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Science Forum Index » Medicine - Vision Forum » Darkened spot and blurriness
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| William Stacy |
 Posted: Sat Jan 06, 2007 2:38 pm |
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True, or move the edge a bit as he said. The most dramatic display of
this is when you manage to hold the eye perfectly still, all detail
vision fades away. The vision system depends on a moving image for
vision to occur. That's the reason we have what's called micronystagmus
or physiologic nystagmus.
w.stacy, o.d.
Don W wrote:
Quote: Mike Tyner wrote:
"Don W" <dwilgus@prodigy.net> wrote
kind of a wonder, because during the normal day, light rays will strike
this area around the vascular tree at various (mostly random) angles,
But it doesn't. Normally the shadow doesn't move and the retina loses all
perception of edges that do not move. Wiggle a light (slit lamp, penlight,
even through the sclera) and the shadows become perceptible.
So what you are saying is that if one modulates the shadows edge, (that
is, turn the edge on and off) the vasculature will appear?
Don W.
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| William Stacy |
| Posted: Sat Jan 06, 2007 2:43 pm |
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to be more correct, I should have said vision requires a changing image,
either in intensity (on/off) or in position (motion). And that fade out
with the still eye only takes a second or so. |
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| Mike Tyner |
| Posted: Sat Jan 06, 2007 3:07 pm |
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"Don W" <dwilgus@prodigy.net> wrote
Quote: So what you are saying is that if one modulates the shadows edge, (that
is, turn the edge on and off) the vasculature will appear?
On-and-off isn't exactly the same as movement. I don't remember seeing
Purkinje's tree in strobe lights, but I suppose you could.
If you immobilize an eye completely, then vision quickly shuts down. The
retina contains layers forming receptive fields, and interconnections
between them. The interconnections function to balance and equilibrate the
neural signal on either side of a dark-light border, with a pretty short
time constant. The Wiki article on "Troxler effect" has a good discussion.
This is one example of the pre-processing that occurs in the retina. It goes
further, such that individual neurons in the optic nerve do not transmit
individual "pixel" information. Instead of telling the brain "there's a dot
in this location", each neuron says something more like "there's a line with
orientation x moving in direction y."
-MT |
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| Don W |
| Posted: Sat Jan 06, 2007 4:04 pm |
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William Stacy wrote:
Quote: to be more correct, I should have said vision requires a changing image,
either in intensity (on/off) or in position (motion). And that fade out
with the still eye only takes a second or so.
Getting back as to what constitutes seeing or not seeing the retinal
vascular. If modulation of the edges (on - off) is the same as moving
the edges, then the eye going from a dark environment to bright light
should produce at least a brief flash of these trees.
I've never noticed this although it seems that should happen.
Don W.
PS. Yes I have read that fixing the retinal image makes the image
disappear. I think also that direct retinal displays need built in
jitter. There has been a paper (Deruaz) that discusses fixation
instability to improve the vision (letters) of people with central
scotomas. I think I know someone that has "improved" their vision from
20/200 to 20/100 by this kind of "trick". |
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| William Stacy |
| Posted: Sat Jan 06, 2007 5:28 pm |
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absolutely it happens. if you haven't seen it, you don't have the light
at the right angle. It is only momentary, which is why moving the light
helps the visualization since a single presentation is not as good as
many in rapid succession, just like in the movies.
Don W wrote:
Quote: William Stacy wrote:
to be more correct, I should have said vision requires a changing image,
either in intensity (on/off) or in position (motion). And that fade out
with the still eye only takes a second or so.
Getting back as to what constitutes seeing or not seeing the retinal
vascular. If modulation of the edges (on - off) is the same as moving
the edges, then the eye going from a dark environment to bright light
should produce at least a brief flash of these trees.
I've never noticed this although it seems that should happen.
Don W.
PS. Yes I have read that fixing the retinal image makes the image
disappear. I think also that direct retinal displays need built in
jitter. There has been a paper (Deruaz) that discusses fixation
instability to improve the vision (letters) of people with central
scotomas. I think I know someone that has "improved" their vision from
20/200 to 20/100 by this kind of "trick".
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| William Stacy |
| Posted: Sat Jan 06, 2007 5:30 pm |
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btw the conditions that I have experienced it is wearing very dark
sunglasses (too dark, really) and catching a ray of sunlight from beyond
the edge of the sun lens. Flashes a very bright P. tree on the darkness
of the sun lens. Don't try this while driving....
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