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Re: guideline 7.1 about screen flickering (fwd)

From: Marja-Riitta Koivunen <marja@w3.org>
Date: Wed, 01 Aug 2001 04:00:20 -0400
Message-Id: <>
To: Anne Pemberton <apembert@erols.com>, "Jonathan Chetwynd" <j.chetwynd@btinternet.com>, "gregory j. rosmaita" <oedipus@hicom.net>, <w3c-wai-gl@w3.org>
I agree we learn to see but many things also develop normally in a certain 
way e.g. in our visual cortex there are cells specialized in detecting 
movement and it is hard keep the eye saccades out of the movement if it is 
strong enough. That is why advertisers want to use movement in the banners 
as it easily get's the user's attention.

I tried to find a good reference from the Web, but it was difficult without 
going too deep and I don't have more time now. The more understandable 
references are similar to this one:

In the visual periphery we can instantly detect even slight movement.
This is an important safety feature, developed long ago to recognize 
stalking predators. The eye's periphery reports a movement, the brain 
thinks it's important, and the vision is automatically centered on the 
threat. It is so imprinted on us that we can't do anything about it, and 

Also Jacob Nielsen refers to it.


At 10:51 AM 7/30/2001 -0400, Anne Pemberton wrote:
>         While your point is well taken, your reason is not based on fact. 
> A cognitive system is not built and dropped in place, it is *built* or 
> developed over one's lifespan, and not all cognitive systems are built to 
> respond the same way to all stimuli whether movement, sound, or images 
> (including images of text, since for perhaps most web user, text comes to 
> them as an image).
>         Most of us trained our cognitive systems before the stimuli under 
> discussion was invented. Children growing up with this type of stimuli 
> will learn to respond to it more appropriately than us old geezers do.
>         Have you tried to read material close to a flickering source? 
> What site? so we can all test it. Or do you mean to say that you have 
> tried to read material close to a moving source, an animation, for 
> example (like that RADAR icon), and you were unable to read the text? How 
> close was the animation? Was text size appropriate to the icon or did one 
> overpower the other? What site did you test it on? Were there any 
> contributing factors preventing your understanding other than the problem 
> graphic? Does this happen whenever you encounter certain type of 
> graphics? Is there any reason that hitting the stop button (or the 
> equivalent in your browser) is insufficient? Does it help to enlarge the 
> text size near a competing graphic?
>         An remember, that being irritated or annoyed is not a disability.
>                                                         Anne
>At 09:52 AM 7/30/01 -0400, Marja-Riitta Koivunen wrote:
>>At 06:49 AM 7/27/2001 +0100, Jonathan Chetwynd wrote:
>>>given that placements are a more subtle way of advertising, perhaps in the
>>>RADAR case and given the client group, it might make sense to advise that an
>>>animated gif is liable to irritate, rather than rely on a medical condition.
>>And not only to irritate. Sometimes it makes it almost impossible to read 
>>a text that is near a flickering image as our cognitive system is built 
>>to pay attention to the movement.
>>>jonathan chetwynd
>>>IT teacher (LDD)
>>>http://www.peepo.com        "The first and still the best picture directory
>>>on the web"
>Anne Pemberton
Received on Wednesday, 1 August 2001 04:03:38 UTC

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