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PSYC 101 (70)


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University of British Columbia
PSYC 101
Barbara Cox

PSYCH 101, 007 CHPT.5 SENSATION  Sense organs & sensory nerves evolved to provide us w/ useful info about the external world  How sensory system do so depends on specific modality of the info, on characteristics of the info, & the state of the brain at the time it receives the info  Sensory systems process info differently for different sources of info I. Sensory Processing  Sensation- detection of elementary properties of a stimulus  brightness, colour, depth, movement, warmth, sweetness  involves innate, “prewired” physiological mechanisms  Perception- detection of the more complex properties of a stimulus, including its location & nature  animate & inanimate objects, their locations, their movements & their backgrounds  involves learning  seeing colour red is sensation but seeing a red apple is perception Transduction  information about the external world that influence & guide our actions are gathered by sense organs located outside the brain  sense organs detect stimuli provided by light, sound, odour, taste, or mechanical contact with environment  info about these stimuli is transmitted to brain through neural impulses (axon potentials carried by axons in sensory neurons)  sense organs’ task is to transmit signals to the brain that are coded to represent that events that have occurred in the environment  brain’s task is to analyze the info & reconstruct what has occurred  Transduction- process by which sense organs convert physical stimuli into changes in the activity of receptor cells of sensory organs  Receptor cells- specialized neurons that directly respond to a physical stimulus, such as light, vibrations  They release neurotransmitters that stimulate other neurons, which alters the rate of neural firing  In somatosenses, dendrites of neurons respond directly to physical stimuli without use of specialized receptor cells Sensory Coding  Info from sense organs must be coded in the activity of the axons carrying info from sense organs to the brain  A code is a system of symbols or signals representing info  Sensory systems transmit info to the brain using in 2 general forms of coding: anatomical coding & temporal coding  Anatomical coding- different features are coded by the activity of different neurons  Brain uses anatomical coding to interpret location & type of sensory stimulus according to which incoming nerve fibres are active  Rubbing your eyes mechanically stimulates the light-sensitive receptors in eyes  The stimulation produces action potentials in axons of the optic nerves which connect eyes with brain  Visual system of brain has no way of knowing that light-sensitive receptors of eyes were activated by a non- visual stimulus  So because of anatomical coding, brain interprets the stimulus as light even though it was just you rubbing your eyes(since the optic nerves were active)  Experiments performed during surgery have shown that artificial stimulation of nerves that convey taste produces sensation of taste, electrical stimulation of auditory nerve produces a sensation of a buzzing noise, etc.  Sensory coding for boding surface is anatomical  Primary somatosensory cortex contains a neural “map” of the skin  Receptors in skin in different parts of body send info to different parts of primary somatosensory cortex  Temporal coding- different features are coded by pattern of activity of neurons  Coding of info in terms of time  Simples form is rate  By firing at faster or slower rate according to intensity of a stimulus, an axon can communicate quantitative info to brain  Light touch to the skin can be encoded by a low rate of firing & a more forceful touch by a high rate  All sensory systems use rate of firing to encode intensity of stimulation Firing of a particular set of neurons (an anatomical code) tells where the body is being touched while the rate at which these neurons fire (a temporal code) tells how intense that touch is. Psychophysics  Branch of psychology that measures the quantitative relation between physical stimuli & perceptual experience  Two methods to reliably measure people’s sensations: just-noticeable difference & procedures of signal detection theory The Principle of the Just-Noticeable Difference  Ernst Weber investigated the ability of humans to discriminate between various stimuli  He measured the just-noticeable difference (jnd)- smallest difference between 2 similar stimuli that can be distinguished  He discovered a principle that was true for many sensory systems: the jnd is directly related to the magnitude of that stimulus  Ex. he presented participants with 2 metal objects & asked whether they different in weight  Participants reported that 2 weights felt the same unless they differed by a factor of 1 in 40  So a person can just barely tell the difference between a 40-gram weight from a 41-gram weight  Difference between the 2 weights one jnd  Weber fraction- ratio between a just-noticeable difference & the magnitude of a stimulus; reasonably constant over the middle range of most stimulus intensities  jnd in sensation is always a constant proportion of the initial intensity  difference senses had difference rations  for the weight of the 2 objects, ratio was 1 in 40  ratio for detecting differences in brightness of white light is about 1 in 60  Gustav Fechner used Weber’s concept of jnd to measure people’s sensations  Assumed jnd as basic unit of a sensory experience, he measured absolute magnitude of a sensation in jnds  Fechner’s law- as intensity increases, larger increases in intensity are needed to produce subjectively equal changes in sensation  Ex. measure the strength of a person’s sensation of light of a particular intensity  It takes larger and larger increases in light intensity to produce the same changes in perception of brightness as it did in the beginning when intensity of the light increased  shape of the curve rises steeply at first but levels off Signal Detection Theory  Psychophysical methods rely heavily on concept of a threshold  Threshold- point at which a stimulus, or a change in the value of a stimulus, can be detected (line between perceiving & not perceiving)  Just-noticeable difference can also be called a difference threshold, minimum detectable difference between 2 stimuli  Absolute threshold- minimum value of a stimulus that can be detected  Threshold is not an absolutely fixed value  By convention, threshold is the point at which a participant detects the stimulus 50% of the time  Because of variability of activity in nervous system  Neurons are never absolutely still even whet not being stimulated; they fire every so often  If a very weak stimulus occurs when neurons in visual system happen to be quiet, brain is likely to detect it  But if neurons happen to be firing, effects of the stimulus are likely to be lost  Signal detection theory- mathematical theory of the detection of stimuli, every stimulus event requires discrimination between signal (stimulus) & noise (consisting of both background stimuli & random activity of nervous system)  Ex. Experiment: When the light flashes, you may hear a faint tone one second later. You are to say “yes” or “no” after each flash of the light about whether you heard the tone or not.  At first, it is easy. You are confident about your yes and no decisions.  As the experiment progresses, the tone gets fainter & fainter until you have doubts about whether you heard it or not, so you are not sure to respond yes or no.  Your response bias is your tendency to say yes or no when you are not sure whether you detected the tone  Hits are saying yes when stimulus is present  Misses are saying no when stimulus is present  Correct negatives are saying no when stimulus is not present  False alarms are saying yes when stimulus is not present  Signal detection theory is best way to determine a person’s sensitivity to occurrence of a particular stimulus  Concept of threshold is not used  Person decides whether a stimulus has occurred, & consequences of making hits or false alarms can bias this decision  Emphasizes that sensory experience involves factors other than activity of sensory systems, such as motivation & prior experience  Receiver operating characteristic curve (ROC curve)- graph of hits & false alarms of participants under different motivational conditions; indicates people’s ability to detect a particular stimulus ROC curve: % of hits & false alarms in judging presence of a stimulus under several payoff conditions. $1 for every hit- say yes on every trial not matter if you heard the tone or not because you have nothing to lose & everything to gain $1 fine for every false alarm- say no every time because you have everything to lose & nothing to gain II. Vision Light  Consists of radiant energy similar to radio waves  Wavelength- distance between adjacent waves of radiant energy  in vision, most closely associated with perceptual dimension of hue  With human eye, different wavelengths of visible light have different colours  All other radiant energy (ultraviolet radiation, X-rays, gamma rays, infrared, radar, radio waves) but visible light is invisible to our eyes  Entire range of wavelengths is the electromagnetic spectrum  Part that human eyes can detect, that we can see as light, is the visible spectrum The Eye & its Functions  Each eye is located in a bony socket & covered by eyelid to keep dust & dirt out  Eyelashes help keep foreign matter from falling into open eye  Eyebrows prevent sweat on forehead from dripping into eyes  Reflex mechanisms also provide protection to eyes  Sudden approach of something toward the face or touch on surface of eye causes automatic eyelid closure & withdrawal of head  Cornea- transparent tissue covering front of eye  Sclera- tough outer layer of eye; “white” of the eye  Iris- pigmented 2 bands of muscles that controls size of pupil (controls amount of light admitted to the eye)  Brain control these muscles & regulates size of pupil  Constrict pupil in bright light  Dilating pupil in dim light  Space immediately behind cornea is filled with aqueous humour (watery liquid)  This fluid is constantly produced by tissue behind cornea that filters the fluid from the blood  In place of blood vessels, it nourishes the cornea & other parts of the front of the eye  Fluid must circulate & be renewed  Glaucoma- disorder in which if aqueous humour is produced too quickly or if passage that returns it to the blood becomes blocked, pressure within eye can increase & causes damage to vision  Our vision would be less clear if transparent cornea was not nourished by aqueous humour but instead a set of blood vessels  Lens- transparent organ situated behind iris of the eye  Special limitation: must remain transparent so contains no blood vessels, therefore is functionally dead tissue  Curvature of cornea & lens causes images to be focused on inner surface of back of the eye (the retina)  Image is upside down & reversed from left to right but brain compensates & interprets the info appropriately  Shape of cornea is fixed but lens is flexible  A special set of muscles can alter shape of lens so that eye can obtain images of either nearby or distant objects  Accommodation- changes in thickness of the lens that adjust for distance, focus images of near or distant objects on the retina  Normally, length of eye matches bending of light rays produced by cornea & lens so that a visual image is sharply focused on retina  Some people need an extra lens in front of their eyes to correct the mismatch & bring the image into focus  Nearsighted people (eyes too long) need a concave lens to correct the focus  Farsighted people (eyes too short) need a convex lens  As people get older, lenses of their eyes become less flexible  Becomes difficult to focus on objects close to them  Need reading glasses with convex lenses  Retina- tissue at back inside surface of eye that contains photoreceptors & associated neurons  Photoreceptor- receptive cell for vision in retina; a rod or a cone  more than 130 million photoreceptors embedded in retina  info from photoreceptors is transmitted to neurons that send axons toward one point at the back of the eye (the optic disc)  Optic disc- circular structure located at exit point from the retina of the axons of the ganglion cells that form the optic nerve  All axons leave the eye at the optic disc & join the optic nerve, which travels to the brain  No photoreceptors directly in front of optic disk, so this portion of retina is blind  Retina has 3 principle layers  Light passes successively through ganglion cell layer (front), bipolar cell layer (middle) & photoreceptor cell layer (back)  Bipolar cell- neuron in retina that receives info from photoreceptors & passes it on to ganglion cells  Ganglion cell- neuron in retina that receives info from photoreceptors by means of bipolar cells & from which axons proceed through the optic nerves to the brain  Visual info passes through a 3-cell chain to brain: photoreceptor -> bipolar cell -> ganglion cell -> brain  Human retina contains 2 general types of photoreceptors: 125 million rods & 6 million cones  Rod- photoreceptor that is very sensitive to light but cannot detect changes in hue  So mainly function in dim light  many rods from different areas may be connected to just one single ganglion cell so visual info provided by them lack sharpness  Cone- photoreceptor that is responsible for acute daytime vision & for colour perception  Function when level of light is bright enough to see things clearly  Fovea- small pit near center of the retina containing densely packed cones but no rods  Responsible for most acute & detailed vision  When we look at a point in our visual field, we move our eyes so that the image of that point falls directly on cone-packed fovea  Number of cones decreases & number of rods increases as move farther away from fovea Transduction of Light by Photoreceptors  A molecule derived from vitamin A is central ingredient for transduction of the energy of light into neural activity  In absence of light, this molecule is attached to another molecule, a protein-> together form a photopigment  Photopigment- complex molecule found in photoreceptors  4 kinds: 1for rods & 3 for cones  When struck by light, it splits into the 2 molecules that make it up, which starts process of transduction  Splitting of photopigment causes series of chemical reactions that stimulate the photoreceptor  Causes photoreceptor to send a message to the bipolar cell with which it forms a synapse  Bipolar cell sends a message to the ganglion cell, which then sends one on to the brain  Rhodopsin- photopigment contained by rods (pink colour)  Once photopigments are split by light, they lose their colour- become bleached  Energy from photoreceptor’s metabolism causes the 2 molecules of photopigment to recombine  can be used to tranduce light into neural activity again  Each photoreceptor contains many thousands of molecules of photopigment Adaptation to Light & Dark  Dark adaptation- process by which eye becomes capable of distinguishing dimly illuminated objects after going from a bright area to a dark one Eye Movements  Vengeance movement- cooperative movement of the eyes, which ensures image of an object falls on identical portions of both retinas  Saccadic movement- rapid movement of the eyes that is used in scanning a visual scene  Pursuit movement- movement that the eye makes to maintain an image of a move image upon the fovea Colour Vision  Hue- perceptual dimension of colour, most closely related to wavelength of pure light  Brightness- perceptual dimension of colour, most closely related to intensity or degree of radiant energy emitted by a visual stimulus  Saturation- perceptual dimension of colour, most closely associated with purity of a colour  Colour mixing- perception of 2 or more lights of different wavelengths seen together as light of an intermediate wavelength  Trichromatic theory- theory that colour vision is accomplished by 3 types of photoreceptors, each of which is maximally sensitive to a different wavelength of light  Opponent process- representation of colours by rate of firing of 2 types of neurons: red/green & yellow/blue  Negative afterimage- image seen after a portion of the retina is exposed to an intense visual stimulus  Consists of colours complementary to those of the physical stimulus  Protanopia- form of hereditary anomalous colour vision, caused by defective “red” cones in retina  Deuteranopia- form of hereditary anomalous colour vision, caused by defective “green” cones in retina  Tritanopia- form of hereditary anomalous colour vision, caused by lack of “blue” cones in retina III. Audition http://webschoolsolutions.com/patts/systems/ear.htm Sound  Consists of rhythmical pressure changes in air  As an object vibrates, it causes the air around it to move  If the vibrating object is moving toward you, it compresses molecules of air  As it moves away, it pulls the molecules of air further apart  Sound waves are changes over time in the pressure of an elastic medium (for example, air or water)  Without air (or another elastic medium) there can be no sound waves, and thus no sound  Changes in air pressure from sound waves move the eardrum in & out  Sound waves can vary in intensity & frequency  The variations produce corresponding changes in sensations in loudness & pitch  Frequency is related to the pitch we perceive, expressed in hertz (Hz)= cycles per second  Low frequency= low pitch; High frequency=high pitch  Lowest frequency that humans can here is 27 Hz & highest is 20,000 Hz  Amplitude is related to the loudness we perceive, expressed in deciBels (dB)  Low amplitude gives soft sound while large amplitude gives loud sound rd  3 sound perceptual dimension is timbre, which corresponds to complexity of the sound vibration  If a tree falls in the forest…  Creates sound waves  Waves received by ears, transduced into neural signals  Without ears to he
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