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Chapter 4

Chapter 4 Notes.pdf

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Department
Psychology
Course
PSY100Y5
Professor
Dax Urbszat
Semester
Fall

Description
Page 131 - 178 Chapter 4: Sensation and Perception  Sensation – the stimulation of sense organs  Involves absorption of energy (light or sound waves) by sensory organs (ears, eyes)  Perception – the selection, organization, and interpretation of sensory input  Involves organization and translating sensory input into something meaningful Psychophysics: Basic Concepts and Issues  Psychophysics – study of how physical stimuli are translated into psychological experience (Gustav Fechner) a) Thresholds: Looking for Limits  Sensation begins with a stimulus (any detectable input from environment)  Threshold – a dividing point between energy levels that do and don’t have detectable effect o Ex. you might not be able to detect a weak odour that is apparent to a dog  Absolute Threshold – for a specific type of sensory input is the min. amount of stimulation that an organism can detect o Defines boundaries of an organism’s sensory capabilities o Defined this as the stimulus intensity detected 50% of the time  Fechner: as stimulus intensity increases  subjects’ probability of responding to stimuli gradually increases b) Weighing the Differences: The JND  Just Noticeable Difference (JND) – the smallest difference in the amount of stimulation that a specific sense can detect  Weber’s Law: states that the size of a JND is a constant proportion of the size of initial stimulus o This constant proportion  Weber fraction o Applies to all our senses  Different fractions apply to different types of sensory input o Ex. Weber fraction for lifting weights is approx. 1/30  should be able to detect difference between 300g and a 310g weight  JND: for 300g is 10g c) Psychophysical Scaling  If 1 light has 2x energy of another  don’t perceive as twice as bright o Must scale the magnitude of sensory experience o Fechner used JND as unit of measurement for scaling  Fechner’s Law: states that the mag. of a sensory experience is proportional to # of JNDs that the stimulus causing experience to above the absolute threshold o Ex. in a dark room with a 3 light bulb lamp of same watts  Constant increase in stimulus intensity produce smaller and smaller increasestin perceived mag. of sensation  When 1 bulb goes on  big difference 1 Page 131 - 178  2 bulb: amount of light  x2 but room doesn’t seem to be 2x bright  3 bulb: it adds just as much light as 1 or 2 , but you barely notice difference d) Signal-Detection Theory  Proposes that the detection of stimuli involves decision processes and sensory processes, which are both influenced by a variety of factors beside stimulus intensity  Ex. monitoring a radar screen, looking for signs of possible enemy aircraft o Mission: detect signals that represent approaching airplanes as quickly and as accurately as possible o 4 possible outcomes: 1) Hits: detecting signals when they are present 2) Misses: failing to detect signals when they are present 3) False Alarms: detecting signals when they’re not present 4) Correct Rejections: not detecting signals when they are absent  Performance also depends on level of “noise” in the system o Noise comes from all the irrelevant stimuli in the environment and the neural activity they elicit o More noise in radio station = harder for you to pick up a weak signal e) Perception Without Awareness  Subliminal Perception – the registration of sensory input without conscious awareness o Limen: another term for threshold o Subliminal – below threshold  Subliminal messages are likely to be persuasive b/c people supposedly are defenceless against appeals operating below their threshold of awareness  Ex. theatre contained hidden message “eat popcorn” in a film that was flashed quickly o Result: popcorn sales increases by 58%  Ex. subliminal presentations of pictures of Coke cans and word “thirsty” o Result: increases participants’ ratings of their thirst f) Sensory Adaptation  A gradual decline in sensitivity due to prolonged stimulation  Ex. notice the garbage in kitchen smells o If you stay in the kitchen without removing the garbage, the stench will soon start to fade o Reality: stimulus intensity of odour is stable but with continued exposure, your sensitivity to it decreases  Ex. jump into pool of cold water  will adapt to the temp. few mins later  Is automatic, built-in process that keeps people tuned in to the changes rather than the constants in their sensory input  Allows people to ignore the obvious and focus on changes in environment that may signal threats to safety 2 Page 131 - 178 Our Sense of Sight: The Visual System a) The Stimulus: Light  Light – a form of electromagnetic radiation that travels as a wave, moving, naturally enough, at the speed of light  Light waves vary in amplitude (height) and in wavelengths (distance between peaks)  Light humans usually see  mixtures of several wavelengths o Thus, light can vary in purity  Purity – how varied the mix is  Influences perception of saturation (shades), or richness of color  Humans can see in ultraviolet spectrums while many animals can see infrared spectrums (longer wavelengths)  For people to see, incoming visual input must be converted into neural impulses that are sent to the brain b) The Eye: A Living Optical Instrument  Eyes serve 2 purposes: 1. Channel light to neural tissue that receives it (retina) 2. House that retina  Light enters the eye through a transparent “window” at the front (cornea) o Cornea + crystalline lens (located behind it): form an upside down image of objects on the retina  Lens – the transparent eye structure that focuses light rays falling on the retina o Made up of soft tissue capable of adjustments that facilitate accommodation  Accommodation: occurs when curvature of the lens adjusts to alter visual focus  Ex. focus on a close object  lens get fatter to give clear image o Common visual deficiencies caused by focusing problems/defects in lens  Nearsightedness – close objects are seen clearly but distant objects are blurry o B/c focus of light from distant objects falls a little short of the retina o Occurs when cornea/lens blends the light too much/eyeball is too long  Farsightedness – distant objects are seen clearly but close objects are blurry o b/c focus of light from close objects fall behind retina o occurs when eyeball is too short  Iris – coloured right of muscle surrounding the pupil  Pupil – the opening in the center of the iris that helps regulate the amount of light passing into the rear chamber of the eye o When it constricts: lets less light into eye but sharpens image (bright light) o Dilates: lets more light in but image is less sharp (dim light)  Image sharpness  sacrificed to allow more light to fall on retina so that more remains visible  Saccades – eye movements as we look at our surroundings o Essential to good vision o Small reduction in voluntary eye movements = vision degrades 3 Page 131 - 178 c) The Retina: The Brain’s Envoy in the Eye  Retina – the neural tissue lining the inside back surface of the eye  Absorbs light, process images, and sends visual info to the brain  Axons run from retina to the brain and converges at optic disk o Optic Disk – a hole in the retina where optic nerve fibres exit the eye  Result: we have blind spots 1. Visual Receptors: Rods and Cones  Retina contains millions of receptor cells that are sensitive to light  Light must pass through several layers of cells before it gets to receptors that actually detect it  Cones – specialized visual receptors that play key role in daylight vision and color vision  Don’t respond well to dim light  don’t see color very well in low illumination (lighting)  Better visual acuity than rods  sharpness + precise detail  Fovea – tiny spot in centre of retina that contains only cones  Visual acuity  greatest at this spot  Rods – specialized visual receptors that play key role in night vision and peripheral vision  More sensitive than cones  handle night vision 2. Dark and Light Adaptation  Dark Adaptation – process in which the eye becomes more sensitive to light in low illumination  Light Adaptation – process whereby eyes become less sensitive to light in high illumination 3. Information Processing in the Retina  Light striking at retina’s receptors triggers neural signals that pass into the networks of cells in the retina  sends impulses along the optic nerve (a collection of axons that connect to eye with the brain)  Then the axons which depart from the eye through optic disk, carry visual info, encoded as a stream of neural impulses to brain  Receptive Field of a Visual Cell – is the retinal area that, when stimulated, affects the firing of that cell  Various shapes and sizes  Light falling in centre has opposite effect of light falling in the surrounding area  Ex. rate of firing of a visual cell might be increased by light in the centre of its receptive field and decreased by light in the surrounding area  Lateral antagonism (Lateral Inhibition) – occurs when neural activity in a cell opposes activity in surrounding cells  Responsible for opposite effects that occur when light falls on the inner vs. outer portions of centre surround receptive fields 4 Page 131 - 178  Allows retina to compare light falling in a specific area against general lighting d) Vision and the Brain a. Visual Pathways to the Brain o Optic Chiasm – point at which the optic nerves from the inside half of each eye cross over and then project to the opposite half of the brain  Ensures that signals from both eyes go to both hems. o Axons leaving each eye form the optic nerves which travel to optic chiasm then the optic nerve fibres diverge along 2 pathways 1) Main pathway: projects into the thalamus  About 90% axons from retina synapse in lateral geniculate nucleus (LGN)  Visual signals are processed in LGN and then distributed to areas in occipital love  primary visual cortext  Divided into 2 specialized pathways: magnocellular + parvocellular Engage in parallel processing (involves simultaneously nd extracting different kinds of info from same input) 2) 2 pathway: leaves the optic chiasm branches off to an area in the midbrain  superior colliculus before travelling through thalamus and on to occipital b. Information Processing in the Visual Cortex o Feature Detectors – neurons that respond selectively to very specific features o After visual input is processed in primary visual cortex, often routed to other cortical areas for additional processing  These signals travel through 2 streams:  Ventral Stream: processes details of what objects are out there  Dorsal Stream: processes where the objects are  As signals move further along visual processing system, neurons become more specialized about what turns them on, and stimuli that activate them become more complex  Ex. discovered cells in temporal lobe of humans that are esp. sensitive to pictures of faces c. Multiple Methods in Vision Research o They use fMRI as well as classic techniques (ex. observing performance of individuals who have suffered specific types of brain damage) o Visual Agnosia – an inablility to recognize familiar objects e) Viewing the World in Colour a. The Stimulus for Colour o Recall: lights people see are mixtures of various wavelengths o 2 kinds of colour mixtures: subtractive and additive 5 Page 131 - 178  Subtractive – works by removing some wavelengths of light, leaving less light than was originally there  Ex. mixing yellow + blue = green  Occurs b/c pigments absorb most wavelengths, selectively reflecting specific wavelengths that give rise to particular colours  Additive – works by superimposing lights, putting more light in the mixture than exists in any one light by itself  Ex. when shining red, blue, green spotlights on white surface b. Trichromatic Theory of Colour Vision o Holds that human eye has 3 types of receptors with differing sensitivities to different light wavelengths o Helmholtz: the eye contains specialized receptors sensitive to specific wavelengths associated with red, green, and blue o Means, humans can see all of the colours of the rainbow b/c the eye does its own “colour mixing” by varying ratio of neural activity among these 3 types of receptors o Colour-Blindness – encompasses a variety of deficiencies in ability to distinguish among colours  Occurs more frequently in males than females  Most colour blinded people are dichromats (make do with only 2 colour channels)  3 types of dichromats  each type is sensitive to a different colour (red, blue, green) c. Opponent Process Theory of Colour Vision o Complementary Colours – are pairs of colours that produce grey tones when mixed together  Can be arranged in a colour circle (see pg 148)  If you stare at a strong colour and then look at a white background  see an afterimage  Afterimage – a visual image that persists after a stimulus is removed  Color of afterimage  the complement colour you originally stared at o If asked people to describe colors but restricted them to using 3 names  Result: run into difficulty  Ex. using only red, blue, green  don’t feel comfortable describing yellow as “reddish green”  If allowed 1 more name  usually choose yellow o Opponent Process Theory – holds that colour perception depends on receptors that make antagonistic responses to 3 pairs of colours o 3 pairs of opponent colors  red vs. green, yellow vs. blue, black vs. white o Can describe some aspects of colour-blindness  Ex. can explain why dichromats typically find it hard to distinguish either green from red or yellow from blue 6 Page 131 - 178 f) Perceiving Forms, Patterns, and Objects  Reversible Figure – a drawing that is compatible with 2 interpretations that can shift back and forth o Idea: same visual input can result in different perceptions o No 1-to-1 correspondence exists between sensory input and what you perceive o Reason people’s experience of the world is subjective  Interpretive process can be influenced by manipulating people’s expectations o Ex. info given to you about the drawing of the “circus act involving a trained seal” created by perceptual set  Perceptual Set – a readiness to perceive a stimulus in a particular way  Creates a certain slant in how someone interprets sensory input  Form perception also depends on selection of sensory input o Means what people focus their attention on o A visual scene may include many objects + forms  can either capture viewers’ attention or not o Ex. Intentional Blindness: video of groups of people tossing a ball around  Observers instructed to focus on 1 of the 2 teams and press a key whenever that team passes the ball  30 secs into the task  woman carrying an umbrella clearly walked through the scene for 4 secs  Result: 44% failed to see the woman o Intentional Blindness - a perceptual set that leads people to focus most of their attention on a specific feature in a scene while neglecting other facets of the scene  Can account for many car accidents  Increases when people talk on cell phones or intoxicated a. Feature Analysis: Assembling Forms o Feature Analysis - process of detecting specific elements in visual input and assembling them into a more complex form  Ex. start off with components of a form (lines, corners) then build them into perceptions (cones, telephones, triangles) o Assumes that perception involves bottom-up processing  Bottom-Up Processing - a progression from individual elements to the whole o Problem: form perception often doesn’t involve botto
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