[visionlist] brightness change detection puzzle

[Olivers, C.N.L. (Christian)]

Dear Arun,

Looking at your demo, if I understand it correctly, you express the intensity changes in bit-space, rather than physical luminance space. Is that the case? My assumption would then be that, as for most monitors, your monitor's bit-luminance relationship follows a positively accelerated gamma-curve. Would that not explain it?
(e.g. https://www.benq.eu/en-eu/knowledge-center/knowledge/gamma-monitor.html).

Sincerely,

Chris Olivers,
Amsterdam

From: visionlist <visionlist-bounces at visionscience.com> On Behalf Of SP Arun
Sent: 24 August 2022 18:59
To: visionlist at visionscience.com
Cc: Thomas Cherian <thomasc at iisc.ac.in>
Subject: [visionlist] brightness change detection puzzle

Dear visionlist,

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

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

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