[Neuroscience] Re: Light and dark adaptation of the eye

r norman via neur-sci%40net.bio.net (by r_s_norman from _comcast.net)
Sat Oct 14 18:07:34 EST 2006


On 14 Oct 2006 15:22:17 -0700, "Tim Allen" <tim.allen from gmx.com> wrote:

>r norman wrote:
>> I am not sure just what you mean.  The receptors can only adapt to the
>> light intensity to which they, themselves, are exposed.  There is no
>> 'eye averaging' system that measures an 'overall' illumination and
>> gets all the receptors to adapt to that level.
>
>Well, thanks - what you explained was exactly what I meant!
>
>> However it is true that generally all the receptors in the eye are subject to an overall
>> exposure to  the same background light intensity with the image
>> producing only a relatively minor change in intensity for specific
>> receptors.
>
>So, to take this a bit further, could it be said that this "locally
>variable adaptation" increases the contrast range that the eye (the
>photoreceptors) is (are) able to deal with?
>

It depends what you mean by contrast range.  Ordinarily, there isn't
that much difference between the brightest and the darkest part of the
visual field.  That is, what looks like "deep shadow" in full sunlight
is actually far brighter than what looks like a bright area in a
moonlit scene.  So the adaptation to the overall illumination allows
the eye to detect those relatively small changes as "high contrast"
events.



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