快猫短视频

PICTURE the following, and prepare to be amazed. You鈥檙e walking across a
college campus when a stranger asks you for directions. While you鈥檙e talking to
him, two men pass between you carrying a wooden door. You feel a moment鈥檚
irritation, but they move on and you carry on describing the route. When you鈥檝e
finished, the stranger informs you that you鈥檝e just taken part in a psychology
experiment. 鈥淒id you notice anything change after the two men passed with the
door?鈥� he asks. 鈥淣o,鈥� you reply uneasily. He then explains that the man who
initially approached you walked off behind the door, leaving him in his place.
The first man now comes up to join you. Looking at them standing side by side,
you notice that the two are of different height and build, are dressed
differently, have different haircuts and different voices.

It sounds impossible, but when Daniel Simons, a psychologist at Harvard
University, and his colleague Daniel Levin of Kent State University in Ohio
actually did this experiment, they found that fully 50 per cent of those who
took part failed to notice the substitution. The subjects had succumbed to what
is called change blindness. Taken with a glut of recent experimental results,
this phenomenon suggests we see far less than we think we do.

Rather than logging every detail of the visual scene, says Simons, we are
actually highly selective about what we take in. Our impression of seeing
everything is just that鈥攁n impression. In fact we extract a few details
and rely on memory, or perhaps even our imagination, for the rest. Others have a
more radical interpretation: they say that we see nothing at all, and our belief
that we have only to open our eyes to take in the entire visible world is
mistaken鈥攁n illusion.

Until the last decade, vision researchers thought that seeing really meant
making pictures in the brain. By building detailed internal representations of
the world, and comparing them over time, we would be able to pick out anything
that changed. Then in 1991, in his book Consciousness Explained, the
philosopher Daniel Dennett made the then controversial claim that our brains
hold only a few salient details about the world鈥攁nd that this is the
reason we are able to function at all.

We don鈥檛 store elaborate pictures in short-term memory, Dennett said, because
it isn鈥檛 necessary and would take up valuable computing power. Rather, we log
what has changed and assume the rest has stayed the same. Of course, this is
bound to mean that we miss a few details. Experimenters had already shown that
we may ignore items in the visual field if they appear not to be
significant鈥攁 repeated word or line on a page of text, for instance. But
nobody, not even Dennett, realised quite how little we really do 鈥渟ee鈥�.

Just a year later, at a conference on perception in Vancouver, British
Columbia, John Grimes of the University of Illinois caused a stir when he
described how people shown computer-generated pictures of natural scenes were
blind to changes that were made during an eye movement. Dennett was delighted.
鈥淚 wish in retrospect that I鈥檇 been more daring, since the effects are stronger
than I claimed,鈥� he says.

Since then, more and more examples have been found that show just how
illusory our visual world is. It turns out that your eyes don鈥檛 need to be
moving to be fooled. In a typical lab demonstration, you might be shown a
picture on a computer screen of, say, a couple dining on a terrace. The picture
would disappear, to be replaced for a fraction of a second by a blank screen,
before reappearing significantly altered鈥攂y the raising of a railing
behind the couple, perhaps. The picture flickers back and forth, and many people
search the screen for up to a minute before they see the change. A few never
spot it.

It鈥檚 an unnerving experience. But to some extent 鈥渃hange blindness鈥� is
artificial because the change is masked in some way. In real life, there tends
to be a visible movement that signals the change. But not always. As Simons
points out, 鈥淲e have all had the experience of not noticing a traffic signal
change because we had briefly looked away.鈥� And there鈥檚 a related phenomenon
called inattentional blindness, that doesn鈥檛 need any visual trick at all: if
you are not paying attention to some feature of a scene, you won鈥檛 see it.

Last year, with Christopher Chabris, also at Harvard, Simons showed people a
videotape of a basketball game and asked them to count the passes made by one or
other team. After about 45 seconds, a man dressed in a gorilla suit walked
slowly across the scene, passing between the players. Although he was visible
for five seconds, 40 per cent of the viewers failed to notice him. When the tape
was played again, and they were asked simply to watch it, they saw him easily.
Not surprisingly, some found it hard to believe it was the same tape.

Now imagine that the task absorbing their attention had been driving a car,
and the gorilla-man had been a pedestrian crossing their path. According to some
estimates, nearly half of all fatal motor-vehicle accidents in the US can be
attributed to driver error, including lapses in attention. It is more than just
academic interest that has made both forms of cognitive error hot research
topics.

Such errors raise important questions about vision. For instance, how can we
reconcile these gross lapses with our subjective experience of having continuous
access to a rich visual scene? Last year, Stephen Kosslyn of Harvard University
showed that imagining a scene activates parts of the visual cortex in the same
way as seeing it. He says that this supports the idea that we take in just what
information we consider important at the time, and fill in the gaps where the
details are less important. 鈥淭he illusion that we see `everything鈥� is partly a
result of filling in the gaps using memory,鈥� he says. 鈥淪uch memories can be
created based on beliefs and expectations.鈥�

Ronald Rensink of the University of British Columbia in Vancouver believes
that our impression of a rich visual world comes from our building internal
representations, though he accepts that they are far less detailed than was once
thought. According to his 鈥渃oherence theory鈥�, the brain first constructs a
temporary layout of the visual scene鈥攏ot much more than the basic geometry
and light distribution. Then attention comes along and pulls out a few of these
鈥減roto-objects鈥� to a higher resolution. More importantly, he explains, 鈥渨hat
attention does is to stabilise these representations so that they form an
individual object, something with continuity in space and in time鈥�. The moment
attention is released, they dissolve back into the volatile, unresolved
landscape. In Rensink鈥檚 view, focused attention is needed to perceive
change.

But while Rensink or Kosslyn would argue that there is some role for internal
images or memory, other researchers argue that we can get the impression of
visual richness without holding any of that richness in our heads. Back in 1992,
Kevin O鈥橰egan, an experimental psychologist at the French National Centre for
Scientific Research (CNRS) in Paris put forward what later became known as his
鈥済rand illusion鈥� theory. He argued that we hold no picture of the visual world
in our brains. Instead, we refer back to the external visual world as different
aspects become important. The illusion arises from the fact that as soon as you
ask yourself 鈥渁m I seeing this or that?鈥� you turn your attention to it and see
it.

According to O鈥橰egan, it鈥檚 not just our impression of richness that is
illusory, but also the sense of having control over what we see. 鈥淲e have the
illusion that when something flickers outside the window, we notice it
flickering and decide to move our eyes and look,鈥� says Susan Blackmore of the
University of the West of England, who supports O鈥橰egan鈥檚 views. 鈥淭hat鈥檚
balderdash.鈥� In fact, she says, we are at the mercy of our change detection
mechanisms, which automatically drag our attention here, there and
everywhere.

At a meeting in Brussels in July this year, O鈥橰egan and Alva No毛 of the
University of California, Santa Cruz, updated the controversial theory.
Sensation, whether it be visual, auditory or tactile, is not something that
takes place in the brain, they argue. Rather it exists in the knowledge that if
you were to perform a certain action, it would produce a certain change in
sensory input. 鈥淪ensation is not something that we feel, but sensation is
something that we do,鈥� says O鈥橰egan.

According to this idea, the sensation of 鈥渞edness鈥� arises from knowing that
moving your eyes onto a red patch will produce a certain change in the pattern
of stimulation in line with laws of redness. In other words, the role of the
brain is to initiate the exploratory action and to hold the knowledge of those
laws: together this give rise to the sensation of redness.

Once you dismiss the need for visual memory, O鈥橰egan says, many of the
problems that vision researchers have grappled with for decades vanish. Namely,
how does the ropy physics of the eye give rise to the largely flawless
experience of visual perception? Leaving aside the blind spot in each retina and
the fact that we view the world through a jerky sequence of eye movements or
鈥渟accades鈥�, we have two upside-down retinal images. If you assume that our
brains build detailed reconstructions from such inadequate, distorted input, you
have to postulate some kind of compensation mechanism in the visual system. In
O鈥橰egan鈥檚 model no such mechanism is required because there is no
reconstruction.

His theory also explains change blindness. Take the example of the dining
couple. The reason you don鈥檛 notice the raising of the railing is because you
didn鈥檛 notice the railing in the first place鈥攊t wasn鈥檛 relevant so it
remained invisible.

O鈥橰egan鈥檚 ideas have not been generally accepted. 鈥淗e鈥檚 pushed the idea that
we lack visual representations farther than most people in the field have been
willing to,鈥� says Simons. But despite their differences, Simons, Rensink and
O鈥橰egan all say that of all the myriad visual details of any scene that you
could record, you take only what is relevant to you at the time.

In the Simons-Levin experiment, for example, even the object to which the
person is attending鈥攖he stranger asking for directions鈥攃an be
swapped without them noticing. Despite the fact that they were looking at him
for around a minute, half the subjects encoded none of the details of his
physical appearance that were later to change. It was not relevant that the
stranger had a certain haircut or that his trousers were a certain colour. What
was relevant was that he was a person in a certain location addressing them with
a certain query. 鈥淧aying attention to an object does not give you all of that
object鈥檚 properties for free,鈥� says Simons. He points out that those who did
notice the switch were students of about the same age as the 鈥渟trangers鈥�. Being
in the same social group, he and Levin speculated, they would be more inclined
to take in individual details, whereas older subjects might categorise the
stranger as 鈥渟tudent鈥� and leave it at that.

The relationships between attention, awareness and vision have yet to be
clarified. But there is one thing on which most researchers agree: because we
have a less than complete picture of the world at any one time, there is the
potential for distortion and error. And that has all sorts of implications, not
least for eyewitnesses. If it is possible to stand less than a metre from a
person and talk to them for a minute without taking in more than a few basic
facts, how reliable is the testimony of a person who witnesses a scene from a
distance, when they were oblivious to its significance and only later came to
recall it?

鈥淚n my view, imagery plays a key role in many sorts of false memories,鈥� says
Kosslyn. 鈥淥ne is `filling in鈥� the gaps and later remembering not only what was
attended to, but also what was filled in.鈥� In retrospect, he says, we don鈥檛 make
any distinction between the two types of information.

For all our experience of a rich visual world, it seems that we take in no
more than a handful of facts about the world, throw in a few stored images and
beliefs, and produce a convincing whole in which it is impossible to tell what
was real and what imagined. As Blackmore puts it: 鈥淭here is a world and a brain
in it, which together are building a construction, a story, a great
肠辞苍蹿补产耻濒补迟颈辞苍.鈥�

When gazing at a painting or photograph your impression
that you are seeing the whole thing in sharp detail is false, say British
researchers. In fact, the only part of it that you see at high resolution at any
given moment is an area about the size of your thumbnail held at arm鈥檚
length.

It is by moving the eye around, and with it that area of high resolution,
that you gradually take in the whole image, says David Wooding from the
University of Derby. Wooding and his colleagues are currently running an
eye-tracking experiment in collaboration with the 鈥淭elling Time鈥� exhibition at
the National Gallery in London. Infrared cameras snap visitors鈥� eyes as they
view pictures, which a special computer program translates into a map showing
where their eyes are pointing, and for how long.

The vast majority of your eye movements occur unintentionally, says Wooding.
They are driven by the scene you are looking at, and by the information you are
trying to extract. For example, when asked to assess how a marriage being
discussed in a picture will turn out, the pattern of viewing is very different
from that when viewing the same picture with no particular purpose.

The researchers hope to gather enough data to find out what it is about
particular features of an image that attracts the visual system towards them.
That may be a tall order, not least because it is already clear that no two
people view an image in the same way. The experiment will run until January.