[Steve West]

Steve West  asked the Naked Scientists:
   
Hi Chris,

I have a quick question for the naked scientists.

Pupils are used to regulate the amount of light that enters the eye, dilating in low light and contracting in bright light. They also may dilate or contract under other influences such as when experiencing extreme emotions, being under the influence of drugs or alchohol, or in the case of my cat, a very rapid dilation just before pouncing on her target.

How come when there is a change in the opening of your pupil but not in the surrounding light level you don't perceive your evironment becoming brighter (like an over exposed photograph), or even discomfort (like leaving the cinema in the day time)?

Thanks for your time

Steve West

What do you think?

[SeanB]

The brain regulates the pupil, and the feedback from pupil dilation is integrated into the image that your brain is creating to give the illusion that you see a smooth world. Thus it compensatesfor differences in brightness almost invisibly to you.

[Steve West]

Hello,

Thanks for taking the time to reply. I still don't really feel quite satisfied with the answer though.

If the brain can easily adapt so that you perceive the same level of brightness, despite a large difference in the opening of the pupil and with no change in the surrounding light level, why does the pupil need to dilate at all? Why can't the pupil just stay the same size and use the brain to correct the image.

Towards the end of this clip the cat's eyes almost double in size. If they are already at the correct size for the surrounding light level, why would the pupils need to adjust?

If I were thinking in terms of photography, I might think this would be similar to using a large aperture and a fast shutter speed, but surely there isn't a "shutter speed" with the human body, is there?

Once again, thanks for replying. Hopefully someone can help me understand what's going on here.

Best

Steve