快猫短视频

THE experiment was simple enough. The subject slid her head into the heart of
the brain scanner and rested one hand on a keypad. Then, by trial and error, she
started to work out an unknown sequence of eight finger taps. A tick would flash
up on a screen whenever she pressed the right key. Once she knew the sequence,
her instruction was to keep drumming out the pattern until it became an
unthinking rhythm. By the end of an hour, her fingers were skipping through the
complex routine almost of their own accord. Like a driver taking a familiar
route home, she was barely conscious of what she was doing.

Such tests may seem trivial, but the difference between what the brain does
during the first tentative taps and the final easy routine tells us more than
how we learn to do things in a habitual, unthinking way. It is a test that some
believe has revolutionary implications for our understanding of what it means to
consciously experience something. As Bernard Baars, a psychologist at the Wright
Institute in California and an editor of the journal Consciousness and
Cognition, explains, it is a beautiful model: 鈥淵ou are comparing two
apparently identical outputs. Externally, the actions look much the same. But in
one case, the actions come with a feeling of being experienced, and in the other
the actions are not experienced. So the level of consciousness becomes your
experimental variable, and how the brain differs between these two states must
tell you a lot about the mind.鈥�

Neuroscientist Dick Passingham of Oxford University and his colleagues from
the Institute of Neurology in London did the finger-tapping experiment last
year. For them the differences are startlingly easy to see. When people are in
the learning phase, having to remember what they have just discovered while
groping for the next step, regions all over the brain are alight with the
effort. A range of high-level cognitive areas in the forebrain, such as those
involved with planning and memory, are at work. But some more lowly brain
centres such as those that control movements are also working overtime,
including the basal ganglia鈥攁 knotted chain of nerve centres deep inside
the brain鈥攁nd the cerebellum鈥攖he cauliflower-shaped bulge off the
back of the brainstem. Vast swathes of the brain light up as if it throws every
vaguely relevant circuit at the task.

Yet within minutes of 鈥済etting鈥� the sequence, this wash of activity begins to
drain away. The job of moving the fingers becomes confined to just a small set
of motor areas. Avi Karni of the Weizmann Institute of Science in Israel has
spotted the same rapid shrinkage in the active area of the motor cortex as
people learn a finger-tapping task. He also found that after a few more days
practice, the activity begins to expand again just a little, so as to strengthen
this highly local motor representation. It seems, having used the whole brain
consciously to establish the individual finger movements, just the bare bones of
the routine are left. The brain now has a template or habit that can produce the
same behaviour 鈥渁s if鈥� it were still going through all the hoops of being
consciously aware.

These results are intriguing, but not that surprising. After all, you鈥檇
expect the brain to be able to 鈥渁utomate鈥� motor tasks such as typing or riding a
bike. However, other imaging studies have shown that a similar process occurs
when we learn more intellectual skills.

The automation of language has been studied intensively by a team led by
Marcus Raichle at Washington University in St Louis. Thinking speech to be a
highly conscious activity, the group didn鈥檛 set out to investigate habit
learning: the intention was merely to show which parts of the human brain 鈥渄id鈥�
language. Volunteers were placed in a scanner and asked to come up with verbs to
match a series of nouns. For example, the noun 鈥渉ammer鈥� should suggest a
response like 鈥渉it鈥�.

Right from the start, the results were unexpected. The team had thought that
just a few isolated language modules would light up. Yet, just as in the
finger-tapping exercise, much of the brain was afire, including wide areas of
the frontal cortex and more low-level 鈥渕ovement鈥� centres. There seemed to be too
much activity for a specific mental task.

Practice run

Then, one day, a man who was about to be scanned worried that he might not be
able to perform the language task fast enough once in the machine鈥攁
reasonable fear because the target list was presented at the rate of almost a
word a second to keep the minds of the subjects busy. He was allowed a few
practice runs with the word list that was to be used in the experiment because,
the researchers thought, exactly the same brain circuits should be used every
time a person made the same association between two words. But when the results
came back, his brain showed barely a flicker of strain.

Raichle calls this the practice effect. It has shown up in other scanner
studies, such as Richard Haier鈥檚 work with people learning to play the computer
game Tetris at the University of California, Irvine, back in 1992, and last
year, with experiments by Steven Petersen of Washington University, St Louis, in
which people learnt the path through a drawn maze. Raichle says that enough
results have accumulated to establish their importance to understanding
consciousness. Exactly how to interpret them is another matter altogether, he
warns.

What these researchers do agree on is that paying focal, effortful attention
to something calls large regions of the brain into action. The brain does not
behave like a collection of isolated pathways, each doing their own thing, but
as a coherent system.

As Passingham notes, there are general-purpose planning centres that seem to
come into play whenever the brain is dealing with any kind of novel or difficult
mental situation, from linking nouns and verbs to working out a key-pad
sequence. These areas seem to hold together a context of ideas for long enough
to guide the more specialist language and motor centres to an appropriate
output. But the surprising level of activation in lower brain areas suggests
that these too are put into exploratory mode, watching and learning from what is
going on.

It is what happens next鈥攖he rapid downloading of a skill from 鈥渂right鈥�
awareness鈥攖hat seems to be the real key to the brain鈥檚 enormous processing
abilities. The imaging experiments suggest that once the brain finds an optimal
way to respond to a certain situation, the wider scaffolding rapidly falls away.
It鈥檚 not that practice makes more efficient use of the pathways that were active
during conscious learning, it鈥檚 that the brain no longer needs to carry a
running memory of its recent performance. Instead, the response can be reduced
to its bare essentials, creating a memory trace in the motor or language areas
which then lies dormant until the right input passes by again.

This general picture of shrinking patterns of activation as the brain learns
from its own fumbling explorations seems well established. But it has only been
in the past year or so that experiments have begun to offer some sense of the
mechanisms behind such shifts, Passingham says.

One step has been to show that the involvement of the prefrontal cortex is
essential for people to be sharply conscious of a mental event鈥攁 fact long
suspected by neuroscientists on the basis of animal studies. With scanning, says
Passingham, demonstrating this could hardly be easier. He simply asked a group
of people to pay close attention to their now established finger-tapping rhythm.
Immediately, the frontal areas of the brain became active again. And the
clincher was that the subjects鈥� performance grew more ragged, as if their brains
were being put back into exploratory mode.

Passingham also showed that the need to use these high-level areas is
probably why there is room for only one thing in the spotlight of attention at
any one moment. He scanned people trying to learn a finger sequence and perform
Raichle鈥檚 verb generation task at the same time. The clash of two novel tasks
resulted in a stuttering performance, as the subjects had to switch between
concentrating on one or the other. But if the finger sequence had already been
mastered, and so no longer made demands on the prefrontal cortex, doing both
became possible.

More controversial is what the difference between high-level attention and
habitual action reveals. For some, such as Raichle, the scanning data suggest
that the brain has two distinct sets of pathways, one for dealing with novelty,
the other for habits. But to others like Passingham, the distinction should be
between the global and the local. Dealing with novelty requires a full brain
response, but once a suitable pattern of response has been established, activity
contracts to whichever pathway most directly connects a given sensory input to
its matching motor or mental output.

The suggestion is that over a lifetime of conscious learning the brain
accumulates thick strata of local routines鈥攈abits of perception and
reaction that allow most things to be processed swiftly and automatically. So
when we are driving a car, changing the gears and even switching of lanes would
be done at a barely conscious level, with familiar or well-anticipated patterns
of input immediately triggering a stereotyped pattern of response. Only when one
of these routines struck a snag鈥攖he gear knob coming off in our hand, say,
or a pothole suddenly looming in the road ahead鈥攚ould we switch to a more
global and exploratory form of reaction.

So what forms our centre of attention is effectively self-selecting鈥攖he
bit of the moment that turns out to be least routine. Our layers of habit form a
mental filter that let only the novel or the difficult grab our attention.
Raichle is intrigued that certain areas of the brain, such as the insula cortex
on the lower face of the frontal lobes, seem regularly to be switched off when
the brain is dealing with novelty.

One controversial guess is that such areas may be critical to the performance
of habits and so would have to be actively suppressed to allow the prefrontal
lobes to mount their more exploratory response. 鈥淲e still need to understand the
negative side of this better鈥攚hat areas of the brain do you have to
inhibit to be able to think about things in a novel way,鈥� says Raichle. But to
work this novelty switch, it seems as though there would still have to be a
central bottleneck to filter the inputs, ensuring that they either find their
way to an appropriate routine or become the focus of our conscious
attention.

Ann Graybiel, a neuroscientist at the Massachusetts Institute of Technology,
believes she might have found this bottleneck. Instead of using scanners,
Graybiel uses electrodes to record the responses of individual neurons in a
monkey鈥檚 brain. For some years, she has been studying the basal ganglia, long
suspected of being a key player in automating complex actions.

Mysterious loops

The basal ganglia are connected to the cortex above by a mysterious set of
looping paths. These seem to collect news of the sensory and motor signals from
wide areas of the cortex, and then funnel it back to the frontal planning
region. When the monkeys were in the early stages of learning that a clicking
noise signalled the availability of a sip of juice, for example, Graybiel found
that the basal ganglia cells fired raggedly in response to all parts of the
event鈥攖he sound of the click, the turning of the monkey鈥檚 body, its
drinking. But quickly there was a dramatic change in activity so that the cells
fired only in response to the salient stimulus鈥攖he click.

Graybiel says these looping circuits are perfectly placed to monitor which
patterns of sensory activity trigger a particular motor response and to
short-circuit the cortex鈥檚 long-winded processing. So when the basal ganglia
cells detect a familiar pattern, they fire themselves, triggering the relevant
motor memory, says Graybiel.

She adds that because all cortex activity, not just motor activity, feeds
through the bottleneck of the basal ganglia, they would presumably do the same
service for any form of mental action. So the basal ganglia could be responsible
for making much of our thinking and speaking rather routine. Graybiel now
intends to find ways of making her monkeys pay fresh attention to the
performance of a habit to see if, as expected, this throws the basal ganglia
back into exploratory mode. 鈥淭hat wouldn鈥檛 be surprising because we all know
what happens when you start thinking about how you are hitting your forehand in
tennis, right?鈥� she laughs.

For researchers such as Graybiel and Raichle, these findings are exciting
because they are pushing neuroscience towards a more systems-level view of
consciousness and the brain. They make it clear that even the most lowly brain
regions can become intricately woven into the production of a moment of
awareness. Raichle criticises those who seek to locate conscious activity in
certain kinds of neural firing codes or within isolated brain areas such as the
visual cortex. 鈥淭hat鈥檚 like trying to understand how a symphony orchestra can
play Beethoven by worrying about the molecular qualities of a violin string.
You鈥檝e got to look at the whole brain in action,鈥� he says.