EEG Intro Forclass

8/8/2019 EEG Intro Forclass

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Intro to neurons and EEGs

Brendan Allison, Ph. D.BCI class lecture #2


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These are neurons. Your brain has hundreds ofbillions of them!

Diagram of a neuron. A group of real neurons.


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Brainwaves (EEGs) reflect the brains electrical activity. Aneuron at rest is like a little battery. Whenever a neuron is

active, its voltage briefly changes. If millions of neurons all

fire at the same time, this produces electrical activity

detectable to an electrode placed on the head.

Diagram of a neuron. Your brain

has hundreds of billions of them!When a neuron is active, its voltage may

change by 100 mV or more


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When millions of neurons fire at the same time, they mayproduce electrical activity detectable to an electrode placed

on the head.

Two illustrations of the brain producing electricity.

Two real human brains. The left image is an MRI.


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For example, if you hear a tone, many different groups ofneurons activate to process 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 100 ms of the

tone) are very consistent. Later

EEG components may varydepending on whether you

ignored the tone, if it was

meaningful to you, if you

expected it, and other factors.


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Macroelectrodes only measure the coordinated activity of

many millions of neurons. Microelectrodes only measure

the activity of one or very few neurons.

The most common recording setup is a scalp macroelectrode.While it is possible to get data from as few as two

electrodes, most labs use an electrode cap. These caps are

specially designed so that each electrode is over a general

region of the brain. This makes it easier to estimate the

source of any EEG activity detected at each electrode.


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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.


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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.


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Limitations of macroelectrode recording from the scalp:

1) Difficult to know exactly what brain region is

underneath each electrode

2) Scalp smears electrical signal

3) Scalp acts as low pass filter

4) Only measures neurons near the scalp

5) Only measures neurons perpendicular to scalp

6) Neurons aligned opposite each other cancel each

other7) Neurons must be active in synchrony to be detected


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Unlike macroelectrodes, microelectrodes can only recordbrain activity if placed in or on the brain. This is only

done when medically necessary. Typical reasons

include finding the source of seizures or preparing to

implant a device into the brain, such a deep brain

stimulation device used to treat Parkinsons disease.

The Utah Intracranial Electrode Array The cone electrode


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Scalp macroelectrode

Safe and easy

Requires minimal training

Measures combined activityof many millions of neurons

Poor localization

Signal is degraded as it

travels through the skull

Susceptible to noise

Chronic use definitely OK

Implanted microelectrode

Requires neurosurgery

Requires neurosurgeon

Measures activity ofone of very few neurons

Excellent localization

Signal not degraded; can learn

more details of neural activity

Less vulnerable to noise

Chronic use probably OK


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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 andother eye movements.

2. The scientist measures the subjects head and then places

the correct sized cap on his head.


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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.


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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 subjects hair, the scientists

skill, type of electrode cap, and other factors.


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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. Thats it! The subject is done, but the scientist now has

data to analyze.


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Event related potentials (ERPs): Brains response to a specificevent, such as a tone or flash.

Spontaneous or free-running EEG: Naturally produced,

rhythmic brainwaves; do not require outside activity.

Commonly studied ERPs include the P300 and N400.

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 eyesMu (8-14 Hz), largest when individual is not moving

Beta (non specific higher frequencies), indicate alertness


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This graph shows about four seconds of EEG from a human subject. Each of the 15 linesrepresents 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.

Remember that alpha waves are a type offree running EEG


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These two figures show responses to flashes. Specifically, theyshow 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 brains

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)


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Filtering: This removes unwanted portions of the EEG. There

are 4 types of filters: highpass, lowpass, notch, bandpass.

Common settings include .1-100 Hz, notch at 60 and 72

Hz.

Amplification: Data are typically amplified 10,000 times.

Digitization (A/D conversion): The analog signals from the

brain must be translated into digital signals. Filters are

often set at 8, 12 bit or 16 bit. 8 bits = 0-256.Epoching: ERP data must be aligned with external events.


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There are many techniques for studying the brain. Many ofthese, such as X-rays, CAT scans, or a standard MRI,

only measure brain structure. They do not measure brain

function. That is, they give you the same picture no

matter what you are thinking or doing. Thus, they would

not be useful in BCIs, because BCIs must be able todistinguish different brain states.

Some people use EEGs in combination with fMRI.T

his canbe a very powerful tool for finding exactly when and

where something occurs.


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Here are three other technologies for studying brain function:

Functional MRI: These measure blood flow. The subject

must be placed inside a huge magnet. Different brain

areas give a different magnetic signal depending on how

much blood they are using.

PET scans: These measure radioactive decay. Radioactive

material is injected into the subjects bloodstream, and

thus areas that use more blood emit more radiation.

Magnetoencephalogram (MEG): This measures the brains

magnetic activity, not electrical activity. It provides

somewhat different information than EEGs.


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Why havent these technologies been used for BCIs?

First, all of these require millions of dollars of nonportable

equipment, and a highly trained technician.

Second, PET and fMRI measure blood flow. When you startusing a brain region more than usual, it takes a few

seconds for blood flow to increase. Thus, only EEG and

MEG can measure brain activity in true realtime.

There are other reasons too; these are the main ones.


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Pure research: EEGs help us learn more

about when, where, and how different brain

areas work together when thinking, speaking,responding to tones, etc.


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Medical: Isolate areas or processes that

respond slowly, improperly, or not at all.

Detect onset of seizures, strokes, psychoticepisodes, 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.


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Entertainment/relaxation: Some people

have used EEG systems designed for alpha

biofeedback. This means that you trainyourself to have more alpha waves in your

EEG, which helps some people relax. EEGs

can be used to play simple games or make

music.


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Passive monitoring: There has been a lot of

progress recently in alertness monitors based

on the EEG. These might warn people theyare dozing off. This has been proposed for

people in attention-critical situations like

nuclear plant technicians, sonar operators,

security guards, and truck drivers. Lie

detection may be possible too.

EEG Intro Forclass

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