On Sat, 30 Jan 1999, ITP_Faculty_Faith_Florer wrote:
> A student wanted to know last week what, exactly, 'night blindness' is. I
> didn't know. Does anyone here know? Specifically, is it loss in the
> ability to perceive form or is it something else? Does it occur because
> people are focussing at the wrong depth in the night? (I read that this
> can happen in the night when driving because of a kind of overconfidence
> that motion perception gives some people.) Is it a problem at the receptor
> level? If it's a problem at the receptor level, what's the problem?
>
I hate to see a good question go unanswered. So, in the absence of a
more authoritative reply, here's mine. The eye contains two different
light sensing systems, one for bright light (daylight), mediated by
the cone photoreceptors, and one for dim light (night vision) mediated
by the rods. So night blindness is undoubtedly a disorder of the rods.
A good idea of normal night vision is to think what it's like to wake
up in the middle of the night in a dark room (i.e. with a dark-adapted
eye). The typical characteristics are vague shapes, no colour, and
better perception just to the side of fixating on objects. That's
_normal_ rod vision, and the rods are able to do it on incredibly
small amounts of light. So if you have night blindness, you would no
longer be able to see like that; that is, you would be blind at night.
However if your cones are still functioning, you would still be able
to see well in daylight, and possibly with good illumination at night
(from car headlights, possibly).
My old medical dictionary says that night blindness "is the condition
of reduced dark adaptation, resulting temporarily from vitamin A
deficiency or permanently from retinitis pigmentosa or other
peripheral retinal disease. Synonym nyctalopia".
Vitamin A, as it happens, is used to make the chemical (a photopigment
called rhodopsin) which allows the rods to capture light and turn it
into nerve impulses.
I'd guess that a web search on some of those dictionary terms,
including "night blindness" would pay off.
-Stephen
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Stephen Black, Ph.D. tel: (819) 822-9600 ext 2470
Department of Psychology fax: (819) 822-9661
Bishop's University e-mail: [EMAIL PROTECTED]
Lennoxville, QC
J1M 1Z7
Canada Department web page at http://www.ubishops.ca/ccc/div/soc/psy
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