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THE EFFECTOF THE GEOMETRICFORMAND MEANING
OFTHE STIMULUS ON THE CONFIGURATION OF THE
VISUALEVOKED RESPONSE
by
SHERRILL JEANSWIFT PURVES
B.Sc, McGill University,1967
M.D. UniversityofB r i t i s h Columbia,1971
A THESISSUBMITTED INPARTIAL FULFILMENTOF
THEREQUIREMENTS FOR THE DEGREE OF
DOCTOROF PHILOSOPHY
i n
THEDEPARTMENT OF PHYSIOLOGY
Weaccept this thesisascxsnforining
totherequired standard
THE UNIVERSITYOFBRITISHCOLUMBIA
May,1976
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In pre sen t in g th is th es is in p ar t ia l fu l f i l me nt of the requirements for
an advanced degree at the U n i v e r s i t y of B r i t i s h Colum bia, I agree that
the Li br ar y sh a ll make it fr ee ly a va il a bl e for r efere nce and study.
I fu rt he r agree tha t perm iss io n for ext ens ive copying of thi s t he s i s
f o r s c h o l a r l y purp oses may be gr an te d by the Head of my Department or
by hi s re pr es en ta t i ve s . I t is understood that copying or pu bl ic at io n
of t hi s th es is for f i n a n c i a l gai n sh al l not be allo wed withou t my
w r i t t e n p e r m i s s i on .
Depa rtment
The U n i v e r s i t y of Br i t ish Columbia
2075 Wesbrook Place
Vancouver, Canada
V6T 1W5
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ABSTRACT
Inthevisual system i t has been established that certain physical
parameters of thestimulusincluding intensity, focus,and sizeof
theelements of apatternhavesignificant effectson the configuration
ofthe evoked response as recorded from theoccipital region. While
claimshavebeenmadei n a fewpublications,tothisdate thequestion
ofwhether anypartof the evoked response i saffectedby higher
perceptualprocessesratherthan onesmoredirectly related tostimulus
inputhas not been resolved.
Evoked responses tofourgeometric shapes (a square,c i r c l e , e l and
omega)wererecorded frommultiple scalp locationsunder two experimental
conditions; i n the f i r s tthe shapeswerepresented atrandom intervals
and inrandomorder with nomeaningassigned tothem. In the second
theywerepresented i n a fixedrhythmic sequence, and two of the shapes
occasionally failed to appear intheir usualtime interval i n the
sequence. The subject"wasrequiredtorapidly signal such omissions by
a buttonpress. The cerebral electrical activityduring this time
intervalof the expected, but omittedstimuluswas recorded and
separatelyaveraged, and these responseswere called emittedpotentia ls.
Thereweresignificant,differencesbetweenthe evoked responses (in the
occipital region) to the square and e l shapes andbetweenthose to the
c i r c l e andomegashapes. These differencesweredemonstrated by three
measurementtechniques: performance of thediscriminant functions
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computed bySWDAi ncla ssi fyi ng single t r i a l responses, aratio
s t a t i s t i c calledXasdescribedbyJohn,Herrington&Sutton (1967),
and amplitude differences i n the N 2 and P 2 components.
Theemitted potentials recorded fromthevertex, containedasmall
and variable early negativecomponent and anobvious late positive
component similarto the oneseeni n theresponses to thepresent square
ande lduringthesecondexperimental condition. Manipulationof the
waveformsfromthesingle t r i a l s priortoaveraging showed that this
negativecomponent was nottime-lockedto the peak.
Theevokedpotential differences foundwere believed to be due to
twoclasses ofvariables; thephysical char acteristi csof thestimulus
(the contoursin thecentral1.5 of thevisual field)and task-related
changesi n the meaning of thestimulus; andthese affected earlierand
laterpartsof the waveform respectively.
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TABLE OFCONTENTS:
Page
t
Abstract i i
TableofContents . i v
L i s tofFigures v i i
L i s tofTables v i i i
L i s tofAbbreviations i x
Acknowledgement x
I. Introduction 1
II. Review of theLiterature 5
2.1 Therelationshipofscalp-recordedevokedresponsesto theunderlying brai na c t i v i t y 5
2.2 PatternVER's i n thestudyofvision 7
2.3 Evokedresponsesi n thestudyofpsychologicalvariablesand meaning 14
2.4 Suitniaryof theliterature review 20
III. Statementof theproblemf orinvestigation 22
IV. Methods 24
4.1 Dataacquisitionmethods 24
4.2 Dataanalysismethods 33
V. Results 38
5.1 Effectofdifferent experimental conditions
ontheevokedresponse 38
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5.2 Effectofdifferentstimulus shapes
ontheevoked responses 42
5.2.1 ResultsofVisu al analysis 43
5.2.2 ResultsofStepwise discriminantanalysis 50
5.2.3 ResultsusingtheXdescript or
computations 60
5.2.4 Summary of theeffecto f the
different stimulus shapeson theevoked responses 64
5.3 Emitted responses 67
5.3.1 Averaged emitted responses 67
5.3.2 Averaged "shifted"emitted responses 72
5.3.3 Control studies: motor potentials,
EOGcontributions,andtopography . . . 76
5.3.4 Summary offindingsfor omitted
stimuli 79
VI. Discussion 81
6.1 Studiesofevoked responsesandstimulusmeaning 81
6.2 Techniques fortheanalysisofevokedresponses 83
6.2.1 Visu al analysi s 85
6.2.2 Xvalues 85
6.2.3 Stepwisediscriminantanalysis 86
6.3 Topographicaldistributionof theevoked
responses 89
6.3.1 Locationofshape effects 89
6.3.2 Distributionofresponsecomponents.. 90
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v i
6.4 Intershape differences related to physical
propertiesof the stimuli 93
6.5 Effectsof stimulusmeaning 99
6.5.1 Present stimuli 99
6.5.2 Omitted stimuli 101
VII Summary& Conclusions 105
Bibliography 110
AppendixA. Computerhardware and data
acquisitionprogram (VER03) 118
AppendixB. Stepwise Discriminant Analysis 120
AppendixC. Termsfrom experimental psychology .. 124
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vii.
LISTOF FIGURES
Figure Page
1. Electrodeplacements 25
2. Dataacquisitionsystem 27
3. Stimulusshapes 29
4. Experimentalparadigms 30
5. Effectofexperimental conditions forcircleand omega . 39
6. Effectofexperimental conditions forsquareande l.... 41
7. Effectofstimulusshapeon theevokedresponses 44
8. Intersubjectvariability; occipitalVER toe l for
13 subjects 45
9. Effectofstimulus size 52
10. Distributionoflatency pointschosenby the4-groupdiscriminant functions 57
11. Distributionoflatency pointschosenby the2-groupdiscriminant functions 62
12. DistributionofXvalues 65
13. Averagedemitted potentialsat C infivesubjects 68
14. Averagedemitted potentialsatmultiple recording sites
inonesubject 69
15. "Shifted"averagedemitted potentialsat C zinfivesubjects 73
16. "Shifted"averagedemitted potentialsatmultiplerecordingsitesin twosubjects 74
17. Responsestoomitted stimuliwithEOGcontrol 77
18. Responsestoomitted stimuliinfrontal regions withabsentbutton press 78
19. Flowchartof VER03program(AppendixA) 119
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v i i i
LISTOFTABLES:
TABLE
I. Latencies of thecomponentso f occipital
evokedresponses to the fourshapes 48
II. Amplitudesof thecomponentsof occipital
evokedresponses to the fourshapes 50
III. Performanceof the "four-shape" discrinunant
functions 54
IV. Percentagesof t r i a l s incorrectly classified
separated in to pa ir s ofshapesconfused witheachother 56
V. Performanceof the "two-shape"discriminant
functions.One subject, five pairs ofshapes 59
VI. Performanceof the "two-shape" discrinunantfunctions. Fiv e subjects, two pai rs of
shapes 61
VII. Mean A values foreachsubject 63
VIII. Amplitude and latency of P^ forresponses
to omitted and present stimuli 70
IX. Amplitude and latency of N 2 forresponses
to omitted and present stimuli 71
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LISTOF ABBREVIATIONS
EEG Electroencephalogram
EOG Electro-oculogram
EP Evoked potential
ER Evoked response
SWDA Stepwise Discriminant Analys
VER VisualEvokedResponse
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X
The author gratefullyacknowledgesthe assistance inthis research
workprovided by anumberof people, especially
Dr.MortLow whoseadvice,encouragement,and t i r e l e s s personal
involvement i n the projectwereinvaluable fo r me
MichaelBakerfor hisinterest and a b i l i t y i n the design and writ ing
ofthecomputerprogramsf or thePDP-11system
JaniceGallowayand RichardFergusonfortheir technicalassistance
with the experimental apparatus and the subjects
JimMcEwenand JohnDoylefromtheDept.of E l e c t r i c a l Engineering
for their helpful instructioni n the use of the UBC Computing
Centre F a c i l i t i e s
Dr. Michael SchulzerfromtheDept.ofMathematicsfor advice on
s t a t i s t i c a l testing and
my committeemembersi n theDept.of Physiology; Drs.Tony Pearson,
HughMcLennanand PeterVaughanfortheir consideration, suggestions
and criticismso f the pro jec t.
Financial support was provided by the Medical ResearchCouncil of
Canadathrough an MRC Fellowship to the author fromSeptember 1972
u n t i lDecember1975 and MRC GrantMT-3313.
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Chapter I
INTRODUCTION
To attempt tounderstandtheenormouscomplexityof neural transactions
within thehumanbra in i s a formidable challenge to invest igato rs i n a l l
of the disciplines of neu rol ogi cal research. One of themethodsof
directly studying the neural processes i n the brai n, i s to record the
electrical activity occurring inresponse tosensory stimu lat ion . Two
classes of electrical activity can be recorded; theminutely localized
potentials of singlenerve c e l l s and themoredi ff us e and slowly changing
potential fieldsdetectable i n the v i c i n i t y of lar ge populations of
neurons. The terms"evokedpot ent ial s" and "evokedresponses" are
coimonlyusedtodenotethe sig nal s of the latter class of cerebral
e l e c t r i c a l a c t i v i t y that are recorded followin g stim ulation of sensory
receptors or afferentpathways.
A stimulus i n i t i a t e s asequenceof physiologicalevents,whichare the
substrates fo r i t s perception, as well as processes leading to an overt
behavioral response. Therefore, analys is of the e l e c t r i c a l activity
occurringbetween stimulus and response should provide clues to the
natureand anatomicalloca tion ofthese physiologicalevents.
Since the late1950's, there has beena largeamountof data published
aboutthe eff ec t of awidevar iet y of stimulus parameters on the evoked
responsesrecorded fromhumansubjects. The stimulus parameters
investigatedhave included bothphysical characteristicssuchas intensity,
1
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2.
durationandsize,andpsychological-contextualonessuchasexpectation,
task-relevance andstimulusmeaning. Withrespectto thelat ter,
conflicting reportshaveappearedi n theliterature. Inspiteofthe ir
importance forthedevelopmentoftheoriesofinformation processing,
somewidely publ iciz ed findingson theeffectofstimulusmeaningon the
evokedpote ntia l configurationhavebeen poorly substantiated.
Theuse ofpatterned visual stimulitoe l i c i t evokedresponsesi s arecent
developmenti nthis f i e l d ofstudy, becauseof therelativetechni cal
complexity of thestimulus presentationapparatus (ascomparedto the
simple stroboscopic flash thatwasusedpreviouslytostimulatethevis ual
system). Withpatterned stimulitheexperimentercanprovokeandexamine
the physiologicaleventsinvolvingmorecomplexaspectsofvisu al
perceptionandrela ted cogn itive processes thanwasformerly possible
using onlytheflas h.
Anotherrecentadvancei n thestudyofcogn itiv e processes rel ate dto
processingofsensory information, appearsi n somerecent reportsof the
recordingof aconsistentwaveform fromthescalpi n theintervalwhen a
subjectexpectsbuti snotac tu al ly presented withastimulus. The
responsesrecorded i n theintervalwhen nostimulushasbeen presented
are called emitted potentials,todistinguishthemfromtheevoked
potentials e l i c i t e d bystimuli thatareact ua ll y presented. Thestudyof
these emitted potentialwaveformsmayprovide important information
concerning bra in processes without inte rferen cefromtheactivity directly
relatedto thestimulus input.
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3.
In recordingsmadefrom thescalp, the evoked response i s ofmuchsmaller
amplitude (l-20yv) than the ongoingelectroencephalographic (EEG)
activitywhich i s 10 - 100 yv i n amplitude. In order to isolate and
studythe activity relatedto the stimulus, repeated responses are averaged
startingat the time of stimuluspresentation. The event-related activity
which, i spresumedt o be relatively constant in the repeated t r i a l si s
enhancedand the background non-event related EEG i scancelledby this
method. Thenumberof t r i a l sper average hasvariedwidely indiffer ent
reports,ranging from 10 to2000ormorerepetitionsof the stimulus.
The variance of these t r i a l s included i n the average i srarelyever recorded.
The resulting averaged evoked responses can beexamined. Thepeaksare
labelledand their latencyand amplitude can bemeasured. Whensimple
stimulusvariablessuch as luminance arealtered,theeffect i susual ly
only seen i n one o f thepeaksorcomponentsof the evoked response and i s
f a i r l yeasy to observe andmeasure. Howeverwhenchangesaremadei n
complexstimulusvariablessuch asmeaning,morethan onecomponentin
the waveformmay be alteredand i tbecomesquite d i f f i c u l t toquantify
differencesi n thewaveformsthatcan be relatedto stimulusvariable s.
Ifthe evoked EEG a c t i v i t y i sdigitizedand thentreatedas aseriesof
discreetobservations,somequite differentmethods,other than averaging
can be used to approach thisproblem. One suchmethodi scall ed
discriminant analysis; i t i s a technique based onmultivariate statistical
theories thatdetermines thevariables (or time points)thatbest account
for thedifferencesbetweentwo ormoresetsof observations (waveforms).
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4.
It requires verycomplexcomputationsand mustbe ca rr ie d out on a large
computer. (AprogramcalledBMD:07MStepwiseDiscriminant Analysis from
theUCLAseries i s availabl e inmostcomputingprogram libraries).
Inorder to investigate the natureand anatomical loca tion of the
(cortical) events that underlie v is ua l perception, a studywas designed
usingcomplexpatterned vi su al sti mu li (geometricshapes)todemonstrate
what,i f any, differences i n the evokedpot ent ial configuration could be
attributed to differences in stimulus meaningor to differences i n the
physical properties of the stimuli. The experimentaldesign also included
aparadigm to record the emittedpotential a c t i v i t y thatappeared during
the intervalwhenthe subject expected but did not see one of the geometric
shapes. A l l of the recordedwaveformsweredigitized and stored as single
t r i a l s so that a variety of techniques, includingaveragingand discriminant
analysis could be usedtomeasureany differencesbetweenthem.
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5.
Chapter II
REVIEWOF THE LITERATURE
2.1 The rela tionship of scalp recordedevokedresponsesto theunderlying brain activity.
A f a i r l y comprehensivereview of the experimental evidenceand current
theories isprovided by Kiloh,McComas&Osselton (1972)for the origin
of spontaneous a c t i v i t y and byRegan (1972)for the o ri gi n of both
spontaneousand evokedEEG a c t i v i t y . The availa bledataabout the origins
of scalp-recorded e l e c t r i c a l potentials are fragmentaryand do not yet
precisely define the sources of eitherspontaneousEEG a c t i v i t y or evoked
potentials.
Early reports suggested that the 0-60 Hz. "slow" a c t i v i t y in the EEG
recorded at the c o r t i c a l surface i s the re su lt of asummationof action
potentials of c o r t i c a l c e l l s . Otherslatershowedconclusively that neuronal
action potentialswerenot e ssential and suggested that thewavesof the
surface EEG resultfroma summationof the exc itatory or inhib ito ry
postsynaptic potentialsdeveloped by the somaand larg er dendrites of the
c o r t i c a lpyramidal c e l l s (Li &Jasper,1953). This suggestion received
further supportfrom studies including intracellular recordings of cortical
neurons (Jasper & Stefanis, 1965; Creut zfeldt,Watanabe& Lux, 1966) that
showeda close timerelationshipbetweenre pet it ive postsynaptic potent ials
andmacropotentials recordedon the surface of the ove rly ing cortex.
Insomemorerecent workE l u l (1968)demonstratedthe existence of slow,
spontaneousmembranepotential shifts i n c o r t i c a lneurons,in thesame
frequencyrangeas the EEG. On the basis of theseobservationshe suggested
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6.
that the EEG may r e f l e c tthese slowerpotentials ratherthanbeing a
temporalsummationof fas tera c t i v i t y , as the e a r l i e r studies of post
synaptic dend ritic pot ential s had implied.
The re la ti onsh ip of the EEG and evokedpotentiala c t i v i t y recorded from
the scalp to the e l e c t r i c a l a c t i v i t y recorded fromthe c o r t i c a l surface
has beeninvestigated by anumberofworkers. Geisl er & Gerstein (1961)
showedbothexperimentally (inmonkey)and i n theo reti cal formulations that
amajor attenuation of e l e c t r i c a lpotentialamplitudeoccurs in the highly
conductivelayers of the CSF and dura but not at the skull or scalp.
Cooper,Winter,Crow&Walter (1965)in studies onhumanpatientswith
simultaneousrecording from the scalp and implanted c o r t i c a l electrodes,
foundthat only "widely synchronized" cort ical activi tywas not obscured
at the scalp and that theamountof attenuationbetweencortex and scalp
dependeduponthe size of the area of cortex involved in the activity.
In a study of the EEG a c t i v i t y followingsensory (flash) stimulation,
Heath& Galbraith (1966),using c o r t i c a l and scalp electrodes i n the primary
visual area showedthat an earlycomponent (65msec.)was evident at the
samelatency i nboth c o r t i c a l and scalp recx)rdings; but for later
(100 to 300msec.)peakstherewerelatency differenc esbetweenthe response
componentsrecorded at the two electrodes. In explanation of thesewaveform
differences, theysuggested that a c t i v i t y generated i nnon-primary cortex
spreadto the o c c i p i t a l scalp lead but not to the c o r t i c a l lead. Hence
they concluded forVER'sto flash, that the earlycomponentsin the response
recorded fromthe s calp electrode directly overtheprimaryvisual cortex
are generatedonly in the immediately underlying brain area, while the
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7.
laterresponsecomponentsrepresentsummationof a c t i v i t y frommore
extensive (including both primary and non-primarycortex as faraway
as thetemporal area)brain regions.
Vaughan (1966)reported similar findings i n a patient explored with cortical
electrodesduring an intracranial surgicalprocedure. However,in studying
the distributionofevokedresponseto flash i n the c o r t i c a l regions
adjacent to the calcarinefissure, he found theresponsedifferedmarkedly
as the recording electrode was movedanteriorly fromthe occipital pole.
He concluded thatmostof the scalp VER usually recorded in the occipital
regions origin ated i n the occipitalpole of the cortex (foveal representation)
and not inmoreanter ior portions of the striate cortex. This findingi s
consistentwith other authors' observations (Devoe,Ripps&vaughan, 1968;
Harter, 1970) that the VER pri ma ri lyreflects foveal stimulation.
2.2 PatternVER'si n the study of vision.
The responserecorded i n theoccipital region e l i c i t e d by spatially
structured stimulus fields i s quitedifferent fromthe one evokedby a flash.
Thismajordif feren ce was f i r s t described bySphelmann(1965)who showed
thata patterned stimulusevokesaresponseofmuchgreater amplitude
and containingpeaksof differentlatency and polarity than a flash of
comparative l i g h t intensity.
The type of pattern presentationusedto e l i c i t the "patternvisualevoked
response" i s important. The stimulus may be abriefpresentation of a
patternon a blank background; i t may be achangeof patternfrom one
formto another (called a "patternreversal" i n the literature) or i t may
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8.
be illuminationof adarkpatternby aflash. Thetheoreticaladvantage
ofapattern reve rsaltypeofstimulationi sthat therei s nochangei n
the t o t a l luminancereaching thesubject's eye, onlyachangei n
distributionofpattern structure withinthevisual f i e l d . Consequently
someworkers (Halliday&Michael, 1970)havesuggestedthattheresponse
to pattern re ver sal contains only "pattern specific"components,and not
any toluminancechangeasdoestheflashVER.
However,thepattern rever salmethodi susableonly for certaintypesof
symmetricalpatterns suchascheckerboardsorgratings. I t hasalsobeen
shownbyEstevez&Sprekreijse (1974)thatthepotentialgeneratedby
pattern reversal includescomponentsfound bothi nresponsesto a pattern
appearanceand apattern disappearance (analgousto an "on" and"off "
response), andhencei snot asuncomplicatedasoriginally suggestedby
Halliday.
The thirdmethodofpresenting thepatterned stimulus (byillurninatinga
darkpatternwithastroboscopic flash) requires le ss elaborate instrument
ationand hasbeenused i nmany studies includingtheoriginalone of
Sphelmann (1965). The VERproducedbythis typeofstimulation consists
ofasummationof theresponsestoboththelargeluminancechangeproduced
bytheflashand to thepatterned stimulus.
For a l lof theabovestimulationtechniquesthestimuliaresuff iciently
widelyspacedi ntimethatthenervous systemhasbeenregardedasreturning
tothe samei n i t i a l statebetweensuccessive stimuli,andhencethese
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9.
responsesaresometimescalled "transient"responsestodistinguishthem
from another typeofevoked pote ntia l described i n theliterature calle d
a "steady state"evoked response. (SeeRegan(1975)fora complete
descriptionofthisphenomenon). Theterm "steadystate"i semployedto
indicate thatthestimuliarepresentedatsucharapid rate (e.g.10 - 60 Hz.)
thatthec o r t i c a lresponsestoindi vidual st imuli overlap. Thesteady state
responsei sthen separated fromthenon-event relatedbackground EEGbecause
i t i scomposedofcomponents (or harmonics) whosefrequencies areexactlythe
same as ormultiplesof thestimulus frequency. AFourier analysisi s usedto
performtherequired computations and theamplitudeandphaseof thefrequency
componentscan bedisplayedandmeasured.
The techniquehas theadvantage ofspeed becausethestimulican bepresented
somuch morerapidlythan theycan beforthestudyoftransient evoked
responses. Regan(1974)hassuggested thatthepropertiesof thecomponents
ofthesteadystateresponseseemtodependonwhetherthestimulation
frequency i s(a)near10 Hz., (b) 13 - 25 Hz., or (c) 45 - 60 Hz. and has
foundthatsomediseasesinvolvingthevisualsystemmayaffectthese
frequencybandsdifferently. Thesteady statetechniquehasalsobeenused
by Campbell &Kulikowski(1972)i nstudiesof therelationshipof a subject's
a b i l i t yto see asinewavegrating patternof lowcontrastto thesizeof the
evoked response e l i c i t e d by the samegrating pattern. Theyseparately tested
thesubject's thresholdbyhavinghimreportwhen thegrati ng pattern
disappearedas thecontrastwasdecreased,andthenbyrecordingtheevoked-
responseto arapid pattern rev ersa lof thegratingatdiffe rent levelsof
contrast. Theyfoundgood agreementbetweenthethreshold determinedby the
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10.
subject's verbal report and the contrast level atwhichthe evoked response
could no longer be detected. The steady state patternevokedresponse
thusprovides an objective indi cator of visual function and perception.
Transient and steadystateevokedresponsesgivecomplementary insights into
brain function. Van Hof (1960)and Tweel& Verduynlunel (1965)showedthat
i t i s not possib le to pred ict the parametersof the steadystateresponses
from the characteristics of the transientresponses. This i s not surp risi ng
i nviewof the very different stimulation techniques, but consequently
results obtained with one methodare not directlycomparablewith those
obtained with the other.
The remainderof the literature on patternVER'sto be reviewed i sconcerned
with the transienttypeofresponse, as i s the experimental work described
i n thisthes is.
The individualpeaksof the t ransi ent pattern VER have been studied using
topographicalmappingtechniques in an attempttodetermine the area of
cortex inwhichthesecomponentsoriginate. Jeffreys(1971)and Je ffre ys &
Axford (1972a& b) foundthat the f i r s t two peaksof the VER occurring
65 - 80msec,and 90 - 110msec,af te r presentation of acheckerboard pattern
(they called thesepeaks "C I" and "C I I" respec tively)weregre atly
influenced by the retinal lo ca tion of the stimulus. The polarity of these
componentsreversedwhentheupperrather thanthe lower f i e l d of vision
was stimulated, and theamplitude of C IIchangeddependingupon the
distance of the recording electrode from the calcarinefissure inboth the
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11.
horizontaland vertical planes. Basedon these and other observations of
responsesto right and l e f t half f i e l d stimu latio n, Je ff re ys & Axford
concluded th at the two peakshad spatially separate sources; the striate
cortex was the source of C I and the ext ra -s tri at e cortex on the outer
surface of the occipital lobes was the source of C I I . Ha ll id ay & Michael
(1970)described a sing lecomponentat 100 msec,i n the response to a
reversingcheckerboard stimulus that alsoshowedpolarity reversaldepending
on the part of the visual f i e l d that was stimulated. Theysuggested that
thiscomponentoriginated i nextra-striatecortex i n the occipital pole
aboveandbelowthe calcar inefissure. A similarcomponentin the responses
to a reversi ng pattern of red lightswas described by Purves& Low (1975)
using separate stimul ati on ofupperand lowervisual fields. The polarities
and latenc ies of thecomponentsdescribed by a l l of these groupsaresomewhat
differentprobably due i n part to differences i n the luminanceof the stimulus
and i n the techniques of stimulus presenta tion. The lack of standardized
stimulation and recording techniquesmakesthe find ings of the different
investigators d i f f i c u l t tocompare.
Datafromintracellular recording in the visual cortex in cats andmonkeys
(Hubel& Weisel, 1965 &1968); Bishop, Coombs&Henry,1971) haveshown
thatmanyc e l l s i nthis brain region aremuchmoresen si ti ve to contoured
thanto unstructured light stLmuli. Suchworkhas r esul ted i n important
advancesi n the understanding of the c e l l u l a r physiology of the occipital
lobe and of the fu nctio nal substrates of vision. Datafrom the study of
VER'srecorded i nawakehumansubjects (see theabove observation,
localizing the origin of these pattern specific responsecomponents to
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12.
striateandextra-striatecortex)inturn,haveprovided supportive
evidenceof theexistenceofthese patternsensitivemechanismsinthi s
regionandtheymayproveto be animportant toolfor further investigation
of visual perceptual processes inman.
In additionto thestudiesconcernedwiththetopographicalmappingof the
responsecomponents,therehave beenmanyothers thathavedemonstrated
the effectof avarietyofpatterned stimulusparameterson theevoked
c o r t i c a l response.
Clynes&Kohn(1967)reportedone of theearliest studies using stimulus
patterns other thantherelatively simpleones (checkerboards andstripes)
thatwereusedbySphelmann (1965). Theystudiedthespatial distribution
oftheevokedpotentialsto alargenumberofvisu al stimuliincluding
complexpatternsandcoloursandfound that therewere "spatially independent
components"thatwerepeculiartoeachtypeofstimulus foragiven individual.
Theyalso noted that whilethelatenciesofthesecomponentswere very
constant, their amplitudes werequite variab le,a finding replicatedi n
manylater studies. Thechanging polaritiesandamplitudes of theresponse
componentstodiffe rent stimuliwhichthey reporteddosuggestthepossib ility
ofchangingsourcesandsinksofcurrenti n thec o r t i c a l a c t i v i t yevokedby
the different stimuli. Howevertheir ratheridiosyncratic electrode
placements (inarosette patternaroundtheoccipitalregions),theiruse
ofbi pola r recording deriv ations,andtheir largenumberofstimulus types
makesi t impossibletocomparetheir resultstothoseof anylat er
investigators,or to make anydefiniteinterpretationsofthemeaningof
their results.
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13.
Rietveld, Tordoir,Hagenouw,Lubbers & Spoor (1967)and Harter &White
(1968)presentedmoredetailed studiesof the responses to a checkerboard
patternandshowedthatthe amplitude of the response from100 to200msec,
post-stimuluswas sensitive to thesizeo f the checks i n the pattern.
Theyshowedthati ncontrastto theflashevoked responses i n which the
luminance of the stimulus i s an important determinant of thelatencyof
thecomponents, (Creutzfeld &Kuhnt,1967; Tepas, Guterus & Klingman, 1974),
thelatencyof thecomponentsi n thepatternresponse did notchangeover a
widerange of luminances. Theysuggestedthat i twas the presence of the
intersecting contrastbordersthatwas essential for the patterned type of
evoked c o r t i c a lresponse. How sharplythe stimuluspatternwas focused was
alsofound tohavea significant effecton the VER amplitude i n these
studies. Manysubsequent reports (Arden & Lewis,1973; Harter & White,
1970; John, 1974) havedemonstrated thatthe amplitude of the VER to
patterns consistingofdifferent sizedchecks i s auseful tool i n the
determinationof therefractive error i nc l i n i c a l ophthalmology. The
patient'sresponse i s oflargestamplitude f or thesmallest sizeof checks
on which he can focus clearly.
Anumberof other stimulus parametershavebeenshownto be reflectedi n
theVER configurationbesides focus, check size and luminance as indicated
above. The overall sizeo f the stimulus for both unstructured(Rietveld,
Tordoir& Duyff,1965) and structured (Harter,1971; Rietveldet a l . , 1967)
f i e l d shasbeenfound to be important with amarkeddifferencebetween
fovealand extra-fovealstimulation. The colourof the stimulus can also
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14.
influence the response (Kinney,McKay,Mensch& Luria, 1972; Clynes &
Kohn,1967; Shipley, Jones & Fry, 1965) as can i t s or ie nt at io n
(Kulikowski, 1974; Yoshida,Iwahara &Nagamura,1975) althoughthere i s
somedispute aboutthe l a t t e r (Kakigi,Miyazaki &Mori,1972). Responses
have beendemonstratedboth to on- and of f- se t of diff use (White &Easton,
1966) and patterned l i g h t (Harter, 1971). The specific findings inthese
reports that are relevant to the presentstudy w i l l be considered i nmore
detail i n the Discussion Section.
2.3 Evokedresponsesin the study of psychological variables and meaning.
A very largenumberof publications i n the evokedpotentialliteraturehave
originated i n laboratories of physiologic al or experimentalpsychology.
A review of the relevantworksof nece ssity includes termsand phrases,
the precise def ini tio ns ofwhichare outside the scopeof th is thes is.
InappendixC a discussion of the terms "psychological variables" and
"stimulus meaning"has been included toprovidesomebackground for the
readerunfamiliarwiththese concepts, and to c l a r i f y th ei r use i n th is text.
The concept of stimulus meaningi s also considered i n relat ionship to the
experimental findings of th is di ss er ta ti on i n section 6.5 of the
Discussion chapter.
A greatmanyreports of the effec ts of a va riet y of psychological variables
on the evoked potential configuration are reviewedby Regan (1972) and
Beck (1975). These studies can be divided into two categories, those that
are concernedwith the entireevokedresponse configuration and those that
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15.
are primarily concerned with a late positive componentof theevoked response
(also called P^ or"P3QQ"becausei t appears as a prominent positive peakat approximately 300msec,after the stimulus).
Chapman&Bragdon (1964)i n one of theearlieststudies of theeffect of
psychologicalmeaningon theevokedresponse not specifically concerned with
the component,comparedevokedresponses to task relevant (blank) and
irrelevant (patterned) stimuli and found that the responses to task relevant
stimuliwerelarger and differentthan those to the irrelevant sti muli.
Howeverthey f a i l e d to consider that therewould have beenadifferencebetween
the responses to theirblank and patternedstimulianywaywhethertheywere
taskrelevant or not. L i f s h i f t z (1966)and Begleiter, Gross &Kissin (1971)
i n similar studies claimed that the VER to pleasant,repulsive and neutral
categorieso fvisual stimuliweredifferent for each category, and that the
effectdisappearedwhenthe stimulus was blurredso as to be unrecognizable.
John, Herrington & Sutton (1967)i n a widely citedstudy onmeaning found
thatthe VER was different fordifferent geometric formsof equal area and
similar fo r versions of thesamegeometric form of unequal area. They
concluded thatthis evidence suggested thewaveform of theevoked response
reflects atleast i n part the symbolicmeaningof the stimulus. Austt,
Buno&Vanzulli (1971)i n a similar study claimed th at both the occipital
visual evoked response and the visualevokedresponse recorded at the
vertex arerelated to the processing of sensory information. The occipital
response was saidto bemorespecificforvisu al stimuli than the one from
the vertex and was modified by changesi nvisualperception, significance
ofthe stimulus and theprogramthe subject was performing.
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16.
Buchsbaum& Fedio (1969) i n a study of the visualevokedresponse to
differentpatterns, includingsomewords,also found differences i n the
VER waveformsthatwererelated tomeaning. Becauseof their relevance to
the findings of the present study these l a t t e r three studiesw i l l be
considered in furtherdetail i n the Discussion section,
Begleiter& Platz (1969) showedthat the responseevokedby a particular
visualstimulus could be modified by c l a s s i c a l conditioning. A negative
peakat 155
msec,to 160
msec,latency was enhancedwhenthe stimulus was
conditionedby asso ciation with a loud trainof clicks and thisenhancement
was reversiblewith extinc tion and reconditioning. In threemorerecent
reports (Begleiter, Porjesz, Yerre &Kissin, 1973; Porjesz & Begl eiter,
1975; Begleiter & Porjesz, 1975) of the effect of the subject'sexpectancy
or perception of a stimulus, these authors haveshownthat the amplitude of
the vertex VER to a flashofmediumintensitywas higher i f the subject
expected orthought he saw a brightflash and significantly smaller i f he
expected orthoughthe saw a dim flash. Thesefindings are reminiscent of
John, Shimokocki&Bartlett's (1969) and Ruchkin&John's, (1966) findings
of "neural read-out frommemoryduring generalizat ion" i n cats . These
investigatorstrai ned cats to discriminatebetweentwo stimulus flicker
frequencies. Theyfound that theevoked potential i n various regions of
the brain to a stimulus of an intermediate flicker frequency closely
resembledthe responseto the appropriate signalfor the behavior which
was displayed on that t r i a l ; the configuration of theresponseevoked by
the intermediate stimulus was differentdependingonwhetherthe catmade
the behavioralresponseassociated with the high or low frequency flicker.
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17.
The authors concluded thatthis const itu ted evidence fo r relea se of a
neural process representing previous experience and provided somefurt her
support fo r the s t a t i s t i c a l theory of lea rning andmemorysuggested by
John (1967).
Investigatorswhoseworkhas centred on the experimental manipulation of the
latepositive (P^)componentofevokedpotentialshavesuggested a large
numberof p ossi ble psychologi cal variab les that may relate to thispeak.
It hasbeenvariablyproposedthat the "P^" i s a physi ol og ic al sig n of
"deliveryo f task-relevan t information" (Donchin&Cohen,1967),of the
"resolutionof prior uncertainty" (Sutton, Tueting, Zubin &John,1967) of
"cognitiveevalu ation of stimulus significance" (Ritter &Vaughan,1969), of
"a decision regarding the stimulus" (Rohbaugh,Donchin &ErUcsen,1974), of a
"reactivechangein the state of arousal" (Karlin, 1970),and of "stimulus
salience" (Jenness, 1972). Someauthors (Karlin, 1970, Naatanen, 1969)
haveargued a non-cognitive explanation f or a l l of these fin din gs, suggesting
thatthe dif fer enc e i n theresponsesto the relevan t or attended stimuli i n
a l l of the experiments can be explained by a "prior-state" hypothesis,
meaningthat a l l of the f ind ing s could be explained on the basis of achange
i n the subject's sta te of arousal or expectancy and not by any specific
parameters of the stimulus i t s e l f . However,Picto n &Hillyard (1974) and
Ford, Roth, Dirks & Kopell (1973)i nmorerecentworkstudying evoked
responsecorrel ates of selective attentionhaveshownthat this i s not true .
A l lhavedemonstrated two separate systemsoperating during selective
attention (for auditory stimuli); one ind ica ted by increased amplitude
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18.
of wavesand invo lvingselectionof a particular input channel and
thesecondindicated by themorewidespread complexand involving
complexinformation processing or decisionmakingbased on information
provided by the stimulus. Harter &Salmon (1972),i n an earlier study,
had also found a consistent "differencewaveform"with an earlier negative
and later positivepeakbetweenresponsesto task relevant and task irrelevant
patterned visual stimuli (although the non-specific"priorstate" of the
subjectwas not as carefully controlled i nthisworkas i t was i n the later
studiesthatusedauditory stimuli).
I t has alsobeenshownthat the componenti s not e l i c i t e d by a l l attended
and task relevantstimulior emitted stimuli (seebelow)that require
decision,but only by those forwhichcertain requirements of detection
confidence and prior subject uncertaintyhave beenmet (Squires,Hillyard &
Lindsay, 1973; Sutton, Braren, Zubin &John,1965; Tueting, Sutton &
Zubin, 1971). In a recent report on the P^ componentduring an auditory
detectiontask withcuedobservation intervals Squires, Squires & Hillyard
(1975a& b)havefurtherc l a r i f i e d the effect of these two majorfacto rs of
decision confidence and expectancy. They suggested that the P^ (becauseof
i t s topographyand sensitivityto the probabilityof the stimulus occurrence
orabsence)i s thesamep hys iol ogi cal bra in event,whetheri t occurs i n a
threshold detection task, omitted stimulusparadigm (Picton,Hillyard &
Galambos,1973) or a single-double stimulus discrimLnation task (Sutton
et a l . ,1967).
A few authorshavereported that a c o r t i c a l responsecan be e l i c i t e d by
omission of an expected stimulus; theseresponsesto stimuli that are
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19.
expected but do not occurhave beencalled "emitted potentials" by
Weinberg,Walter &Crow(1970)and this termhasbeenadoptedbymostother
investigatorsof thisphenomenon. Sutton, Ruchkin& Tueting (1974) reviewed
mostof the relevant studies of the componentof the emitted potential
and the experimental paradigmsusedfor i t s production. Squires etal.(1975b)
providedsomefurther experimental evidence that c l a r i f i e ssomeof the
questions on theappearanceof the P^ i n threshold detection tasks raised
i nthe presentation of Sutton et a l .(1974) and they alsodemonstratedthat
the essential c r i t e r i a fo r a stimulusabsenceto e l i c i t a P^waveare that
i t i sclearly recognizable and relatively improbable.
Therehasbeensomecontroversyaboutthe s p e c i f i c i t y of the emitted response.
Sutton et a l .(1974)stated that there was no evidence available indicating
thatthe P^componentof the emitted potential i sspecific for the modality
or intensity of the stimulus. Ritter,Simson&Vaughan (1974)howeveri n
another paperfromthesamesymposiumd id providesomeexperimental evidence
thatthe negativecomponente l i c i t e d by the missing stimulus was different
forthe two modalities theyused (visualand auditory) but they did not
investigate s p e c i f i c i t ywithin a modality. Otherauthors who havedescribed
emitted responses have been investigating effectsof stimulus timing
(Klinke,Fruhstorfer & Fin kenzell er, 1968) selective attenti on (Picton et a l . ,
1973) or the subject's expectancy i n a guessing paradigm (Weinberget a l . ,
1970) andnoneof these experiments provides any evidence on the specificity
of the response to the modality ormeaningof the omitted stimulus.
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20.
24 Summaryof the literature review.
It hasbeenestablished that sensory evokedresponsesrecorded from the
scalp r e f l e c t den dri tic post-synaptic pote nti ala c t i v i t y i n the underlying
c o r t i c a l c e l l s . I t has alsobeenshownthat only widely synchronized
cortical activityappearsi n scal p recordingsbecauseof the att enuation
by the intervening layers ofmeninges,cerebrospinal f l u i d and skull.
The EEGwaveformrecorded after presentation of a sensory stimulus consists
(after theon-going,non-eventre lat ed EEG a c t i v i t y hasbeenextracted) of
a ser ies of well definedpeaksorcomponents. Experimental evidence
suggeststhat at leastsomeof thesecomponentsoriginate i n different
c o r t i c a l regions. I t has al sobeenshownthat these differentcomponents
are se nsi ti ve tochangesi n a var ie ty of stimulus parameters.
The use of patterned visual stimuli toproduceevokedresponsesi s a
relatively recentdevelopmentthat allows the study ofmuchmorecomplex
aspects of stimulusmeaningthandid the oldermethodof stroboscopic
flashpresentation. Componentsof the pattern VER have beenshown to be
sensitive to focus, to thenumberof lines per uni t area (contour density)
and to a les ser extent t o the luminanceof the presented stimulus. Steady
stateevokedresponses (related i nprinciple to transientevoked responses,
but e l i c i t e d by a different stimulationtechnique) to grating patterns of
varying contrast levels and spatial frequencieshave beenshownto begood
indicatorsof the subject's thresh old fo r these patterns asdetermined by
separate psychophysical threshold testing.
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21.
Thereis also a largebodyof literature concernedlesswith the physical
parametersof the stimulus andmorewith the ef fe ct of psychological
variables on theevokedresponseconfiguration; bothwithnon-specific
variablessuchas attention, and withmorespecific cognitive variables such
as stimulus meaningor decision-makingbasedon information provided by a
stimulus. I t has beenshown,forexample,that a late positivecomponent
of the responsei n any modality i s related to this decisionmaking process,
but that thewaveappearsi n theevokedresponseonly i f the subject i s
certainhe detects the "stimulus" and i f the stimulus occurs relatively
infrequently during the paradigm.
I t has alsobeenclaimed that theevokedresponseconfiguration i s affecte d
by the symbolicmeaning of the stimulus. However,the few published studies
purporting todemonstratesucha relationshiphave failed to define clearly
either the experimentalchangesmadein stimulus meaningor the effects
theyproduce on the configuration of theevokedresponse.
Therehave been a few reports of the recording of c o r t i c a l responses (at
the scalp) e l i c i t e d by s ti mu li that areexpectedby the subject but do not
occur. Thesehavebeencalled emitted potentials since they are not evoked
by an external stimulus butpresumably r e f l e c tsomebrainprocessrelated
to therememberedstimulus. I t has beensuggestedthat this typeofresponse
recorded at the scalpmightprovide a usef ulmethod of examiriing brain
processes rela te d to stimulus rteaningwithoutcmtaminationby activity
that isdirectlyrelated to the sensory input. To thisdatehowever,
neither the techniques for obtaining emitted pot ent ial s, nor the configuration
and topographyof thesewaveformshave beenwelldocumented in the
literature.
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22.
Chapter I I I
STATEMENT OF THE PROBLEM FORINVESTIGATION
The questionofwhetheranypartof theresponse evokedbypatterned stimuli
reflectshigher perceptualratherthan simple sensory processi ss t i l l
unresolved. In tworecent, comprehensive reviewsof thef i e l d (Ciganek,1975;
Regan,1972),theclaimsofJohneta l . (1967)weresingledout ascr ucial
becauseoftheir significance forthestudyofevoked response correlates
of higher processes. Both reviewers urged thatthesefindings require
independent validationanddevelopment. Tothisdate therehasbeenno
reportofsuch studies.
Thereforeaseriesofexperimentswasdesignedtostudytheeffectof the
geometric formandmeaningof thestimuluson theconfigurationof the
visual evoked response. More s p e c i f i c a l l y i twas proposed:
(1) Toestablishwhethertherearereliableorcons istent
differencesbetweenevoked responsestogeometricshape
stimuliand todeterminethespatial (scalp distribution)
of anydiffere nces.
(2) Todemonstrate theeffecton theevoked responseof
assigningameaning(ascuesi n atask)to thediff erent
shapes.
(3) Torecordthe"emittedpotentials" occurringwhen asubject
expectsbutdoesnotactuallyseethese shapes,andthus
determine i f i t i spossiblet orecordEEGcorrelatesof
brainprocessesrelatedto themeaningof astimulusi n
the absenceof thestimulus i t s e l f .
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23.
( 4 ) Toapplyavarietyofanalytical techniquesto theevoked
potentialsobtainedtoexploremeansofquantifyingany
differencebetween thecomplexwaveformsrecordedi n the
differentexperimentalparadigms.
I n a l lof theexperimentalwork,data collectionandanalysiswere performed
byad i g i t a lcomputerwherepossible. Singlet r i a l samplesweresavedso
that'a variableandsmallnumber oft r i a l s couldbeusedi n theaverages
andsothat alternate analysis techniques suchas astepwise discriminant
analysis,manipulationofsingle t r i a l s priortoaveragingand computation
of theratio s t a t i s t i c calledtheAdescriptor (Johneta l . , 1967)could
becarriedout. (Seesection6.2 for adetailed discussionofthese
analyticaltechniquesandAppendixB for adescriptionofstepwise
discrimiiiant analysis.
Becauseinaveragesofsmallnumbers o ft r i a l s ,anya c t i v i t yo frelatively
highvoltage presenteveni nonlyone t r i a l affectstheaveragedwaveform
to adisproportionate extent,anautomatic artefact rejectionroutinewas
includedi n thedataacquisitionprogram (seeMethods).
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ChapterIV
METHODS
4.1 DataAcquisitionMethods.
Forthef i r s t part (paradigm1)oftheexperimentt hir tee n subjectswere
used. Ten ofthesewerepaidwomenstudent volunteers betweentheages
of19 and28. Theother threeweremembersoftheLaboratory St af f. A l l
ofthesubjectsweretestedatleastonceandsomeweretested repeatedly
but alwaysondifferentdays separatedby atleastaweek. At o t a lof
26runsofparadigm1wereusedfortheanalysi s.
Fiveofthethirt een subjects, includingtwo ofthestaffmembers,were
usedforthesecondpartoftheexperiments (paradigm2),i nwhichfrom
threetotenseparate recording sessions foreachsubjectwerecarriedout.
The subjectssatalonei nashieldedroomwithlowintensityoverhead
lighting (.5ft-candles)andwatchedatelevision screen1meterdista nt
forthepresentationofthestimulus.
The electrodeswereGrassgold discs applied with Grasselectrode paste
accordingtotheInternational10-20system (Jasper,1958)forelectrode
placement. (SeeFig.1fordiagramatic representation). Fora l lof
paradigm1thestandard locationsof 0 2, 0^ P 4, P , C 4, C 3and C zwereused.
Inparadigm2theselocationswerealsousedbut in someexperiments
0 , P , C , F and twolocati ons designated CP 0and CP.locatedhalfway
Z Z Z Z j 4betweenCj and P^ and C^ and P respectivelywereused. A l l recordingwas
referredtothecontralateralearor tothel e f tear i n thecaseof
midline locations.
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25.
Fig. 1 Diagramaticrepresentationof thepositionof the
electrodes on thescalp.
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26.
The EEG was recorded on aBeckmanDynograph TypeR with a time constant
settingof .3 sec andhigh frequencycut-off f i l t e r a t 30 Hz (aset ting
of 3 on thepoweramplifier f i l t e r ) givingan effectivesystem band width
of 0.5 -30Hz (3dB points).
FromtheDynographthe EEG signalson seven channelswereled to A-D
convertersof the computer (adetailed descript ionof the computer system
used i n these experiments i sgiveni n Appendix A) and sampled every 2msec,
for a 512msec,epoch, beginning 56msec,before thestimuluspresentation.
Single t r i a l s (256pointsper trial) werestoredon thediscof the PDP 11
computer system and a l ldata was transferredto Dectapes forpermanent
storageat the end of each paradigm.
The experimental set-up i sshowni n the diagram i nFig. 2. The experiments
were a l l controlled i nrealtime by a program calledVER03which ran on the
PDP 11 system. The flowchartof the program i sgiveni n Appendix A with
someexplanation. An automaticartefact reject ionroutinewas included.
This routinechecked each t r i a l sample to determine i f i t exceededsome
presetamplitude limits, and i f i t di d the t r i a lwas not saved. The program
thencontinued the experimentu n t i l therequirednumberof t r i a l s had been
collected i n each group to be sampled. (Thenumberwas 30 t r i a l sof each
shape i n paradigm 1, and 10 t r i a l so f each shape in paradigm 2). I t was
found fromobservingthe paperwriteouto f the EEG a c t i v i t y that this
technique effectively excluded any t r i a l swheretherewas eye blink
or subjectmovement. Insomeexperiments the EGG (electro-oculogram) was
recordedon one o f the seven channels to check f or any event-related slow
eyemovementswhich might nothavebeen rejectedby the system.
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D a t a A c q u i s i t i o n S y s t e m .
Fig. 2 Experimental se t up.
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The stimuliwere the fourdifferentgeometricshapesshownin Fi g. 3.
They included two familiarshapes, (one with corners and one without),
a square, and a c i r c l e , and two unfamilia r and le sseasilynamedshapes
calledhere the e l and theomega. Theywerea l lshownaswhiteoutli nes
on a blackbackground. A l l had approximately equal length of contrasting
border. Theywerepresented as slidesby aKodakCarousel pr ojecto r.
The imagesfromthe projectorweremonitoredby a televisioncamera and
were transmitted to the T.V. monitor placed one meteri n front of the
subject. An electromagnetic shutter (Gerbrant) on the lens of the
projector controlled the brief 20msec,presentation of the stimulus.
Advancementof the carousel and the shutteropeningwerecont rol led by
thecomputer.
The size of the shapeson the T.V. screen was approximately 6 cm. by 6 cm.;
therefore the shapessubtendedan angle of 3.6 degrees at the centre of
the subject's visual f i e l d . The luminancemeasuredat the screen was
10 ft-candle s fo r thebackgroundand approximately25 ft -candles for a l l
four stimuli. The subject was instructed to fixate visually on a point on
the centre of the screen throughout the recording period.
Forparadigm1 (see Figure 4 for a diagramatic representation), the four
stimuliwerepresented i n arandomsequenceat from 3-5 sec. in te rv al s
u n t i l 30 t r i a l s ofeachshapewerecol lec ted . The number30 was chosen
becausei t was the lowestnumberper averagethatproducedconsistent
evokedpotentialwaveformsand was the highestnumberof si ngle trials
i twouldbe p ossibl e to store in thecomputermemoryduring a sin gleexperiment.
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29.
Fig.3 StiraulijsShapes 1.circle 2.square 3.omega 4.el
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Stimulus presentation
20 msec
fc==l-*H
56
msec*
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31.
The subjectswereinstructedt owatchthepresentationof thediffer ent
shapesand totrytostay alertandattentive. Total time f or paradigm1,
includingone or twobrief interruptions,wasfrom10 to 15minutes. The
ongoingEEG wasmonitoredon thepaper writeoutby theexperimenterand
when theAlpha rhythmbegantodominatetherecordor anyTheta activity
appeared,theexperimentwasinterruptedand thesubjectaskedtorefocus
her attention afterabrief rest.
Inparadigm2 (seeFigure4foradiagramaticrepresentation), five shapes,
a c i r c l e , e l ,cross, squareand omegawerepresentedi n the sameordera t
regular intervalsof 1.4sec. Inapproximatelyonethirdof thesequences
eitherthesquareor thee lwasomitted (theshutterdidnotopen). The
experiment continuedu n t i lthe EEGduringtenomissionsofeach shapewas
collectedandstored. Withnorejectionsdue toartefact this required
63 sequencesoffiveshapes,or 315stimuli, which tooklhminutes.
Ten responsestoeachof thefourshapes usedi nparadigm1 whenthey d id
appearwerealso saved.
The subjectwasinstructedtolearnthesequenceoffive shapes, thenwas
told that occasionallythesquareore l would f a i ltoappear. She wasgiven
two buttons (handheldthumb-switches) andinstructed thati fthesquare
f a i l e d toappearat theexpected time,shemustpresstheleft-handbutton
andwhen thee lwasemittedshemustpresstheright-hand button. Failure
torespondcorrectlywithin400msec,of when thestimulus shouldhave
appearedi n therhythmic sequence,orpressingthebuttonwhen thestimulus
did appear resultedi n awarning tone being sounded (controlledby the
computer).
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The rhythmicpresentation sequence was not broken by the warning tone.
Button presseswererecorded as squarewavepulses on one of the seven
datachannels sothatthe exacttimingof the responsecouldbecompared
to the EEG a c t i v i t y . I talsoprovided a way of counting thenumberof
correctresponses at the end of the experiment. The subjectswere allowed
to practise this f a i r l y d i f f i c u l t taskpriorto therecordingsession and
a l lbecamequiteproficienti nmakingthecorrectresponsewithin400msec,
in from70 - 100%of the t r i a l swith the missing stimulus, and i nmaking
veryfew responseswhenthe stimulus was present.
At the end of an experimentalsessionthe datawereaveraged, 30 t r i a l s per
average for paradigm 1,and 10 t r i a l s per average for paradigm 2and the
averageswereimmediatelywrittenout. A l l of the data (assingle trials)
was thentransferredto Dectapes f orpermanentstorage andcould be
accessedfrom there by aprogramcalledGASPf orfurther analysis.
Q
Calibrationof the dataacquisitionsystem was doneat the beginning of
each experiment by insertinga5 Hz. 50yv squarewavesignalfrom a Grass
calibrator intoone pairof inputs at the headboard. A l l seven recording
channelswerecalibratedusingthispairofelectrodes. The VERprogram
was then run and a writeout ofsomesingle t r i a l smade. The gainswere
adjustedon the EEG amplifiers sothatthe50 yv signal i n a single t r i a l
appeared with an amplitude of25mm. on theplotter. Sincea l l averaged
EEG signalson the f i n a l plotweretruemeans (ratherthansums)a50 yv
signalappeared with a25mm. amplitude i n the final plotwhetheri t was
the signalof a single t r i a l or the signal i n the averaged response.
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4.2 DataAnalysisMethods.
The differencesbetweenthe evokedpotentialwaveformse l i c i t e d by the
different shapeswereanalyzed by a va ri et y of techniques. They include
visual inspection foridentification andmeasurementof the prominent
peaksof thewaveformsand twomorecomplexquantitativeapproaches.
The simpler of the two i s the computationof a descriptors t a t i s t i c called X
that was described by Johnet a l .(1967). This s t a t i s t i c gives a
quantitativemeasureof how different two waveforms are and the formula
andmethodsusedtocomputei t are givenbelow.
The secondmethodof determining the dif fer enc esbetweenthe responsesi s
called discriminant anal ysis , a techniquewhichi sbasedon multi varia te
s t a t i s t i c a l theory. A desc riptio n of the technique and the assumptions
onwhichi t i sbasedi s given i nAppendixB. The discriminant ana lys is
performedon this experimental data was carried out by theBMD:07Mprogram
i n the UBC computingcenter. The input data fo r theprogramconsi sts of
two ormoregroupsof observations (inthis experimental workeach "group"
was ashapeand eachobservation was the EEG a c t i v i t y recorded during a
single presentation of thatshape). Eachobservation i n turn consists of
anumberof variableswhichi nthis studywerethe digitized values of the
EEG voltage at different timepoints.
The programproceedstochoosea subset of the var iab les (i .e . voltages
at different timepoints) and cal cul ate s a discriminant function using these
variables thatw i l l discriminatebetweenthe differentgroupsthatwere
entered in to theprogram. Thismathematical functio n can be evaluated fo r
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34.
anewobservation (asingleEEGsample)andi tw i l l provideapredict ion
ofwhich group (shape) thisnewobservationwasmost l i k e l ytohavecome
from. Themethodtheprogramusestosel ect the varia blesand tocompute
thisdiscrimin ant function i s described i nAppendixB. Certainparameters
for thisprocessmustbesetatthe timethe data i s enteredandthese are
also given in thatappendix. Anotherre late d, butmuchsimpler program
calledUBC:CLASSevaluates the discriminant function fo rnewdata.
In ordertosubmittheEEGdatatotheUCLABMD:07Mprogram (runonthe
UniversityIBM 370computer) the selecteddigitized t r i a l s (one channelat
a time)weretransferredfromDectapetoeitherpapertapeormagnetictape
on thePDP 11system. Duringthe transfe r the digitized t r i a l representation
was reduced from256pointsto 67pointsbyomitting the f i r s t26points
(whichrepresented the52msecs.ofthe pre-stimulus bas eli ne) ,then taking
the averageofeachofthe67setsof 3poin ts t hat followedandomitting
the l a s t29points. Thedata fromonechannelat atimewerethentransferred
to f i l e sonthe Uni versi tyIBM 370system. Each67point single t r i a l
representation includeda 5character label indicatingwhichshapei tcame
from. The BMD:07Mprogramwasrun for ei th er2 or 4groups (each group
consistingof 30t r i a l s e l i c i t e d by adifferent shape). On a fewruns
60 t r i a l s per groupwere included but the separationofthegroupsbythe
programdi d not appearto beimprovedbythis increase in thenumberof
observations per group. Thenumberofvaria ble s per observationwas 67 as
indicatedin the previous paragraphandtheprogramselected6 or 7 ofthese
(i.e.6 or 7latency points)asthe b asisofthe discriminant function.
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35.
P i l o t studiesshowedthat allowing theprogram to run throughmorethan
6 or 7steps (i. e. tochoosemorevariablesthanthis) did not improve
the discrimination and so the programwas instructed to stopatthisnumber
of steps. This discriminant function producedby each run of the program
was stored i n aseparate f i l e and thenamesof the variab les i t selected
wererecorded separately (e.g. the 5th, 21st, 35th,42nd, 51st and 60th
variable)..
For test ing one of thesefunctions, datafroma l l of the experiments for
thesamesubject fo r thesamechannel that had not beenused fo r input to
theBMD:07Mprogramthatdetermined the function,wereput i n a sing le f i l e .
The programUBC:Classwas run on this f i l e with the discrhntlnant function
and aformat statement indicatingwhichwere the six or seven variables
(latency points) to be selected fromthe responsefor thi s partic ular
function as it s input. The r es ul ts of th is testing wereexpressed as
percentagesof correc tlyc l a s s i f i e d t r i a l s and i n the case of the 4group
choices, thewrongly c l a s s i f i e d t r i a l swerealsogrouped as towhichshape
theyweremistakenfor and thesepercentageswere tabulated.
Tocomputethe X rati os as described by Johnet al .(1967)averages of
10 t r i a l swereprepared on thePDP-11systemwiththeGASPprogram. These
weretransfer red bymagnetictapeto the IBM 370 wherea Fortr an program
computed the X values for the 4averagesgiven i tfrom the following formula:
= ( dl , 3 + d2,4 } / ( dl , 2 + d3,4 )
whered.. i s the absolute value of the rootmeansquare (r.m.s.) difference
between waveformsi and j . /"r.m.s.= x. /nwherex. i s the i sample
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36.
of thewaveformwith the valueof thebaselineascomputedfromthef i r s t
3points(18msec.)of theresponsesubtracted fromi t V Thesubscript s
1and 2denotethe tworeplicatedresponsesto oneshapeand 3 and 4
denotethereplicatedresponsesto thesecond shape.
Analysisof theresponsestothe omittedstimuliwas madesomewhat
d i f f i c u l tbytherelatively smallnumberofthese responses available.
The squareore lwasomitted from i t spositioni n thesequence only
occasionally. Anexperiment consistingofpresentationof atlea st
320 stimuliprovidedonly20"emitted" responses, ten each forthesquare
and e l . In theaveragesofonlytent r i a l sthebackgroundEEG(noise)
levelwassometimeshighenoughto makei td i f f i c u l ttovisuallyseparate
the componentsofevent-related a c t i v i t y , especially i f thesewere
relatively smalli n amplitude. However,combining data frommorethanone
experimenttoprovidemoreresponses fo r averagingintroduced added
v a r i a b i l i t yto theresponses becauseofchangesi n thesubject's performance
level, stateofalertnessor i ntechnical elements suchasminor differences
i n electrodeplacement. Inviewoftheseconsiderations, forpresentat ion
of theresults, themeasurementsmade byvisual inspection fo r compiling
tablesVTI andVTIIweremade onaveragesof tent r i a l s from each single
experiment,butforthei l l u s t r a t i o n sof theresponses30t r i a l speraverage
wereusedsothattheevoked responsecomponentscouldbemore easily
visualized.
Aftertheconventional averageswereproducedandexamined,analternate
techniquesimilartotheoneusedbyRuchkin (1974)wast r i e d tocompensate
fortheeffectof avaryingestimateof thetimeofoccurrenceof the
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37.
omittedstimulus by thesubject. Its purpose was tohave a l lof the
peaks from thesingle t r i a l s coincidei n timeprior to averaging, so that
any activity thatwas time-locked to the P peaks would be enhanced.
The single t r i a l s at C were f i r s t filtered by a 0 - 6 Hz band-pass
d i g i t a l f i l t e r tomakethepeakof the slow positive activity clearerfo r
measurement. The filtered trial swerethenmovedto adisplayscreen.
Aclear positivewavecouldbe seen i n about 90% of the responsesexamined,
between240 and 450msec,afterthe stimulus shouldhaveoccurred.
(If nopeakcouldbe seen inthat interval the t r i a l was notincludedfo r
the "shifted" average). The latencyof themostpositivepeakwasmeasured
and thesizeo f the shift along the timeaxis (inmsec.)thatwas required
tomovethepeakof thelargest positivewaveto an arbitrarily chosen
centralpoint (350msec.)was calculated. As a simplecontrol, this
shiftingprocedure was also appliedtosomeresponses from single trials
elicited by the square and e lwhentheywerepresent. Thisi sfurth er
discussed i nsection5.3.2.
TheGASPprogram mentioned included routinesforretrieving the single
t r i a l s from any experiment from Dectapes, averaging anynumberof responses,
switchingaround the seven channels i f necessary i n caseswhere different
channelshadbeenused for a givenscalp location, displayingaverages or
single t r i a l son thedisplay screen and f orplottingany of these responses
beforeo raftermanipulation on aprinter-plotter. The program also had
the capacity fordigital filter ing of data through whatever bandpass was
required.
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38.
Chapter V
RESULTS
5.1 Effectof the DifferentExperimental Conditions on theEvokedResponse.
Before examiningthe effectof the stimulusshapeon the configuration of
theevokedresponse, i t i s necessary to assess the contribution of the
differentpresentation conditions of the two experimental paradigmsi n
order to decidewhetherthe datafromthe two could becombinedfor
subsequent analysis.
In the stimulussequenceofparadigm 2, the c i r c l e andomegawerealways
present and served only as cue stimuli. For the square and e l , on the
otherhand,since theappearanceof the appropriateshapemeantthat no
button press was to bemade (detected omissionsweresignalledby pressing
one of the two switches) and since there was someuncertainty as towhether
the square or e lwouldappeari n i t s time interval in any given stimulus
sequence, the presence of these stimulusshapeswas informational!/more
important for the subject.
In theevokedpotentialse l i c i t e d by the c i r c l e andomegathere was
essentiallyno differencebetweenthe responses recorded i n the two paradigms
at any of the five recordingsitescommonto both experiments. Figure 5
showsthese responses for one subject in the two paradigmsand i t can be
seenthat theevokedpotentialsfromparadigm1 and 2 are very similar.
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39.
100msec
Fig.5 Effectofexperimental conditionsfo r thec i r c l eand
omega. Paradigm1 (notask) responsesare shown bys o l i d linesandparadigm2 (task)responsesare shownby dotted lines. n=30trials/average. (SubjectSC
Exp. 18A,34,35)
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In theevokedpotentialse l i c i t e d by the squareand e lhowever, there
wereconsistent differenc esbetweenthe two paradigms, but only in the
componentslater than 220msec. Inparadigm 2, but not inparadigm 1,
abroadpositivecomponentwith amaximumamplitudeof 8 to 18 pv
(depending on the subject)between 220 and 320msec,post-stimulus was
seenin the responses e l i c i t e d by the squareand e lshapes. This diff ere nce
i s i l l u s t r a t e d i n Figure 6 for one of the subjects. Thi s lateposit ive
waveappearing i nparadigm 2 was la rg es t at C , smaller i namplitude but
definitely present i n the right and l e f t parietal regions, and small and
inconsistentlypresent i n the occipital regions. For a given subject this
positivewavewas of thesameamplitudeand config uratio n fo rboth the
squareand e l responses. Therewereno amplitudeasymmetriesbetween
and C^, CP^ a n a- CP^ (notshowni n Fig. 6) and P^ and P .
Insummary,the effect of the task imposed i nparadigm 2 on theevoked
responsesto the four present shapeswas as follows:
1. The effectwas to introduce abroadpositivewavei n theevoked
responses at C ,whichwas only varia bl y present and of smaller
2
amplitude i noccipital regions.
2. Thiswavewas only seeni n theresponsese l i c i t e d by the square
and e lshapes and not to the c i r c l e andcmegashapes.
3. I t was seenonly i ncomponents220msec,or later post-stimulus.
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41.
+100 msec
F i g . 6 E f f e c t o f e x p e r i m e n t a l c c n d i t i o n s fo r the square and e l .
Paradigm 1 (no task) respon ses ar e shown by d o t t e d l i n e sand paradigm 2 (task) respon ses a r e shown by s o l i d l i n e s .
n=30 t r i a l s / a v e r a g e . ( Su b je c t MP, Ex p. 7,53,54 )
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Inview of theabove findings, i nconsideringthe data foranalysisof
theeffectof thedifferentshapes on the evoked responses, the circle
andonega t r i a l s from paradigm 1 and 2weremixed. For the square and
e l shapes there was a differencebetweentheparadigms whichwas maximal
atC and startedonly 220msec,post-stimulus. Since i t was very small
in occipital regions theresultsof computerized analysisof the
differentresponses at the occipital locationsfor these two paradigms
werealsocombinedforconsiderationi n the sectionto follow.
5.2 Effectofdifferentstimulus shapes on the evoked responses.
Consistentdifferencesbetweenthe evoked responses to the four different
shapeswereobserved. In order to best define and quantify these
differences,three techniqueswereused. F i r s t ,the data from the
standard paradigm 1condition (no task) was examinedvisually, the
componentslabelled and amplitude and latencymeasurementsmadeas
describedbelow. Secondly, thedigitized single t r i a l s from occipital
channels of one experimentweresubmitted to theBMD:07MStepwise
DiscariminantAnalysisProgram; a function todiscriminatebetween shapes
foreach subject was computedand thentestedon largenumbersof single
t r i a l s from other experiments with this subject. Finally,the A
s t a t i s t i c as described by John et a l . (1967)was computedusing
averages of ten t r i a l s formostof the experiments.
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5.2.1 Resultsofvis ual analysis.
For v is ua l analysistheevoked responses fromthirteen subjects (who
had eachbeen run i nparadigm 1 atleastoncewitho c c i p i t a l , parietal,
central,andvertex electrodes) wereused. Someof thesubjectswere
testedup tofive timesi nthisparadigmbut theaveragenumber of
runspersubjectwas two.
Thirty t r i a l s foreach stimulus shapewereaveragedtoobtainthe
VER's forexamination. With subjects thatweretestedmorethan
oncetheresponseswerefoundto benearly identical fromone
experimenttoanother. A typicalset ofresponsesto thefour shapes
in seven channelsfor onesubject i s shown i nFig.7. Theresponses
showedsomevariationi nlatencyandconfigurationbetweensubjects,
butdidexhibit es sential lythesameseriesof components fora l l
subjects. Fig.8 shows theresponseat 0 to the e lshapefora l l
thirteen subjects.
Ina fewsubjectsmid (T^ or T ) orpo sterio r (Tj-or T )temporalor
frontal (F^,F^ or F^) electrodeswereused instead of therightand
l e f t centrals. Noclearresponsewasseenatmid-temporal or frontal
locations. Theresponseat theposteriortemporals appeared very
similartothat seenat 0, and 0.
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100 msec
Fig. 7 Evokedresponsese l i c i t e d by thefour di ff er en t stirnulusshapes at the 7electrode locations used i n paradigm 1.
n=30trials/ aver age. (Subject SC, Exp. 1)
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45.
LP
msec
Fig. 8 Intersubject v a r i a b i l i t y . Responserecordedfrom
^1 - A2 e l i c : i - t e d DY t b ee lshape for each of the 13 subjects.n=30trials /avera ge.
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For further desc ript ionof theresponsesthelabellingshowni n
Fig.7 wasperformed. The f i r s tmajorpositivepeakseenatabout
95 msec., i n most subjects,was calledPp Thiswas sometimes
precededby adefi ni te small negative peakcalledN . Following P^
the nextlarge negative peak (whichwas sometimesdoublei nsome
channels)was calledN,,, thebroaderpositivewavefollowingN 2 was
calledP 2, and alater positivepeak (wheni toccurred)was called
P . It wasnotedthattherelative amplitudesofthesecomponents
variedi n theresponsestodifferentshapes andamongsubjects,but
i twasusually possibletoidentifythem ina l lof therecorded channels
for a l l subjects.
For a l l subjectstheamplitudeof the P 1 component was approximately
equali noccipitalandparietal recordings and slightly smaller than
thisi n thecentr al leads.
The N 2 component in six of thethirt een subjectswas largesti n the
occipital regions andeit her reversed p ol ar it ycompletelyorbecame
biphasic (i.e. partofi t reversed i npolarity) anter iorly. The
reversaltookplace ei the r jus t anter iorto orposteriorto the
parietal electrodes (asi tdoes i nFig.9). In theremainingseven
subjects thiscomponentbecameprogressively smaller anter iorto the
occipital locations rather thanreversingi npolarity,(asi tdoes i n
Fig.7).
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The ?2 conponent had abroadconfigurationand thelatencyof this
peak wasmorevariablebetween thedifferent recording sites than
was thelatencyof theothercomponents. Insevenofthirte en
subjects thiswavewas ofhighest amplitudei n theoccipitaland
parietal regions,and i n six ofthirteen i twas largesti n the
centralorvertex channels.
Becausethe components were most clearly definedi n theoccip ital
regions therightandl e f t occipital channelswereusedto measure
the latencyandamplitudeof thethreecomponents fora l lof the
subjects. Theaveragesoft h i r t y t r i a l swereusedforthese
measurements, althoughi nsomesubjects i twasnotedthattheaverage
of justtent r i a l s showed the componentsequally well.
Itwas notedthatthelatencyof thethreecomponents for a
particular subjectwas quite consistentfora l l fourshapes.
Table Ishowsthemeanlatenciesof thethreecomponents for the
thirteen subjects' averageresponsesto thefour different shapes.
Itcan beseenbyinspectiono f the magnitude of thestandard deviations
ofthemeanvalues i nthis Table that therewere no significant
differencesin thelatencyofthesethreecomponents between the
different shapes. Themeanlatencyfora l l fourshapesanda l l
thirteen subjects for P- was 95 msec, for N 2 was 148 msec, and for
P~ was 219 msec.
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48.
TABLEI:
Latenciesof thepeaksof thethree principalcomponentsofthe occipitalevoked responsesto thefourshapespresented
inparadigm1. Eachvalue (inmsec.) i s themeanand
standarddeviation for13subjects.
N.2
square 95- 17 145- 20 212- 25
e l 92 - 20 150- 21 225- 18
circle 101- 23 150- 26 219- 21
omega 93- 24 148- 25 221- 24
Mean of 4shapes 95 148 219
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49.
Therewerenoconsistent differencesi n thelatencyof thecomponents
fo r any of theshapesbetweenrightandl e f t occipital regions. I f any
small differenc ebetweenrightandl e f twasseenforagiven subject,
themean o f the two wastakenas thevalue used fo r preparing TableI.
The amplitudeofthese threecomponentswasmeasuredfromthe pre-
stimulusbase l i n e . Tocomparetheamplitudeof thea c t i v i t yat
0- and0^thesubjectsweredivid ed accordingtohandedness. There
wereeight right-handers andfour left-handers (datafromonesubject
wereomitted becausethe EEGfrom 0^wastechnically inadequate).
Ameanamplitudedifference foreachof thefourshapeswascomputed
for eachsubject. For theright-handed groupno meandiffere nces
between0- and 0 2amplitudesofgreaterthan1 yvwereseenforany
ofthecomponentsandthesewerenotevenconsistentlyi n the same
direction. For thefour left-handed subjects therewas aconsisten t
tendencyforN 2to bes l i g h t l y higher overtherighthemisphere(mean
differenceof 2 uv)for a l l fourof theshapes.
Themeanamplitudeofeachof thethreecomponentsrecorded at for
the fourshapesfor12 of thesubjectsi sshowninTableII(datafrom
one subjecthad to bediscardedbecauseoftechnicalproblems). The
amplitudesof the N 2componentof thesquarevs. e l ,c i r c l e vs.omega
and c i r c l e vs.e l and of the P 2componento f thesquarevs. e l ,
c i r c l e vs.omega andsquarevs.omegawerea l lfoundto be significantly
different usingtheScheffe's (1959)methodoftestingmult iple
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TABLE II :
Amplitudesof thethreeprincipalcomponentsof theresponses
at 02to thefourshapes presentedi nparadigm1. Each value
(inyv) i s the mean andstandarddeviationfor12 subjects.*
P
lN
2P
2
square 2.9 2.4 6.4 4.0 6.2 3.1
circle 3.4 -2.4 5.2 2.8 7.2 4.5
cmega 3.8 3.1 8.1 3.8 10.5 5.5
e l 4.8 3.4 10.4 4.8 9.5 5.0
ByScheffe's(1959)methodoftesting contrasts(asprovidedi n the UBCComputingCenterProgramMFAV),theamplitudesof thecomponentsof thefollowingpairsofshapesweresignificantly different (p= .01):forthe Ncomponentthesquareandel ,thec i r c l eandomega and thec i r c l eande l were a l ldifferent,andfo rthe P~componentthesquareandel ,thec i r c l eand omegaandthesquareand omegapairswere a l ldifferent. For
the P^component noneo f the6possiblepairsofshapesweresignificantly different.
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51.
contrasts (p = .01). Therewereno significant differences (p = .01)
in the amplitude of the component. I t i s notedthattheScheffe's
methodi s based on a two way analysisofvarianceand thus i t i s
essentially testingthedifferencebetweenthe shapes for each of
the12 subjects.
Fortwo subjects,paradigm 1 was repeated wi th the T.V. camera lens
adjustedsothatthe shapes appeared on the viewing screen at
approximatelyone half theirusual size. The evoked responses to
thesesmallerstimulus shapeswerenotdifferent (byvisual inspection)
from the ones evoked by the standardsized shapes. The responses to
thesmallandlargesquare, e l ,c i r c l e andomegafor one subjectare
showni nFig. 9.
5.2.2 Resultsof StepwiseDiscrirninant Analysis.
Inap i l o t study with theBMD:07Mprogram a sampling of data from a l l
five subjectsfrom a l l of therecording locations (occipitals, parietals
and centrals)was t r i e d to determine which ones would bebestfo r
ccnpletetesting. Using the U s t a t i s t i cof the program as an indicator
ofhow wellthe groupswereseparated(see,Appendix B) i t was foundthat
one of theoccipital locationsalways had the lowest U value ( i . e .
showedthebest separation) for anypairo f groups from agivenexperiment.
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52.
Fig. 9 Effectofstimulus size. Responses tostandard sized
stimuli (visualangleof 3.6) areshownassol id
lines, those to thehalf-sizedones asdotted lines,
(subjectML, Exp. 8,21) n=30 trials/average
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Therefore, fortestingtheabilityofdiscriminant analysistoclas sify
newdataforeachsubject, onlytheoccipital datawere completely
testedandarereported here.
TheworkwiththeBMD:07Mand thetestingof thefunctionsproducedby
i t (withtheprogramUBC:CLASS)wasdonei n twophases. In thef i r s t
phase,data collected fromfour subjectsi nparadigm1wereused. Single
t r i a l s froma l lfour stimulus shapesweresubmittedto theBMD:07Mprogram
at once,hencetheresulting discriminant functionswereintendedto
classifyanysingle t r i a l (orobservation)asbelongingto one of the
fourpossiblegroups (shapes). In thesecondphasethedata fromonly
twostimulus shapesat atimeweresubmittedto theprogram.
In TableIIItheresultsoftestingthediscriminant functions
classifying a l l fourshapesforthefour subjectsondata fromrightand
l e f t occipital locationsarepresented. Thepercentages giveni n the
Tablearetheproportionsof thetotalnumberoft r i a l sthefunction
correctly classified. Theclassificationsmade on thet r i a l s usedto
computethefunction (i.e.ap o s t e r i o r i c l a s s i f i c a t i o n )wereincluded
inthepercentages andtotalnumbersoft r i a l s giveni nthis Table.
120 t r i a l s ,30 ofeach shape,wereusedtocomputeeachfunction.
Itcan beseen fromtheTable thatthefunctionsclassifiedfrom35 to 44%
of the waveformsfromthesingle t r i a l s correctly, proportionswhichare
significantly greater thanthelevelof 25%expected witharandom
c l a s s i f i c a t i o n (z =3.58f or35%whichi sgreater thanzn 1 =2.57).
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TABLEII I:
Performanceof the "Fourshape"Discriminant Functions.
Percentageof t r i a l s correctly c l a s s i f i e d by theBMD:07M
StepwiseDiscriminant Analysis technique on data from right
and l e f t occipital locations. Totalnumberof trials
c l a s s i f i e d foreach subject i s given in parentheses. A l l
percentages given are significantly* greater than the 25%
level expected with arandom c l a s s i f i c a t i o n (p = .01).
For 35% z = 3.58 whichi s greater than z m = 2.57.
Subject 0 2 (right) 0-j (left)
MP (260) 41% 44%
SC (480) 37% 37%
VS (240) 39% 40%
ML (360) 35% 41%
meanof 4 subjects 38% 40%
* significancedeterminedby evaluating the test statisticz = p - p//pq/n-wherep i s the observed proportion,
p i s the expected proportion, q = 1 - p and n i s thesamplesize, z i s considered to be normally distributed.(Bahn, 1972).
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TABLEII I:
Performanceof the "Fourshape"Discriminant Functions.
Percentageof t r i a l s correctly c l a s s i f i e d by theBMD:07M
Stepwise Discriminant Analysis technique on data from right
and l e f t occipital locations. Totalnumberof trials
c l a s s i f i e d foreach subject i s given i n parentheses.
Subject 0 2(right) O-^left)
MP (260) 41% 44%
SC (480) 37% 37%
.VS (240) 39% 40%
ML (360) 35% 41%
meanof 4 subjects 38% 40%
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55.
To ascertainwhichof thefourshapesthefunctionswerebest ableto
identify,thepatternof theincorrect classificationswas studied
(i.e.whentheresponse fromoneshapewas c l a s s i f i e d