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Chapter 5

Ch. 5 - Sensation and Perception.docx

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Psychology
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Psychology 1000
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Dr.Mike

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Chapter 5: Sensation and Perception Sensation is when our sensory organs respond to and translate environmental stimuli into nerve impulses that are then sent to our brain. Perception is how we give meaning to the stimulus input 1) The Five stages of Sensory Processing and Perception of Information 1) The afferent nerves receives stimulus 2) Receptors translate stimulus properties into nerve impulses 3) Feature detectors analyze stimulus features 4) Stimulus features are reconstructed into neural representation 5) Matching process results in recognition and interpretation of stimuli The perception of characters is also influenced by their context, that is we can the same thing but perceive them differently due to their context. 2) Sensory Processes - Transduction is the processes whereby the characteristics of a stimulus are converted into nerve impulses Example: Sharks sense electric currents leaking through the skins of fish hiding in the set - Humans have a range of stimulus detectors. For instance, the sense of touch can subdivided into pressure, pain and temperature. - Human sensory systems extract information from the environment on what we need to function and survive - Psychophysics studies the relationship between physical characteristics of stimuli and sensor capabilities. Example: what is the smallest different in brightness can humans detect? - Fechner is the founder of psychophysics and developed the jnd (just noticeable difference) a) Stimulus Detection: The Absolute Threshold - The absolute threshold is the lowest intensity at which a stimulus can be detected 50% of the time. - The lower the absolute threshold, the more sensitive - In humans, vision is the most sensitive - The absolute threshold is not a set value, rather people have their range of uncertainty and decision criterion, a standard of how certain they are that the stimulus is present before they detect it - The criterion is affected by factors such as fatigue, expectations and the significance of the stimulus, also known as signal detection theory - b) Difference Threshold - The smallest different between two stimuli that people can detect 50% of the time - Weber’s law states that the just noticeable difference threshold (or jnd) is directly proportional to the intensity of the physical stimulus Example: If I = 50 db and JND is reported at 55 db. Calculate C - However, Weber’s law is linear and the value of JND is not constant. It doesn’t work for everything - Fechner’s law is sensation is higher with the log intensity. It is more general and more cognitively economic. - Steven’s Power Law is more applicable to a variety of sensations c) Sensory Adaptation - The diminishing sensitivity to an unchanging stimulus is called sensory adaption. Example: when you first enter a swimming pool, it may feel cold at first but your sensors adapt to the temperature. d) Subliminal Perception - Subliminal perception suggests that it’s possible to perceive things unconsciously if it is presented below the absolute threshold - James Vicary falsified that popcorn sales increased by 50% when subliminal cuts to use popcorn was used - However, there is no evidence that subliminal cuts can influence consumer behaviour - It can prime behaviour as proven by Bruce & Valentine (1986) o The experiment tested if people can recognize famous people o A picture relevant to the famous person was shown for 30 msec beforehand o Suggests that as long as it was related, it primed the person - Fitzmons et. Al. (2008) subliminally exposed people to Apple and IBM logos and found that people who were exposed to the Apple logo were more creative with bricks - Therefore, it does not affect consumer behaviour, but can prime behaviour in general 2) Visual System - Cornea is transparent cover that protects and is responsible for focusing. Light enters through the cornea - Behind the cornea is the pupil, an adjustable opening that dilates or constricts to control the light entering the eye. The pupil is controlled by the iris - Lens focuses back to the eye. Becomes thinner to focus on distant objects and thicker to focus on nearby objects - The lens focuses toward the light-sensitive area called the retina o Myopia (nearsightedness) the lens focuses the visual of the image in front of the retina, too near, resulting in blurred images faraway o Hyperopia (farsightedness) is when the lens focuses the image behind the retina; it is difficult to see things up close - There are 3 layers in the retina, with each layer coming closer to the optic nerve (output  optic nerve): o Ganglion First layer, outer edge. Respond to spots of light, but within the cortex they respond to lines. In the cortical cells, they are orientation specific and motion sensitive (ex: bars, edges, lines) o Between the ganglion and bipolar cell is the horizontal cell; inhibitory and responsible for cleaning up the image o Bipolar Second layer o Between the bipolar and receptor is the amacrine; inhibitory and cleans up the image as well o Receptor Last layer a) Photoreceptors - Duplex theory; 2 ways to see - Rods function best in dim light and are receptors of black-and-white brightness - Cones are color receptors which function best in bright light - Humans have a mixture of rods and cones found within the retina. - Rods are not found in fovea, the most sensitive part of the retina - Rods and cones input messages to the brain via bipolar cells which connect to ganglion cells, whose axons are collected into a bundle to form the optic nerve - Visual acuity is the ability to see things in fine detail is the greatest visual objects connected onto the fovea - The optic nerve exits through the back of the eye to the blind spot, an area where there are no photoreceptors b) Visual Transduction - Rods and cones translate light waves into nerve impulses through the action of protein molecules called photo pigments which are released onto the synapse o Rhodopsin breaks down and creates an AP at the axon in rods o Chloroloe, erthryolobe and cynalobe break down selectively and create an AP down the axon - If a nerve response is created, a message is sent instantly to the visual relay station in the thalamus, then to the visual cortex of the brain - Rods more sensitive to the entire color spectrum than cones, with the exception of red. - Dark adaptation is the progressive improvement of brightness sensitivity in dim environments o Photopigments are bleached if exposed to bright light o Cones start to recover first, then the rods recover afterwards by being replenished with photo-pigments - Spectral sensitivity is being used to coloured right, exposed to fovea - Single-Cell Recording The isolation of a single retinal ganglion cell o Attach microelectrodes record output o Project spot of light on screen and move until output maximum o This retinal area is served by the ganglion cell o Movement in any direction will lower firing rate  Inhibition can lower firing rate o Stimulus in the middle and outside maximizes response - Lateral Inhibition Is the inhibitory horizontal cell running from receptor A to bipolar B o If you stimular the receptor, it will increase firing o Lateral inhibition creates a receptive field o Extreme intensity on other sides of receptor decreases the frequency of firing in central receptor  Low intensity will increase frequency of firing o Thus causing phantom spots o Inhibition in adjacent columns will caused perceived intensity to drop c) Colour Vision - Trichromatic theory suggests that there are 3 types of color receptors in the retina and are all stimulated by different wavelengths. However, it had failures: o Yellow is produced by red and green receptors, yet people who are red-green color blind can see yellow o Poses a problem with afterimage, a phenomenon in which staring intently at a colour will produce the same image in a different colour afterwards - Opponent-process theory suggests that each cone responds to two different wavelengths each. For instance, one responds to red or green, blue or yellow, black or white. - Dual-process theory combines the previous 2 theories to account for the colour transduction process. First stage involves the reactions of cones that are maximally sensitive to RGB wavelengths. In the second stage, colour information from the cones is coded through an opponent-process mechanisms further along in the visual system - Colour-deficient vision occurs to people who are color-blind and deficient to either R-G or Y-B systems (dichromat) or B-W (monochromat) d) Analysis and Reconstruction of Visual Scenes - After the transformation of light energy to nerve impulses occurs, the process of combining the messages received from the photoreceptors into the perception of visual scene begins - Optic nerve sends nerve impulses to the thalamus, which is then routed to the primary visual cortex in the occipital lobe - Groups of neurons called feature detectors in the visual cortex receive and integrate sensory nerve impulses originating in specific regions of the retina o Fires selectively in response to stimuli that have specific characteristics o Example: one neuron may fire if it sees a horizontal line - Parallel processing is the ability of our senses to take in a variety of information about an object and construct a unified image of its properties. o Example: a multi-colored beach ball - The information analyzed and recombined by the primary visual cortex is then sent to the visual association cortex - Complex features of a visual scene are combined and interpreted - Simple Cell Fires to a slit at a particular orientation on a specific retinal location o If the line does hit the on-centre, it will generate the maximal response o Ex: A vertical slit at a particular retinal area would “hit” all on centres o Visual cortex can only respond to organized lines - Complex Cell Respond to moving slits at a particular orientation - Hypercomplex Respond to the combination of stimulus features o Cells may be specific and generate a response to a certain stimulus, such as familiar faces 3) Audition - Our sense of hearing is sound waves, a form of mechanical energy. It is pressure waves in air, water or any other conducting medium. - Frequency the number of sound waves, or cycles, per second. Measure in hertz; 1 hertz equals one cycle per second. Pitch and frequency are directly proportioned to one another. o The higher the frequency, the higher the perceived pitch o The lower the frequency, the lower the perceived pitch. - Amplitude vertical size of the sound waves; the amount of compression and expansion of the molecules in the conducting medium. Determines the loudness and expressed in decibels (db). a) Auditory Transduction: From pressure Waves to Nerve Impulses - Transduction system of the ear is made up of tiny bones, membranes and liquid-filled tubes designed to translate pressure waves into nerve impulses i) Transduction within the inner ear - Within the inner ear is the cochlea; a snail-shaped tube filled with fluid and he basilar membrane. Resting on the membrane is the organ of Corti which contains tiny hair cells that act as sound receptors - When sound waves strike the eardrum, the pressure created at the oval window by the bones of the middle ear sets the fluid inside the cochlea in motion. - These waves cause the basilar membrane to vibrate, bending the hair cells in the organ of Corti. - In turn, the hair cells trigger a release of the neurotransmitter into the synaptic space between the hair cells and the neurons of the auditory nerve, sending impulses via an auditory relay station in the thalamus to the auditory cortex in the temporal lobe. o There are feature detector neurons in the auditory cortex which response to specific auditory input b) Coding of Pitch and Loudness - Auditory system transforms the sensory qualities of loudness and pitch into the language of nerve impulses o Ex: High-amplitude sound waves cause the hair cells to bend more, thus releasing more neurotransmitter substances at they synapse, resulting higher rate of firing within the auditory nerve - Loudness is coded in terms of rate of firing in the axons and which specific hair cells. - Frequency Theory states that nerve impulses sent to the brain match the frequency of the sound wave o Disadvantage is that neurons are limited to the rate of firing o Therefore, individual impulses fired by groups of neurons cannot produce frequencies high enough to match sound wave frequencies above 1000 hertz - Place Theory of pitch perception suggests that the specific point in the cochlea where the fluid wave peaks and most strongly bends the hair cells serves as a frequency coding cue o Bekesy (1957) discovered that high-frequency sounds produced an abrupt wave that peaked close to the oval window o Low frequency vibrations produced a slower fluid wave that peaked farther down the cochlear canal - Much like the theories of colour vision, both frequency and place theory compliment each other o At low frequencies, frequency theory is true o At higher frequencies, place theory is true c) Sound Localization and Hearing Loss - The nervous system uses information concerning the time and intensity difference of sounds arriving at the two ears to locate the source of sounds in space - Sounds arrive first and loudest at the ear closest to the sound - Conduction deafness is caused by the problems involving the mechanical system that transmits sound waves to the cochlea o Ex: Punctured ear drum o Hearing aids can help - Nerve deafness Caused by the damaged receptors within the inner ear or damage to the auditory nerve itself o Can’t be cured by the hearing aid o Exposure to loud
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