Nancy Melucci wrote:
> Are there any explanations for variations in the acuity of night
> vision from
> person to person (other than owning night vision goggles)?
See: http://web.utk.edu/~wverplan/biblio10.html for the full article, by
William S. Verplanck from University of Tennessee, Knoxville.
*****
"Night Blindness and Nyctalopia.
"Some persons report consistent difficulties in seeing at night, even
when they are fully dark-adapted. They cannot detect objects
readily visible to others and show both confusion and slow recovery
after brief exposure to relatively bright light sources.
Maneuvering in dimly illuminated spaces and driving or flying at night
present serious problems to these individuals. The
presence of such a history, whether the disturbance in sight is of
recent appearance or long-standing, is usually taken as prima
facie evidence of night blindness.
"However, a sharp distinction must be made between night blindness as
indicated by such reported difficulties and nyctalopia,
or true night blindness, which may be diagnosed only on the basis of an
accurate measurement of retinal sensitivity. Many, if not
most, of those individuals who report difficulty in seeing at night
prove to be psychoneurotic. Many who have unusually
insensitive retinas, on the other hand, do not report special
difficulties in seeing at night, either because they assume that others
have the same difficulties, or because they fail to note them in out
well-illuminated urban culture, which offers few situations in
which intact rod function is required. To establish the presence of
nyctalopia, it is essential to use an instrument of established
validity for the measurement of retinal sensitivity.
"Incidence of Nyctalopia.--No definitive data on the occurrence of
nyctalopia in the population are available, since
measurements have never been made on a representative sample of the
population. From the studies which have been made of
selected groups (e.g. school children, service men), it is known that
the normal population will include a small percentage of
persons of low visual sensitivity whose performance will be as poor as
or poorer than that of many individuals whose nyctalopia
is associated with disease or degenerative processes. About 2 per cent
of the Navy men were disqualified for night duties as
"night blind" on this basis. Those so disqualified seldom if ever showed
symptoms other than a relatively high absolute terminal
threshold, and their reduced sensitivity must be taken as the
consequence of the normal variability in the density in the retinal
rods and the efficiency of the process whereby rhodopsin, the visual
purple, is regenerated.
"The incidence of nyctalopia as part of a distinct clinical pattern is
not well understood. It has been observed frequently in several
diseases, and may appear in certain unusual conditions such as:
"(1) Idiopathic Nyctalopia.--Idiopathic nyctalopia is an hereditary
absence of rod function, which has been traced through
several generations of certain families. Although typically it appears
alone, it may be associated with color blindness and myopia.
There is no effective treatment.
"(2) Oguchi's Disease.--This rare hereditary syndrome, first reported in
Japan and later observed in Europe, has its primary
symptom nyctalopia with marked contraction of the visual field under low
levels of illumination. Ophthalmoscopic examination
shows a remarkably gray appearance of this fundus which disappears with
dark-adaptation. Day vision is not affected. No
treatment as proved of value.
"(3) Retinitis Pigmentosa.--Nyctalopia is the first and invariable
symptom of retinitis pigmentosa. In the early stages of the
disease, dark adaptation takes place, but at a retarded rate. As the
disease advances, rod function is progressively lost, and the
absolute terminal threshold is elevated. Diagnosis of retinitis
pigmentosa is based upon ophthalmoscopic examination.
"(4) Glaucoma.--Early impairment and progressive loss of rod sensitivity
is observed in glaucoma.
"(5) Retinitis Punctata Albescens.--The earliest symptom of this disease
is the complete absence of rod function. Often
nyctalopia is the only symptom associated with the altered state of the
retina.
"(6) Other syndromes of the Visual System.--Nyctalopia has been observed
as one symptom of each of the following
pathologic conditions: myopia, disseminated chorioretinitis, pregnancy,
nicotine poisoning, the Lawrence-Moon-Biedl
syndrome, gyrate atrophy of the choroid and retina, choroideremia and
atrophy of the optic nerve. Nyctalopia may be simulated
by opacities of the ocular media.
"(7) Overexposure to Sunlight.--Mild transient nyctalopia may appear in
persons who have been overexposed to bright
sunlight for several days. It will disappear within a few days if the
persons will protect their eyes from the sun wither by
remaining indoors or by the use of dark sun glasses.
"(8) Avitaminotic Nyctalopia.--Epidemics of night blindness have been
observed in populations subject to a vitamin A
deficient diet. This phenomenon has been related to the important role
played by vitamin A in the cycle of regeneration of
rhodopsin. Nyctalopia has been considered the classical symptom of this
deficiency and the presence of an elevated terminal
threshold has been erroneously taken by some as sufficient basis for
diagnosis, even in the absence of other signs of the
deficiency.
"Although much work has been done on experimentally induced vitamin A
deficiency and the terminal rod threshold, the data are
still inconclusive. Some investigators have been able to produce night
blindness experimentally (See Fig. 101) and to reproduce
remarkably rapid recoveries through the administrations of rapid of
massive doses of vitamin A, e.g., 100,000 I.U. Others have
found recovery slow or absent in some cases after several months of its
administration. Others have not been able to produce
nyctalopia in all subjects through the manipulation of the diet. The
conclusions are that vitamin A deficiency may or may not
produce nyctalopia and that administration of vitamin A may or may not
lead to recovery from it. Ingestion of the provitamin
carotene was conspicuously unsuccessful in improving night visual
performance of American service personnel, although it
seems to have somewhat improved that of less well-nourished armies, as
the Japanese. If an individual complains of difficulty in
seeing at night and such pathologic conditions as retinitis pigmentosa
have been ruled out, the administration of massive doses of
vitamin A may lead to improvement, and hence to the conclusion that the
nyctalopia was produced by a vitamin A deficient diet.
"(9) Faulty Vitamin A Metabolism.--In such diseases as cirrhosis of the
liver, reduced retinal sensitivity may appear. Massive
dosage with vitamin A is followed by partial or complete return of
sensitivity to the normal range. It is believed that such
nyctalopia may be referred to altered intermediary metabolism of vitamin
A."
*****
--
Sue Frantz, Asst Prof of Psych [EMAIL PROTECTED]
Faculty Office Bldg, 2400 Scenic Drive Office: (505)439-3731
New Mexico State Univ. - Alamogordo Fax: (505)439-3802
Alamogordo, NM 88310 USA http://alamo.nmsu.edu/~frantz
