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you as your conscious perceptions. Typically, what you experience as “now”
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corresponds to what was actually happening some tens or hundreds of mil-
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liseconds in the past.
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Estonian- Canadian psychologist Endel Tulving suggested the term
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chronesthesia, or “mental time travel.” One of Tulving’s contributions was
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the distinction between two different kinds of memory: semantic memory,
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which refers to general knowledge (Gettysburg was the site of an important
29
battle in the American Civil War), and episodic memory, which captures
30
our recollection of personal experiences (I visited Gettysburg when I was in
31
high school). Mental time travel, Tulving suggested, is related to episodic
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memory: imagining the future is a similar conscious activity to recalling
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events in the past.
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Recent work in neuroscience has lent credence to this idea. Researchers
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have been able to use functional magnetic resonance imaging (fMRI) and
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positron emission tomography (PET) scans to pinpoint regions in the brain
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that are active while subjects are conducting various mental tasks. Interest-
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ingly, the tasks of “remember yourself in a particular situation in the past”
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and “imagine yourself in a particular hypothetical situation in the future”
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are seen to engage a very similar set of subsystems in the brain. Episodic
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memory and imagination engage the same neural machinery.
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Memories of past experiences, it turns out, are not like a video or film
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recording of an event, with individual sounds and images stored for each
09
moment. What’s stored is more like a script. When we remember a past
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event, the brain pulls out the script and puts on a little performance of the
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sights and sounds and smells. Part of the brain stores the script, while oth-
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ers are responsible for the stage settings and props. This helps explain why
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memories can be completely false, yet utterly vivid and real- seeming to us—
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the brain can put on a convincing show from an incorrect script just as well
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as an accurate one. It also helps explain how our chronesthetic ability to
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imagine future events might have developed through natural selection.
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Evolution, always looking to work with existing materials, constructed our
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powers of imagination out of our existing capacity to remember the past.
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While a capacity for mental time travel is important for some aspects of
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consciousness, it certainly isn’t the whole story. Kent Cochrane was an am-
21
nesiac, famous in the psychology literature as the patient “K. C.” When he
22
was thirty years old, K. C. suffered a serious motorcycle accident. He sur-
23
vived, but during surgery he lost parts of his brain, including the hippocam-
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pus, and his medial temporal lobes were severely damaged. Afterward, he
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retained his semantic memory but completely lost his episodic memory. His
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ability to form new memories was almost completely absent, much like the
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character of Leonard Shelby in the movie Memento. K. C. knew that he
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owned a particular car, but had no recollection of ever driving in it. His
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basic mental capacities were intact, and he had no trouble carrying on
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a conversation. He just couldn’t remember anything he had ever seen
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or done.
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There’s little question that K. C. was “conscious” in some sense. He
33
was awake, aware, and knew who he was. But consistent with the connec-
34
tion between memory and imagination, K. C. was completely unable to
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contemplate his own future. When asked about what would happen tomor-
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row or even later that day, he would simply report that it was blank. His
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personality underwent a significant change after the accident. He had, in
01
some sense, become a different person.
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There is some evidence that episodic memory doesn’t develop in chil-
03
dren until they are about four years old, around the time they also seem to
04
develop the capacity for modeling the mental states of other people. At
05
younger ages, for example, children can learn new things, but they have
06
trouble associating new knowledge with any particular event; when quizzed
07
about something they just learned, they will claim that they have always
08
known it. Tulving has argued that true episodic memory, and the associated
09
capacity for imagination and mental time travel, might be unique to hu-
10
mans. It’s an intriguing hypothesis, but the current state of the art doesn’t
11
let us say for sure. We know that rats, for example, after trying and failing
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to reach some food, will continue to think about how to reach it after the
13
food has been removed, which might be interpreted as a kind of planning.
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Their mental activity involves the hippocampus, which is associated with
15
episodic memory in humans. Our ability to imagine the future is incredibly
16
detailed and rich, but it’s not hard to imagine how it might have evolved
17
gradually over the span of many generations.
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•
20
There’s so much we don’t know about the development of consciousness, it’s
21
easy to be dubious of any particular theory. Was crawling out of the water
22
and onto land a pivotal step along the way, as Malcolm MacIver suggests,
23
or is that just another fish story?
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We should be skeptical; that’s our job. There are aquatic animals that
25
seem to be much smarter than your average goldfish. Whales and dolphins,
26
of course, but those are mammals that descended from land animals— so
27
their intelligence actually provides evidence for the hypothesis, not against
28
it. Octopuses are quite intelligent by many standards. T
hey have the biggest
29
brains of any invertebrate (animals without spinal cords), although still
30
only about one- thousandth the number of neurons that a human has. An
31
octopus might not be able to do crossword puzzles, but it can solve certain
32
simple challenges, such as opening a jar to get at food that’s inside.
33
MacIver notes that octopuses, while underwater creatures, seem to max-
34
imize the extent of their sensory capacities. They have very large eyes, and
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tend to remain still while executing complex tasks. It’s dangerous being an
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octopus; from the point of view of a predatory sea- dweller, you are a vulner-
02
able bag of delicious nutrients. To survive, they have had to develop innova-
03
tive defensive strategies, camouflaging themselves by changing skin color
04
and emitting clouds of ink when forced to flee. Intelligence is a part of that
05
defensive arsenal; an octopus will hide among rocks and coral when it
06
sleeps, often arranging pieces so as to better shield itself from view. Perhaps
07
the evolutionary pressure that led to large octopus brains was of a com-
08
pletely different type from that which led to land- dwelling animals.
09
Whatever the importance of climbing onto land might have been, it did
10
not lead immediately to animals that could write sonnets and prove math-
11
ematical theorems. Four hundred million years is a long time. The evolution
12
of consciousness as we now know it took many steps. Chimpanzees can
13
think and execute a plan, such as building a structure in order to get to a
14
banana that is out of reach. That’s a kind of imaginative thought, though
15
certainly not the whole story.
16
We can conceive of many moments in the evolutionary history of con-
17
sciousness ultimately leading to the exquisite complexity of our current
18
mental capacities. As the reducibly complex mousetrap reminds us, we
19
shouldn’t let the intimidating sophistication of the final product trick us
20
into thinking that it couldn’t have come about via numerous small steps.
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The Babbling Brain
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I
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t’s an image familiar from countless TV hospital dramas: the patient
14
lying on their back, head placed inside an intimidating- looking medi-
15
cal apparatus meant to peer inside their brain. Most often it will be an
16
MRI machine, which will produce beautiful images of brain activity by
17
tracking the flow of blood. In my case it was an MEG machine: magneto-
18
encephalography. By measuring the appearance of magnetic fields just out-
19
side my skull, this beast was going to test whether or not I had a brain, and
20
whether my brain could indeed have thoughts.
21
I passed. I like to think the outcome wasn’t really in doubt, but it’s good
22
to have these things verified by science.
23
My brain scan was carried out by neuroscientist David Poeppel in his lab
24
at New York University. Unlike fMRI, which makes beautiful pictures but
25
doesn’t have great time resolution, MEG isn’t very good at telling you where
26
processes are located in the brain, but it can distinguish when they happen
27
down to a few milliseconds.
28
That’s important, because our brains are intricately connected multi-
29
level systems that take time to do their work. Individual neural events hap-
30
pen several times per millisecond, but it takes tens of milliseconds for
31
several of them to accumulate to sufficient strength for your brain to sit up
32
and say, “Hey! Something’s happening!”— a conscious perception.
33
In the brain, most of the hard work of thinking is done by the neurons.
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They are joined by glial cells, which help support and protect the neurons.
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Glial cells may play a role in how neurons talk to one another, but the
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Isofield Contour Map
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L
R
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Sink Source
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20 fT/Step
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A map of the magnetic fields just outside my brain, generated by listening to
a beeping sound. (Courtesy of David Poeppel lab, New York University)
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information- carrying signals in the brain are carried by the neurons. A
19
typical neuron will come equipped with two types of appendages: a large
20
number of dendrites, which receive signals from outside, and (usually just
21
one) axon, down which signals are sent. The body of a neuron is less than a
22
tenth of a millimeter across, but axons can range from one millimeter all
23
the way up to a full meter long. When a neuron wants to send a signal, it
24
“fires” by pumping an electrochemical signal down its axon. That signal is
25
received by other neurons at connection points known as synapses. Most
26
synapses consist of a dendrite connecting to an axon, but the brain is a
27
messy place, so various other kinds of connections are possible.
28
So neurons talk to each other by squirting electrically charged mole-
29
cules from the axon of one to a dendrite on another. As any physicist will
30
tell you, charged particles in motion generate magnetic fields. When a
31
thought happens in my brain, this corresponds to charged particles hop-
32
ping between neurons, creating a faint magnetic field that extends just a bit
33
outside my skull. By detecting these magnetic fields, an MEG machine can
34
pinpoint exactly when my neurons do their firing.
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Poeppel and his colleagues are using this technique to study perception,
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cognition, and the workings of language in the brain. Sitting there in the
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MEG, I listened to various meaningless beeps and boops, and the techni-
01
cian was able to track how long it took before I consciously perceived the
02
auditory signal as a sound— tens of milliseconds, in a cascade of interre-
03
lated cortical responses.
04
I was most impressed by something much more prosaic— these probes
05
attached to my skull could sense me thinking. What we call a “thought” cor-
06
responds directly and unmistakably to the motion of certain charged par-
07
ticles inside my head. That’s an amazing, humbling fact about how the
08
universe works. What would Descartes and Princess Elisabeth have thought?
09
Very few people today would deny that thinking is somehow related to
10
what goes on in the brain. The divide is between those who believe that
11
“thinking” is just a way of talking about the physical processes in the brain
12
like the ones my MEG detected, and those who believe that we need to add
13
some additional ingredients over and above the physical. It’s worth doing a
14
little thinking of our own about how brains actually work, to help under-
15
stand why the physical picture is so compelling.
16
17
•
18
The brain is a network of interconnected neurons. We talked briefly in
19
chapter 28 about how complex structures can arise by gradual agglomera-
20
tion of smaller units into ever- larger ones, preserving the existence of inter-
21
esting structure on all scales. The brain is a great example.
22
The conventional view of what happens in the brain is that it’s not
23
the neurons themselves that encode information but the way they are
24
connected to one another. Every neuron is connected to some other neu-
25
rons, and not to others; that’s what defines the network structure of the
26
brain, known as its connectome.
The Big Picture Page 55