Mind Hacks™: Tips & Tools for Using Your Brain (15 page)

Find out why static pictures can make up a moving image on your TV screen.

The easiest way to experience this effect is, of course, to turn on your television or
go to the cinema. Movie projectors show 24 frames (pictures) a second, and that’s good
enough for everyone to perceive continuous motion in the change from one frame to the
next.

Television and computer screens are more complex cases, because the refresh doesn’t
happen for the whole image at once as it does with cinema but the principle is the
same.

To demonstrate the effect to yourself in a more low-tech way, try this old child’s
game. Take a notebook and in the page corners draw the successive frames of a moving
scene. I’m not very good at drawing stickmen, so when I did it I just tried drawing small,
filled circles moving up from the bottom corner to the top of the page. Alternately, you
may find a flip book in your local bookshop.

Flip through the pages of the book using your thumb and — at a particular speed — you’ll
see the scene come to life. They’re not just single pictures any more; together they form
an animation. In my case, I see the little dot shoot up the side of the page. If I flip
through the pages more slowly, the dot moves more slowly — but still continuously, as if it
moves through every position on its path. Then, as I slow down even more, there comes a
certain point at which the feeling of watching a single moving circle disappears and I’m
just looking at a bunch of pages populated with slightly different shapes in slightly
different positions.

This apparent motion effect is also sometimes called the phi phenomenon. The simplest
form in which you’ve probably encountered it before is two lights flashing at such an
interval that you see one light moving from the first position to the second, as on an LED
ticker display. Imagine only two lights from such a display. If the delay between the
lights flashing is too short, the lights seem to flash on simultaneously. If it is too
long, you just see two lights flashing on, one after the other. But if just right, you’ll
be treated to some apparent motion.

Although the optimum time varies with circumstance, 50 milliseconds is approximately
the delay you need between the first light blinking out and the second light flashing on,
in order to get a strong illusion of a single light moving between the two locations. Note
that that’s 20 flashes a second, close to the
rate of image change in cinema. (Just so you know, as the physical distance
between the two light flashes increases, so does the optimum time delay.
¹
)

The effect is most powerful when you see the light appearing at several locations,
making a consistent movement — exactly like LED tickers, on which a message appears to
scroll smoothly across despite really being made out of sequentially flashing lights. In
fact, it isn’t just that we feel there’s an illusion of movement: the apparent motion
effect activates a region called MT (standing for
middle temporal
gyrus
, a folded region on the temporal lobe) in the visual cortex, one
primarily responsible for motion processing. Apparent motion is just as valid as real
motion, according to the brain.

And this makes sense. The only difference with apparent motion, as far as visual
perception is concerned, is that some of the information is missing (i.e., everything that
happens in the locations between the flashing lights). Since there’s no way to detect
motion directly — we can’t
see
momentum, for example — and visual
information is all we have to go on, apparent motion is just a legacy of our tolerance for
missing data and our ability to adjust.

A visual system that wasn’t susceptible to the effect would be overdesigned. The
capacity to perceive apparent motion lets us see consistency in images that are moving too
rapidly for us to comprehend individually.

The obvious benefit of the phenomenon is that we can sit back and watch television and
movies.

It also explains why wheels can look as if they are going backward slowly when they
are actually going forward extremely quickly. Remember that apparent motion is strongest
when adjacent lights, or images, flash up approximately 50 milliseconds apart. Caught on
film, a wheel rotating forward may be turning at such a speed that, after 50 milliseconds
(or a frame), it’s made almost a full turn, but not quite. The apparent motion effect is
stronger for the wheel moving the short distance backward in that short time rather than
all the way round forward, and so it dominates: We see the wheel moving slowly backward,
rather than fast and forward.

The phi phenomenon also seems to say something important about the relationship of
real time to perceived time. If you show two flashing lights of different colors so as to
induce the phi phenomenon, you still get an effect of apparent motion.
²
For some people, the light appears to change from the first color to the
second as it moves from the first spot (where the first light was shown) to the second
spot (where the second light was shown).

Now the thing about this is —
how did your brain know what color the
light was going to change to?
It seems as if what you “saw” (the light
changing color) was influenced by something you were about to see. Various theories had
been put forward to explain this, either about the revision of our perceptions by what
comes after or about the revision of our memories. Philosopher Daniel Dennett
³
says that both of these types of theory are misleading because they both
imply that conscious experience travels forward in time along a single,
one-step-forward-at-a-time-and-no-steps-back track.

Instead, he suggests, there are multiple drafts of what is going on being continuously
updated and revised. Within an editorial window (of, some have suggested, about 200
milliseconds of real time), any of these drafts can out-compete the others to become what
we experience.
⁴

If there’s a flash of light on a moving object, the flash appears to hang a
little behind.

Watch Michael Bach’s Flash Lag demo at
http://www.michaelbach.de/ot/mot_flashlag1
(Flash). A still from it is shown in
Figure 2-23
. In it, a blue-filled circle orbits
a cross — hold your eyes on the cross so you’re not looking directly at the moving circle.
This is to make sure the circle is moving across your field of view.

Occasionally the inside of the ring flashes yellow, but it looks as if the yellow
flash happens slightly behind the circle and occupies only part of the ring. This is the
flash-lag illusion. You can confirm what’s happening by clicking the Slow button (top
right). The circle moves slower and the flash lasts longer, and it’s now clear that the
entire center of the circle turns yellow and the lag is indeed only an illusion.

The basic difficulty here is that visual perception takes time; almost a tenth of a
second passes between light hitting your retina to the signal being processed and reaching
your cortex (most of this is due to how long it takes the receptors in the eye to
respond). The circle in Bach’s demo moves a quarter of an inch in that time, and it’s not
even going that fast. Imagine perpetually interacting with a world that had already moved
on by the time you’d seen it.

So we continuously extrapolate the motion of anything we see, and our brain presents
us with a picture of where the world most likely is now, rather than where it was a
fraction of a second ago. This applies only to moving objects, not to stationary ones, and
that’s why the disparity opens
up between the moving blue circle and the static yellow flash — one is being
extrapolated; the other isn’t.

Figure 2-23. In the movie, the circle orbits the cross and flashes from time to time

Straightforward extrapolation of the path of moving objects is one way in which this
effect can take place, and this happens as early as the retina itself during visual
processing. The cells in the eye compensate for its slow response by being most active at
the front edge of a moving object. (Without this, the most active cells would be the ones
that had been exposed to the object the longest, that is, the ones at the back.
²
)

That’s one way in which the flash-lag effect could come about, because the delay for
visual processing is compensated for with moving objects, but flashes still pay the
penalty and are seen later. But that doesn’t explain the demonstration movies constructed
by David Eagleman and Terrence Sejnowski (
http://neuro.bcm.edu/eagleman/flashlag/
; QuickTime). Essentially the same as Bach’s demo, these movies have an
erratically moving ring that should confuse the brain’s motion prediction.

In Experiment 1 (
http://neuro.bcm.edu/eagleman/flashlag/demos/r1.mpg
; QuickTime), the ring abruptly changes direction at the same time as the
flash. Still we see the flash lag behind the moving ring, even though prediction of the
future motion of the ring could not have occurred.

Eagleman and Sejnowski’s explanation is that vision is
postdictive
. They argue that the brain takes into account changes
in the scene that occur after the flash, for a very short time (less than a tenth o f a
second), and the motion preceding the flash isn’t relevant at all. This is similar to the
way two flashing dots can appear to be a single dot apparently moving
[
Show Motion Without Anything Moving
]
smoothly from one position to another, if the timing is right. Your brain must have filled
in the interim motion retrospectively, because you can’t know what in-between would be
before the second dot appears. Similarly, the circle in this flash-lag experiment and the
following fraction of a second comprise a period to be assembled retrospectively. The ring
is moving smoothly after the flash, so you have to see it moving smoothly, and the flash
appears slightly behind, because by the time you’ve mentally assembled the scene, the ring
has moved on.

The situation is muddied because flash lag isn’t unique to motion. One experiment
³
found the same effect with color. Imagine a green dot slowly becoming red
by passing through all intermediate shades. At a certain point, another dot flashes up
next to it, with the same color for that time. Looking at it, you’d see the flash-lag
effect as if the changing dot were moving along the color dimension: the flashed dot would
appear lagged. That is, the flashed dot would appear greener than the changing dot.

That flash lag appears for phenomena other than motion supports that post-diction
position. It could be the case that we don’t see the world at an instant, but actually as
an average over a short period of time. The moving ring appears to be ahead of the flash
because, over a very short period, on average it
is
ahead of the
flash. The colored dot appears to be redder than the flashed dot because it
is
redder, over the averaged period.

The effect was first noticed with the taillights of a car in the dark (the car being
invisible except for the rear lights). A flash of lightning lets you see the car, and the
lights appear to be halfway along it: the car — which is flashed — lags behind the
taillights — which are extrapolated.

It should also be evident in reverse. If you’re photographed from a moving car, the
flash of the camera should appear a little behind the car itself.