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4031 Exam 2

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PSYC 4031
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CH 3Darkadaptation differencesConesrods are both differentially sensitive to lightdiffer in the speed with which they adapt to changes in illuminationAdaptation in the context of light energy adaptation is a change in sensitivity to a particular light intensity Lamb 1990Dark adaptation an increase in sensitivity as the eyes remain in the darkLight adaptation a decline in sensitivity as the eyes remain in the lightOur species evolved in a world in which the transition from light to dark took place slowly However due to technological innovation we can greatly speed up the transition from light to dark or vice versa at the flick of a switchEx if you enter a matinee movie after the movie has already begun the dark adaptation is not an instantaneous process Entering the dark theater from the sunlight leaves us temporarily blindedonly after youve been in there for a while can you determine the wide range of behaviors that your fellow movie goers are engaging inMany lab studies have been conducted to investigate dark adaptationIn a typical study the observer is first exposed to an intense light the adaptation stimulusAdaptation stimulus In dark adaptation studies the intense light to which observers are exposed prior to the darknessThen the observer is placed in total darknessthe threshold for detecting a small spot of light is measured the test stimulusTest stimulus In dark adaptation studies the small spot of light for which the threshold is measured after the lights have been turned offThe threshold may be measured using an ascending series of light intensities CH 2The experimenter records the intensity at which the observer reports seeing the test stimulus the thresholdThreshold the intensity of light that an observer can barely detectThen the experimenter repeatedly measures the threshold after different time periods until the observer experiences complete dark adaptation Dark adaptation curve shows the relationship between time in the darkthresholdFigure 313 A dark adaptation curve for one eye Keep in mind that a low threshold is indicative of high sensitivity Notice the kink in the curve at about 10 minutes of dark adaptation which is clarified in Figure 314Notice the portion of the curve for a few minutes after the light has been turned offKeep in mind that the light must be relatively intense for a person to detect it However after about 30 minutes dark adaptation is nearly complete sensitivity is highthreshold is low an observer can detect a low intensity light Further the eye is now about 100000 times as sensitive as it was in the bright lightTry website demonstration 35 Dark AdaptationWhy are we talking about dark adaptation in the context of conesrodsIn a typical dark adaptation experiment researchers shine the test stimulus on a part of the retina where it might fall on both conesrods Notice the kink in the curve that occurs about 510 minutes in the darkThe first part of the curve represents the activity of conesthe second part represents the activity of rodsHow were researchers able to map the differential dark adaptation of conesrods1 First consider the cones If you wanted to look at the dark adaptation of cones what could you do You could go to the one place on the retina where only cones are found the center of the fovea If you used a really small test stimulusensured that it fell only on the center of the fovea you would obtain results similar to Figure 314 a a dark adaptation curve for cones Alternatively you could use a light stimulus ex one near 700 nm that would excite cones but not rods Notice that the threshold drops quickly during the first few minutes However sensitivity levels off at this pointthe threshold remains relatively high2 Testing dark adaptation for rods is more difficult Ideally you could locatetest a person who has only rods in his her retina a monochromatMonochromat person who requires only one color to match his her perception of all other colors every hue looks the same to this person Extremely rarethe person is truly colorblind Rather than search for a monochromat you could minimize the presence of cones by shining the test stimulus only on the periphery of the retina about 20 degrees out from the fovea where many rods but few cones are found In either case youd obtain a dark adaptation curve similar to Figure 314 b where the threshold remains high for about 5 minutes then decreases sharplylater levels out at a very low threshold Figure 314 ab The cones adapt fairly rapidly but after about 10 minutes they no longer gain in sensitivity the lower the threshold the higher the sensitivity The rods also adapt rapidly however their sensitivity is less than the cones until about 10 mins Rods continue to gain in sensitivity until about 20 minsDark adaptation is usually complete after about 30 mins but a number of factors influence its rate NortonCorliss 2002 sizeshape of the test stimuluswhether it is exposed continuously or in a flashing onoff patternsize of the pupilintensity colorduration of the adaptation stimulusEx an intense adaptation stimulus may prolong the dark adaptation process to 40 mins HoodFinkelstein 1986 LambPugh 2004 2006So the process of dark adaptation actually involves two stagesearly stage cones adapt rapidly to the change in lighting conditions However because the cones are not as sensitive as the rods we would not be able to detect weak light stimuli The rods also begin adapting to the change in lighting conditions but are slower to adapt than cones
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