Re: Night vision (Student question)

2000-11-06 Thread Karl L. Wuensch




Lots of possibilities -- distribution/density of rods, 
level of and rate of regeneration of rhodopsin (possibly related to diet -- eat 
carrots), clarity of any tissue through which light must pass on way to 
retina.  Or, maybe the brother and husband are just better guessers on the 
SAT.  ;-)  What most people describe as guessing yields 
better performance than would be expected by truely random 
guessing, and this applies to perceptual phenomena too (signal detection theory 
and all that). 

  - Original Message - 
  From: 
  [EMAIL PROTECTED] 
  
  To: [EMAIL PROTECTED] 
  Sent: Monday, November 06, 2000 6:15 
  AM
  Subject: Night vision (Student 
  question)
  Folks, Are there any explanations for 
  variations in the acuity of night vision from person to person (other than 
  owning night vision goggles)?   



Night vision (Student question)

2000-11-06 Thread Claudia Stanny

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 PsychologyPhone:  (850) 474 - 3163
University of West Florida  FAX:(850) 857 - 6060
Pensacola, FL  32514 - 5751 

Web:http://www.uwf.edu/psych/stanny.html



Re: Night vision (Student question)

2000-11-06 Thread Deborah Briihl

What I have read is the opposite - Males appear to have better visual
acuity under photopic conditions, while females have lower absolute
thresholds under scotopic - and can be seen in childhood. Females also
may dark adapt faster. There are other gender differences as well related
to acuity. I'm pulling this information from:
Coren, Ward, & Enns (1994) Sensation and Perception (4th ed.)

At 06:15 AM 11/6/00 -0500, [EMAIL PROTECTED] wrote:
Folks, 

Are there any explanations for variations in the acuity of night vision
from 
person to person (other than owning night vision goggles)?  


A student  asked what the explanation would be, according to her,
her brother 
and husband see better in the dark than she does . I also wonder since
this 
is a subjective and ancedotal account, if night vision has been studied
with 
this in mind (finding out who might have better than average night vision
and 
what neuroanatomical or neurochemical conditions would make this so.)


Thanks for any help you can give me. 


Nancy Melucci 
Los Angeles Harbor College 
Deb

Dr. Deborah S. Briihl
Dept. of Psychology and Counseling
Valdosta State University
Valdosta, GA 31698
(229) 333-5994
[EMAIL PROTECTED]

Well I know these voices must be my soul...
Rhyme and Reason - DMB




Re: Night vision (Student question)

2000-11-06 Thread Sue Frantz

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 hav

Night vision (Student question)

2000-11-06 Thread Drnanjo
Folks,

Are there any explanations for variations in the acuity of night vision from person to person (other than owning night vision goggles)?  

A student  asked what the explanation would be, according to her, her brother and husband see better in the dark than she does . I also wonder since this is a subjective and ancedotal account, if night vision has been studied with this in mind (finding out who might have better than average night vision and what neuroanatomical or neurochemical conditions would make this so.)

Thanks for any help you can give me.


Nancy Melucci
Los Angeles Harbor College