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PSYC Week 9.docx

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University of Toronto Scarborough
Ingrid Johnsrude

Week 9 Perception Perception is the study of the higherlevel processes that the brain uses to interpret organize and select sensations Perception depends not just on the sensation ie the sensory input but also on knowledge and experience Perception often feels immediate Were not conscious of all the processes involved in creating our perception However the fact that a single stimulus can be perceived in different ways was compelling evidence to the Gestalt psychologists youll learn more about them in a bit that sensation and perception were different Here we will discuss auditory perception of pitch loudness and location and explore visual perception in some detail Do bear in mind though that the principles you are learning in the section on visual perception apply in other sensory domains as well Relating Sensation to Perception PsychophysicsThis term comes from psycho referring to psychological sensation and physics referring to the physical properties of the world In the late 19th century researchers like Fechner and Von Helmholtz were interested in describing the properties of human sensation The psychophysicists goal was to measure the relationship between the physical characteristics of the stimulus and the sensory experience They thought that by doing so they would learn how the human brain transduces environmental stimulation and we would be able to understand all of perception in terms of elementary sensation Although we now accept that that perception is rather more complex than just the sum of sensations Chapter 6 the techniques for measuring sensory thresholds that were developed by the psychophysicists in the 19th century are indispensable tools in psychologyFechner published The Elements of Psychophysics in 1860 laying the foundation for the systematic investigation of sensory processes He was quite a fellowa polymath who was a physicist a physiologist an experimental psychologist and a mystic and he combated debilitating illness much of his life You can read about him here httpserendipbrynmawreduMindConsciousnesshtmlA difference threshold is the just noticeable difference between two stimulithe minimum amount of change required in the intensity of something in order to detect that it is now stronger or weaker than it was Ernst Weber who was a colleague of Fechners but worked a few years before he did was interested in how sensitive people were to changes in weight Weber made the observation that it was easier to detect a very smallincrease in weight when hefting two lightweights than when hefting two heavy weightsnote the small weight added to weight on thefellows left hand at right Weber alsoinvestigated other sensory domains askinghow much longer does one line have to be thananother for someone to just notice that they aredifferent How different in loudness must onetone be from another for someone to just hearthat they are different In general across allsensory domains that he tried the justnoticeable difference threshold for a stimulusthe minimum change in intensitylengthweight that can just be noticed goes up as its intensity goes up In fact the just noticeable difference JND divided by the initial intensity weight loudness length is a constant This is Webers Law Different sensory domains and tasks have different constantsfor sound frequency the JND is 3 for sound intensity its 15 for light intensity its 1 and for taste it is as high as 20 Why might knowing JNDs for different sensory domains be helpfulAs an example lets pretend that you are working as a consultant for a company that makes sweet drinks The head of Research says People love our bestselling drink Sunny G but its very sweet Too much sugar is not good for children who are our best customers and besides sugar is quite expensive Consultant Psychologist thats you how much sugar we can remove from our drinks before people will notice Knowing Webers law and that the JND for taste is about 20 you just need to know how much sugar there is in a litre of Sunny G Lets say it is 10 tablespoons So you know that you can remove up to 2 tablespoons per liter ie 20 of 10 before people will notice the differenceFurthermore Fechner had the insight that although different domains may have different JNDs in every domain each JND might represent an equal step in the psychological magnitude of a sensation This is an amazing ideanow we can relate how strong a stimulus seems to be to how strong it actually iswe can relate the psycho to the physics and do this across sensory domains Fechner tells us that this insight came to him with dramatic suddenness on the morning of Oct 22 1850 before he got out of bedSo going back to our Sunny G example a version with 8 tablespoons of sugar per litre would be one step in magnitude less sweet than classic Sunny G and a version with 64 tablespoons ie 8 tablespoons minus 20 of 8 would be TWO steps in magnitude less sweetFigure 51 in your textbook page 131 shows this fundamental relationship between the physical intensity of a stimulus and its psychological magnitude in units of JND This is Fechners LawWebers law does not hold at the extremes of intensityWeber himself noted this As some stimuli like painful stimuli become intense a smaller change in energy is actually required for a JND This results in Fig 52 in your textFechner law is a theoretical constructFechner himself did not think that it could be tested because he didnt think people could consistently report the psychological magnitudes of sensation But SS Smitty Stevens a psychophysicist working in the middle of the 20th century developed a technique in which participants were asked to assign different numbers to sensations of different magnitudes He showed that people were remarkably consistent in the numbers they assigned and that over a narrow range of intensities Fechners Law held However it did not always hold at high stimulus valuesStevens developed Stevens Power Law to account for the sensation over a greater range of physical stimulus intensitiesIn addition to difference thresholds described thus far we can study absolute thresholds Fechner defined an absolute threshold as the minimum intensity of stimulation that must occur before we can experience a sensation For example the faintest sound a person can hear the gentlest touch a person can feel the weakest odor someone can smellSignal Detection Theory SDTThe observers absolute threshold is not always the same If I present a very very faint sound to you one you can only just hear 100 times you will not say yes I heard it every single timeyoull miss a few In fact the absolute threshold is usually defined as the stimulus level eg concentration of sugar intensity of sound or light magnitude of force where observers say they detected the stimulus on about 50 of the trials This is rather arbitrary One might ask why not make the threshold the stimulus level where they are 25 correct or 75 correct
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