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sense notes.docx

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Western University
Psychology 2015A/B

Chapter 1Principles of perceptual measurement Perception is difficult to measure because it is only experienced by one personWe personally are able to measure our own perceptionsSome psychologists argue that making an attempt at measuring perception is fruitless because it isnt measurable Information obtained across individuals Is generally consistent therefore data can be consistently validatedScientific basis of perceptual measurement Why take a scientific approach o Satisfies a natural curiosity for knowledgeo First step in quantifying the relationship between 2 variables o May offer clues about the nature of the brain Quantitative relationships and their benefit Allows for measurement of stimuli and comparison with other stimuliAllow comparisons between individuals and species Crossmodal comparisonscomparison among different sensory modalitiesGeneral relationship between physical stimulus and perception Early scientists looking for a function that could relate the intensity of a stimulus to its perceived magnitude Increasing functionas the physical intensity of the stimulus increases so will our perception of it this could happen in a number of ways Linear functionproportion between the two remains constant across the whole rangeExponential relationshipperceived sensation intensity changes very slowly at low values of physical intensity but after a certain point changes such that small changes in stimulus intensity produce a dramatic increaseLogarithmic functionslope is very large at the beginning such that perceived intensities can change dramatically with small changes in stimulus intensity 2 general approaches to obtaining the precise relationship between events and experience o Simply ask subjects to rate the perceived intensity of a certain stimulus at various physical intensities and then plot the answers to determine the functiono Measure the smallest change in stimulus that causes a just discriminable change in sensation Experimental psychologists understood that a mathematical relationship could be established by obtaining 2 characteristicsstarting point and slopeClassical psychophysics All 3 functions are displaced to the right rather then at the originThis is because we are unable to detect very low intensitiesThe intensity has to reach a certain detectable levelabsolute threshold Stimulus intensities below this point are subthreshold Suprathreshold region is where sensation takes place For this wed have to determine how the slope changes as a function of physical intensity We find this out using the difference thresholdhow small a change in stimulus intensity is required to produce a discriminable change in sensation Fechner wanted to determine the relationship between the mind and bodycalled psychophysics how physical and mental world interact o He believed that absolute and difference thresholds were fundamental parameters in the formula for perceptionPsychophysical methods 3 general methods developed by Fechner to obtain absolute and difference thresholds Method of adjustmentwhere a human subject is told to simply adjust the physical intensity of a stimulus until it is barely detectable Method of limits preferable if speed isnt an issuesubject is presented with a stimulus whose intensity is chosen from an ascending or decending series if ascendingthe intensity is increased until the subject reports that it was perceived descending the intensity is decreased until it isnt perceived Both these methods make it so that the next intensity value is prediactble Methods of constant stimuli changes thisintensity values are randomly chosen from a preset range and presented to the subjecto Subject replies whether the sensation occurred and frequency chart is established o Typical setup for experiments on vision Absolute threshold Response curves would be expected to look like a step functionAll intensities below a certain point are too weak to produce a detectable sensation Subject is presumed to be an ideal detectorall subthreshold intensities fail to produce a detectable sensory event whereas all suprathreshold ones produce a positiveHumans arent ideal detectors Response curves would realistically look more like an ogive sshape Intervening intensity levels cause uncertainty about whether or not the event occurredThis is a psychometric functionReasons the sensory system deals with uncertaintyo Stimulus itselfusually some variability in the intensity because no physical device can provide perfect delivery at a specific intensityo Our nervous system is inherently noisyo Judgment about detection can be affected by physical emotional and cognitive factorsConventional approaches to threshold detection No welldefined point that can serve as a threshold We therefore adopt an arbitrary response level that can be used to obtain thresholdsusually 50 but can be other numbers No allorone condition for stimulus detection Difference thresholdsErnst Weberworked mainly with discrimination of object weights smallest detectable change for a series of different starting weightsDifference threshold experiment on the visual system If we ask people to discriminate whether a light is getting brighter or dimmer at some points the difference will be so small that they cant detect it The intensity that produces 50 brighter responses can be taken as the point of perceptual equivalenceo At this point the participant couldnt decide whether the light was brighter or dimmer JND just noticeable differenceextra bit of intensity that needs to be added or subtracted from the target light intensity at this perceptual equivalence pointo The amount will depend on what level of noticeable difference we want as our criterion ex 75Incremental thresholdwhen the target light intensity is raised the difference between the 2 intensities b minus a is taken as the difference thresholdDecrement thresholdThe intensity is lowered to point c and the difference between the 2 intensities a minus c is a difference threshold They are often averaged to find a composite valueWebers lawlook up law in book pg 10 Multiple discrimination threshold experiments The greater the intensity level at which we have to make a JND judgment the greater the difference threshold needed to attain that JNDThe difference threshold is therefore not constant but increases in a linear fashion with stimulus intensityImplications of webers law The requirement for a JND is that the incremental or decremental amount be scaled to the stimulus intensity Difference threshold is therefor not a constant value but some proportion k of the stimulus intensity I The proportion k is webers fraction Find Webers fraction by determining the difference threshold at a number of intensities then plot them against the different values of intensity The slope of this line is the Weber fraction Theres no universal Weber fraction that applies to all sensory systems Some are very sensitive to change some of our most acute is pitch perceptionAt the extreme situations of high or low intensities value of k can change dramatically so much so that Webers law doesnt applylimited to a range of intensitiesFechners assumption Proposed that all JNDs were produced by equal increments in sensation regardless of the operating level Fechners law can be integrated into Webers law JND event is presumed to occur through identical changes in sensation the assumption that are brought about by progressively greater changes in intensity Deriving the stimulussensation relationshipAccording to Fechner stimulussensation relationship must necessarily follow a logarithmic functionFechner concluded that the logarithmic function is the only one that will allow for difference threshold to increase according to Webers law but still retain the same values of changes in sensation Fechners lawat low intensity levels the magnitude of our sensations can change quite rapidly with small changes in stimulus intensity whereas we become less sensitive at higher intensitiesModern psychophysics Fechners lawcornerstone of perceptual psychology Stevensproposed a set of direct methods for studying sensationbegan era of modern psychophysicsMagnitude estimation under the power law Whereas Fechner thought sensations could only be measured indirectly through difference thresholds Stevens thought it could be obtained directlyInstrumental in establishing set of procedures known as scalingasked subjects to provide a direct rating of the sensation that they experiencedcame to be known as magnitude estimationExperimental design and outcomes
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