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<p class="MsoNormal">Threshold contrast increases as background luminance decreases (e.g., Blackwell, and others). If your steps are increases in luminance by specific amounts regardless of the background luminance and not percent increments, then even though
the luminance contrast may be larger for the faintest stimuli when the background luminance is low, it may still be below the higher threshold contrast (and hence, invisible) at that lower background luminance. Could that help explain what’s happening?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">John<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">--<o:p></o:p></p>
<p class="MsoNormal">John D. Bullough, Ph.D., FIES - <a href="mailto:John.Bullough@mountsinai.org">
<span style="color:#0563C1">John.Bullough@mountsinai.org</span></a><o:p></o:p></p>
<p class="MsoNormal">Program Director - Population Health Science and Policy<o:p></o:p></p>
<p class="MsoNormal">Light and Health Research Center, Icahn School of Medicine at Mount Sinai, Albany, NY<o:p></o:p></p>
<p class="MsoNormal">Tel.: +1.518.368.5418, Web: <a href="http://icahn.mssm.edu/lhrc">
<span style="color:#0563C1">http://icahn.mssm.edu/lhrc</span></a><o:p></o:p></p>
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<p class="MsoNormal"><b>From:</b> visionlist <visionlist-bounces@visionscience.com>
<b>On Behalf Of </b>SP Arun<br>
<b>Sent:</b> Wednesday, August 24, 2022 12:59 PM<br>
<b>To:</b> visionlist@visionscience.com<br>
<b>Cc:</b> Thomas Cherian <thomasc@iisc.ac.in><br>
<b>Subject:</b> [visionlist] brightness change detection puzzle<o:p></o:p></p>
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<p class="MsoNormal"><span style="color:black">USE CAUTION: External Message.</span><o:p></o:p></p>
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<p class="MsoNormal"><span class="xelementtoproof"><span style="font-family:"Arial",sans-serif;color:black;background:white">Dear visionlist, </span></span><span style="font-family:"Arial",sans-serif;color:black">
<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">We have a brightness perception puzzle in the image linked below. <o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span class="xxelementtoproof"><span style="font-family:"Arial",sans-serif;color:black"><a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__drive.google.com_file_d_1QpWT48uA9R1ZG48ERy4F3osvxPgoy4qT_view-3Fusp-3Dsharing&d=DwMFAw&c=shNJtf5dKgNcPZ6Yh64b-ALLUrcfR-4CCQkZVKC8w3o&r=tjDepLyu7dPCApYnRGWy2Dd5cBMpBsngr3rOVKMVeQs&m=mK4qFJDWTF3Kkj-Md5p59DehfWgjdVIYeIn0gbW1j2nLnaz6pJWTgOE6wQWRtHlj&s=Bocm2T-Oj25eudp2DbgdVy--exKOrBWhmOjkRFJsugk&e=">https://drive.google.com/file/d/1QpWT48uA9R1ZG48ERy4F3osvxPgoy4qT/view?usp=sharing</a></span></span><span style="font-family:"Arial",sans-serif;color:black">
<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">When we tried to find the smallest change in brightness detectable against a uniform background, we found something strange: for dark backgrounds, we found
that only large brightness changes could be detected, whereas for brighter backgrounds, we found that much smaller brightness changes could be detected. <o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">This was puzzling to us because this is the opposite of Weber's law -- by Weber's law we would expect that the threshold brightness change for brighter backgrounds
should be larger than for darker backgrounds. But we observe the opposite. <o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">All this must be because of contrast detection somehow. But we are unable to think of reasons why this is happening. Any ideas? <o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">Regards, <o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">Arun<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">---<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">SP Arun<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">Professor <o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">Centre for Neuroscience<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">Indian Institute of Science<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">Bangalore 560012<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">T: +91 80 22933436 <o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><span style="font-family:"Arial",sans-serif;color:black">W: <a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__sites.google.com_site_visionlabiisc&d=DwMFAw&c=shNJtf5dKgNcPZ6Yh64b-ALLUrcfR-4CCQkZVKC8w3o&r=tjDepLyu7dPCApYnRGWy2Dd5cBMpBsngr3rOVKMVeQs&m=mK4qFJDWTF3Kkj-Md5p59DehfWgjdVIYeIn0gbW1j2nLnaz6pJWTgOE6wQWRtHlj&s=V8v2o79WW5FkgkDIVM0uYNu6oqyCbRZ51k3PKJC06BA&e=" title="https://sites.google.com/site/visionlabiisc">https://sites.google.com/site/visionlabiisc</a><o:p></o:p></span></p>
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