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Chapter 4

Chapter 4

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PSYC 100
Jens C Pruessner

4.1 How do we sense our world? Experience divided into Sensation: sense organs‟ detection and response to external stimulus energy, transmission to brain. Perception: brain‟s processing of detected signals internal representations of stimuli, forms conscious experience. Sensation=detection; perception=construction of useful and meaningful information about environment. What we sense is result of how we perceive.  Stimuli must be coded to be understood by brain: sensory coding: sensory organs translate stimuli‟ physical propertiesneural impulse patterns. Transduction: sensory receptors‟ translation of physical/chemical stimuli into electrical/chemical signals (neural impulses). Brain needs qualitative and quantitative info. Coarse coding: qualities are coded by a few types of receptors, each responding to broad range of stimuli; combined responses by different receptors firing at different rates tells us difference between stimuli.  Psychophysics measures relationship between stimuli and perception o Sensory thresholds: absolute threshold: min stimulation intensity before you experience sensation. Difference threshold: min. change required to detect difference. Weber’s law: based on proportion of original stimulus rather than fixed amount. More intense stimulusbigger change needed to notice. o Signal detection theory: detection of faint stimulus requires judgment (not all-or-none). Stimulus only presented in some trials - hit, miss, false alarm, correct rejection. Response bias: tendency to report signal. Beliefs, expectations, situations influence how person experiences sensations from environment o Sensory adaptation: decrease in sensitivity to constant level of stimulation 4.2 What are the basic sensory processes?  Taste buds detect chemicals: gustation: sense of taste. Taste buds: sensory organs in oral cavity with taste receptors (sweet, sour, salty, bitter, umami). Preference based on varying sensations, also culture (womb).  Nasal cavity gathers odorants: ancestors‟ reliance on visionless developed olfaction: sense of smell, direct route to brain. Chemical particles (odorants) go into nose, nasal cavity, contact olfactory epithelium: thin tissue layer, contains smell receptors, which transmit info to olfactory bulb: brain center for smell. Info bypasses thalamus, prefrontal cortex decides pleasing/aversive, intensity processed in same areas as emotion, memory. Sense of smell important for communication, e.g. pheromones:  Sensors in skin detect pressure, temp, pain: haptic sense. Skin contact = tactile stimulation. Skin is largest sensory reception organ. Sensory neurons reach to skin‟s outer layer, long axons enter CNS through spinal or cranial nerves. o Two types of pain: warning system that stops you from continuing harmful activities. Fast fibers are for sharp, immediate pain and slow fibers are for chronic, dull pain (difference in myelination).  Ear detects sound waves: audition: sense of sound perception. Movement and vibrations of objects cause air molecule displacement, producing change in air pressure, which travels through air. Sound wave: pattern of changes in air pressure, produces percept of sound, arrive at outer ear, travel down auditory canal to eardrum: thin membrane, marks beginning of middle ear, vibrates due to sound waves, translates vibration to ossicles (hammer, anvil, stirrup), transfer to oval window in inner ear. Cochlea: fluid-filled tube with membrane at end called round window. Basilar membrane runs through center of cochlea. Oval window‟s vibrations create pressure waves in cochlear fluid, prompting basilar membrane to oscillate, which stimulates hair cells to bend, send info to auditory nerve (hair cells are primary auditory receptors). Sound waves hit eardrum, converted to neural signals, travel to brain along auditory nerve, producing sensation of sound. o cochlear implant: first neural implant used successfully in humans, helps those with loss of hair cells in inner ear by stimulating auditory nerve.  Eye detects light waves: min. „seeing‟ takes place in eyes; what we see results from brain processes. Light passes through cornea: clear outer covering of eye, focuses incoming light into lens, then light is bent farther inward, focused to form image on retina: thin inner surface back of eyeball, contains photoreceptors that transduce light into neural signals. Pupil: dark circle in eye that opens to let in light waves. Iris: opaque, circular muscle (eye color), control‟s pupil‟s size. Dilates in dim light and when we see something we like. Accommodation occurs behind iris: muscles change lens shape; flatten/thicken to focus on distant/close objects. Lens and cornea form upside-down image on retina o Retinal cells: rods: respond to low light (black-and-white perception); cones: respond to higher light (color perception). Photopigments: light-sensitive chemicals in rods and cones, initiate transduction of light waves into electrical neural impulses. Fovea: center of retina, cones dense, rods at edge. o Transmission from eye to brain: bipolar, amacrine, horizontal cells converge on retinal ganglion cells: first neurons in visual pathway with axons, generate action potential during seeing process, send signal to thalamus, axons gathered into optic nerve bundle, exits eye at back of retina (no rods/cones blind spot). Optic chiasm, half of axons in optic nerves cross, causes info to be projected from L-R hemisphere, vv. Info reaches visual areas of thalamus, travels to primary visual cortex (occipital lobes). o Wavelength determines color of light: color is product of visual system; no color in physical world. 400-700nm. In retina, cones transduce lightneural impulses. Trichromatic theory: color vision results from activity in three types of cones sensitive to different wavelengths (S, M, L), perception of different colors due to ratio of activity among three types of receptors. Opponent-process theory: some colors are “opposites,” when we stare at one color, we see after-image. Color categorized across three dimensions: hue (depends on dominant wavelength), saturation (purity/vividness, based on unity of wavelength), brightness (perceived intensity/luminance, total amount of light reaching eye). Lightness: brightness of stimulus relative to surroundings. o Subtractive color mixing: occurs within stimulus itself (physical, not psyc). E.g., mixing paints. Paint colors determined by pigments, chemicals which absorb different wavelengths and prevent them from being reflected to eye, so its color is wavelength it does not absorb. Red, yellow, blue = subtractive primary colors, mixed togetherblack (absorb nearly all colors of visible spectrum) o Additive color mixing: different wavelengths of light interact within eye‟s receptors, psyc process. Three primaries law of color: any color can be created by combining three wavelengths from different ends of spectrum. Additive primary colors are red, green blue.  Other sensory systems: kinesthetic sense: perception of position in space, movement of bodies and limbs, comes from receptors in muscles, tendons, joints, helps coordinate voluntary movement. Vestibular sense: perception of balance from receptors in semicircular canals in inner ear. 4.3 How does perception emerge from sensation? Experience is construction of brain; neurons talk to each other conscious experience of world. Sensation happens when sensory receptors transduce stimuli into electrical impulses, nerves transmit impulses to brain. Except olfaction, all sensory info is rela
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