[visionlist] brightness change detection puzzle

[Anshul Jain]

Hi Arun, 

This might be a something you have already looked at but is your monitor gamma corrected for nonlinearity? The same change in input for darker background results in a smaller change in brightness than for brighter background (See below), which might explain better detection at higher brightness. 



Regards,
Anshul 

Sent from my iPhone

> On Aug 24, 2022, at 1:29 PM, SP Arun <sparun at iisc.ac.in> wrote:
> 
> 
> 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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