has been viewed steadily and then withdrawn  Subtractive Colour Mixture o Mixing pigments or paints produces new colours by subtraction—that is, by removing (i.e., absorbing) other wavelengths o Paints absorb (subtract) colours different from themselves while reflecting their own colour o Ex. blue paint mainly absorbs wavelengths that correspond to nonblue hues. Mixing blue paint with yellow paint (which absorbs wavelengths other than yellow) will produce a subtractive mixture that emits wavelengths between yellow and blue (i.e., green). o Theoretically, certain wavelengths of the three primary colours of red, yellow (not green, as in additive mixture), and blue can produce the whole spectrum of colours by subtractive mixture Opponent-Process Theory  Assumed that there are three types of cones  Proposed that each of the three cone types responds to two different wavelengths  One type responds to red or green, another to blue or yellow, and a third to black or white  For example, a red-green cone responds with one chemical reaction to a green stimulus and with its other chemical reaction (opponent process) to a red stimulus  Ex. Staring at a coloured-image o Stared at the black and green colours, the neural processes that register these colours became fatigued o Cast gaze on the white surface, which reflects all wavelengths, a ―rebound‖ opponent reaction occurred o Each receptor responded with its opposing white or red reactions Dual Process – Colour Transduction  Combines the theories to account for the colour transduction process  Trichromatic theorists were right that cones do indeed contain one of three different protein photopigments that are most 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  Opponent processes do not occur at the level of the cones, as he maintained  Certain ganglion cells in the retina, as well as some neurons in visual relay stations and the visual cortex, respond in an opponent-process fashion by altering their rate of firing Colour-Deficient Vision  Trichromats o Normal colour vision o Sensitive to all three systems  Red-green  Yellow-blue 8  Black-white o Dichromat  Person who is colour-blind in only one systems (red-green or blue- yellow) o Monochromat  Sensitive to only black-white system Analysis & Reconstruction of Visual Scenes  Once the transformation of light energy to nerve impulses occurs, the process of combining the messages received from the photoreceptors into the perception of a visual scene begins Feature Detectors  From the retina, the optic nerve sends nerve impulses to a visual relay station in the thalamus, the brain's sensory switchboard  Input is then routed to various parts of the cortex, particularly the primary visual cortex in the occipital lobe  Point-to-point correspondence between tiny regions of the retina and groups of neurons in the visual cortex  Fovea o One-to-one synapses of cones with bipolar cells produces high visual acuity, is represented by a disproportionately large area of the visual cortex o More than one cortical ―map‖ of the retina; there are at least 10 duplicate mappings  Groups of neurons within the primary visual cortex are organized to receive and integrate sensory nerve impulses originating in specific regions of the retina  Feature Detectors o Fire selectively in response to stimuli that have specific characteristics o Certain neurons fired most frequently when lines of certain orientations were presented o One neuron might fire most frequently when a horizontal line was presented; another neuron would fire most frequently to a line of a slightly different orientation o Some cells respond to bars, slits, edges in certain positions o Respond to colour, movement, depth  Parallel processing o Subdivide a visual picture into components and process them at the same time o Construct a unified image of its properties  Visual Association Cortex o Final stages in the process occur o Information is analyzed and recombined by the primary visual cortex and is routed to the VAC o Complex features of visual image are combined and interpreted in light of memories and knowledge Audition  Stimuli for sense of hearing – sound waves – mechanical energy  Sound 9 o Pressure waves in air, water or conducting medium o Vibrations of sound caused by successive waves of compression and expansion among the air molecules surround source o Two Characteristics  Frequency  Number of sound waves, or cycles per second  Hertz (Hz) - technical measure of cycles per second  Related to the pitch (higher pitch: higher frequency)  Human Range: 20 Hz – 20,000 Hz  Amplitude  Vertical size of sound waves  Amount of compression and expansion of molecules in conducting medium  Primary determinant of the perceived ―loudness‖  Differences expressed as decibels (db) – measure of physical pressures that occur at eardrum  Increase tenfold – decibel Auditory Transduction: From Pressure Waves to Nerve Impulses  Transduction System o Made up of tiny bones, membranes, and liquid-filled tubes designed to translate pressure waves into nerve impulses o Sound waves travel into an auditory canal leading to the eardrum – movable membrane that vibrates in response to waves (1,200 km/h)  Middle Ear o Beyond eardrum o Cavity housing three smallest bones  Hammer (malleus)  Attached to eardrum  Anvil (incus)  Stirrup (stapes)  Attached to the oval window (membrane) – boundary between middle and inner ear o Amplifies sound waves 30x  Inner Ear o Contains cochlea  Coiled, snail-shaped tube  Filled with fluid  Contains basilar membrane o Basilar Membrane  Sheet of tissue  Runs its length o Origin of Corti  Resting on basilar membrane  Contains 16,000 tiny hair cells (sound receptors) o Hair Cells  Tips are attached to tectorial membrane that overhands the basilar membrane along the entire length of cochlea 10  Synapse with neurons of the auditory nerve – sends impulses via an auditory relay station in thalamus to auditory cortex – temporal lobe  Process o Sound waves strike eardrum o Pressure created at the oval window by hammer, anvil and stirrup o Sets fluid inside cochlea into motion o Fluid waves vibrate the basilar membrane and the membrane above it o Bending of hair cells o Triggers a release of neurotransmitter substance into synaptic space between hair cells and neurons of auditory nerve o Nerve impulses result and sent to brain o Auditory cortex – feature detector neurons that respond to specific kinds of input Coding of Pitch & Loudness  Transforms sensory qualities of loudness and pitch into nerve impulses  Loudness o High-amplitude sound waves cause hair cells to bend more o Release more neurotransmitter substance o Resulting in a higher rate of
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