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psy100 Ch5

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University of Toronto St. George
Dan Dolderman

CHAPTER 5: SENSATION, PERCEPTION, AND ATTENTION Sensation: how our sense organs respond to external stimuli and transmit responses to brain - DETECTION Perception: processing of detected sensory signals that result in internal representation of stimulus - CONSTRUCTION of useful info about environment Study of sensation and perception: study of bodily systems that convert stimulus energy into useful info. - many times, perception based on PRIOR EXPERIENCE, shapes our expectations during identification of sensory experience - what we see or hear is result of brain processes that actively construct perceptual experience to allow adaption for env. Can get details wrong HOW do we sense our world? - convert forms of physical energy into signals brain can understand - b/c faced different adaptive challenges, EACH species is sensitive to different types of physical energy o e.g. nocturnal animals: poor vision, superb hearing o insects see ultraviolet, reveals food source sensory coding: how sensory organs translate stimuluss physical properties into neural impulses - diff features coded by DIFF patterns receptors are specialized neurons - transduction: sensory receptors produce neural impulses when receive physical/chemical stimulation - AFTER o Connecting neurons in sense organs transmit info to brain Most go first to thalamus (except smell) From thalamus to cortex, neural impulses interpreted as sight, smell, sound, touch, or taste. Sensations: transduced messages carried by nerve impulses SENSORY CODING divided into TWO: quantitative, qualitative - Quantitative: o Intensity, brightness, loudness o Indexed by neural firing FREQUENCY o Also more intense stimuli tend to recruit more neurons - Qualitative o Color, taste o Different sensory receptors respond to different qualities of a stimulus o In most sensory systems (EXCEPT sense of SMELL) receptors provide coarse coding: sensory qualities are coded by only a few receptors, each responds to broad range of stimuli. By comparing and integrating activity get final percept. Psychophysics: examines PSYCHOLOGICAL experiences of PHYSICAL stimuli - test limits of human sensory systems absolute threshold: minimum intensity of stimulation that must occur before we can experience a sensation, ABOVE CHANCE -e.g. for hearing it is the faintest sound you can hear Difference threshold: the just noticeable difference between two stimuli, minimum amount of change required to detect a difference - e.g. minimum change in volume required for you to notice difference - Webers law: the size of a just noticeable difference is based on a relative proportion of difference rather than a fixed amount of difference o e.g. exam score 6/10 vs 96/100. percentage, not absolute size, is imp o picking up one-ounce letter vs two-ounce easy to detect VS one pound package and one pound + 1 ounce Classical psychophysics based on idea of threshold: either see smthg or not - had ignored important variable: HUMAN JUDGEMENT - absolute threshold was WRONG signal-detection theory: detecting a stimulus requires making a judgment about its presence or absence, based on a SUBJECTIVE interpretation of AMBIGUIOUS info o this is daily tasks of radiologist looking for early signs of cancer o knowledge of patients age/sex affects judgment, level of motivation and attention - involves series of trials in which stimulus is presented on some and not on others. Research participant states whether they observed it or not. Each trial ONE of FOUR outcomes: o signal present, observer detects it: HIT o signal present, failed detect: MISS o signal not present, erroneous detection: false alarm o signal not present, no detection: correct rejection *** SENSITIVITY measured by comparing HIT RATE to FALSE ALARM rate - correct for any bias - response bias: participants tendency to report detecting the stimulus on ambiguous trials o e.g. radiologist checking for tumor is extra cautious about accepting abnormality b/c treatment is dangerous vs checking for signs of broken bone willing to make more positive diagnosis because treatment does not endanger life o also expectations affect bias: soldier expecting imminent attack likely to err on side of responding, many false alarms Sensory Adaptation - when an observers sensitivity to a stimuli decreases over time - when aspect of environment changes, it is important to be able to detect change than for unchanging stimulus. - NOTE: when continuous stimulus stops, large response as well Sensory Processes No neurons beyond sensory organs respond DIRECTLY to the world. Gustation (sense of taste) - job: keep poisons out of digestive system, letting good food in - stimuli: chemical substances that dissolve in saliva - taste receptors part of taste buds of tongue and mouth - people differ greatly in how many taste buds they have, from 500 to 10000 - mostly on tongue, but also mouth and throat - each taste bud has 50 receptors - microvilli: short, hairlike structures at tip of each taste bud, come into direct contact with saliva - when stimulated, send electrical signals to brainstem region called medulla thalamus cortex experience of taste - many kinds of taste receptors, respond to four primary sensations: sweet, sour, salty, and bitter. - Every experience composed of mixture of these 4 - In general, near-infinite variety of perceptual experience from activation of unique combinations of small number of receptor types using coarse coding - Different taste buds spread relatively uniformly throughout tongue and mouth - Taste relies heavily on smell - Also texture, extent of discomfort (spicy), cultural beliefs - Entire taste experience happens in BRAIN not mouth - Supertasters: people who experience intense taste sensations (genetic), more likely to feel pain when eating spicy food, picky eaters Olfaction (sense of smell) - MOST DIRECT route to brain, but also least understood - Sensing chemicals from outside the body - Odor particles nose upper and back portions of nasal cavity olfactory epithelium, thin layer of tissue embedded with olfactory receptors particles dissolve cause reaction that triggers chemical receptors nerve impulses to olfactory bulb: brain centre for smell below the front lobes - FROM HERE GOES DIRECTLY to other brain areas, bypass thalamus - Prefrontal cortex: process info smell is pleasant or aversive - Amygdala: process intensity of smell - These regions involved in emotion and memory formation o Olfactory stimuli can evoke powerful memories and feelings - thousands of receptors, each respond to different chemical group - two possibilities: either each detects specific unique odor, or odors each stimulate several receptors and pattern of activation gives smell - humans sense of smell vastly inferior to other animals - for Helen Keller, called smell potent wizard, able to smell oncoming storm - Pheromones: chemicals released by animals and humans that trigger physiological or behavioral reactions in other animals o Dont smell but processed in same way - specialized receptors in nasal cavity respond - major role in sexual signaling - may explain why menstrual cycles of women living together synchronize Haptic sense (touch) - pain, temperature, pressure - anything that makes contact with skin provides tactile information, gives rise to integrated experience of touch - haptic receptors: sensory neurons that terminate outer layer of skin o their long axons enter CNS by spinal or cranial nerves - for temp, separate receptors for hot and cold, both can be triggered at same time by intense stimuli can produce strange experience like wetness - integration of various signals produces haptic experiences. E.g. tickling sensation, can be pleasant or unpleasant depending on mental state - why cant you tickle yourself? Brain areas involved in touch sensation respond LESS to self-produced tactile stimulation than to external Pain - warning system that prevents you from continuing harmful activities - experience of pain created by brain o e.g. phantom limb: person really feels pain but pain occurs because of misinterpretation of neural activity - most pain experiences from damaging skin - nerve fibers than convey pain info are THINNER than those for temperature and touch and are found in all body tissues that sense pain: skin, muscles, membranes around bones and joints, organs, etc. - TWO kinds of nerve fibers for pain: o 1) fast fibers fo
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