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

feedbackdroid via neur-sci%40net.bio.net (by feedbackdroid from yahoo.com)
Mon Oct 16 10:16:59 EST 2006

Tim Allen 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?

Much of the neurophysiology of this was worked out back in the 70s, by
people like Frank Werblin at Berkeley, Werblin and Larry Thibos, and
many others whose names escape me now.

IIRC .... basically, not counting adaptation, the retinal cells respond
from zero to full-response over about 2-3 orders of magnitude of light
intensity change. The response curve is S-shaped on a log scale of
intensity. Then, adaptation causes these S-curves to shift over many
orders of magnitude, but they basically retain the same shape. So
basically, the same relative contrast will produce about the same level
of response, regardless of ambient light levels.

Also, regards your earlier question about individual photoreceptors
adapting, remember that the eye is in constant motion, so over a short
period of time a typical photoreceptor will be exposed to pretty much
all the light intensities in the local scene, to produce an average
level of adaptation.

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