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Visual phenomena

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Gray
25258.  Thu Sep 29, 2005 6:07 am Reply with quote

A thread for things that force you to accept that your eyes and brain aren't necessarily giving you an accurate picture of the world that you're sure is there.

Simple one to start with - and one of my favourites. Were you aware that there is a huge hole in the vision of each of your eyes - a whole area that returns no information about the outside world? You probably weren't, because your brain helpfully 'fills in the details for you'.

Where the optic nerve exits the retina on its way to the brain, there are no photoreceptors. This means that if light from outside is focused by the eye's lens onto that area - 'the blind spot' - you will not be able to see it. A simple demonstration:

Close your left eye, and focus your right eye on the red square on the left. Now, slowly move your head towards the screen but never take your right eye off the square. When you're about 10 inches away, the red circle on the right will vanish - its image is being projected onto your blind spot, where you can't detect it.



That's the interesting thing about blind spots - you can't see anything that is in them, but also, the fact that you have a blind spot is hidden from you. You're blind to even the existence of your blind spots.

And here's you, walking around with no problems - for decades - and you'd never even noticed. I love that. :-D

 
Mr Grue
25268.  Thu Sep 29, 2005 7:37 am Reply with quote

If you've got a TV that can tune into static without going blue, try gently affixing a square of masking tape to one of the corners of the screen, about an inch or two in from each side. Then stare at the middle of the screen and wait.

The masking tape should disappear, and if it works really well, you won't even determine when or how the disappearance takes place.

This is the brain effectively noticing something which seems to be out of context with the rest of the visual information and "filling in" the blank, in this case the masking tape, as it sees fit. It's exactly the same process that allows the human visual field to fill in the gap of the blind spot.

Gray, are you going to do the blind spots in the middle of horizontal and vertical lines bit? That's jolly good fun too.

 
john.birch
25271.  Thu Sep 29, 2005 7:57 am Reply with quote

What you describe, Mr Grue, is also the reason why we see "faces" and similar things in clouds, and flames in the fire, and so on.

The brain is constantly trying to make order out of chaos - for the very simple reason that order is far easier for the brain to process than randomness. The human brain's processing power is remarkably limited and it only copes by simplfying what the input, grouping things and matching them against recognised and understood concepts.

This also means that once you see an object in a cloud or whatever you never seem to be able to stop seeing it.

In practice our vision processes remarkably little - most of what we see is the brain filling in. You think you see far mor ethan you do - if you doubt that simply close your eyes and try to recall everything that was in front of you. Come on - you were looking at it only a split second ago!

There have been some quite interesting tests for this. If someone is speaking to a total stranger and - in mid-converstion that person is replaced by someone else (the viewer being distracted by - say - a large board moving between them) they rarely notice the substitution.

With our brains making so many assumptions, fillling in and simplyfying so much, it does make you wonder how much of the world really is as we see it.

 
Jessica
25274.  Thu Sep 29, 2005 8:07 am Reply with quote

what beats me though is people who will pay good money for supposed images of the Virgin Mary or Jesus on bits of mouldy wallpaper or half-eaten sandwiches.

 
Mr Grue
25280.  Thu Sep 29, 2005 8:27 am Reply with quote

I think I saw something on the test you describe on the box.

If memory served the problem with the experiment was that they wanted to catch people when they were off-guard, or rather when they were in a normal rather than an experimental situation, so they ran the test as soon as the subject had signed the agreement to participate in the experiment. The administrator ducked down behind the reception desk "to get the second form" and was replaced by administrator number 2, with the person's response videotaped.

There was a rather fun collation of different degrees of response, from the majority who were oblivious to the switch, through the "something's wrong, but I'm not sure whats" to those who practically climbed the walls. Fewer people noticed when the administrators dressed alike, but the number who failed to notice when the administrators were completely different was still surprisingly high.

 
Gray
25282.  Thu Sep 29, 2005 8:39 am Reply with quote

Well, that's belief for you...

This site has many excellent illusions. I particularly like this one (which has cropped up before here). Click the 'Cover 1' button to prove to yourself that those two squares are the same actual colour.

 
laidbacklazyman
25541.  Sat Oct 01, 2005 1:36 am Reply with quote

There's a group of pavement artists that specialise in what can only be described as visual phenomena, heres some of their work to be going on with.




 
Okapi
25544.  Sat Oct 01, 2005 3:19 am Reply with quote

When glancing at my watch, the second hand often seems to freeze or hang for what feels like considerably more than one second.

Is that perhaps down to the brain's concentration on one task (determining the time) at the expense of other tasks such as perceiving motion?

 
laidbacklazyman
25547.  Sat Oct 01, 2005 3:28 am Reply with quote

do you not think it may be similar to watching motor racing on the tele, where as the speed of the wheels slow down they meet the frequency of the screen and begin to go backwards
a bit like this
BTW that effect is similar to a strobe light, I don't need to tell you to be careful if that sort of thing affects you do I?

 
Gray
25589.  Sat Oct 01, 2005 12:03 pm Reply with quote

There was a series of letters in The Times about this very topic some years back. The only explanation that seemed to satisfy everyone is that when your eye is moving to the watch, it's extremely poor at picking out the tick of the second had if it happens before your eye has become used to focusing on the thing.

Therefore it can tick without you noticing it, and this makes it seem like the next tick - that you notice, as your eye has sorted itself out after moving - has taken ages to come.

Moving eyes are pretty poor at picking up motion, and this accounts for why some lizards and almost all birds have this strange head-bobbing walk - their body can move while their head stays utterly still (to catch as much movement in its field of vision as possible). Then, when the neck is stretched as far as it will go, the head is whipped forward to catch up.

Watch any bird on a swaying branch - its head will almost always be completely still. Better still, pick up a chicken and rotate its body - again, the head will stay put - very comical. Detecting predators is very, very important if you're tasty.

 
brackett
26395.  Sun Oct 09, 2005 7:19 am Reply with quote

This is quite nice, Paul McCartney's new album (Special Edition cover)

Look at the design of Paul's name. Notice anything interesting?

Flip the image upside down - This might be hard for those of you with a desktop to do.

 
Anna
26397.  Sun Oct 09, 2005 8:57 am Reply with quote

Featured rather prominently in Angels and Demons by Dan Brown...

Tiny bit of info here

And here's some more...

 
Ned Trifle
26539.  Mon Oct 10, 2005 6:20 pm Reply with quote

Does laidbacklazyman have a link for those artists?
They are amazing.

 
Flash
26540.  Mon Oct 10, 2005 6:58 pm Reply with quote

Go to http://www.coolopticalillusions.com/optical_illusions_images_2/optical_illusion_images.htm and click on the "Artist Chalk Drawing" links in the yellow box on the left-hand side.

LBLM, sorry if that was poaching, but I don't know when you might pick this up.

 
laidbacklazyman
26546.  Mon Oct 10, 2005 11:36 pm Reply with quote

No that's fine I only stopped by last evening to collect a message so it would have been overlooked by me.

 

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