OUTLINE
• Introduction of BCI
–EEG signal
• SSVEP BCI System
–Application
1
Let watch the Video …..!
One Giant Bite: Woman with Quadriplegia Feeds
Herself Chocolate Using Mind-Controlled Robot Arm
http://www.youtube.com/watch?v=76lIQtE8oDY
2
Brain Computer Interface
• A system which connects the human brain
with external devices
• BCI acquire electroencephalography (EEG)
data from the human brain, then recognize
and translate the specific brain signal patterns
into the device control command through the
signal-processing algorithm [1].
3
What is EEG ?
• An electroencephalograph (EEG) is the recorded
electrical activity generated by the brain. In general,
EEG is obtained using electrodes placed on the
scalp with a conductive gel.
• The amplitude of an EEG signal typically ranges
from about 1 uV to 100 uV in a normal adult, and it
is approximately 10 to 20 mV when measured with
subdural electrodes such as needle electrodes
Electroencephalography (EEG)
•
•
Measures the brain’s electric
activity from the scalp
Measured signal results from
the activity of billions of neurons
•
•
Amplitude:
Bandwidth:
•
Errors:
0.001-0.01 mV
0.5-40 Hz
– Thermal RF noise
– 50/60 Hz power lines
– Blink artifacts and similar
•
Typical applications:
– Sleep studies
– Seizure detection
– Cortical mapping
Different types of normal brain waves
• Beta waves occur at a frequency of 13 to 30 cycles per second.
They are usually associated with anxiety, depression, or the use
of sedatives.
• Alpha waves occur at a frequency of 8 to 12 cycles per second in a
regular rhythm. They are present only when you are awake but
have your eyes closed. Usually they disappear when you open
your eyes or start mentally concentrating.
• Theta waves occur at a frequency of 4 to 7 cycles per second. They
are most common in children and young adults.
• Delta waves occur at a frequency of 0.5 to 3.5 cycles per second.
They generally occur only in young children during sleep.
EEG measurement setup
• 10-20 Lead system is most
widely clinically accepted
• Certain physiological
features
are used as reference
points
• Allow localization of
diagnostic features in the
vicinity of the electrode
• Often a readily available
wire or rubber mesh is
used
• Brain research utilizes
even 256 or 512 channel
EEG hats
Electrodes – Basics
• High-quality biopotential measurements require
– Good amplifier design
– Use of good electrodes and their proper placement on the patient
– Good laboratory and clinical practices
• Electrodes should be chosen according to the application
• Basic electrode structure includes:
–
–
–
–
–
The body and casing
Electrode made of high-conductivity material
Wire connector
Cavity or similar for electrolytic gel
Adhesive rim
• The complexity of electrode design often neglected
Electrodes - Basics
•
•
•
•
Skin preparation by abrasion or cleansing
Placement close to the source being measured
Placement above bony structures where there is less muscle mass
Distinguishing features of different electrodes:
–
–
–
–
How secure? The structure and the use of strong but less irritant adhesives
How conductive? Use of noble metals vs. cheaper materials
How prone to artifact? Use of low-junction-potential materials such as Ag-AgCl
If electrolytic gel is used, how is it applied? High conductivity gels can help reduce the junction
potentials and resistance but tend to be more allergenic or irritating
Baseline drift due to the
changes in junction
potential or motion artifacts
Choice of electrodes
Electromagnetic
interference
 Shielding
Muscle signal
interference
 Placement
Ag-AgCl, Silver-Silver Chloride Electrodes
• The most commonly used electrode type
• Silver is interfaced with its salt silver-chloride
• Choice of materials helps to reduce junction potentials
– Junction potentials are the result of the dissimilar electrolytic
interfaces
• Electrolytic gel enhances conductivity and also reduces
junction potentials
– Typically based on sodium or potassium chloride, concentration in
the order of 0.1 M weak enough to not irritate the skin
• The gel is typically soaked into a foam pad or applied directly in
a pocket produced by electrode housing
• Relatively low-cost and general purpose electrode
• Particularly suited for ambulatory or long term use
Gold Electrodes
•
•
•
•
Very high conductivity  suitable for low-noise meas.
Inertness  suitable for reusable electrodes
Body forms cavity which is filled with electrolytic gel
Compared to Ag-AgCL: greater expense, higher
junction potentials and motion artifacts
• Often used in EEG, sometimes in EMG
Conductive polymer electrodes
•
•
•
•
•
•
Made out of material that is simultaneously conductive and adhesive
Polymer is made conductive by adding monovalent metallic ions
Aluminum foil allows contact to external instrumentation
No need for gel or other adhesive substance
High resistivity makes unsuitable for low-noise meas.
Not as good connection as with traditional electrodes
Metal or carbon electrodes
• Other metals are seldom used as high-quality noble
metal electrodes or low-cost carbon or polymeric
electrodes are so readily available
• Historical value. Bulky and awkward to use
• Carbon electrodes have high resistivity and are noisier
but they are also flexibleand reusable
• Applications in electrical stimulation and impedance plethysmography
Needle electrodes
•
•
•
•
Obviously invasive electrodes
Used when measurements have to be taken from the organ itself
Small signals such as motor unit potentials can be measured
Needle is often a steel wire with hooked tip
Current technique of BCI
Input
Output
13
Based on EEG signal :
1. Event–related de-synchronization / synchronization
(ERD/ERS)
2. Steady state visual evoke potentials (SSVEP)
3. P300 component of event related potentials (ERPs)
4. Slow cortical potentials (SCPs)
14
Different features of each BCI system [2]
15
SSVEP is an EEG signal response to the flickering
visual stimulus with a frequency higher than 6 Hz [1]
16
Journal: Development of a Low-Cost FPGA-Based
SSVEP BCI Multimedia Control System [1]
• User target:
patients suffering from severe motor
disabilities, such as amyotrophic lateral
scleroses (ALS), spino-cerebellar ataxia (SCA),
and other paralyzed patients, may have
limited motion while constrained on a hospital
bed
17
System configuration
18
Acquisition module
Pre amplifier:
Instrumentation
amplifier, INA 128,
(Gain setting, Gain:
1000)
19
ADC Module
MicroChip MCP3201
• the 8-pin dual in-line
package (DIP) with
serial control
interface [serial
peripheral interface
(SPI)] devices,.
• The resolution and
maximum sampling
rate of the ADC are
12 b and 100 k
samples/s.
20