Transcript Document

EEG
These are neurons. Your brain has hundreds of
billions of them!
Diagram of a neuron.
A group of real neurons.
Brainwaves (EEGs) reflect the brain’s electrical activity. A
neuron at rest is like a little battery. Whenever a neuron is
active, its voltage briefly changes.
When a neuron is active, its voltage may
change by 100 mV or more.
Electrical activity in a single neuron.
When millions of neurons fire at the same time, they may
produce electrical activity detectable to an electrode placed
on the head.
Two real human brains. The left image is an MRI.
Two illustrations of the brain producing electricity.
For example, if you hear a tone, many different
groups of neurons fire as your brain processes
that tone. EEGs can tell us when and where
these groups of neurons fire. Doctors often
use this technique to diagnose hearing
disabilities, since EEGs can reveal which
groups of neurons are damaged.
This figure shows some of the EEGs evoked by a tone.
Early responses (within 0.1 seconds of the tone) are very
consistent. Later EEG components may vary depending on
whether you ignored the tone, if it was meaningful to you, if
you expected it, and other factors.
Most EEG studies use an electrode cap. This is a special cap
that contains electrodes at certain locations over different
areas of the head.
On rare occasions, doctors may need to use surgery to implant
an electrode inside the skull to get better recordings. This
is only done when medically necessary. For example,
doctors may need to know exactly what area of a patient’s
brain is creating seizures.
Most EEG recordings use an electrode cap that contains a
large number of electrodes. Many labs use between 16
and 64 electrodes, but caps with 256 or more electrodes
have been used in scientific and medical studies.
Different electrode caps.
Most electrode caps are designed with electrodes over
specific areas of the skull (and thus specific areas of the
brain). Otherwise, you would be recording from different
brain areas each time you use a cap.
These are standardized electrode locations, called the International 10-20 system.
Scientists run subjects all the time in EEG experiments.
Subjects may be paid, they may volunteer, or they may
receive class credit for participating.
Before preparing a subject for EEG recording, s/he is shown
the lab and the equipment, and is asked to sign a consent
form agreeing to be in the study. This is very important.
Scientists are required to get “informed consent” from
subjects. After this, the preparation begins….
1. It is often necessary to place an electrode on or behind the
ear before donning the electrode cap. Scientists often
clean the area behind the ear with rubbing alcohol. Some
people put electrodes near the eye to detect blinking and
other eye movements.
2. The scientist measures the subject’s head and then places
the correct sized cap on his head.
3. Electrode gel is then placed between each electrode and
the scalp to get a good connection. Everyone agrees that
electrode gel in your hair is a wonderful experience.
Two types of electrode gel.
Squirting gel under an electrode.
4. The scientist checks the cap to make sure there is a good
connection between each electrode and the brain.
5. The subject is now ready for recording! A typical
recording session lasts about an hour. It takes roughly 30
minutes to prepare a subject for recording, depending on
the number of electrodes, the subject’s hair, the scientist’s
skill, type of electrode cap, and other factors.
6. After recording, the cap is removed. Electrode gel washes
out easily with water, so many subjects rinse or wash their
hair after a recording session. Of course, smart people
know that electrode gel in your hair makes you cool.
7. That’s it! The subject is done, but the scientist now has
data to analyze.
Most people have heard of free-running EEGs. These are
naturally produced, rhythmic brainwaves that do not require
outside activity.
Well known free running EEGs include:
Delta (1-4 Hz), found in deep sleep
Theta (4-8 Hz), found in sleep, meditation, hypnosis
Alpha (8-14 Hz), indicate relaxation and closed eyes
Mu (8-14 Hz), largest when individual is not moving
Beta (non specific higher frequencies), indicate alertness
This graph shows about four seconds of EEG from a human subject. Each of the
15 lines represents a different electrode site. This has a lot of alpha activity (about
10 waves per second), meaning the subject was probably awake but drowsy with
eyes closed. Again, alpha waves are a type of free running EEG
However, people sometimes are interested in the brain’s
response to a certain event. For example, if someone touches
your hand or plays a tone, your EEG will change as your
brain processes that event.
The technical term for EEG activity based on a specific event
is an event related potential (ERP). One common bump is
called the P300, named because it starts about 300
milliseconds after an event.
These two figures show responses to flashes. Specifically, they
show how the brain responds differently to flashes people
notice compared to flashes they choose to ignore. In each
graph, the relatively flat lines (red or dashed) show the brain’s
response to ignored flashes. Notice the large bump starting
around 300 milliseconds in the other lines (blue or solid),
showing the response to flashes people counted. These are
examples of the P300, a type of Event Related Potential (ERP).
Top figure: from Allison
thesis (Allison 2003)
Bottom figure: from Bayliss
thesis (Bayliss 2001)
There are other technologies for studying brain activity. Two
well known techniques are PET and fMRI. These
approaches provide different information than EEGs.
EEGs are very good at telling when a brain area was
active, but are poor at finding exactly where in the brain
the signal came from. EEG recording equipment is also
much cheaper, easier to use, and more portable that the
tools needed for PET or fMRI.
Some people use EEGs in combination with fMRI. This can
be a very powerful tool for finding exactly when and
where something occurs.
Richard Caton was the first person to record
electrical activity from the brain in 1875.
Dr. Caton placed electrodes on the surface of
rabbit and monkey brains. He found that this
activity changed in response to flashing lights.
The first person to record brainwaves from
humans without surgery was Hans Berger.
Dr. Berger began studying the
EEG in 1924, and first
published his results in 1929.
Doctor Hans Berger
Though Berger lacked modern electronics, he
made many important discoveries. He showed
that attention affects alpha and beta waves.
Dr. Berger’s EEG recording apparatus
Dr. Berger’s electrode cap
Pure research: EEGs help us learn more
about when, where, and how different brain
areas work together when thinking, speaking,
responding to tones, etc.
Medical: Isolate areas or processes that
respond slowly, improperly, or not at all.
Detect onset of seizures, strokes, psychotic
episodes, or other problems. Enable
communication for severely disabled with
brain computer interface systems. Train kids
with ADD to pay attention longer. Study the
effects of drugs over time.
Entertainment/relaxation: Some people
have used EEG systems designed for alpha
biofeedback. This means that you train
yourself to have more alpha waves in your
EEG, which helps some people relax. EEGs
can be used to play simple games or make
music.
Passive monitoring: There has been a lot of
progress recently in alertness monitors based
on the EEG. These might warn people they
are dozing off. This has been proposed for
people in “attention-critical” situations like
nuclear plant technicians, sonar operators,
security guards, and truck drivers.
Any questions ?