Chapter 6-Sensation & Perception PDF

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Chapter 6: Sensation & Perception BASIC CONCEPTS OF SENSATION & PERCEPTION  

Perception – processes by which her brain organizes & interprets sensory input In everyday experiences, sensation & perception blend into 1 continuous process: o Bottom-up processing starts at sensory receptors & works up to higher levels of processing o Top-down processing constructs perceptions from sensory input by drawing on our experience & expectations  Interpret what our senses detect

Transduction  

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Our sensory systems convert 1 form of energy into another All our senses: o Receive sensory stimulation, often using specialized receptor cells o Transform that stimulation into neural impulses o Deliver neural information to our brain Transduction - process of converting 1 form of energy into another that our brain can use Psychophysics studies relationships between physical energy we can detect & its effects on our psychological experiences

Thresholds 

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Absolute Thresholds o The minimum stimulation necessary to detect a particular light, sound, pressure, taste, or odor 50% of the time o Detecting a weak stimulus, or signal depends on its strength & our psychological state – our experience, expectations, motivation, and alertness o Signal detection theory predicts when we will detect weak signals  Measured as our ratio of “hits” to “false alarms” o Stimuli you can’t detect 50% of the time = subliminal  Below our threshold o Under certain conditions you can be affected by stimuli so weak that don’t consciously notice them  An unnoticed image or word can reach visual cortex & briefly prime response to a later questions  We can evaluate a stimulus even when we aren’t aware of it & even when we’re unaware of our evaluation o Much of our information processing occurs automatically, out of sight, off the radar screen of our conscious mind Difference Thresholds o To function effectively, we need absolute thresholds low enough to allow us to detect important sights, sounds, textures, tastes, and smells o Need to detect small differences among stimuli o Difference threshold (or the just noticeable difference – jnd) = minimum difference a person can detect between any 2 stimuli ½ the time  Increases with the size of the stimulus o Weber’s Law:  States that for an average person to perceive a difference, 2 stimuli must differ by a constant minimum % (not a constant amount)  Exact proportion varies, depending on the stimulus

Sensory Adaptation  

When we are constantly exposed to an unchanging stimulus, we become less aware of it bc our nerve cells fire less frequently Our eyes are always moving – reason why things don’t disappear when we look at it for awhile

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Allows freedom to focus on informative changes in our environment without being distracted by background chatter We perceive the world not exactly as it is, but as it is useful for us to perceive it

Perceptual Set  

A set of mental tendencies & assumptions that affects (top-down) what we hear, taste, feel, and see Through experience we form concepts, or schemas, that organize & interpret unfamiliar information

Context Effects  

A given stimulus may trigger radically different perceptions because of our differing perceptual set and of the immediate context Phenomenon suggests that brain can work backward in time to allow a later stimulus to determine how we perceive an earlier one

Motivation & Emotion 

Perceptions are influenced by motivation & emotions

VISION: SENSORY & PERCEPTUAL PROCESSING Light Energy & Eye Structures  

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Our eyes receive light energy & transduce (transform) it into neural messages that our brain then process into what we consciously see The Stimulus Input: Light Energy o Visible light is a thin slice of the whole spectrum of electromagnetic energy, ranging from imperceptibly short gamma waves to the long waves of radio transmission o 2 physical characteristics of light help determine our sensory experience:  Wavelength – distance from 1 wave peak to the next  Determines its hue (colour we experience)  Intensity – amount of energy in light waves  Determined by a wave’s amplitude, or height  Influences brightness The Eye o Light enters eye through cornea  Cornea bends light to help provide focus o Light passes through pupil  Pupil – small adjustable opening o Iris – surrounds the pupil & controls its size  Coloured muscle that dilates or constricts in response to light intensity  Responds to our cognitive & emotional states  Each is v distinct o Lens – behind the pupil  Transparent  Focuses incoming light rays into an image on the retina  Retina – a multilayered tissue on eyeball’s sensitive inner surface  Focuses rays by changing its curvature & thickness in accommodation (a process) o Retina receives upside-down images of the world  Millions of receptor cells in retina converts particles of light energy into neural impulse & forward those to the brain  In brain, impulses are reassembled into a perceived, upright-seeming image  Along the way, visual information processing percolates through progressively more abstract levels  Happens at astonishing speed  Light signals go from retina to visual cortex, which informs the motor cortex, which then sends out orders to contract muscles

Information Processing in the Eye & Brain 

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Retinal Processing o Path of a single light energy:  Through retina’s sparse outer layer of cells to back of eye  At back of eye, encounter its buried receptor cells (rods & cones)  Would see light energy trigger chemical changes  Chemical reaction would spark neural signals, activating nearby bipolar cells  Bipolar cells activate neighboring ganglion cells  Ganglion cells axons twine together to form optic nerve  Optic nerve = information highway to brain  Thalamus stands ready to distribute information it receives from eyes  Optic nerve can send nearly1 million messages at once through its nearly 1 million ganglion fibers  Where optic nerve leaves eye, there are no receptor cells, creating blind spot o Rods & cones have different locations & functions  Cones cluster in & around fovea (retina’s area of central focus)  Many cones have their own hotline to brain  Each cone transmits its message to single bipolar cell o That cell helps relay cone’s individual message to visual cortex, which devotes large area to input from fovea o These direct connections preserve the cones’ precise information, making them better able to detect fine detail  Rods share bipolar cells which send combined messages  Cones enable you to perceive colour  In dim light they become ineffectual, so you see no colours  Rods enable black-&-white vision; they remain sensitive in dim light  Several rods will funnel their faint energy output onto a single bipolar cell  Rods & cones provide special sensitivity:  Rods to faint light  Cones to detail & colour o Summary: retina’s neural layers pass along electrical impulses, encode & analyze sensory information o In human eyes, information follows this pathway:  After processing by your retina’s nearly 130 million receptor rods & cones, information travels forward again, to bipolar cells  It moves to eye’s million or so ganglion cells, & through their axons making up the optic nerve to your brain  After a momentary stop-off in thalamus, information travels to visual cortex  Any given retinal area relays its information to a corresponding location in your visual cortex in the occipital lobe at the back of your brain o Same sensitivity that enables retinal cells to fire messages can lead them to misfire  Retinal cells are so responsive that even pressure triggers them Colour Processing o Young-Helmholtz Trichromatic (three-colour) theory implies that the eye’s receptors do their colour magic in teams of three  Retina has 3 types of colour receptors, each especially sensitive to one of three colours (red, green, blue) o Most people with colour-deficient vision aren’t actually “colour-blind”  They lack functioning red- or green-sensitive cones, or sometimes both  Their vision is monochromatic (1-colour) or dichromatic (2-colour) o Hering’s opponent-process theory:  3 sets of opponent retinal processes (red-green, yellow-blue, white-black) enable colour vision  Some neurons are turned “on” by red, but turned “off ” by green in the retina & thalamus  “Red” & “green” messages can’t travel at once o Therefore they are opponents o Mystery of colour vision is roughly this: Colour processing occurs in 2 stages:

The retina’s red, green, and blue cones respond in varying degrees to different colour stimuli (Young-Helmholtz Trichromatic theory)  Cones’ responses then process by opponent-process cells (Hering’s theory) Feature Detection o Brain’s computing system deconstructs visual images & then reassembles them o Feature detectors – nerve cells in brain that respond to a scene’s specific features; to particular edges, lines, angles, and movements  Located in occipital lobe’s visual cortex  Receive information from individual ganglion cells in retina  Pass this specific information to other cortical areas, where teams of cells (supercell clusters) respond to more complex patterns Parallel Processing o Doing many things at once o To analyze a visual scene, brain divides it into subdimensions – motion, form, depth, colour - & works on each aspect simultaneously  We then construct our perceptions by integrating separate but parallel work of these different visual teams o To recognize a face, brain integrates information projected by your retinas to several visual cortex areas, compares it to stored information & enables you to recognize the face: Grandmother!  Debated whether this stored information is contained in single cell or (more likely) distributed over vast network of cells  Some supercells – grandmother cells – do appear to respond v selectively to 1 or 2 faces in 100  Facial recognition requires 30% of cortex o After stroke or surgery has damaged brain’s visual cortex, some experience blindsight 

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Perceptual Organization 

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When given a cluster of sensations, people tend to organize them into a gestalt (German for “whole” or “form”) o We can’t separate perceived scene into our left & right fields of view o Our conscious perception is an integrated whole “In perception, the whole may exceed the sum of its parts, rather as water differs from its hydrogen & oxygen parts.” Our brain does more than register information about the world We filter incoming information & construct perceptions Form Perception o Figure & Ground  In our eye-brain system, our first perceptual task is to perceive any object (figure) as distinct from its surroundings (ground)  Sometimes the same stimulus can trigger more than 1 perception o Grouping  Some basic features of a scene (i.e. colour, movement, light-dark contrast) we process instantly & automatically  Our minds bring order & form to stimuli by following certain rules for grouping  Rules:  Proximity – we group nearby figures together  Continuity – we perceive smooth, continuous patterns rather than discontinuous ones  Closure – we fill in gaps to create a complete, whole object  Rules usually help us construct reality Depth Perception o Enables us to estimate an object’s distance from us o Binocular Cues  Important in judging distance of nearby objects  Our eyes are ~2.5 inches apart; retinas receive slightly different images of the world  By comparing these 2 images, the brain can judge how close an object is to you  The greater the retinal disparity (difference between the 2 images), the closer the object

Monocular Cues  Depth cues available to each eye separately  Examples:  Interposition – if one object partially blocks our view of another, we perceive it as closer  Relative size – if we assume 2 objects are similar in size, most people perceive the one that casts the smaller retinal image as farther away  Light & shadow – shading produces a sense of depth consistent with our assumption that light comes from above  Linear perspective – parallel lines appear to meet in the distance; the sharper the angle of convergence, the greater the perceived distance Motion Perception o Normally our brain computes motion based partly on its assumption that shrinking objects are retreating (not getting smaller) and enlarging objects are approaching, people are imperfect at motion perception o When large & small objects move at same speed, large objects appear to move more slowly o Our brain perceives rapid series of slightly varying images as continuous movement (stroboscopic movement) o Lighted signs exploit phi phenomenon with succession of lights that creates impression of moving arrow Perceptual Constancy o Top-down process o Recognize objects without being deceived by changes in their colour, brightness, shape, or size o Colour & Brightness Constancies  Our experience of colour depends on an object’s context  Perception of consistent colour – colour constancy  We see colours thanks to brain’s computations of the light reflected by an object relative to the objects surrounding it  Bc we construct our perceptions, we can simultaneously accept alternative objective & subjective realities  Brightness constancy (aka lightness constancy) depends on context  We perceive an object as having constant brightness even while its illumination varies o Depends on relative luminance – amount of light an object reflects relative to its surroundings  We perceive objects in their environmental context, not in isolation  Comparisons govern perceptions o Shape & Size Constancies  Object whose actual shape can’t change seems to change shape with angle of our view  Shape constancy helps us perceive form of familiar objects as constant even while our retinas receive changing images of them  Our brain manages this with visual cortex neurons that rapidly learn to associate different vies of an object  Size constancy helps us perceive objects as having constant size, even while our distance from them varies  Close connections between perceived distance & perceived size  Perceiving an object’s distance gives us cues to its size  Knowing an objects general size provides us with cues to its distance  In size-distance judgments we consider an object’s context  Perception is not merely a projection of the world onto our brain; our sensations are disassembled into information bits that our brain then reassembles into its own functional model of the external world  Our assumptions can leas us astray  Our brain constructs our perceptions o

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Perceptual Interpretation  

We learn to link an object’s distance with its size Experience & Visual Perception o Restored Vision & Sensory Restriction

Experience guides, sustains, and maintains the brain neural organization that enables our perceptions  Studies suggest that for normal sensory & perceptual development, there is a critical period – an optimal period when exposure to certain stimuli or experiences is required  Early nurture sculpts what nature has endowed  Our visual experience matters Perceptual Adaptation  Within a couple days of new glasses we adjust  Perceptual adaptation to changed visual input makes world seem normal again  Humans adapt to the context and learn to coordinate movements 

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THE NONVISUAL SENSES Hearing   

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Audition (hearing) helps us adapt & survive We are remarkably attuned to sound variations The Stimulus Input: Sounds Waves o Vary in shape o Amplitude of sound waves determines their loudness o Their length (frequency) determines the pitch we experiences o Long waves have low frequency & low pitch o Short waves have high frequency & high pitch o Measure sounds in decibels  Zero decibels represents absolute threshold for hearing  Every 10 decibels corresponds to a tenfold increase in sound intensity The Ear o Process that transforms vibrating air into nerve impulses, which our brain decodes as sounds, begins when sound waves enter outer ear o Intricate chain reaction begins as visible outer ear channels the waves through the auditory canal to the eardrum (tight membrane) causing it to vibrate o In middle ear, piston made of 3 tiny bones (hammer, anvil, stirrup) picks up vibrations & transmits them to the cochlea (snail-shaped tube in inner ear) o Incoming vibrations cause cochlea’s membrane (oval window) to vibrate, jostling fluid that fills the tube o Motion causes ripples in the basilar membrane, bending the hair cells lining its surface o Hair cell movement trigger impulses in adjacent nerve cells o Axons of those cells converge to form auditory nerve, which sends neural messages (via thalamus) to auditory cortex in brain’s temporal lobe o From vibrating air to moving piston to fluid waves to electrical impulses to the brain o Cochlea has 16,000 hair cells o Deflect tiny bundles of cilia on top of a hair cell by width of an atom & alert hair cell, thanks to special protein at its tip, triggers a neural response o Damage to cochlea’s hair cell receptors or their associated nerves can cause sensorineural hearing loss (or nerve deafness)  More common than conduction hearing loss – caused by damage to the mechanical system that conducts sounds waves to cochlea o As pain alerts us to possible bodily harm, ringing of the ears alerts us to possible hearing damage o Only way to restore hearing for people with nerve deafness is a cochlear implant (sort of bionic ear)  Electronic device translates sounds into electrical signals that, wired into cochlea’s nerves, convey information about sound to the brain Perceiving Loudness, Pitch, and Location o Responding to Loud & Soft Sounds  A soft, pure tone activates only the few hair cells attuned to its frequency  Given louder sounds, neighboring hair cells also respond  Brain interprets loudness from # of activated hair cells  If a hair cell loses sensitivity to soft sounds, it ma still respond to loud sounds

Hard-of-hearing people don’t want all sounds amplified; like sound compressed (harderto-hear sounds are amplified more than loud sounds) Hearing Different Pitches  Current thinking of how we discriminate pitch combines 2 theories:  Hermann von Helmboltz’s place theory presumes that we hear different pitches bc different sound waves trigger activity at different places along the cochlea’s basilar membrane o Brain determines a sound’s pitch by recognizing the specific place (on membrane) that is generating neural signal o Problem with this theory: can explain how we hear high-pitched sounds but not low-pitched sounds  Frequency theory (aka temporal theory) suggests: o Brain reads pitch by monitoring the frequency of neural impulses traveling up the auditory nerve o Whole basilar membrane vibrates with incoming sounds wave, triggering neural impulse to brain at same rate as sound wave o Problem: an individual neuron can’t fire faster than 1000 times per second  Volley principle: neural cells can alternate firing o By firing in rapid succession, they can achieve a combined frequency above 1000 times per second, thus:  Place theory best explains how we sense high pitches  Frequency theory best explains how we sense low pitches  Some combination of place & frequency theories seems to handle pitches in the intermediate range Locating Sounds  Placement of our 2 ears allows stereophonic (“three-dimensional”) hearing  A noise can reach one ear sooner than the other 

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The Other Senses 

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Touch o Some spots on skin are especially sensitive to pressure, others to warmth, others to cold, others to pain  Sense of touch is a mix of these 4 basic & distinct skin senses  Our other skin sensations are variations of pressure, warmth, cold, and pain o Cognition influences our brain’s sensory response Pain o Body’s way of telling you something has gon...