[visionlist] brightness change detection puzzle

daniele.zavagno at unimib.it daniele.zavagno at unimib.it
Thu Aug 25 04:35:32 -04 2022

Hi, I looked at the file you linked on the web and can make out all the rows starting right to left til the second last included for the top and middle stripes, and the complete row for the bottom one. 

When I downloaded the file and opened it with photoshop, I get that the top row background measures 84 in RGB values, the second last dot measures 85 in RGB values, the last dot does not exist, as readings are 84 RGB

This pattern repeats for the middle row: background=105, second last dot from the right=106; last dot does not exist.

However, for the brightest row, the bottom one: background=131, the second last and the last dot both=132.

Maybe during the transformation subtle changes have gone lost, but it wouldn’t hurt to check the digital readings of your actual display to see if what you think you have put there actually exists. Just a thought.

Daniele Zavagno, Ph.D.
Associate Professor
Department of Psychology
University of Milano-Bicocca
P.zza dell’Ateneo Nuovo, 1
20126 Milano, Italy
tel. (+39)0264483814

Il giorno 24 ago 2022, alle ore 18:58, SP Arun <sparun at iisc.ac.in> ha scritto:

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? 


SP Arun
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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