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Psych 1000: Lecture Notes on Chapter 5

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Western University
Psychology 1000

CHAPTER 5: Sensation and Perception Sensation Psychophysics ~ Subliminal Perception ~ Visual Systems What do Weber fractions tell you about sensation? Can we perceive subliminal messages? How does the visual system work? Sensation Getting the information in How do we perceive the world around us? Psychophysics and Psychophysiology Psychophysics Psychophysics: The relation between the physical stimulus and the psychological response Ex) Vision. We can measure the physical levels how light comes in, but it is very difficult to measure the response. Fechner: “Father” of Psychophysics Can determine a just noticeable difference  JND – Just noticeable difference. This can be measured, we can measure an intensity difference Threshold: - Value of stimulus characteristic required to produce some response - Absolute: Lower limit - Difference: Amount of change for JND Absolute Thresholds Designated as the lowest intensity at which a stimulus can be detected 50% of the time. Vision is the most sensitive system Vision You can see a candle flame on a dark clear night at 50km away Hearing You can detect a watch tick at 6m Taste You can taste a Tsp. of sugar at 8L of water Smell You can detect 1 drop of perfume in a 6 room apt Touch Wing of a fly falling on a check from 1m away Difference Threshold -Amount of change for JND - The relationship between brightness and perceived brightness - Not a 1:1 relation Weber’s Law - Size of a difference threshold relative to physical intensity of test is constant. Must be able to calculate this  I = C I I - is the change of intensity I - is the physical intensity given C - is the constant number If I = 50db and a JND is reported at 55db… then the change of the intensity (I) is 5. (Because the change is 55-50=5) Now we simply plug this information into the formula 5 = C 50 can be reduced to 1 = C 10 Note: 1. Now we can predict other JND‟s eg. What‟s a JND @ 100 db? To do this we look at the given information. We know I is 100db and C is 1/10 (just calculated above)  I = 1 100 10 From here we simply cross multiply (10)(I) = (100) I = 10 - A JND would occur @ new test + 10 = 120 db OR new test – 10 = 90 db - Value of JND is NOT CONSTANT The relative difference is 2. Can compare the sensitivity of different system The smaller the fraction is, the better the system The Weber‟s Constant – The amount of change in order for humans to detect that change Vision (Brightness) 1/60 Kinesthesis (Weights) 1/50 Pain (Thermal) 1/30 Audition (Mid Pitch; Mod. Loud) 1/10 - Pressure (Skin) 1/7 Smell (India Rubber-most sensitive smell) ¼ - change intensity by 25% to notice a difference Taste (Salt) 1/3 The human vision system is the most sensitive system because the change is very small (Small fraction – larger number)) The human taste system is the least sensitive because the change has to be very large (larger fraction –smaller number) in order for a human to notice *Fehner‟s Law (Wont have to calculate this. Only Weber constants) - Sensation increases with the logarithm of intensity.  S = k log I  Compare: delta I = CI  This is more general and cognitively economic Steven‟s Power Law (Don‟t have to calculate this either)  S = k log I^n  Most predictive across a wide variety of sensation Sublimation Perception  Even though you are not consciously aware of it, you simply have to experience it and it will affect your unconscious.  Can we perceive stimuli that are below threshold?  Is our behavior going to be affected by subliminal stimuli? James Vicary (1957)  Claimed the 50% increase in popcorn sales o Didn‟t really. Nobody questioned him because they wanted to believe it.  Concern about the use of subliminal “cuts” o Embedding subliminal cue „BUY POPCORN DRINK SODA‟ o People became worried because maybe you will make them do stuff that they don‟t want to do. Subliminal Messages  Often have hidden sexual images in advertising, saying “SEX”  To be more likely to like a product and want to buy because you think of sex, and want it more Subliminal Perception  In general no evidence that subliminal cuts influence consumer behavior  But consider Bruce & Valentine (1986) o On priming o If you don‟t know what happened in the subliminal cue it made you 100 msec faster to register the image.  Fitzsimons et al (2008) o 30 msce exposure to the apple symbol or the IBM symbol o Then they were asked to think of creative uses of a brick. Those people who were shown the apple generated more creative ideas. o There must be something that influences a subsequent task, not necessarily consumer decisions. o Sensory Systems 1. Accessory Structures (eg. The outer ear) 2. . Transductions -> Receptors 3. Coding (eg. Frequency) 4. Interaction -> Physiological and Psychological The Visual System Iris  Coloured area on the eye  Sphincter muscle to control the size of the pupil Pupil  Controls light level by contracting or expanding Cornea  The transparent covering outside of the eye  Protect the eye  Focusing the light Lens  Helps focus information to the back of the eye. Vitreous Humor  A gel that inflates the eye and keeps the shape inside Retina  Across the back of the eye  Light sensitive receptors are located at the back of the eye  There are three distinct layers in the retina o Ganglion Cells (Output stage) Amacrine (Between ganglion and bipolar)  Inhibitory o Bipolar Cells (Between the ganglion and receptor) Horizontal Cells (Between Bipolar and Receptors)  Exclusively inhibitory. Some sort of inhibition, responsible for cleaning up the image) o Receptor Cells (On the very far back on the eye, to output to the optic nerve) Visual Receptors:  Rods: Thin  Cone: Thick Fovea  The most sensitive part of the retina, almost in the middle of the retina The Optic Nerve  A bundle of axons that escapes from the back of the eye Blind Spot  No visual receptors  axons, blood vessels leave they eye here Visual Processing The retina ~ Single Cell Recording ~ Lateral Inhibition The Retina Rods and Cones - The duplex theory - 120 million rods - 7 Million cones - Rods and cones function very differently RODS  operate at low intensity  sensitive to brightness  none in the fovea (low light source – stars- can only be picked up in peripheral vision because the fovea is center  MONOCHROME – decode in black and white only CONES  Operate at higher intensities o “Insensitive  Cones are located in the fovea, more sensitive to images strait on  Full colour But how do rods and cones work?  Visual Pigments are chemicals that are photosensitive – break down when light hits then  Rods: Rhodopsin – breaks down to create an action potential and generate a pigment. Cones: Chlorolabe, Erythrolabe, Cyanolabe How to demonstrate this 1. Dark Adaptation Curve 2. Spectral Sensitivity But what about “phantom spots” ? –the weird gray spots you seen in an optical illusion Single Cell Recording  Isolate single retinal ganglion cell  Attach microelectrode and record the output.  Project spot of light on screen and move until output maximum  This is retinal area served by the ganglion cell  Movement in ANY direction decreases firing rate Receptive Field  Every ganglion cell has a receptive filed  How a receptive ganglion cell responds.  Centre: Maximum Response area that turns it on  Off-Centre: Minimum response area turns it off.  But not every cell may be on center, some are off center  Doesn‟t have to be a perfect circle  Horizontal cells are inhibitory McCullough Effect Perception Lateral Inhibition ~ Visual Cortex ~ Gestalt Laws Lateral Inhibition – This is not in the text book but a very important concept  Caused by inhibitory horizontal cells that run side ways  Phantom spots can also be explained this way (gray smudges in between black lines)  Off center and on surround which is why we see them at intersections  Lateral inhibition
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