27
The connectome is simply the list of every single neuron in the brain,
28
along with all of the connections between them. It’s a system of impressive
29
complexity: the human brain contains roughly 85 billion neurons, each of
30
which is connected to a thousand or more other neurons, so we’re talking
31
about a hundred trillion or more connections in total. It’s hard to look into
32
a real human brain and discern all of those connections— but that’s exactly
33
the goal of several ongoing neuroscience research projects. Fully character-
34
izing the human connectome would require something like a million mil-
S35
lion gigabytes of information.
N36
329
Big Picture - UK final proofs.indd 329
20/07/2016 10:02:52
T H E B IG PIC T U R E
01
Every neuron gleans input from other neurons, and occasionally from
02
the outside world. Given that input, it decides whether to fire. Firing is a yes
03
or no question— it either happens or it doesn’ t— but the input the neuron
04
receives can be quite rich. Very roughly, a neuron will “listen” to its input
05
for about 40 milliseconds at a time, and each incoming signal takes one
06
millisecond to transfer. That’s a huge amount of information. Forty sepa-
07
rate inputs, from a couple of thousand different synapses, resulting in
08
roughly 40 x 2,000 = 80,000 “bits” of information, or about 280,000 possible
09
messages a neuron could receive before it decides whether to fire or not. It’s
10
not simply “If I get more than the appropriate number of input signals, I
11
will fire”; some signals increase the chance of firing, some decrease it, and
12
the signals interact in complicated ways.
13
Knowing the complete human connectome wouldn’t, by itself, come
14
close to telling us everything we want to know about how human brains
15
think. Not all neurons are the same, so knowing how they are connected
16
isn’t everything there is to know. Scientists have completely mapped the
17
connectome of one multicellular organism: the tiny C. elegans nematode, a
18
flatworm whose most common form has precisely 959 cells, 302 of which
19
are neurons. We know how all of those neurons fit together— about 7,000
20
connections in total— but that doesn’t tell us what the flatworm is think-
21
ing. It’s like we know the highway map, but not the traffic patterns. Maybe
22
someday we’ll be able to read the nematode’s mind.
23
24
25
26
27
28
29
30
31
32
33
34
35S
The connectome of the C. elegans nematode, as represented in a computer model
36N
from the OpenWorm project. (Courtesy of Chris Grove, Caltech)
330
Big Picture - UK final proofs.indd 330
20/07/2016 10:02:53
t h E b A b b l I n g b R A I n
People change over time, and our connectomes change along with us.
01
The strength of the connections evolves, as the repeated firing of certain
02
signals increases the chances that specific synapses will fire again in the
03
future. We believe that memories are formed in this way, by synapses grow-
04
ing and shrinking in strength in response to stimuli. Neuropsychiatrist
05
Eric Kandel shared the 2000 Nobel Prize in Medicine for his detailed in-
06
vestigation of how this happens in a particular organism, the humble sea
07
slug. Slugs aren’t great at remembering things, but Kandel trained them to
08
recognize certain simple stimuli. He then showed that these new memories
09
were connected to a change in the synthesis of proteins in the neurons,
10
which led to alterations in their shape. Short- term memories were associ-
11
ated with synapses being strengthened, while long- term memories came
12
from entirely new synapses being created.
13
More recently, neuroscientists have been able to directly observe neu-
14
rons in mice growing and connecting as they learned how to perform new
15
tasks. Impressively (or disturbingly, depending on your perspective), they
16
have also been able to remove memories from mice by weakening specific
17
synapses, and even artificially implanting false memories by directly stimu-
18
lating individual nerve cells with electrodes. Memories are physical things,
19
located in your brain.
20
A connectome is like a map of the countries of the world. It’s not nearly
21
enough to allow us to understand politics, but knowing the information
22
contained therein is an important part of the bigger task. Having a good
23
map won’t stop you from getting lost, but it might help you find your
24
way home.
25
26
•
27
One of the most crucial features of the brain is that it’s not simply an undif-
28
ferentiated mess of connected neurons. The connectome is a network, but
29
it’s a hierarchical network— groups of neurons are connected together, and
30
those groups are then connected, and so on up to the entire brain. The
31
babble of consciousness, with different mental modules offering input and
32
being stitched together to make our aware self, is reflected in the workings
33
of the brain. Different parts have their own jobs to do, but it’s only when
34
they come together that we find a conscious person.
S35
There are various pieces of evidence for this, some of which come from
N36
331
Big Picture - UK final proofs.indd 331
20/07/2016 10:02:53
T H E B IG PIC T U R E
01
studies of what happens when we lose consciousness: when we sleep, or
02
when we’re under anesthesia. One study, for example, gave a small magnetic
03
stimulation to local regions of patients’ brains. Effects of the signal were
04
then measured as they propagated through the brain. When the patients
05
were conscious, the signal induced responses all over the brain; in uncon-
06
scious subjects the responses were confined to a limited r
egion near the
07
initial stimulus. Results like this are of much more than academic interest:
08
doctors have long sought a way of telling whether a patient under anesthesia
09
or suffering from brain damage was truly unconscious, or merely unable to
10
move and communicate with the outside world.
11
To say that the connectome is a hierarchical network is to say that it lies
12
somewhere between being maximally connected (every neuron is talking
13
to every other neuron) and minimally connected (every neuron talks only
14
to its immediate neighbors). As far as we can tell, the connectome is what
15
mathematicians call a small- world network. The name comes from the fa-
16
mous six- degrees-of-separation experiment by psychologist Stanley Mil-
17
gram. He found that randomly chosen people in Omaha, Nebraska, were
18
linked to a specific person living in Boston, Massachusetts, by an average of
19
about six first- name relationships. In network theory, we say that a network
20
has the small- world property when most nodes are not directly connected
21
to one another, but each one can be reached from any other one by a small
22
number of steps.
23
That’s what we find in the connectome. Neurons tend to be connected
24
to nearby neurons, but there are also connections relatively far away. Small-
25
world networks show up in many contexts, including connections between
26
websites, electrical power grids, and networks of personal friendships.
27
That’s not an accident: this kind of organization seems to represent an op-
28
timum of efficiency for certain tasks, allowing processing to be done locally
29
and results to spread quickly throughout the system. It is also robust to
30
damage; knocking out a few connections doesn’t appreciably alter the sys-
31
tem’s capacity. It’s a perfect fit for the squabbling modules inside our brains.
32
One way of thinking about a small- world network is to say that it has
33
“structure at all scales.” It is not simply a bunch of neurons grouped into a
34
ball, with those balls connected to one another. Rather, it’s neurons con-
35S
nected into groups, connected into bigger groups, into even bigger groups,
36N
and so on. There is some indication that this kind of arrangement describes
332
Big Picture - UK final proofs.indd 332
20/07/2016 10:02:54
t h E b A b b l I n g b R A I n
not only the spatial organization of the connectome but also how signals in
01
the brain come and go in time. Small signals happen relatively frequently,
02
medium- sized ones less often, and very big ones relatively rarely.
03
Physicists say that systems with this kind of hierarchical behavior are at
04
a critical point. It’s a ubiquitous phenomenon in the study of phase transi-
05
tions, since systems become critical right as they are about to change from
06
one phase to another. When water boils, there are many small bubbles,
07
fewer larger ones, and so on. Criticality can be thought of as a sweet spot
08
between boring order and useless chaos. As neurophysiologist Dante
09
Chialvo put it, “A brain that is not critical is a brain that does exactly the
10
same thing every minute, or, in the other extreme, is so chaotic that it does
11
a completely random thing, no matter what the circumstances. That is the
12
brain of an idiot.”
13
In both space and time, then, the evidence we have to date indicates that
14
our brains are complex systems organized in such a way as to take maxi-
15
mum advantage of their complexity. Given how impressive our brains are
16
at carrying out complicated tasks, that should come as no surprise.
17
18
•
19
We could study the brain in exquisite detail, characterizing every neuron
20
and mapping every connection, and still not convince ourselves that the
21
brain accounts for the mind, the actual thinking of a human being. Back in
22
chapter 26 we talked about Princess Elisabeth’s objections to Descartes’s
23
picture of an immaterial soul interacting with the physical body, perhaps
24
through the pineal gland. As interesting as those objections were, they don’t
25
necessarily close the deal until we can directly connect what happens in the
26
brain to what we think of as our identities as persons. Over the years psy-
27
chology and neuroscience have made great strides in doing just that.
28
We’ve already seen that memories are physically encoded in the brain.
29
It’s unsurprising, then, that our sensory perceptions are likewise encoded
30
there. This is obviously true in some crude way, as the magnetic fields stick-
31
ing out of my head demonstrated. But scientists have made advances re-
32
cently in extracting quite detailed images of what patients are seeing, just
33
by looking at what their brains are doing. By using fMRI images to deter-
34
mine what parts of the brain are firing when subjects are looking at images,
S35
or watching videos, neuroscientists can construct a template from which
N36
333
Big Picture - UK final proofs.indd 333
20/07/2016 10:02:54
T H E B IG PIC T U R E
01
they can reconstruct images directly from the fMRI data, without “cheat-
02
ing” by knowing what the subjects are watching. It’s not mind reading, at
03
least not yet; we can make crude representations of what people are looking
04
at, but not what they are imagining inside their heads. Perhaps that’s just a
05
matter of time.
06
None of this will necessarily convince a determined Cartesian dualist
07
who wants to believe in immaterial souls. Of course, they will admit, some-
08
thing happens in the brain as we think and perceive the world. But that’s not
09
all that happens. The experiencing, the feeling, the actual soul of a person—
10
that’s something else entirely. Perhaps the brain is like a radio receiver. Al-
11
tering it or damaging it will change how it plays, but that doesn’t mean that
12
the original signal is being created
inside the radio itself.
13
That idea doesn’t really hold up either. Damaging a radio might hurt our
14
reception, making it hard to pick up our favorite station. But it doesn’t turn
15
that station from heavy- metal music into a smooth- jazz format. Damag-
16
ing the brain, on the other hand, can change who a person is at a fundamen-
17
tal level.
18
Consider what’s known as the Capgras delusion. Patients suffering from
19
this syndrome have damage to the part of the brain that connects two other
20
parts: the temporal cortex, associated with recognizing other people, and
21
the limbic system, which is in charge of feelings and emotions. A person
22
who develops Capgras delusion will be able to recognize people they know,
23
but will no longer feel whatever emotional connection they used to have
24
with them. (It is the flip side of prosopagnosia, which involves a loss of the
25
ability to recognize people.)
26
You can imagine what this would do to a person. One patient, “Mrs. D,”
27
began to suffer from Capgras delusion at the age of seventy- four. Whenever
28
she would see her husband, she would recognize this person, including all
29
of the mental associations that said “this is my husband”— but she no lon-
30
ger felt any affection or love toward him, merely indifference. But she knew
31
that she should have feelings for him, so her brain came up with a clever
32
reconciliation of the inconsistency: this man wasn’t really her husband, he
33
was an impostor who looked just like him.
34
Mrs. D was not a unique case. There are many other examples of people
35S
suffering from some sort of brain damage, and having their emotional states
36N
or personalities dramatically altered thereby. That doesn’t prove beyond any
334
Big Picture - UK final proofs.indd 334
20/07/2016 10:02:54
t h E b A b b l I n g b R A I n
possible doubt that the mind is nothing more than a way of talking about
01
what happens in the physical brain. But it should work to lower our cre-
02
dence in old- fashioned Cartesian dualism to a very small value indeed.
03
That leaves us either with physicalism— the world, including people, is
04
purely physical— or some newfangled form of non- Cartesian dualism. To
05
clean up that final question, we need to think more about what it means to
06
be a conscious, experiencing person.
The Big Picture Page 56