The document summarizes the history and development of clinical neurophysiologic diagnostic tests such as the electroencephalogram (EEG). It discusses early recordings of electrical brain activity in animals in the late 19th century. Hans Berger recorded the first human EEG in 1929. The development of evoked potentials and far-field potentials advanced the diagnostic utility of EEG. Digital technology and advances like dipole localization, magnetoencephalography, and computer analysis have further improved EEG as a diagnostic tool, though imaging modalities also provide competition. Overall the document traces the history and progress of EEG and related fields.

1. HISTORY AND PERSPECTIVE
OF CLINICAL
NEUROPHYSIOLOGIC
DIAGNOSTIC TESTS
BY IAN GONZALES

2. EARLY HISTORY OF
ELECTROENCEPHALOGRAM AND RELATED
FIELDS
IN 1875, RICHARD CATON, A PHYSIOLOGIST FROM
THE ROYAL INFIRMARY SCHOOL OF MEDICINE,
LIVERPOOL, ENGLAND, SUCCESSFULLY RECORDED
ELECTRICAL ACTIVITY FROM AN ANIMAL BRAIN.
• HIS PAPER STATED, THAT, “... THE GALVANOMETER
HAS INDICATED THE EXISTENCE OF ELECTRICAL
CURRENTS. THE EXTERNAL SURFACE OF THE GRAY
MATTER IS USUALLY POSITIVE IN RELATIONSHIP TO
THE SURFACE OF SECTION THROUGH IT…”
• THIS WAS THE FIRST DESCRIPTION OF ELECTRICAL
ACTIVITY FROM ANIMAL BRAINS.

3. IN 1890, ADOLF BECK, FROM THE JAGIELLONSKI
UNIVERSITY IN KRAKOW POLAND, FOUND
OSCILLATORY POTENTIAL WHEN RECORDING
BETWEEN TWO ELECTRODES PLACED ON THE
OCCIPITAL CORTEX OF A RABBIT.
• UNAWARE OF CATON’S EARLIER WORK, HE
CLAIMED TO BE THE FIRST TO DISCOVER
ANIMAL BRAIN ELECTRICAL ACTIVITY.

4. FLEISCHL VON MARXOW, FROM THE UNIVERSITY OF VIENNA, WHO ALSO
DESCRIBED SIMILAR BRAIN ELECTRICAL ACTIVITY IN ANIMALS AND DEPOSITED
HIS FINDINGS IN A SEALED ENVELOPE AT THE IMPERIAL ACADEMY OF SCIENCE
OF VIENNA IN 1883.

5. HANS BERGER WAS A PSYCHIATRIST FROM HENA, GERMANY, AND WAS
INTERESTED IN OBJECTIVE MEASURES OF HUMAN BRAIN FUNCTION AND THE
MIND, HE BECAME THE FIRST TO DESCRIBE ELECTRICAL ACTIVITY FROM
ELECTRODES PLACED ON THE HUMAN SCALP.
• HE POSTULATED THAT THERE WOULD BE LOCALIZED INCREASE OF BLOOD FLOW
AND INCREASED HEAT BY CHEMICAL BREAKDOWN IN THE CORTEX IN RESPONSE
TO MOVEMENT OR SENSORY STIMULATION OF EXTREMITIES.

6. HIS HYPOTHESIS TURNED OUT BE AMAZINGLY CORRECT AND CAN BE NOW
DEMONSTRATED BY POSITRON EMISSION TOMOGRAPHY (PET), SINGLE-PHOTON
EMISSION COMPUTERIZED TOMOGRAPHY (SPECT) AND FUNCTIONAL MAGNETIC
RESONANCE IMAGING.
• HE WAS ABLE TO SUCCESSFULLY RECORD OSCILLATORY POTENTIALS OF
AROUND 10 HERTZ (HZ), WHICH HE CALLED “ALPHA RHYTHM.”
• HIS FIRST PAPER APPEARED IN 1929 AND WAS TITLED, “ELECTRENKEPHALOGRAM
DES MENSCHEN.”

7. • BERGER’S PIONEERING DISCOVERIES WERE AT FIRST RECEIVED WITH SKEPTICISM,
HOWEVER, IN 1935, PROMINENT PHYSIOLOGIST ADRIAN AND MATHEWS, FROM
ENGLAND, FINALLY APPROVED BERGER’S WORK AND APOLOGIZED FOR THEIR LONG
DISBELIEF, AND CALL THE EEG WAVES “BERGER RHYTHM.”
• IN 1936, THERE WERE SIX EEG LABORATORIES IN THE UNITED STATES.
• AS EARLY AS 1935, GIBBS ET AL. DISCOVERED 3-HZ SPIKEWAVE DISCHARGES IN
ASSOCIATION WITH ABSENCE SEIZURES.
• IN 1936, JASPER FOUND FOCAL SPIKES IN FOCAL SPIKES IN FOCAL SEIZURE AND
WALTER FOUND FOCAL SLOWING CORRESPONDING TO THE SITE OF A BRAIN TUMOR.

8. DEVELOPMENT OF EVOKED POTENTIALS
THE NEXT MAJOR STEP FORWARD IN
DIAGNOSTIC UTILITY OF CLINICAL
NEUROPHYSIOLOGY WAS THE DEVELOPMENT
OF EVOKED POTENTIAL (EP) RECORDING.
• THE EP IS AN ELECTRICAL POTENTIAL
RECORDED IN RESPONSE TO AN EXTERNAL
STIMULUS: VISUAL, AUDITORY, OR
SOMATOSENSORY.
• THE AMPLITUDE OF MOST EPS DETECTED
FROM SCALP ELECTRODES IS USUALLY
VERY SMALL [<10 MICROVOLTS (ΜV)] AND
CONSIDERABLY SMALLER THAN THE
ONGOING SPONTANEOUS EEG ACTIVITY
(GENERALLY FROM 30 TO 100 ΜV OR

9. GEORGE DAWSON FROM LONDON, ENGLAND, FIRST INTRODUCED A METHOD OF
EXTRACTING THE EP FROM THE ONGOING EEG ACTIVITY.
• HE CONCEPTUALIZED THAT THE SMALLER THE EP RESPONSE AND THE LARGER
THE NOISE , THE GREATER THE NUMBER OF SUMMATIONS IS REQUIRED TO
EXTRACT A MEASURABLE AND RELIABLE RESPONSE.

10. DATA GAINED FROM THE VARIOUS EPS, NAMELY VISUAL (VEP), AUDITORY (AEP),
AND SOMATOSENSORY (SSEP), ADVANCED THE CLINICAL APPLICATIONS OF EPS
FOR VARIOUS NEUROLOGICAL DISORDERS.
• EPS HAVE BEEN ESPECIALLY USEFUL IN THE DIAGNOSIS OF MULTIPLE SCLEROSIS
(MS), WHERE EPS TEND TO SHOW ABNORMALITIES EVEN IF THERE ARE NO
CLINICAL SIGNS OR SYMPTOMS RELATING TO THE EXAMINED EP MODALITY.
• EPS HAVE BECOME IMPORTANT TOOLS FOR MONITORING PNS AS WELL AS CNS
FUNCTIONS DURING SURGERY (INTRAOPERATIVE MONITORING)

11. DISCOVERY OF THE FAR-FIELD POTENTIAL
THE CONCEPT OF FFP (FAR-FIELD POTENTIAL) WAS FIRST DEVELOPED BY JEWETT
WHO SUCCESSFULLY RECORDED THE SMALL AMPLITUDE ACTIVITY (<1ΜV WHEN
RECORDED FROM THE SCALP) IN RESPONSE TO AUDITORY STIMULATION.
• THE RESPONSE CONSISTED OF SEVERAL WAVELETS OCCURRING WITHIN 10 MS
FOLLOWING AN AUDITORY CLICK.

12. HE POSTULATED THAT THE
IMPULSES PASSED THROUGH THE
VARIOUS STRUCTURES OF THE
BRAINSTEM AUDITORY PATHWAY
AND THEY WERE PICKED UP AT THE
SCALP VIA A VOLUME CONDUCTION
SPREAD, RATHER THAN BEING
TRANSMITTED, VIA THE
ANATOMICAL AUDITORY PATHWAY
WITHIN THE BRAIN.
• THE SCALP ELECTRODES
DETECTED THE POTENTIALS THAT
WERE GENERATED AT A
DISTANCE.

13. THE CONCEPT OF FFP WAS REVOLUTIONARY BECAUSE UP TO THAT TIME, THE
POTENTIAL RECORDED FROM A GIVEN ELECTRODE WAS ASSUMED TO BE
GENERATED DIRECTLY UNDER OR NEARBY THAT ELECTRODE (THE SO-CALLED
NEAR-FIELD POTENTIAL).
FFP RECORDING TECHNIQUES MADE IT POSSIBLE TO MEASURE THE POTENTIALS
ARISING FROM DISTANT SITES OR DEEP WITHIN THE BRAIN TISSUE, WHICH WERE
OTHERWISE NOT ACCESSIBLE BY WITHOUT INVASIVE RECORDING TECHNIQUES.

14. THE SHORT LATENCY AUDITORY RESPONSE WAS AT FIRST CALLED THE “JEWETT
BUMP” AND IS NOW REFERRED TO AS THE “BRAINSTEM AUDITORY-EVOKED
POTENTIAL (BAEP)” OR “AUDITORY BRAINSTEM RESPONSE (ABR).”
• BAEP REFLECTS VARIOUS LEVELS OF AUDITORY PATHWAY WITHIN THE
BRAINSTEM AND ALLOWS US TO EVALUATE BRAINSTEM FUNCTIONS
NONINVASIVELY BY SURFACE ELECTRODES.

15. THE DIGITAL ERA
IN THE EARLY 1970S, COMPUTERIZED TOMOGRAPHY (CT) SCAN BROUGHT A
REVOLUTIONARY METHOD FOR DETECTING AND LOCALIZING BRAIN LESIONS
NONINVASIVELY.
• WITH THE APPEARANCE OF THE CT SCAN, SOME THOUGHT THAT CT WOULD
REPLACE EEG AND EVENTUALLY CLINICAL EEG WOULD VANISH AS A
NEUROLOGICAL DIAGNOSTIC TEST, BUT THIS ASSUMPTION WAS INCORRECT.
THE EEG PRIMARILY REFLECTS BRAIN FUNCTION WHILE CT REPRESENTS
ANATOMICAL BRAIN STRUCTURES.

17. • IN ORDER FOR EEG TO COMPETE WITH CT SCANS, TOPOGRAPHICAL MAPPING WAS
DEVELOPED IN ORDER TO IMPROVE THE ANATOMICAL ACCURACY IN LOCALIZING
LESIONS.
• TOPOGRAPHICAL MAPPING IS ACCOMPLISHED BY MEASURING THE AMPLITUDE VALUES OF
GIVEN WAVES AND INTERPOLATING VALUES WHERE THERE ARE NO ELECTRODES FROM
THE KNOWN VALUES OF NEIGHBORING ELECTRODES.
• HOWEVER, WITH THE EMERGENCE OF MAGNETIC RESONANCE IMAGING (MRI) AND
THE DISTORTION OF WAVES DUE TO CSF, IT BECAME EVEN MORE DIFFICULT TO
COMPETE.
• RECENT AND FUTURE ADVANCES IN EEG DATA AND TECHNOLOGY MAKE THESE
GOALS LESS AND LESS HARDER TO ACHIEVE.

18. DIGITAL EEG
THERE ARE FOUR ADVANTAGES TO DIGITAL EEGS
1. EEG IS STORED IN DIGITAL FORMAT ALLOWS FOR CHANGING THE DISPLAY OF
THE RECORDED ACTIVITY (I.E., MONTAGES, FILTER SETTINGS, SWEEP SPEED,
THE AMPLITUDES SCALE, ETC.) AS AN OFFLINE PROCEDURE AS NEEDED UPON
EEG INTERPRETATION.
2. THE CAPABILITY OF RECORDING CONTINUOUSLY AT THE BEDSIDE WITHOUT
THE ATTENDANCE OF AN EEG TECHNOLOGIST.
• LONG-TERM BEDSIDE EEG MONITORING ALLOWS DETECTION OF EPISODIC EVENTS
SUCH AS INTERMITTENT OR CLINICALLY UNRECOGNIZABLE SEIZURES.

19. 3. THE CAPABILITY OF TRANSMITTING DATA INSTANTANEOUSLY TO A DISTANT
REVIEW SITE.
4. APPLICATION FOR FURTHER ADVANCING EEG TECHNOLOGY IS COMPUTERIZED
DATA ANALYSES USING TOPOGRAPHIC MAPPING, ADVANCED STATISTICAL
AND QUANTITATIVE ANALYSIS, DIPOLE ANALYSIS, ETC., WHICH MAY DISCLOSE
FINDING NOT READILY RECOGNIZED BY VISUAL INSPECTION AND MAY
EVENTUALLY LEAD TO A MORE ACCURATE ASSESSMENT OF THE
DYNAMICALLY CHANGING BRAIN FUNCTION, INCLUDING COGNITIVE
FUNCTION.

20. DIPOLE LOCALIZATION
IT IS POSSIBLE TO ESTIMATE AN ELECTRICAL SOURCE WITH INVERSE CALCULATION BY
MEASURING THE POTENTIAL FIELD (I.E., DIPOLE FIELD DISTRIBUTION), WHICH SPREADS
BETWEEN THE ELECTRICAL SOURCE AND THE PICK UP ELECTRODE VIA VOLUME
CONDUCTION.
DIPOLE LOCALIZATION IS BASED ON TWO PRINCIPLES:
1. THE CURRENT FLOWS FROM A POSITIVE SOURCE TO A NEGATIVE SOURCE,
CREATING POSITIVE AND NEGATIVE FIELDS ON THE ELECTRO-CONDUCTIVE
MEDIA.
2. ASSUMING THAT THE ELECTRO-CONDUCTIVE MEDIA ARE HOMOGENEOUS AND OF
A SPHERICAL SHAPE, MEASURING THE CURRENT VALUES AT A MINIMUM OF THREE
SITES IN 3-D SPHERES CAN ESTIMATE THE LOCATION OF THE CURRENT SOURCE
BY THE INVERSE CALCULATION.

21. • HOWEVER, THE MEASURED ACTIVITY AND DISTORTED BY INTERVENING TISSUE
BETWEEN THE ELECTRODE AND CORTEX (CSF, SKULL, AND SCALP).
• FURTHERMORE, THE SKULL IS NOT AN EXACT SPHERICAL SHAPE, BUT THIS CAN
BE REMEDIED BY USING MRI OR CT SCANS WHICH RECONSTRUCTS THE
INDIVIDUAL HEAD AND FACE SHAPES IN 3-DIMENSIONS.
• ELECTRODE PLACEMENTS ARE MEASURED BY A STEREOTACTIC SENSOR DEVICE
ONTO THE MRI.

22. MAGNETOENCEPHALOGRAPH
MAGNETOENCEPHALOGRAPH (MEG) DETECTS
MAGNETIC FIELDS CREATED BY CURRENT FLOW OVER
THE CORTEX.
• MAGNETIC FIELDS APPEAR VERTICALLY IN
RELATION TO THE DIRECTION OF CURRENT FLOW
OVER, MAGNETIC SENSORS PLACED OVER THE
HEAD CAN DETECT MAGNETIC FIELD CREATED BY
THE TANGENTIALLY DIRECT CURRENT FLOW OVER
THE CORTEX, BUT NOT RADIALLY ORIENTED
CURRENT FLOW.
• MAKING THE RECORDED POTENTIALS CLEARER
BECAUSE THE ELECTRICAL CURRENT IS NOT
ATTENUATED AND DISTORTED DUE TO THE
INTERVENING TISSUES.

23. ADVANTAGE OF MEG
• MEG SELECTIVELY DETECTS TANGENTIALLY ORIENTED CURRENT, AVOIDING ARTIFACT
INHERENT TO EEG RECORDINGS
DISADVANTAGE OF MEG
• THE COST AND MAINTENANCE OF THE EQUIPMENT IS 100 X GREATER THAN REGULAR EEG
EQUIPMENT
• MEG CANNOT BE USED FOR LONG RECORDINGS, AND IT IS ALMOST IMPOSSIBLE TO RECORD A
CLINICAL SEIZURE.
DESPITE THESE DISADVANTAGES, RECENT RESEARCH IS ALLOWING MEG TO BECOME A USEFUL
TOOL IN THE DIAGNOSTIC TEST FOR CLINICAL PATIENTS.

24. CLOSING REMARKS
EEG AND ITS RELATED FIELDS HAVE PROVIDED VAST CONTRIBUTIONS TO THE
ADVANCEMENT OF NEUROLOGICAL AND NEUROPHYSIOLOGIC SCIENCES AS A
DIAGNOSTIC TEST AS WELL AS RESEARCH TOOL.
• HOWEVER, INTRODUCTION OF NEUROIMAGING STUDIES (CT AND MRI) AND
DEVELOPMENT OF FUNCTIONAL IMAGING STUDIES (FMRI, SPECT, AND PET) HAS
WEAKENED THE ROLE OF EEG AS A NEUROLOGICAL DIAGNOSTIC STUDY AND BRAIN
FUNCTION MEASUREMENT TOOL.
RECENT PROGRESS IN COMPUTER APPLICATIONS HAS BROUGHT HOPE FOR FURTHER
ADVANCEMENT OF EEG AS A BRAIN FUNCTION DIAGNOSTIC TEST.
“EEG IS THE WINDOW OF THE HUMAN MIND”