This document discusses electroencephalography (EEG) and evoked potentials. It begins by describing EEG as a method for recording electrical brain activity using electrodes placed on the scalp. It then discusses how EEG is used to diagnose neurological conditions like epilepsy and brain tumors. The document outlines different brain wave patterns observed in EEG like alpha, beta, theta, and delta waves. It also discusses how EEG is performed and interpreted, potential artifacts, and different electrode montages. Finally, it describes evoked potentials as the electrical response of the brain to sensory stimulation, and summarizes different types of evoked potentials including visual, somatosensory, and auditory potentials.

1. Electroencephalogram
(EEG): Measuring Brain Waves

2. Electroencephalogram
• Instrument for recording electrical activity that
generates in the individual neuron of the brain.
• Effective method for diagnosing many neurological
illness and diseases such as epilepsy, tumor,
sleeping patterns, mental disorders, etc.
• Electrodes around the head (scalp or cerebral
cortex) measures and records activity of the brain.

3. Where and why is EEG used?
EEG is performed predominantly in Neurology/Neurophysiology, but as it
provides a measure of cerebral function, it is also used in other
specialities;
Neurology
• Epilepsy
•Head Injury
• Brain Tumor/Lesions
• Cerebrovascular
disorders
• Neurosurgery
Psychiatry
• Epilepsy & Related
Disorders
• Organic brain diseases
• Developmental
Disability
• Head Injury
Pediatrics
• Pediatric Epilepsy
• Down’s Syndrome
• Other
diseases/syndromes
Internal Medicine
• Endocrine/Metabolic
Diseases
• Hepatic Diseases
• Toxemia

4. EEG waveforms
 Alpha waves - 8-13 Hz
 Beta waves - Greater than 13 Hz
 Theta waves - 3.5-7.5 Hz
 Delta waves - 3 Hz or less
 Gamma – 22-30 Hz

5. EEG waveforms

7. Alpha Wave
 Characteristics:
- frequency: 8-13 Hz
-amplitude: 20-60 µV
 Easily produced when
quietly sitting in relaxed
position with eyes closed
(few people have trouble
producing alpha waves)
 Alpha blockade occurs with
mental activity
Rhythm > Alpha
Frequency
Component
8.1 to 13 Hz
Amplitude Infant: 20µV Child:
50µV Adult: 10µV
Main Scalp Area Occipital - Parietal
Patient Condition Resting, Eyes Closed

8. Beta Waves
 Characteristics:
-frequency: 14-30 Hz
-amplitude: 2-20 µV
 The most common form
of brain waves. Are
present during mental
thought and activity
Rhythm > Beta
Frequency
Component
Over 13 Hz
Amplitude 10 to 20 µV
Main Scalp Area Frontal
Patient Condition Resting, Eyes Open

9. Theta Waves
 Characteristics:
-frequency: 4-7Hz
-amplitude: 20-100µV
 Believed to be more common in
children than adults
 Walter Study (1952) found
these waves to be related to
displeasure, pleasure, and
drowsiness
 Maulsby (1971) found theta
waves with amplitudes of
100µV in babies feeding
Rhythm > Theta
Frequency
Component
4.1 to 8 Hz
Amplitude Child: 50µV
Adult: 10µV
Main Scalp Area Temporal
Patient Condition Drowsiness

10. Delta Waves
 Characteristics:
-frequency: upto 4Hz
-amplitude: 20-200µV
 Found during periods of
deep sleep in most people
 Characterized by very
irregular and slow wave
patterns
 Also useful in detecting
tumors and abnormal brain
behaviors
Rhythm > Delta
Frequency
Component
Up to 4 Hz
Amplitude 100µV
Main Scalp Area Frontal
Patient Condition Deep Sleep
(Adult)

11. Gamma Waves
 Characteristics:
-frequency: 36-44Hz
-amplitude: 3-5µV
 Occur with sudden sensory stimuli

12. EEG Electrodes
• Transform ionic currents from cerebral tissue into electrical
currents used in EEG preamplifiers.
• Types of electrodes :
 Peel and stick electrodes
 Silver plated cup Scalp electrodes
 Intracerebral needle electrodes

13. EEG Electrodes Chatracteristics
One of the keys to recording good EEG signals is the type of
electrodes used. Electrodes that make the best contact with a
subject's scalp and contain materials that most readily conduct
EEG signals provide the best EEG recordings.
 Small
 Easily affixed to scalp
 Remain in place (glue, hat, strap)

14. The EEG be recorded with Scalp electrodes through the
unopened skull or with electrodes on or in the brain.
A normal EEG

15. Basic Terminology
 Montage: patterns of connection between electrodes;
usually 16 or more electrodes
 Referential: background rhythm interpretation

16. Electrode position

17. Recording the EEG
In clinical EEG recording, the
waveforms are recorded in a
series of Montages.
What is a montage?
Each EEG trace is generated
from an active and a reference
electrode. Different patterns of
electrodes are selected and the
traces grouped to provide data
from different areas of the
scalp.

18. Types of Montage
Common
Reference
All active electrodes
are referred to a single
point, e.g. Linked Ears
(LE), or Cz.

19. Types of Montage
Average
Reference
Here, all active
electrodes are referred
to a calculated average
of other electrodes.
Electrodes which are
likely to show artefacts
are generally not
included in this
average calculation

20. Types of Montage Bipolar
Each trace is made
up of an electrode
referred to a
neighboring
electrode. Traces are
typically grouped in
patterns such as
anterior-posterior
(front to back of
head) or transverse
(side to side)

21. Bipolar leads
 More popular.
 Voltage between the two scalp
electrode is recorded.

22. Types of Montage Laplacian
Also known as
Source Derivation,
the Laplacian
montage shows each
active electrode
referred to a
weighted average of
all of its neighbors.
Laplacian montages
are very good for
localising the focus
of abnormal activity

23. Artifacts in EEG Recordings
EEG activity is very small
in voltage terms – it is
therefore very easy to
record artifacts in the
EEG. It is important to be
able to determine real
EEG from many kinds of
interference.

24. Artifacts in EEG Recordings

25. Artifacts
 Three sources
– 60-cycle noise
– Muscle artifact
– Eye Movements

26. Movement in reference lead

27. Chewing

28. Vertical eye roll

29. Excessive muscle – notice saturation of T5

30. Talking and moving head

31. Yawn

32. Eye Closure and reopening

33. Blink and
Triple blink

34. Dealing with artifacts
 60-cycle noise
– Ground subject
– 60 Hz Notch filter
 Muscle artifact
– No gum!
– Use headrest
– Measure EMG and reject/correct for influence
– Statistically control for EMG
 Eye movements
– Reject ocular deflections including blinks
– Computer algorithms for EOG correction

35. High and low pass filtering
 Do not eliminate frequencies of interest
 Presence of a notch filter tuned at 50Hz,
eliminates mains frequency interference.
 Undesirable property of Notch filter is
‘Ringing’.
 Typical range of frequency of standard EEG
machines : 0.1-70Hz

36. Noise
 EEG Amplifiers are selected for minimum
noise levels.
 2uV is stated as the acceptable figure for
EEG reading.
 Recorded noise increases with the BW of the
system.

37. Display Device
Strip Chart Recorder
 Cheaper
 Limited hours of recording possible
Monitor
 Accuracy & Precision
 Post Processing
 Changes can be done through software

38. EEG interpretations
•Sudden development of delta waves
coincident with surgical manuver 
injury warning
•In penumbra region, EEG poorly
predict brain damage
•Anesthetics & hypothermia causes EEG
changes  multifactorial interpretation

39. Why Evoked potentials…
The EEG machine records tiny
electrical voltages from the brain,
representing the averaged electrical
activity of millions of neurons.
……… non specific.
But evoked potentials are triggered.

40. Evoked potentials
What are evoked potentials:
•The electrical potentials produced after
stimulation of specific neural tracts.
•The recorded plot of voltage versus time
Initial artifact representing the stimulation
of the tract followed by the neuronal
response, which is recorded as a series of
peaks and valleys

41. Evoked potentials
Many times signals are small  an
amplifier reduces the electrical noise by
subtracting the signal at a reference
electrode from the recording electrode.
•Filtering of this signal we focus on the
evoked response of interest.

42. Evoked potentials
•The evoked response always occurs at a set
time after stimulation.
•Summating all those responses increases
the time-locked response, whereas the
background activity acts as a random signal
and averages out to zero.
•So we get only the evoked potential
response.

43. Recording of Evoked potential
 If an external stimulus is applied to the a sensory area of brain, it responds by producing an
electrical potential known as the ‘evoked potential’. The most frequently used evoked
potentials for clinical testing include brainstem auditory evoked responses, visual evoked
responses and somatosensory evoked potentials.
 Evoked potential, recorded at the surface of the brain, is the integrated response of the
action of many cells. The amplitude of the evoked potential is of the order of 10 microvolts.
The evoked potentials are generally superimposed with electroencephalograms. Therefore,
it is necessary to remove the EEG by an averaging technique while making evoked
potential measurements. Since the background EEG and other unwanted signals often
appear irregular, or do not synchronize to stimuli, they are markedly reduced by averaging
across multiple trials. In general, averaging reduces noise proportional to the square route
of the number of trials. Most of the improvements in signal-to-noise ratio occur within 40 to
100 trials.
 Since many evoked potential components are of short duration, about 2 ms to 1 sec., rapid
sampling rates are needed to digitally record such low level potentials. Usually, the
sampling rate is 1000/second. The amplitude of evoked potentials are normally measured
on a vertical scale with sample points measured as bits on a logarthmic resolution scale.
Resolution of voltage is usually sufficient with a 8-bit recording, although 10-to 16-bit A/D
systems are becoming available.

44. CLASIFICATION OF EVOKED
POTENTIALS

45. Visual Evoked potential:

46. Visual evoked potential:

47. Visual evoked potential:
Primary visual system is arranged to
emphasize the edges and movements so
shifting patterns with multiple edges and
contrasts are the most appropriate method to
assess visual function.
Flash VEP
•Greater variability of response with multiple
positive and negative peaks
•Primarily used when an individual cannot
cooperate or for gross determination of visual
pathway. Ex in infants / comatose patients

48. Somatosensory evoked
potentials (SSEP) :

49. SSEP :
•Evoked potentials of sensory nerves in
the peripheral & central nervous system
• Used to diagnose nerve damage or
degeneration in the spinal cord
•Can distinguish central Vs peripheral nerve
lesion

50. Auditory evoked potentials

51. • Breif electrical pulse “ click”
• Intensity – 65 – 70 dB above threshold
• Rate – 10 – 50 clicks / sec
• Averaging of 1000 – 4000 Hz range
• Read after first 10 msec of stimulus
AUDITORY BRAINSTEM RESPONSES

52. Biofeedback Instrumentation
 Feedback is a common engineering term and
refers to its function to control a process.
When this concept is applied to biological
processes within the body, it is known as
biofeedback.

53. Biofeedback Instrumentation
 Biofeedback is a means for gaining control of the body processes to
create a specially required psychological state so as to increase
relaxation, relieve pain and develop healthier and more comfortable
life patterns. The technique involves the measurement of a variable
produced by the body process and compares it with a reference value.
Based on the difference between the measured and reference value,
action is taken to bring the variable to the reference value.
 Biofeedback is not a treatment. Rather, biofeedback training is an
educational process for learning specialized mind/body skills. Through
practice, one learns to recognize physiological responses and to
control them rather than having them control us.
 The objective of biofeedback training is to gain self-regulatory skills
which help to adjust the activity in various systems to optimal levels.

54. Different physiological processes
 Many different physiological processes have been evaluated for
possible control by biofeedback methods. However, the
following four neural functions are commonly employed:
• Emotions or Electrodermal Activity (Galvanic skin response
measurements)
• Muscle tension or EMG (Electromyograph measurements)
• Temperature/sympathetic pattern (Thermistor readings)
• Pulse (Heart rate monitoring)

55. Electrodermal activity
 Electrodermal activity is measured in two ways: BSR (basal
skin response) and GSR (galvanic skin response) is a measure
of the average activity of the sweat glands and is a measure of
the phasic activity (the high and low points) of these glands.
BSR gives the baseline value of the skin resistance where as
GSR is due to the activity of the sweat glands. The GSR is
measured most conveniently at the palms of the hand, where
the body has the highest concentration of sweat glands.
 The measurement is made using a dc current source. Silver-
silver electrodes are used to measure and record the BSR and
GSR.

56. Block diagram for measurement and record of Basal
Skin Resistance (BSR) and Galvanic Skin Response
(GSR)
The BSR output is connected to an RC
network with a time constant of 3 to 5
seconds which enables the measurement
of GSR as a change of the skin resistance.

57. Applications
 Biofeedback instrumentation for the measurement of EMG,
temperature and pulse/heart rate is not different from other
instruments used for the measurement of physiological variables.
 Transducers and amplifiers are employed to measure the variable that
is to be controlled by the feedback process. The magnitude of the
measured variable or changes in the magnitude are converted into a
suitable visual or auditory stimulus that is presented to the subject.
 Based on the stimulus, the subject learns to control the abnormal
conditions. Reports have appeared in literature regarding applications
of biofeedback to control migraine headaches, to slow down heart rate,
etc.
 Biofeedback techniques have been greatly refined and computerized
biofeedback training and psychological computer-assisted guidance
programs in the privacy of one’s home are now a reality.

58. EEG Telemetry system (Joseph.J.Carr)