Re: Photic Sneeze syndrome
- From: Rushtown <Rushtown@xxxxxxx>
- Date: 26 Apr 2007 08:40:53 -0700
On Apr 26, 1:51�am, Scaly Lizard <scalyliz...@xxxxxxxxxxxxxxxxxxx>
wrote:
On 25 Apr 2007 23:36:42 -0700, Rushtown <Rusht...@xxxxxxx> wrote:
About 15% of people have a condition where they sometimes sneeze when
bright sunlight suddenly hits there eyes.
The condition is more common in Caucasians.
Why did evolution develop such a trait?
The key is the above mentioned ethnic difference.
In the E of E D (Era of Evolutionary Development) more Caucasians
lived in caves. There being no running hot water the caves got really
smelly
and the caveman's nose filled up with cave stink, thereby rendering
him unable to smell the bear lurking outside the cave when he (the
caveman) first stepped out into the sun.
However those cave people who sneezed when first leaving the cave
cleared their noses (while momentarily startling the bear, who paused
for a moment). With their now cleared nose they were then able to
smell the bear or other threatening animal. This increased the
chances of survival of those with photic sneeze syndrome. There being
more dark caves at northern latitudes Caucasians inherited this trait
in a greater proportion than other races.
That's a great story. You should design sets for insurance
commercials, with that wonderful imagination.
The truth, however, is much more mundane.
Photic sneeze syndrome is part of the human defense
mechanism, but has nothing to do with dark caves and
odiferous bears, lol.
Exposure to high intensity light activates the tear ducts,
which then secrete a cooling and soothing liquid over the
surface of the eye. This action is initiated by squinting,
which squeezes muscles around the tear ducts. Tears
also provide an extra layer of diffusion, protecting the retina
against discrete sources of intense light.
Why tears in response to light? Evolution has 'taught' or
'selected' those animals with a protective response to
high-intensity light because such light can cause blindness,
which is a severe disadvantage in courting and breeding.
So we cry when it's bright. But here's where the neurology
get really interesting! In the 1800's, medical researchers
(when dissecting cadavers of sometimes illicit origin) found
that the tear ducts also leak into the nasal cavity. It's
called a nasolacrimal duct, and i didn't have to look that
up, honest. It's a great word. Say it aloud a couple times,
and you can't help but chuckle.
This cleared up something which had been a big mystery
until the 1800's... why do we get a runny nose when we
cry, and why do our eyes water when we sneeze? It was
all neatly explained by Triumphant Victorian Science, as a
reaction between tears and snot, cross-irritating both the
eyes and the nasal membrane.
So the case would seem to be closed, but there was still
one niggling unanswered question. Some folks sneeze
immediately as a response to bright light, before their eyes
have had a chance to produce tears and leak them into
the nose!
Experimental evidence proved the phenomenon, and the
science establishment counter-argued that some lesser-
minded people might have been trained, like a Pavlovian
dog, to sneeze when they encounter a situation where
they usually expect to sneeze. So that was that.
Now, enter the science of the modern day. Today, we
know that sneezing as a result of visual stimuli has a
physical basis in the workings of the human brain. In
some people, the proximity of the ophthalmic nerve to
the main optic nerve creates a biochemical short-circuit,
to say it simply.
To say it more succinctly, high activation in the optic nerve,
as a response to a high stimulus, has evolved a cross-path
to the lacrimal nerve. No big deal. That's the response to
bright light that we all experience. We tear up. All of us.
But the lacrimal nerve (which makes tears flow), is part of
the trigeminal nerve, which also includes the ophthalmic
nerve, which makes us respond by squinting immediately
in response to bright light.
Around 20% of humans have developed a pathway from the
ophthalmic nerve to the optic nerve, through the backdoor
of the lacrimal nerve. Since the optic nerve operates quite
fast, an immediate response is sent to the trigeminal nerve,
which tells the trigeminal to go into full defense mode.
Full defense? The trigeminal instructs the face to clench
the jaw, close the eyes, and expel the nasal cavity at
explosive speed.
In short, a sneeze.
Regrets for blowing your theory open, but sneezing due to
bright light has nothing to do with cavemen and hungry
bears. Instead, some folks have a system which responds
to sudden bright light by cross-exciting the trigeminal nerve
into full defense mode.
By the way, if you can produce any evidence that the
phenomenon is more prevalent in caucasians, then please
do so. It's been my belief that such claims were only an
'urban legend'.
SL- Hide quoted text -
- Show quoted text -
Great answer. And it is more explanatory than the Wikipedia article.
http://en.wikipedia.org/wiki/Photic_sneeze_reflex
This article mentions that white folks do sneeze more when light hits
their eyes. Maybe only because they have lighter colored eyes.
BTW based on another analysis of the E o ED I predict that Koreans
don't feel like taking siestas at 3pm. Why? Because the drowsy
feeling at 3 in
the afternoon developed in populations who evolved in warmer climates.
The survival benefit was that it kept
them out of the sun at the hottest time of the day, 3pm (in Africa).
Caucasians still show this tropical heritage with their linear built.
Northeast
Asians do not---they are cold adapted, one of those adaptations being
"no need
for a siesta."
.
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