[visionlist] brightness change detection puzzle

[Andrew Anderson]

Hi Arun,

Thank you for sharing this. Have you considered whether this might reflect there is a non-linear relationship between your nominal intensity steps and the change in luminance on the monitor? If the function is an accelerating one (like a gamma function), the incremental luminance steps from your darker background will actually be a bit smaller than those from the brighter background, even though the change in pixel values are identical.

Kind regards,

Andrew

Assoc. Prof. Andrew Anderson
Department of Optometry & Vision Sciences
The University of Melbourne, 3010
AUSTRALIA
Ph: (+613) 9035 9916

From: visionlist <visionlist-bounces at visionscience.com> on behalf of SP Arun <sparun at iisc.ac.in>
Date: Thursday, 25 August 2022 at 3:32 am
To: "visionlist at visionscience.com" <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<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<https://sites.google.com/site/visionlabiisc>

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