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During dark adaptation, a large amount of Vitamin A (retinol) is
converted into rhodopsin and reconversion of retinene and opsin into
rhodopsin also takes place. Because of these reconversions, the visual
receptors (rods) become so sensitive that even a minute amount of light
causes excitation. The regeneration of rhodopsin in the retina is
dependent on Vitamin A (figure 5). In night blindness, the retina contains
less rhodopsin than a normal retina and the rate of regeneration
of rhodopsin is much slower, and this condiLion could be reflected on
dark adaptation ability of individuals (Guyton, 1976), "
62
The relationship between the ability to see in the dark and Vitamin A
status was demonstrated by Dowling and I.iald (1958). Rats were maintained
on Vitamin À Ceficient diet. The liver Vitamin À level began to
fall steadily within the first week of the start of the Vitamin À deficient
diet, within three weeks reached about 5% of the normal value.
Then Lhe blood Vitamin À level was depleted and reached 0% which was
followed by a depletion in the rhodopsin concentration of the retina.
These conditions, fina1ly, resulted in an increase in the visual threshold
(final dark adapted threshold), marking the onset of night blindness.
Final dark adapted threshold is the minimum amount of light which
illuminates a given surface in order to render it visible (Hecht and
Mandelbaum, 1939). Final dark adaptaLion threshoLd was defined as the
average of three ascending and lhree descending thresholds obtained
after 35-40 minutes in the dark (Russel1 et aI., 1973).