Read Mind Hacks™: Tips & Tools for Using Your Brain Online
Authors: Tom Stafford,Matt Webb
Tags: #COMPUTERS / Social Aspects / Human-Computer Interaction
There’s a veritable electrical storm going on inside your head: 100 billion brain
cells firing electrical signals at one another are responsible for your every thought and
action.
A
neuron
, a.k.a.
nerve cell
or
brain
cell
, is a specialized cell that sends an electrical impulse out along fibers
connecting it, in turn, to other neurons. These guys are the wires of your very own personal
circuitry.
What follows is a simplistic description of the general features of nerve cells, whether
they are found sending signals from your senses to your brain, from your brain to your
muscles, or to and from other nerve cells. It’s this last class, the kind that people most
likely mean when they say “neurons,” that we are most interested in here. (All nerve cells,
however, share a common basic design.)
Don’t for a second think that the general structure we’re describing here is the end
of the story. The elegance and complexity of neuron design is staggering, a complex
interplay of structure and noise; of electricity, chemistry, and biology; of spatial and
dynamic interactions that result in the kind of information processing that cannot be
defined using simple rules.
1
For just a glimpse at the complexity of neuron structure, you may want to
start with this free chapter on nerve cells from the textbook
Molecular Cell
Biology
by Harvey Lodish, Arnold Berk, Lawrence S. Zipursky, Paul Matsudaira,
David Baltimore, and James Darnell and published by W. H. Freeman (
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowSection&rid=mcb.chapter.6074
), but any advanced cell biology or neuroscience textbook will do to give you
an idea of what you’re missing here.
The neuron is made up of a cell body with long offshoots — these can be very long (the
whole length of the neck, for some neurons in the giraffe, for example) or very short (i.e.,
reaching only to the neighboring cell, scant millimeters away). Signals pass only one way
along a neuron. The offshoots receiving incoming transmissions are called
dendrites
. The outgoing end, which is typically longer, is called the
axon
. In most cases there’s only one,
long, axon, which branches at the tip as it connects to other neurons — up to
10,000 of them. The junction where the axon of one cell meets the dendrites of another is
called the
synapse
. Chemicals, called
neurotransmitters
, are used to get the signal across the synaptic
gap. Each neuron will release only one kind of neurotransmitter, although it may have
receptors for many different kinds. The arrival of the electric signal at the end of the
axon triggers the release of stores of the neurotransmitter that move across the gap (it’s
very small, after all) and bind to receptor sites on the other side, places on the neuron
that are tuned to join with this specific type of chemical.
Whereas the signal between neurons uses neurotransmitters, internally it’s electrical.
The electrical signal is sent along the neuron in the form of an
action
potential
.
2
This is what we mean when we say
impulses
,
signals
,
spikes
, or refer, in brain imaging
speak, to the
firing
or
lighting up
of brain areas
(because this is what activity looks like on the pictures that are made). Action potentials
are the fundamental unit of information in the brain, the universal currency of the neural
market.
The two most important computational features are as follows:
Together these two features mean that the real language of the brain is not just a
matter of spikes (signals sent by neurons), but spikes in time.
Whether or not a new spike, or impulse, is generated by the postsynaptic neuron (the one
on the receiving side of the synapse) is affected by the following interwoven
factors:
All of this short-term information is affected by any previous history of interaction
between these two neurons — times one has caused the other to fire
and when they have both fired at the same time for independent reasons — and
slightly adjusts the probability of interaction happening again.
3
Spikes happen pretty often: up to once every 2 milliseconds at the maximum rate of the
fastest-firing cells (in the auditory system; see
Chapter 4
for more on that). Although the
average rate of firing is responsive to the information being represented and transmitted
in the brain, the actual timing of individual spikes is unpredictable. The brain seems to
have evolved an internal communication system that has noise added to only one aspect of
the information it transmits — the timing, but not the size of the signals transmitted.
Noise is a property of any biological system, so it’s not surprising that it persists even
in our most complex organ. It could very well also be the case that the noise
[
Neural Noise Isn’t a Bug; It’s a Feature
]
is
playing some useful role in the information processing the brain does.
After the neurotransmitter has carried (or not carried, as the case may be) the signal
across the synaptic gap, it’s then broken down by specialized enzymes and reabsorbed to be
released again when the next signal comes along. Many drugs work by affecting the rate and
quantity of particular neurotransmitters released and the speed at which they are broken
down and reabsorbed.
Hacks such as
Why People Don’t Work Like Elevator Buttons
and
Get Adjusted
show some of the other consequences for psychology of using
neurons to do the work. Two good introductions to how neurons combine on a large scale can
be found at
http://www.foresight.gov.uk
. This is a British government Department of Trade and Industry project that
aimed to get neuroscientists and computer scientists to collaborate in producing reviews of
recent advances in their fields and summarize the implications for the development of
artificial cognitive systems.
When you think really hard, your heart rate noticeably increases.
The brain requires approximately 20% of the oxygen in the body, even during times of
rest. Like the other organs in our body, our brain needs more glucose, oxygen, and other
essential nutrients as it takes on more work. Many of the scanning technologies that aim to
measure aspects of brain function take advantage of this. Functional magnetic resonance
imaging (fMRI)
[
Functional Magnetic Resonance Imaging: The State of the Art
]
benefits from the fact
that oxygenated blood produces slightly different electromagnetic signals when exposed to
strong magnetic fields than deoxygenated blood and that oxygenated blood is more
concentrated in active brain areas. Positron emission tomography (PET)
[
Positron Emission Tomography: Measuring Activity Indirectly with PET
]
involves being injected with weakly radioactive glucose and reading the subsequent signals
from the most active, glucose-hungry areas of the brain.
A technology called
transcranial Doppler sonography
takes a
different approach and measures blood flow through veins and arteries. It takes advantage of
the fact that the pitch of reflected ultrasound will be altered in proportion to the rate of
flow and has been used to measure moment-to-moment changes in blood supply to the brain. It
has been found to be particularly useful in making comparisons between different mental
tasks. However, even without transcranial Doppler sonography, you can measure the effect of
increased brain activity on blood flow by measuring the pulse.
For this exercise you will need to get someone to measure your
carotid
pulse
, taken from either side of the front of the neck, just below the angle
of the
jaw. It is important that only very light pressure be used — a couple of
fingertips pressed lightly to the neck, next to the windpipe, should enable your friend to
feel your pulse with little trouble.
First you need to take a measure of a resting pulse. Sit down and relax for a few
minutes. When you are calm, ask your friend to count your pulse for 60 seconds. During
this time, close your eyes and try to empty your mind.
With a baseline established, ask your friend to measure your pulse for a second time,
using exactly the same method. This time, however, try and think of as many species of
animals as you can. Keeping still and with your eyes closed, think hard, and if you get
stuck, try thinking up a new strategy to give you some more ideas.
During the second session, your pulse rate is likely to increase as your brain
requires more glucose and oxygen to complete its task. Just how much increase you’ll see
varies from person to person.
Thinking of as many animals as possible is a type of
verbal
fluency
task, testing how easily you can come up with words. To complete the
task successfully, you needed to be able to coordinate various cognitive skills, for
example, searching your memory for category examples, generating and using strategies to
think up more names (perhaps you thought about walking through the jungle or animals from
your local area) and checking you were not repeating yourself.
Neuropsychologists often use this task to test the
executive
system
, the notional system that allows us to coordinate mental tasks to
solve problems and work toward a goal, skills that you were using to think up examples of
animals. After brain injury (particularly to the frontal cortex), this system can break
down, and the verbal fluency task can be one of the tests used to assess the function of
this system.
Research using PET scanning has shown similar verbal fluency tasks use a significant
amount of brain resources and large areas of the cortex, particularly the frontal,
temporal, and parietal areas.
1
Interestingly, in this study people who did best used less blood glucose than people
who did not perform as well. You can examine this relationship yourself by trying the
earlier exercise on a number of people. Do the people who do best show a slightly lower
pulse than others? In these cases, high performers seem to be using their brain more
efficiently, rather than simply using more brain resources.
Although measuring the carotid pulse is a fairly crude measure of brain
activity compared to PET scanning, it is still a good indirect measure of brain activity
for this type of high-demand mental task, as the carotid arteries supply both the middle
and anterior cerebral arteries. They supply blood to most major parts of the cortex,
including the frontal, temporal, parietal, and occipital areas, and so would be important
in supplying the needed glucose and oxygen as your brain kicks into gear.
One problem with PET scanning is that, although it can localize activity to certain
brain areas, it has poor temporal resolution, meaning it is not very good at detecting
quick changes in the rate of blood flow. In contrast, transcranial Doppler sonography can
detect differences in blood flow over very short periods of time (milliseconds).
Frauenfelder and colleagues used this technique to measure blood flow through the middle
and anterior cerebral arteries while participants were completing tasks that are known to
need similar cognitive skills as the verbal fluency exercise.
2
They found that the rate of blood flow changed second by second, depending
on exactly which part of the task the participant was tackling. While brain scanning can
provide important information about which areas of the brain are involved in completing a
mental activity, sometimes measuring something as simple as blood flow can fill in the
missing pieces.
— Vaughan Bell