Wirt Atmar writes:

>It's called the "motion-after effect," or more commonly, the "waterfall
>effect." See:
>
>    http://www.biols.susx.ac.uk/home/George_Mather/Motion/MAE.HTML
>
>...and you're correct, it is related to habituation.
>Your brain, your CNS, as
>well as all of your peripheral nervous system, and in fact all of
>evolutionary adaptation, operates in the same manner, in a constant mode
>of prediction of the next symbols in a string. In all of these systems, a
>"model" is constructed/evolved in either neuronal or genic memory of the
>next
>most likely symbol in a string of symbols to appear. If the actual symbol
>that is presented to you is different than the expected, a difference
>signal of some sort is created, indicating the intensity of the novelty of
>the event.

This effect is the source of one of the most dangerous sensory illusions
for pilots, one that has led to a number of fatal accidents, including
one in a commercial airliner. It's one that you can experience for
yourself next time you're flying at night.

If you're in an airplane that's turning, you can feel the airplane roll
into a bank. But in a properly-executed turn, you won't feel any turning
motion after the initial roll because of the way that the forces act on
the airplane: they press you straight into your seat, just as if you were
in level flight. After ten or fifteen seconds, you (and the pilot) become
habituated to this. So when the airplane rolls out of the bank back to
level flight, it seems to you (and the pilot) that the airplane is now
banked in the opposite direction.

If the pilot acts to "correct" the attitude of the airplane based on
his/her perception, the result will be that the airplane ends up slightly
nose-down, banked in the original direction. Unless the pilot corrects
this mistake (which requires ignoring very strong and convincing sensory
input), the airplane ends up in an ever-tightening downward spiral, which
all the while feels like level flight. One of the hardest things for a
pilot to learn during instrument training is to ignore this very
compelling illusion and believe the instruments instead. (There are
redundant indications from multiple instruments, so it's almost always
obvious when one of them is lying.)

The reason that the illusion is strongest at night is that during the
day, the visual cues from the horizon overcome the motion cues from the
semicircular canals. But at night or in clouds, there is no horizon to
recalibrate the motion-sensing system.

Wirt also writes:

>...Novelty is an especially effective way to break
>habituated responses, but what the [HP3000-L] tag lines need to be
>is made into more of
>an "irritant" (a technical term; I'm not kidding). By placing the tag
>lines as the first two lines of every post, I suspect that they would be
>truly much harder to ignore and dismiss, even for the most jaded.

This may not be the case. Several years ago, a good friend of mine who
runs the Internet operations for a large financial services company was
having an argument with her web designers. The new requirement was to
display some particularly important user-specific notifications on the
page that the user sees immediately after login. The designers wanted to
put it at the top of the page. My friend said that the top of the page
was a very silly place to put it, because the profusion of banner ads has
conditioned most people to ignore everything above any horizontal visual
break near the top of the page.

Luckily, this financial services ferm has a usability lab complete with
Orwellian visual-attention-tracking cameras. When they pulled a bunch of
people into the lab and sat them down in front of their web site, the
designers discovered that my friend was right: the gaze almost never went
above the bottom of the banner, no matter what they had there.

So putting two fixed lines at the top of every 3000-L post would likely
just result in everyone reading from the third line on.

-- Bruce


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