静止的动画

alexqxp

静止的图画，点击放大后再看变成动画。

alexqxp

其实也是单张图画。如果盯着一张叶子看，图画就变静止了。

不是风动，也不是旗动，而是心动。

alexqxp

How is a motion-detecting neuron in the brain “wired up”
to detect the direction of motion?
To explore motion perception, scientists of-
ten employ test patterns of very short movies
(two frames in length). Imagine in frame one a
dense array of randomly placed black dots on a
gray background. If, in frame two, you displace
the entire array slightly to the right, you will see
the patch of dots moving (jumping) to the right,
because the change activates multiple motion-de-
tecting neurons in your brain in parallel. This
phenomenon is termed apparent motion, or phi.
It is the basis for “motion” pictures in which no
“real” motion exists, only successive still shots.
But if in the second frame you displace the
dots to the right and also reverse the contrast of
all the dots so that they are now white on gray
(instead of black on gray), you will see motion in
the opposite direction—an illusion discovered by
psychologist Stuart M. Anstis, now at the Uni-
versity of California, San Diego. This effect is
known as reversed phi, but we shall henceforth
call it the Anstis-Reichardt effect, after the two
vision scientists who first explored it. (The sec-
ond person was Werner Reichardt, then at the
Max Planck Institute for Biological Cybernetics
in Tübingen, Germany.) We now know that this
paradoxical reverse motion occurs because of
certain peculiarities in the manner in which mo-
tion-detecting neurons, called Reichardt detec-
tors, operate in our visual centers.
Wired for Motion
How is a motion-detecting neuron in the
brain “wired up” to detect the direction of mo-
tion? Each such neuron or detector receives sig-
nals from its receptive field: a patch of retina (the
light-sensing layer of tissue at the back of the
eyes). When activated, a cluster of receptors in,
say, the left side of the receptive field sends a sig-
nal to the motion detector, but the signal is too
weak to activate the cell by itself. The adjacent
cluster of retinal receptors on the right side of the
receptive field also sends a signal to the same cell
if stimulated—but, again, the signal is too weak
on its own.
Now imagine that a “delay loop” is inserted
between the first patch and the motion-detecting
neuron but not between the second (right) patch
and the same neuron. If the target moves right-
ward in the receptive field, the activity from the
second patch of retina will arrive at the motion-
detecting neuron at the same time as the delayed
signal from the left patch. The two signals togeth-
er will stimulate the neuron adequately for it to
fire. Such an arrangement, akin to an AND gate,
requires the circuit to include a delay loop and
ensures direction as well as velocity specificity.
But this is only part of the story. In addition,
we have to assume that for some reason we have
yet to understand, stationary displays such as a
and b produce differential activation within the
motion receptive field, thereby resulting in spuri-
ous activation of motion neurons. The peculiar
stepwise arrangement of edges—the variation in
luminance and contrast—in each subregion of
the image, combined with the fact that even when
you fixate steadily your eyes are making ever so
tiny movements, may be
critical for artificially acti-
vating motion detectors.
The net result is that your
brain is fooled into seeing
motion in a static display.
Enhancing Motion
Finally, it is also known
that patterns with a cer-
tain amount of regularity
and repetitiveness will ex-
cite a large number of mo-
tion detectors in parallel,
very much enhancing your
subjective impression of
motion. A small section of
a display such as c is insufficient to generate no-
ticeable motion, although the massively parallel
signals from the highly repetitive patterns togeth-
er produce strong illusory motion. Readers may
want to conduct a few casual experiments them-
selves: Is the illusion any stronger with two eyes
than with one? How many almondlike shapes or
snakes are necessary to see them moving?
The manner in which stationary pictures
work their magic to create tantalizing impres-
sions of motion is not fully understood. We do
know, however, that these stationary displays ac-
tivate motion detectors in the brain. This idea has
also been tested physiologically, by recording
from individual neurons in two areas of the mon-
key brain: the primary visual cortex (V1), which
receives signals from the retina (after being re-
layed through the thalamus), and the middle
temporal area (MT) on the side of the brain,
which is specialized for seeing motion. (Damage
to the MT causes motion blindness, in which
moving objects look like a succession of static
objects—as if lit by a strobe light.)
The question is, Would static images like the
rotating snakes “fool” motion-detecting neu-
rons? The initial answer seems to be yes, as has
been shown in a series of physiological experi-
ments published in 2005 by Bevil R. Conway of
Harvard Medical School and his colleagues.
Thus, by monitoring the activity of motion-
detecting neurons in animals and simultaneously
exploring human motion perception using cun-
ningly contrived displays such as a, b and c, sci-
entists are starting to understand the mecha-
nisms in your brain that are specialized for seeing
motion. From an evolutionary standpoint, this
capability has been a valuable survival asset as an
early warning system to attract your attention—
whether to detect prey, predator or mate (all of
which usually move, unlike stones and trees).
Once again, illusion can be the path to under-
standing reality. M
VILAYANUR S. RAMACHANDRAN and DIANE ROGERS-
RAMACHANDRAN are at the Center for Brain and Cogni-
tion at the University of California, San Diego. They serve
on Scientific American Mind’s board of advisers.
(Further Reading)
◆  Phi Movement as a Subtraction Process. S. M. Anstis in Vision Research,
Vol. 10, No. 12, pages 1411–1430; December 1970.
◆  Perception of Illusory Movement. A. Fraser and K. J. Wilcox in Nature,  
Vol. 281, pages 565–566; October 18, 1979.
◆  
Neural Basis for a Powerful Static Motion Illusion. Bevil R. Conway, Aki-
yoshi Kitaoka, Arash Yazdanbakhsh, Christopher C. Pack and Margaret S.
Livingstone in Journal of Neuroscience, Vol. 25, No. 23, pages 5651–
5656; June 8, 2005.
◆  Stuart M. Anstis’s Web site for “reversed phi” effect:  
http://psy.ucsd.edu/~sanstis/SARevMotion.html