Nancy Melucci asks:
>
>Are there any explanations for variations in the acuity of night vision?
Visual sensitivity under low light conditions depends on several factors,
all of which might vary among individuals.
Dark adaptation (recovery of sensitivity following exposure to bright
light) depends on the speed with which the visual pigment rhodopsin is
regenerated in the rods. Rhodopsin is formed from vitamin A (which is why
severe deficiencies of vitamin A produce deficits in night vision). I
don't know whether individual differences in vitamin A would be manifested
as differences in quality of night vision - it generally takes a severe
deficit to show up as impaired night vision. Any one know whether there
are differences in the speed of the regeneration process?
Several other factors might vary enough between individuals to create
differences in quality of night vision:
Clarity of the cornea (people with cataracts will have worse night vision),
the lens, or of the viteous humor will determine how much light actually
stimulates the retina. Less light transmitted, less to reach the retina to
be detected.
The size of the pupil when it is fully dilated might vary - the bigger the
opening, the more light the eye can gather. Animals that have excellent
night vision (like owls) have large eyes and pupils that are capable of
great dilation.
As we age, we accumulate pigments in the foveal region of the retina (which
also reduce our sensitivity to blue light) and these will filter out light
and affect vision under low-light conditions. (This has been suggested as
the reason why little old ladies like those blue rinses - to them their
hair looks white, without them their hair might have a dingy, yellowed cast.)
Two other possibilities that are far more speculative:
Are there large enough individual differences in amount of convergence
among receptors in the periphery to produce differences in sensitivity?
There are some variations in the chemical composition of the photopigments
for color vision and these have slightly different sensitivity contours
(thus, there are different varieties of red-green anomolous color vision,
depending on which variant of the red/green photopigment the person has).
I suppose it is possible that there might also be variants in rhodopsin,
but I don't know of any research on that.
Claudia Stanny
________________________________________________________
Claudia J. Stanny, Ph.D. e-mail: [EMAIL PROTECTED]
Department of Psychology Phone: (850) 474 - 3163
University of West Florida FAX: (850) 857 - 6060
Pensacola, FL 32514 - 5751
Web: http://www.uwf.edu/psych/stanny.html
