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Chapter 5

Chapter 5: Sensation & Perception

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Psychology 1000
Shauna Burke

Chapter 5: Sensation & Perception 1 Introduction • sense organs: experience light waves as brightnesses & colours, air vibrations as sounds, chemical substances as odours or tastes • synthesia: mixing of the senses; women=more likely - pruning of neural connections that occurs in infancy has not occurred in people w/ synthesia; support: diffusion tensor imaging - deficit in neural inhibitory processes in brain that ordinarily keep input from sensory modality from “overflowing” into other sensory areas & stimulating them • A.R. Luria (1968)- studied successful writer/musician; report experiences while listening to electronically generated music • binding problem: how do we bind all our perceptions into one complete whole while keeping its sensory elements separate • sensation: stimulus-detection process by which our sense organs respond to & translate environmental stimuli into nerve impulses that are sent to brain • perception: active process of organizing this stimulus input & giving it meaning Sensory Processes • transduction: specific types of stimuli activate specialized sensory receptors; transported to; nerve impulses; specialized neurons (feature detectors) break down & analyze specific features of stimuli; numerous stimuli pieces are reconstructed into a neural representation that is compared w/ previously stored information (ex. knowledge of what objects look, feel like) • psychophysics: studies relations b/w physical characteristics of stimuli & sensory capabilities; concerned w/ 2 kinds of sensitivity - absolute limits of sensitivity; softest vs loudest sound - differences b/w stimuli Stimulus Detection: TheAbsolute Threshold Chapter 5: Sensation & Perception 2 • absolute threshold: lowest intensity at which a stimulus can be detected correctly 50% of time • lower the absolute threshold=greater the sensitivity Signal Detection Theory • decision criterion: standard of how certain they must be that a stimulus is present before they will stay they detect it • factors: fatigue, expectation, potential significance of stimulus • signal detection theory: concerned w/ factors that influence sensory judgements • ex. experiment- warning light appears, barely perceptible tone may or may not be presented - influenced by rewards & costs for giving correct/incorrect answers - participants become more conservative in their “yes” responses as costs for false alarms are increased, resulting in higher detection thresholds Difference Threshold • smallest difference b/w 2 stimuli that people can perceive 50% of the time • also just noticeable difference • Ernst Weber- some degree of lawfulness in range of sensitivities within sensory systems - Weber’s law: difference threshold is directly proportional to magnitude if stimulus with which comparison is being made; expressed as Weber fraction - ex. weight: 500 g, second weight: 510 g (1/50 difference) - smaller the fraction, greater sensitivity to differences SensoryAdaptation • sensory systems are attuned to changes in stimulation Chapter 5: Sensation & Perception 3 • also habituation • ex. jumping into cold pool, eventually get used to temperature • were it not for tiny involuntary eye movements that keep movements that keep images moving about the retina, stationary objects would simply fade from sight if we stared at them • experiment (1961)- tiny projector on contact lens; guaranteed that visual images presented through projector would maintain constant position on retina, even when eye moved The Sensory Systems Vision • normal stimulus for vision: electromagnetic energy, or light waves • measured in nanometers; ROY G. BIV (colour order) Human Eye: • light waves enter eye through cornea- transparent protective structure at front of eye • pupil- dilates or constricts to control amount of light enters eye • iris- surround pupil • low lights=pupils dilate, more light=improve clarity • lens- elastic structure, thinner to focus on distant objects, thicker to focus on nearby objects • retina- multi-layered tissue; flips image • myopia: nearsightedness; focuses on visual image, blurred image for faraway objects • hyperopia: farsightedness; focuses on faraway objects, blurred image close up; aging Photoreceptors: rods: black & white brightness receptors; dim light; 500x more sensitive than cones • • cones: bright illumination • rods in humans, except in fovea- small area in centre of retina that contains only cones • periphery of retina contains mostly rods • bipolar cells: synaptic connections w/ rods & cones; synapse w/ layer of 1 million ganglion cells- axons are bundled to form optic nerve Chapter 5: Sensation & Perception 4 • light-sensitive ends actually point away from direction of entering light so they receive only fraction of light energy that enters eye visual acuity: ability to see fine detail; visual image projects directly into fovea; results in firing • of large number of cones & their private line bipolar cells • blind spot- no photoreceptors; ordinarily unaware b/c perceptual system fills in missing part of visual field Visual Transduction: From Light to Nerve Impulses • photopigments: rods & cones translate light waves into nerve impulses through action of these protein molecules • absorption of light produces chemical reaction that changes rate of neurotransmitter release at receptor’s synapse w/ bipolar cells • greater the change in transmitter release, stronger signal passed onto bipolar cell & in turn, to ganglion cells • nerve responses at 3 levels (rod or cone, bipolar cell, ganglion cell)=message received instantaneously Brightness Vision & Dark Adaptation • depends on wavelength • rods more sensitive, except at red area • cones more sensitive at yellow-green area • dark adaptation: progressive improvement in brightness sensitivity that occurs under conditions of low illumination - sunlight=photopigments depleted; regenerated & sensitivity increases - after complete adaptation, rods able to detect light intensities only 1/10000 as great as those that could be detected before dark adaptation began - ex. WWII- pilots using red goggles, always be adapted to dark Colour Vision • 1800- any colour in visible spectrum can be produced by some combo of wavelengths that correspond to colours blue, green, red (additive colour mixture) Chapter 5: Sensation & Perception 5 • Young-Helmholtz; Trichromatic theory- 3 types of colour receptors in retina; visual system combines signals to recreate original hue; all 3 activated equally=pure white colour Problems: - yellow produced through activity of red & green receptors; people w/ red-green colour blindness still saw yellow - colour afterimage: image in different colour appears after colour stimulus has been viewed steadily & withdrawn; ex. looking at image for set time, then looking at blank page • Opponent-Process Theory: Ewald Hering (1870)- each of 3 cones responds to 2 different wavelengths; 1=red or green, 2=blue or yellow, 3=black or white - ex. (pg. 154)- green turned red, black turned white; rebound opponent reaction occurred as each receptor responded w/ opposing white or red reactions • Dual Processes in Colour Transduction: combines trichromatic & opponent-process theories to account for colour transduction process - micro-electrodes: record from single cells in visual system, discovered certain ganglion cells in retina respond in opponent process fashion by altering rate of firing • Colour-deficit vision: trichromats=sensitive to all 3 systems (red-green, blue-yellow, black- white); dichromat=person who is colour blind in one system; monochromatic=sensitive only to black-white system • Analysis & Reconstruction of Virtual Scenes: • Feature detectors: - thalamus- brain’s sensory switchboard - primary visual cortex: receives input; in occipital lobe at rear of brain - fovea=disproportionally large area of visual cortex - more than 1 cortical map of retina - feature detectors: organize & integrate sensory nerve impulses originating in specific regions of retina; fire selectively in response to stimuli that have specific characteristics - Hubel & Wiesel: tiny electrodes; record activity of individual cells of visual cortex of animals; certain neurons fired most frequently when lines of certain orientations were presented; ex. different line shapes (crooked, straight) - modules: subdivide visual scene into its component dimensions, process them simultaneously Chapter 5: Sensation & Perception 6 - parallel processing: separate but overlapping modules within brain simultaneously analyze colours, shape, distance & movement; ex. beach ball coming at you - visual association cortex: final stage; recognize beach ball for what it is Audition • sound waves- form of mechanical energy frequency: number of waves/cycles per second; hertz=pitch; 20-20000 Hz • • amplitude: vertical size of sounds waves; compression/expansion; sound’s perceived loudness; decibels- measure of physical pressures take place at eardrum Auditory Transduction: From Pressure Waves to Nerve Impulses: • speed: 1200km/hr, sound waves travel into auditory canal leading to eardrum vibrating activity of bones- hammer, anvil, stirrup amplifies sound waves more than 30 times • • cochlea: contains basilar membrane- > organ of Corti- 16000 hair cells=actual sounds receptors • sound waves strike eardrum- pressure created at oval window by hammer, anvil, stirrup sets fluid inside cochlea into motion • fluid waves that result vibrate basilar membrane & membrane above it, causing bending of hair cells in organ of Corti; triggers release of neurotransmitters into synaptic space b/w hair cells & neurons of auditory nerve - > nerve impulses sent to brain Coding of Pitch & Loudness • high amplitude=cause hair cells to bend more - > more neurotransmitter, synapse w/ auditory cells; higher rate of firing; which specific hair cell is sending message • 2 processes: frequencies below 1000 Hz & for higher frequencies • frequency theory: nerve impulses sent to brain match frequency of sound wave; problem- limited to lower frequencies • George von Bekesy (1957): cut tiny holes in cochlea of guinea pigs & cadavers; higher frequency=abrupt wave peaked close to oval window; lower frequency=slower fluid wave, peaked further down cochlear canal - place theory: specific point in cochlea where fluid wave peaks & most strongly bends hair cells serves as a frequency coding hue - auditory cortex has tonal frequency “map” corresponds to specific areas of cochlea Chapter 5: Sensation & Perception 7 Sound Localization: • time & intensity differences in sounds arriving at 2 ears locate source of sounds in space • sounds arrive first & loudest if closest to ear Hearing Loss: • 3 million Canadians, 23 million NorthAmerica; 90% born w/ normal hearing • conduction deafness: caused by problems involving mechanical system that transmits sound waves to cochlea; use of hearing aids; punctured eardrum • nerve deafness: caused by damaged receptors within inner ear, damage to auditory nerve; cannot be helped by hearing aid; repeated exposure to loud sounds Taste & Smell: The Chemical Senses • gustation: taste • olfaction: smell • common chemical sense: sensitive to chemical molecules, not ene
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