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Psychology 20 All Chapters.doc

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Psychology 1000

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Chapter 5 Sensation and Perception pgs 158173SensationStimulus is received by sensory receptorsReceptors translate stimulus properties into nerve impulses transductionFeature detectors analyze stimulus featuresStimulus features are reconstructed into neural representationNeural representation is compared with previously stored information in brainMatching process results in recognition and interpretation of stimuliperceptionSynesthesiamixing of the senses a sensory impaired person may experience sounds as colours or tastes as touch that have different shapesmixed up sensationsSometimes our senses and perceptions mix togetherSensationstimulus detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brainPerceptionmaking sense of what our senses tell usis the active process of organizing this stimulus input and giving it meaning Sensory process can be perceived in different ways at different timesInterpretations are influenced by contextsthe characters that follow them by your learned expectation of what normally follows the number or letterperception is a step beyond sensationSensory ProcessesMore than just the five familiar senses vision audition hearing touch gestation taste and olfaction smellThere are also senses for balance and body position and touch is subdivided into separate senses of pressure pain and temperatureImmune system has sensory receptors that detect invasion and receive stimulation from the brainHuman sensory systems are designed to extract the information from the environment needed to function and survive Psychophysicsstudies relations between the physical characteristics of stimuli and sensory capabilitiesconcerned with 2 kinds of sensitivityAbsolute limits of sensitivityEx what is the dimmest light softest sound humans can detectDifferences between stimuliEx what is the smallest difference in brightness that we can detect How much difference is there between two tones before we can tell that they arent identical Stimulus Detection The Absolute ThresholdHow intense must a stimulus be before we can detect its presenceCan be examined through tests of varying intensitiesto see if they can be detectedAbsolute thresholdthe lower the absolute threshold the greater the sensitivitythe lowest intensity at which a stimulus can be detectedSignal DetectionPeoples apparent sensitivity can fluctuate quite a bitno fixed absolute threshold Theres a range of uncertainty people set their own decision criteriona standard of how certain they must be that a stimulus is present before they say they detect itthis can also change from time to time depending on factors fatigue expectation stimulis significanceSignal detection theoryconcerned with the factors that influence sensory judgments At low stimulus intensities both the participants and the situations characteristics influence the decision criterion if a stimulus is perceived or notIf the reward of perceiving a stimuli is high the results in claiming to perceive a stimuli is high and vice versaThus signal detection research shows that perception is a decision Research Foundations Subliminal Stimuli Can They Affect BehaviourSubliminal stimulus is so weakbrief that although its received from the senses it is not perceived consciouslystimulus is well below absolute thresholdCan subliminal stimuli affect our attitudes and behaviours without our knowledge oYes but to a limited extentStudies have shown that subliminal messages can affect ones perception but the belief of this will increase the effects the messages can have on a persons perceptionThe Difference Threshold pg164 Difference thresholdthe smallest difference between two stimuli that people can perceive 50of the time oCalled JND just noticeable differenceWebers Lawthe difference threshold is directly proportional to the magnitude of the stimulus with which the comparison is being made Can be expressed as a Weber fraction oThe smaller the fraction the greater the sensitivity to differencesoHumans have greater visual sensitivity than their sense of smelloHumans are highly sensitive to differences in pitch of sounds and far less sensitive to loudness differencesSensory AdaptationSensory systems are finely attuned to changes in stimulationSensory adaptationSensory neurons constantly respond to a stimulus by decreasing their activity and diminishing sensitivity to an unchanging stimulusAdaptation habituation is part of everyday lifeoMonotonous background noises eventually disappear the feeling of a watch on your wrist goes away and the coldness of water becomes more tolerableAdaptation occurs in all sensory modalities including visionoIf tiny involuntary eye movements that keep images on the retina didnt occur stationary objects would fade from sight if we stared at themThe Sensory SystemsVisionStimulus for vision is measured in nanometers nm and is usually electromagnetic energy or light wavesElectromagnetic spectrum that humans can perceivexrays tv and radio signals infrared and ultra violet rays The Human EyeLight waves enter eye through corneatransparent protective structure at the front of the eyeBehind cornea is the pupiladjustable opening that can dilate or constrict to control the amount of light that enters the eyePupil size is controlled by muscles in the coloured iris that surrounds the pupilLow levels of illumination cause pupil to dilate more light is let into the eye to improve optical clarity bright light constricts the pupil to limit the amount of light enteredBehind pupil is a lenselastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objectsLens of the eye focuses on the lightsensitive retinamultilayered tissue at rear of the fluidfilled eyeballThe lens reverses the images from right to left and top to bottom when its projected on the retina and the brain reconstructs the visual input into the image perceivedAbility to see clearly depends of the lenss ability to focus the image directly onto the retinaMyopia nearsightednessgood vision for nearby objects difficulty seeing faraway objectsoLens focuses the visual image in front of the retina too close to the lens which results in a blurred image of faraway objectsoUsually occurs because the eyeball is longer front to back than normalHyperopia farsightedness Excellent distance vision difficulty seeing objects close up oOccurs when lens does not thicken enough and the image is focused on a point behind the retina too far from the lensoAging process causes eyeball to become shorter over time which aids in Hyperopia developmentEyeglasses or contact lenses correct natural lenss inability to focus visual images directly onto retinaPhotoreceptors The Rods and ConesThe retinamultilayered screen that lines the back surface of the eyeball and contains specialized sensory neurons is an extension of the brainRetina contains 2 types of lightsensitive receptors cells rods and cones 120 mill rods 6 mill cones in human eyeRodsfunction best in dim light black and white brightness receptorso500 times more sensitive to light than cones but do not give rise to colour sensationsoAn Owls eye contain only rods exceptional vision in dim light but not colour visionConescolour receptors function best in bright illuminationoPigeons and chipmunks only have cones because theyre only active by day see the world in colour have poor night visionHumans and animals that are active by day and night have a mix of rods and conesIn humans rods are found everywhere in the retina except the foveasmall area in the centre of the retina that contains only conesThere are less cones as one moves away from the center of the retina and more rods in the periphery of the retinaRods and cones send messages to brain through 2 additional layers of cellsBipolar cells have synaptic connections with the rods and cones they synapse with a layer of about one million ganglion cells whose axons are collected into a bundle to form the optic nerveInput from more than 126 million rods and cones funnel into 1 million traffic lanes leading out of the retina toward higher visual centersRods and cones form the rear layer of the retina and their light sensitive ends point away from the direction of the entering light so that they receive only a fraction of the light energy that enters the eyeIn the fovea all the cones have their own private line to a single bipolar cellIn result our visual acuityour ability to see in fine detail is greatest when the visual image projects directly on the foveaPredator birds such as hawks and eagles have two foveas in each eye their visual acuity allows them to see small prey on the ground from thousands of feet above the earth Optic nerveformed by ganglion cell axons are in the back of the eye not far from the fovea producing a blind spot with no photoreceptorsWe are usually unaware of our blind spot because our perceptual system fills in the missing part of the visual field Visual Transduction From Light to Nerve ImpulsesTransductionprocess where the characteristics of a stimuli are converted into nerve impulsesRods and cones translate light waves into nerve impulses through the action of protein molecules called photopigmentsLight absorption by the photopigment molecules produces a chemical reaction that changes the rate of neurotransmitter release at the receptors synapse with the bipolar cells The greater the change in transmitter release the stronger the signal passed on to the bipolar cell and to the ganglion cells whose axons form the optic nerve If nerve responses trigger each of the 3 levels rods or cones bipolar cell and ganglion cell the message is sent to the visual relay station in the thalamus and then to the visual cortex of the brainBrightness Vision and Dark AdaptationRods are more sensitive than cones are to low lighting Brightness sensitivity for both rods and cones depends on the wavelength of the lightRods have a much greater brightness sensitivity than cones throughout the colour spectrum except at the red endrods are very insensitiveCones are most sensitive to low illumination in the greenishyellow range of the spectrumDark adaptationthe progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination After absorbing a light a photoreceptors pigment molecules are depleted for a period of time During dark adaptation photopigment molecules are regenerated and the receptors sensitivity increases greatly
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