Table Of ContentClinical Electroencephalography and
Topographic Brain Mapping
F.H. Duffy V.C.Iyer WW SulWillo
Clinical
Electroencephalography
and Topographic
Brain Mapping
Technology and Practice
With 192 Illustrations, 14 in Color
Springer-Verlag New York Berlin Heidelberg
London Paris Tokyo
FRANK H. DUFFY, M.D.
Associate Professor of Neurology, Harvard Medical Schoo!, Director of Developmental Neuro
physiology, Director of the BEAM Laboratory, The Children's Hospital, Boston, MA 02115, USA
VASUDEVA C. hER, M.D., D.M.
Chief of Division of Clinical Neurophysiology and Professor of Neurology, University of Louis
ville School of Medicine, Louisville, KY 40292, USA
WALTER W. SURWILLO, PH.D.
Professor and Director of Psychophysiological Research, Department of Psychiatry and
Behavioral Sciences, University of Louisville School of Medicine, Louisville, KY 40292, USA
Library of Congress Cataloging-in-Publication Data
Duffy, Frank H.
Clinical electroencephalography and topographic
brain mapping.
Includes bibliographies.
1. Electroencephalography. 2. Brain mapping.
I. Iyer, Vasudeva, C. II. Surwillo, Walter W.
III. Title. [DNLM: 1. Brain Mapping-methods.
2. Electroencephalography-methods. 3. Evoked
Potentials. WL 150 D858cl
RC386.6.E43D84 1989 616.8'047547 88-32699
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DOl: 10.1007/978-1-4613-8826-5
Preface
Electroencephalography is truly an interdisciplinary endeavor, involving concepts and
techniques from a variety of different disciplines. Included are basic physics, neuro
physiology, electrophysiology, electrochemistry, electronics, and electrical engineer
ing, as well as neurology. Given this interesting and diverse mixture of areas, the train
ing of an EEG technician, a neurology resident, or an EEG researcher in the basics
of clinical electroencephalography presents an uncommon challenge.
In the realm of technology, it is relatively easy to obtain a technically adequate
EEG simply by learning to follow a protocol and by correctly setting the various
switches on the EEG machine at the right time. But experience has shown that the
ability to obtain high-quality EEGs on a routine, day-to-day basis from a wide variety
of patients requires understanding and knowledge beyond what is learned by rote.
Likewise, knowledge above and beyond what is gained by simple participation in
an EEG reading is necessary to correctly and comprehensively interpret the record.
Such knowledge comes from an understanding of the basic principles upon which
the practice of clinical EEG is founded - principles that derive from the various
disciplines cited.
While it is clear that some understanding of each of these disciplines plays an
important role in the successful training of an EEG technologist, neurology resident,
or EEG researcher, the depth and extent of the understanding pose a dilemma. How
much and what kind of material should go into an introductory text?
The authors have attempted to meet the challenge of this dilemma in three ways.
First of all, the text emphasizes concepts. For example, in discussions of Ohm's law the
reader is not burdened with irrelevant computations of circuit parameters. Instead,
the text shows how Ohm's law provides the basis for understanding EEG-based
problems such as why low impedance leads are essential and how a modern
impedance meter is able to measure the impedance of a single electrode by hooking
it up in series with all the other EEG electrodes connected in parallel. Secondly, the
text leaves out all but essential detail. Aside from material that is of historical interest,
the criterion for inclusion is whether the material is essential for understanding the
concepts. At the same time, special care was taken to avoid the risk of becoming sim
plistic. Thirdly, the text focuses on topics that are directly relevant to the recording of
EEGs and to the understanding of the fundamental principles upon which EEG
interpretation is based. A case in point is the Chapter 12 discussion of the principles
of localization.
The text is intended for a wide and varied audience interested in electroen
cephalography. It requires no special knowledge beyond a familiarity with simple
algebra and the elements of biology. To simplify presentation of the material, refer-
vi Preface
ences to relevant literature, for the most part, have been left out. Chapter 19 and
Appendix 7 are notable exceptions, mainly because they deal with newly-emerging
areas of interest for which reference sources may not be readily available.
Chapters 1-9 comprise a useful primer for the beginner learning to record clinical
EEGs for the first time. Those with some previous experience in EEG should also find
these chapters useful, primarily as a review of essentials or a fresh way of thinking
about the topics concerned. EEG technologists preparing for Board Examinations
will find Chapters 10, 11, and 12 and Appendix 2 particularly helpful.
Chapters 10-16 and 21 were written with neurology residents who are just starting
their EEG training in mind. The normal features of the EEG and some of the better
known, most-frequently encountered abnormalities are discussed and illustrated in
Chapters 14 and 15. Chapter 21 briefly discusses the use ofEEG in clinical diagnosis
and its relationship to other neurologic tests; but detailed questions of interpretation
are not taken up. Such matters are considered in more advanced, specialized texts.
However, taken together with Chapters 1 and 2 - which deal with background
material and basic electrical concepts - this material provides the essentials for train
ing of neurology residents as well as the elementary basics for training of EEG fellows.
The text includes a chapter each on the topics of seizure monitoring and average
evoked potentials. In recent years, these have become important tools for the neuro
diagnostician. A unique feature of the text is the inclusion of two chapters dealing with
the topographic mapping of brain electrical activity. These chapters, which take up
the basics of this exciting new method, will be of interest to anyone doing research in
EEG and cortical evoked potentials as well as to clinical neurologists seeking new
ways of interpreting the electrical activity of the brain.
Frank H. Duffy
Vasudeva G. Iyer
Waiter W. Surwillo
Contents
Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . v
1. Brain Electrical Activity: An Introduction to EEG Recording ......... . 1
Historical Perspective. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Recording Bioelectric Activity .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Some Characteristics of the EEG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
The EEG Frequency Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Fourier Series and Power Spectral Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Recording EEGs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The EEG Machine: An Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Computerized EEG Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2. Basic Electrical Concepts ........................................ . 11
Electrical Currents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Atomic Structure .................................................. 11
Conductors and Insulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Potential Difference and Voltage ...................................... 12
Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Electrical Circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Ohm's Law ....................................................... 13
Series and Parallel Circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Circuit Parameters ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Capacitance ...................................................... 15
Transient Response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Series R-C Circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Time Constant .................................................... 17
AC and DC....................................................... 18
AC Circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Impedance ................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Frequency Response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3. The Differential Amplifier ........................................ . 21
Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Single-Ended Amplifier ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Amplifying Bioelectric Activity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
The Differential Amplifier-Basic Concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Common-Mode Rejection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Sensitivity or Gain ................................................. 26
viii Contents
Amplifier Noise ................................................... . 26
Input Impedance ................................................. . 27
Special-Purpose Connections ........................................ . 27
The EEG Amplifier as a Whole ...................................... . 28
4. Filters .......................................................... . 29
The Need for Filtering ............................................. . 29
Basic Concept and Function ........................................ . 29
Low-Frequency Filter .............................................. . 30
Low-Frequency Response-Asymptote Plot ............................. . 31
High-Frequency Filter ............................................. . 31
High-Frequency Response-Asymptote Plot ............................ . 32
High-and Low-Frequency Response Combined-The True Curve ........... . 32
60-Hz Notch Filter ................................................ . 34
Interpreting the Frequency-Response Curve and the Use of Filters .......... . 34
Summary ........................................................ . 36
5. The Writer Unit ................................................. . 37
Penmotors ....................................................... . 37
Penmotor Frequency Response ...................................... . 37
Pens ............................................................ . 38
Error of the Arc .................................................. . 38
Pen Mounts ...................................................... . 38
Inking System .................................................... . 39
Inking-System Maintenance ......................................... . 39
Chart Drive ...................................................... . 40
Marker Pens ..................................................... . 40
6. Calibration and Calibration Methods .............................. . 42
Purpose and Basic Concept ......................................... . 42
Voltage Calibration - Deflection Sensitivity ............................. . 42
Linearity ........................................................ . 43
Frequency Response ............................................... . 43
Biological Calibration .............................................. . 44
Noise Level ...................................................... . 45
Postcalibration ................................................... . 45
7. Recording Electrodes ............................................ . 46
Basic Concepts ................................................... . 46
Ions ............................................................ . 46
The Electrical Double Layer ........................................ . 47
Polarization and the Double Layer .................................... . 47
Electrode Potentials ............................................... . 47
Residual Potentials ................................................ . 48
Types of Electrodes ................................................ . 48
Application of Surface Electrodes .................................... . 48
Electrode Impedance .............................................. . 49
Factors Affecting Electrode Impedance ................................ . 50
Electrode-Induced Artifacts ......................................... . 51
Detection of Electrode Artifacts ..................................... . 51
Impedance-Measuring Devices ...................................... . 51
8. Electrical Safety ................................................. . 54
Macroshock and Microshock ........................................ . 54
Ground and Grounding ............................................ . 54
Leakage Currents ................................................. . 56
Patient Grounding ................................................ . 56
Contents ix
Effect of Patient Impedance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
The EEG Technician's Role in Patient Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,58
Isolated Ground and Biopotential Isolator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Ground Loops. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
9. Elementary Practical Troubleshooting Methods ..................... . 60
Basic Principles ................................................... 60
Single-Channel Problems -The Principle of Substitution. . . . . . . . . . . . . . . . . . . 60
Single-Channel Prohlems Observed During EEG Recording. . . . . . . . . . . . . . . . 61
Electrode-Board Artifacts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Prohlems Common to All Channels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Connector and Switch Contacts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Chart-Drive Malfunctions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
10. Neurophysiology................................................. 68
Structure of the Neuron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Membrane Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Nernst Equation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Goldman Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
The Sodium-Potassium Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Action Potential. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Synaptic Potentials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Membrane Equivalent Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
Membrane Potential Fluctuations and the EEG ..... . . . . . . . . . . . . . . . . . . . . . 72
The Role of Different Types of Neurons in the Generation of the EEG . . . . . . . . 73
Rhythmicity of the EEG Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
11. Recording Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Electrodes as Field Samplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
The 10-20 International System ...................................... 78
Derivations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Montages - Rationale .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Commonly Used Montages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Reformatting of Montages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Extension of the 10-20 System ....................................... 82
Special Electrodes ................................................. 82
12. Localization and Polarity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Volume Conductors ................................................ 84
Concept of a Dipole: Fields and Equipotential Contours . . . . . . . . . . . . . . . . . . . 84
Theory of Localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Problems of Polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
The Five Principles of Localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Cancellation, Summation, and the Determination of Polarity. . . . . . . . . . . . . . . . 89
Phase Reversal .................................................... 90
Localization in Referential Recording .................................. 91
Commonly Seen Localizing Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Contaminated Average Potential Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
13. Introduction to EEG Reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Reading EEGs-An Analogy ......................................... 94
Learning to Read .................................................. 94
Terminology ...................................................... 94
Describing the EEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Interpreting the EEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
x Contents
More on Artifacts - Physiological Artifacts .............................. . 97
Writing the EEG Report ........................................... . 98
14. The Normal EEG ............................................... . 99
Features of the Awake EEG in Adults ................................. . 99
Features of the EEG During Drowsiness and Sleep in Adults ............... . III
The EEG in Deeper Stages of Sleep .................................. . 120
Age-Related Differences: The EEG in Relation to Maturation .............. . 124
Age-Related Differences: The EEG in Old Age .......................... . 132
15. Abnormal EEG Patterns .......................................... . 135
General Considerations ............................................ . 135
Abnormalities of the Background Rhythms ............................. . 135
Abnormal Sleep Patterns ........................................... . 143
Abnormal Slow Activity ............................................ . 144
Paroxysmal Epileptogenic Abnormalities ............................... . 151
Abnormal Periodic Paroxysmal Patterns ................................ . 181
16. Activation Procedures ............................................ . 190
Hyperventilation .................................................. . 190
Intermittent Photic Stimulation ...................................... . 194
Sleep ........................................................... . 201
Sleep Deprivation ................................................. . 202
Pharmacological Activation ......................................... . 202
17. Average Evoked Potentials ........................................ . 203
Historical Background ............................................. . 203
Method of Superimposition ......................................... . 204
Signal Averaging .................................................. . 204
Coherent Averaging ............................................... . 205
Instrumentation .................................................. . 206
Display Systems .................................................. . 207
Practical Clinical Methods .......................................... . 207
General Principles of Interpretation .................................. . 209
Visual-Evoked Potential (VEP) ....................................... . 210
Brain-Stem Auditory-Evoked Potential ................................. . 212
Short-Latency Somatosensory-Evoked Potential (SSEP) ................... . 214
18. Seizure Monitoring and Ambulatory EEGs ......................... . 218
Seizure Monitoring ................................................ . 218
Ambulatory EEG Monitoring ........................................ . 220
19. Clinical Use of Brain Electrical Activity Mapping .................... . 222
Why Brain Electrical Activity Mapping? ............................... . 222
The Use of Topographic AnalysiS in Interpretation of EEG and EP .......... . 223
Tips For the Conduct of Successful BEAM Studies ....................... . 224
The Application of BEAM to Clinical Practice .......................... . 226
A Cautionary Note: Potential Errors in Clinical Usage of Topographic Analysis .. 227
Topographic Analysis in the Clinical Setting ............................ . 228
20. Recommended Standards and Practices for Brain Electrical
Activity Mapping and Topographic Analysis ......................... . 238
Personnel ....................................................... . 239
Recommended Standards for Topographic Mapping Environment
and Equipment ................................................... . 240
Summary ........................................................ . 242
Contents xi
2l. EEG in Clinical Diagnosis and Its Relationship
to Other Neurological Tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 243
Computerized Tomography .......................................... 243
Magnetic Resonance Imaging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 244
Positron-Emission Tomography ....................................... 244
Neurosonography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 244
Brain Electrical Activity Mapping ..................................... 244
Evoked Potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 244
Role of EEG in Relation to Other Neurodiagnostic Tests . . . . . . . . . . . . . . . . . .. 244
Seizure Disorders - General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 245
Febrile Seizures ................................................... 246
Infantile Spasms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 246
Lennox-Gastaut Syndrome ......................................... " 246
Primary Generalized Epilepsy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 246
Partial (Focal) Epilepsy ............................................ " 247
Nonconvulsive Status Epilepticus (NCSE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 248
Sleep Disorders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 248
The Comatose Patient .............................................. 248
Electrocerebral Silence ............................................. 249
Diffuse Encephalopathies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 249
Metabolic Encephalopathies ......................................... 250
Infectious Encephalopathies ......................................... 250
Dementia ................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 250
Focal Encephalopathies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 251
Cerebrovascular Disorders ......................................... " 251
Head Trauma ..................................................... 252
Appendices
l. Glossary of Major Terms Used in the Text 253
2. Neuroanatomy for EEG Technologists. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 262
3. Grounding Checks ............................................... 269
4. Measurement of Chassis Leakage Current. . . . . . . . . . . . . . . . . . . . . . . . . .. 270
5. The 10-20 International System of Electrode Placement. . . . . . . . . . . . .. 271
6. A Glossary of Common Artifacts in the EEG. . . . . . . . . . . . . . . . . . . . . . . .. 274
7. EEG Recording in Patients with Infectious Diseases. . . . . . . . . . . . . . . . .. 282
Index .............................................................. " 285
