[visionlist] brightness change detection puzzle

[Bullough, John]

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?

John

--
John D. Bullough, Ph.D., FIES - John.Bullough at mountsinai.org<mailto:John.Bullough at mountsinai.org>
Program Director - Population Health Science and Policy
Light and Health Research Center, Icahn School of Medicine at Mount Sinai, Albany, NY
Tel.: +1.518.368.5418, Web: http://icahn.mssm.edu/lhrc



From: visionlist <visionlist-bounces at visionscience.com> On Behalf Of SP Arun
Sent: Wednesday, August 24, 2022 12:59 PM
To: visionlist at visionscience.com
Cc: Thomas Cherian <thomasc at iisc.ac.in>
Subject: [visionlist] brightness change detection puzzle

USE CAUTION: External Message.
Dear visionlist,

We have a brightness perception puzzle in the image linked below.
https://drive.google.com/file/d/1QpWT48uA9R1ZG48ERy4F3osvxPgoy4qT/view?usp=sharing<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=>

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.

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.

All this must be because of contrast detection somehow. But we are unable to think of reasons why this is happening. Any ideas?

Regards,
Arun

---
SP Arun
Professor
Centre for Neuroscience
Indian Institute of Science
Bangalore 560012
T: +91 80 22933436
W: https://sites.google.com/site/visionlabiisc<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=>

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