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EGG-State Dependent Processing of Visual Information
Psychophysiological Approaches to Human Information Processing F. Klix, R. Naatanen, and K. Zimmer (Editors) 0 Elsevier Science Publishers B.V. (North-Holland), 1985
=-STATE
21 1
DEPENDENT PNlCFSSING CF VISUAL INE'OFMATICN
Rudolf Schmitt, Eckard Schulz and E l k e van der M e e r Department of Psycholcgy Humbldt-University of Berlin Berlin GDR
The e f f e c t of daninant alpha waves i n the EM; on reaction times i n visual stimulated task w a s studied i n two on-line closed-loop experiments. The real t i m e processing of the EM; consisted i n detectirq dcminant alpha waves. Parameters of stochastic models, periods extracted fran Walsh and Fcurier special pmer are features to decide the hypotheses.
The present paper deals w i t h the controlling of the human motor action i n connection w i t h visual processing. The main attention is directed t o the so called elementary operatims which are assumed t o have a constant t h e consumption. The measurements were received by means of two on-line closed-loop experiments. The trails were f u l l y automatized and controlled by a desk top canputer HP9845t. Stochastic model building meth& are used. Features f o r further analysing were produced by time series analysis. I n p t s t o the visual system of healthy subjects were alpha-numeric s t r i n g s where the f i r s t figure should be detected and chessboard l i k e pairs of patterns which were connected by a transformation. The subject w a s inclined t o identify these transformations. Feaction times and identification e r r o r s were the outputs of the motor system. States q i are defined by the spontaneously and endogenous produced waves on the human brain c l a s s i f i e d by frequency ranges of the EEG (Electroencephalogram). The so called alpha waves q1 were chosen f r m the set Q of typical EEG patterns and used as a trigger f o r the input. Alpha waves are characterized by the frequency range f r a n 8 till 13 Hz and r e l a t i v e l y high mplitudes modulated by lower frequencies. Assuming there are measurements of the t r i p e l (x,q,y) the i q l i c a t i v e r u l e s
212
R Schmitt el aL
are the first approximation of the system under study. The indices i , j , k indicate one element of the state set Q, i n p t set X and output set Y. The state set Q contains a l l possible spectral w i n d m of the EM; (alpha, beta, ganma, delta waves) , X - t h e visual s t i r m l i and Y - p a r a m t e r s in time and frequency or sequency danain derived fran time series of the reactim times. The e f f e c t s of daninant alpha waves on the i n p t output relations are demonstrated by parameters of stochastic models f i r s t and second by s h i f t s of periods i n the histograms of reaction times f i l t e r e d by Walsh and F o x i e r analyses.
The alpha waves here mentioned i n t h e basic work of Norbert Wiener very often. H e assurd that a so called master clock produced these waves and that all infarmation processing in the human brain took place between these periods. An analogy between the discontinims work of a d i g i t a l canputer and the hypothetical work of the brain w a s discussed. N o there are m e than one res u l t showing us that a large amount of interactsubsystems w i t h the oscillating property exist in the human body. Fwthermore t h e r e s u l t s of d i f f e r e n t experbents led t o the conclusion that t k infarmation could be proceeded on a continious and discontinicus way. Ccgnitive researchers have suggested that the order of processing incaning information may be either serial or parallel. Evidence f o r both types of processing has been found (Burgess a d Spoor, 1982). T i l l naw no kind of information processing can be excluded. There are saw mare or less convincimj hypothetical assumptions c o n c e m i g the origin of the alpha waves (Cseutzfeld, 1977) hut t h e influence of the temporary daninant alpha waves i n the EEG on the i n p t o u t p t r e l a t i o n of equation (1) is not yet clear despite much publications done i n this f i e l d (Mler and S i f f t , 1981). CXU: research w a s aimed a t finding a w e l l masurable state containing the past of the system and shawing its influence on the chosen i n p t output r e l a t i o n both in time and frequency or sequency domain. The periods i n the reacticn times shauld be lengthened and more pronounced i n the 8-13 Hz range in case of using alpha waves as trigger and sequential informaticn processirag can be assumed. The folloging hypotheses m e proposed:
EEG-State Dependent Processing
213
1. Using the daninant alpha waves as trigger f o r the input presentation leads t o significant change i n the reaction times. The degree of changes depends on the type of the input (stimulus), the order of the s t i n u l u s presentation and the &ration of the alpha waves. 2. Processing incanhg sensory informatian is f o l l u e d by decisions procedures based on elementary operations. These operations take constant time intervalls and can be roeasured by periods i n the reaction times. The periods are fuzzy distributed i n the amplitudes and frequencies and they can be divided by frequency and sequency analysis. 3 . The p e r of the rhythms i n deperdence of q show higher parts i n the lcmx frequency ranges, i n the 8-13 $z range expecial-
lY.
Canputer controlled experimental equipnent (hardware) : The used hardware is described by means of Figure 1. The subject is placed i n an e l e c t r i c a l l y and acoustically shielded Faraday cage. T m terminals display the tasks and instructions f o r each trial through a window. The keyboard with five switches is connected with the central u n i t HP9845T of the autaMtic data aquisition system 3052A and serves for the on-line measurements of reaction times and errors. The on-line processing of the EM; is based on the electrode position 02 of the international 10-20 system. The linked mastoids w e used as reference electrodes. The signal is amplified t o the maxinun amplitude of 1 V and f i l t e r e d by a bandpass with the upper frequency of 11 Hz and the lcmx frequency of 8 Hz. Cne of the four counters of the real time clock receives pulses f r a n the Schmitt-Trigger i n case of the amplitude of the EM; signal is greater than a subject de@ent limit. The sum of the pulses are used by the ccmputer to recognize a production of daninant alpha waves in the EM; by the subject with eyes open and t o react according a fixed experinent a l design. The length of the EEGmeasuren-ent interval is one second. The criterion for the daninance of alpha waves i s
with qu = 8 amplitude different l i m i t and q, limit. This criterion cqmkzing of d d n a n t quick enough t o react
values greater than the interindividually = 13 q l i t u d e values greater than the can be shown t o be sufficient for the realpha waves. The procedure i s very easy and a t once. W i n g the test of the amplitude
214
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l i m i t a canparison is made betwa?n the detected alpha waves by a pcwzx spectrum of the F a s t Fourier Transfonnation (E'F'I!) and our procedure. The incorporation of a further c a p r a t o r with connected counter made it impossible t o overlook waves in case of strong modulation by counting the zero crossings of the f i l t e r e d EEG signal. Daninant alpha waves mans in tern of spectral p r t h a t mre o r equal 70 % of the whole pmer should belong t o the alpha range. T i l l now he could successfully test a further improver k n t of the detection of daninant alpha waves. This new mthod consists i n the application of a moving Slow Fourier Transform (Dehlke, 1984). The algorithn w a s programed i n machine code and is f a s t enough f o r real t i m e processing.
string scanning (experiment I ) : The input o r s t h l u s f o r the 17 healthy subjects aged 18 up to 36 years wre a l p h a n w r i c strings consisting of 4 0 elements. Each s t r i n g w a s presented on a carpxlter controlled screen i n a horizontal line. The instruction infonred the subject to scan the string fran left t o r i g h t and t o press the button of the keyboard i n case of the f i r s t feature is detected. The sequential display of the features f r a n 0,l to 9 followed a f t e r the measurement of the reaction time by the real time clock of the canputer. The t a s k of the subject consisted i n pressing the ixtton in case of the feature of the s t r i n g and the presentation w a s identical. This procedure served the aquisition of errors. The positions of both the f i r s t feature ard the letters were randmized by an autocorrelation criterion. Exactly three i t e m s with the same position of 4 0 p o s s i b i l i t i e s were contained i n each of the 120 strings. The beginnings of presentations were controlled by the EEG pattern state. Half of the 240 strings were produced after detection of state q1 and the second half a f t e r 92, t h a t means not 91. This factor of the design w a s randanized also. The CaTIputer ccanpared the masured state w i t h the upper and lower baund according the equation (2) a f t e r the display of the sentence "please relax". In case of the equatian (2) is f u l f i l l e d the next i t e m is presented inn-ediately.
...
In a second trial the subjective estimations of the d i f f i c u l t i e s t o discriminate letters and features were scaled. Table 1 demonstrates the two f a c t o r i a l design of this experiment. The temporal order of the values of the two factors were r d a n i z e d and the values were balanced.
2 16
R. Schmitt et aL
92
Table 1
-
Design experiment I:p . p o s i t i m i ; q., waves q2 = GI ( d e s y n h m i s a t i o n )
.
- daninance of
alpha
P a t t e r n t r a n s f o m t i o n (experiment 11): The inputs m e 8 x 8 matrices of black and white squares. They here selected fran patterns w i t h horizontal (HS) or v e r t i c a l (VS) or diagonal synmtry (El. Figure 2 illustrates one typical pair of patterns used. The right pattern was produced by turning the l e f t pattern w i t h horizontal symnetry around the v e r t i c a l axes.
Figure 2
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EEG-State Dependent Aocessmg
Three possible turnings (transformations) arourtd the diagonal axes (El'), the vertical axes (VT) and the horizontal axes (HT)
m e incorporated. Table 2 shows the three factor design w i t h 3 x 3 x 2 factors. The abbreviation HSVT is used for the pattern pair of Figure 2 for example. The possible nine canbinations were presented seven times. The states q, and q were used as trigger and the 4-3 division of t h i s third fact& was balanced on t m subjects i n each case. The subjects had t o recognize the relation be-en the two patterns and t o press the reserved button irrmediately. Transf.
IHSHT
m Hsm
... Ern
Syrrmetry
Hs
7
7
7
VS
7
7
7
Ds
7
7
7
3
4
3 4
I Table 2
mprimentaldesign: V h t 11. HS - horizontal syrrmetry, - vertical symtletry, J B - diagonal symnetry, HT - horizontal transformation, W. vertical transformation, m diagmal transformation, q, - daninance of alpha waves (synchronisation), q2 - state of desynchronisatim.
VS
-
-
Stochastic model building and t i n e series analysis: The procedures and models shown in Figure 3 were used for a suitable transformation of the data t o prepare decisions concerning the hypotheses.
The m i n orientation for experiment I was the pure s e r i a l processing. we assumed that the relatively great length of the strings, the f a c t s that more than one feature are present in the strings and small distances betvieen the l e t t e r s exist, are sufficient t o force the subject t o scan. The reaction tiroes should consist of a time constant a(qk) needed for accumdation, muscular response, etc. and a canponent f (xi,q ) depending on the position xi of the first feature and the s h e q,. That means, the reaction time yi is
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w kakh
f mnsjbrma6bur
sequtey power spednx
of v a h e s I
J
+
Figure 3 Applied algorithms and their data base derived f m experiment I.
219
EEG-State Dependent Processing
where is the normal d i s t r i b u t e t error. The sinplification
is reasonable i f the f o l l m i n g assmptions are made: ( 1 ) Each further position implicates
a further wnparison,
( 2 ) each further position implicates a further perception of the letter as a r u l e individual, (3) it is a self-terminating search. After an unsuccessful cunparison one new i t e m is perceived as a rule. It follaws
where bl (q ) is the tim f o r the perception of one new i t e m and b (q,) is $he c a q x @ o n t i m e . estimation of a ( q ) I 6 ( q ) w a s performed by the least square method. The splittingkof % dcording eq. (5) was received by time series analysis. The assuqtions 2 and 3 are statistical by nature. After creating the hypotheses on the basis of 8 subj e c t s (Ankert, Schorradt, 1982) the experiments were carried cut w i t h 17 subjects ( G i e s e , 1983). P a r m t e r estimation and residual analysis were used t o clean the data by the c r i t e r i o n
de
I 'i I
-3's
Y
>0
n- 1 i=l A where z . = yi - y . I Ayi - predicted reaction tirnes and s Y standarh d e v i a t i d of the measured values.
-
To ccmpare the f i t n e s s an approximation by polynans
... + $ ( q k ) . x y l + E (7) w i t h the normal distributed error e , k = 1,2, N = 1 , 2 , . ..4, y . = a ( qk ) 3
+
b(qk)-xi
+ k l ( q k ) * xi2 +
w a s p e r f o d . The structure of eq. ( 4 ) led to the mst favour-
220
R.Schmitt et al
able approximation measulfed by
The state dependency of the regression lines w a s tested by a mthod of Spurrier et al. (1982). With
E(y/xqi) = y
=
a(q2)+b(q2) 'x
=
a(ql)+b(q,)-x+de. (l-x)+ga.x (9)
To split b(q) according to eq. (6) a histogram (H(At,q) was canputed with the time interval at = 20 m s . The first 256 values of Hi( ~ t , q )can be sham to form stationary time series. A f t e r canputing the autocarrelation function r (z,q) with 7 = 128 on the first 256 values thedensity function 0,l was performed by Fourier transformation with the maXirmm frequency of 25 Hz. The obvious non-sine-consine function form of the histograms convinced us to produce the sequmcy ordered pmer spectrum p,(s,q) of the s a w interval to ccmpare the results. The equations
...
are used with fc - cal coefficients and fS
-
sal coefficients.
22 1
EEG-State Dependent Processing
RESULTS The follawing r e s u l t s cauld be obtained:
Experimnt I:
1. The approximation of the data by the regression lines led t o satisfactory values of the used criterion B: 16 times was B 7 0.9 , 11 times was 0.85sB 5 0.9 and 7 tires was B < 0.89. 2. The regression lines of 14 subjects m e placed over the second line or crossed each other after alpha wave daninance. The difference betwen the l i n e s of 8 subjects was significant, according t o H of quation (12). The arithmetic man m and the standaxd deviations s of t h e estimated parmter OF1 a l l subjects m e ( i n ms): P
ma (q1 )=174,19, sa1=47,78, ma(q2)= 173,21, sa2= 48,4
%(ql ) =1010,34, sbl=357 ,07 ,% (q2)= where m a ( q l ) '17
1
l7
845,26,
sb2= 352,61
A
ai(ql) and sal is the standard i=l deviation t h a t belongs t o ma(ql). The regression on the data of a l l subjects led to similar parameter values (in ms) :: a ( q , ) = 168,81, a(q2) = 168,11, b(ql) = 1143,72, b(q2)=933,81. 3. There were a t least one period in the range 4@60 m s ( = 5 0 1 ~ ) and one period in the range 90 - 110 ms ( e100 ms) found in b & hspectra after Walsh and Fmiertransformation. 4. Periods of greater length could be derivated f r a n circa 100 m s periods.
5. In accordance w i t h the hypothesis 3 periods i n the range 77 - 125 a= m e pronounced a f t e r alpha wave daninance. Stage q2 led t o doninant periods of 4 0 60 ms.
-
6. The sequency ordered pak~1'spectra discriminate sharper betW S e n d i f f e r e n t tyPeS Of kiStOgramS H( At,ql) and H( At,q2).
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Fxperiment I1 7. The effects of the symnetry of the patterns and the transformations between the two patterns on the reaction time could be sham to be significant on the 5 % level by Friedman's range analysis of variance (Kirchner, 1983).
8. In contradiction t o the r e s u l t s of experiment I the state q, led to shorter reaction times in mst cases. DISCUSSION
In conclusion it should be eqhasized that the effects of daninant alpha waves are evident despite the ccntradictive results: greater times in experiment I and 1reaction times i n experimt 11. The following statenwts on the basis of psychophysiological processes of sensory perceptions could explain the results. Sane
of them are hypothetical by nature.
1. The training of the feature discrimination f r a n the string lead to the orienting of the visual system of the subject t o the range 'string'. There are a t least two nerve f i b e r s that contrilxltes t o the visual disaimination. The main pass
leads fran the receptive f i e l d s of the retina through the lateral geneculate body t o the cortical neurons i n the Area 17. At the s a m time the unspecific axes of Thalamus are excited and pulse ccded informations reach the s a w Area 17. This bypass makes the excitation of the cortex neurons easier or possible a t a l l (tracing e f f e c t or 'Bahnungs-effect' ) A s e d information of the same orienting range can be proceeded quicker.
2. The advantage in processing an input of the sam! orienting
range (tracing effect) is destroyd i n case of alpha waves are daninant. This results i n lengthening of the reaction tinM2.5.
The reasm for shorter reaction t h s i n experiment I1 a f t e r q1 can be explained on the basis of the s a m psychophysiological llOd€?l.
1. Each pattern induced an other orienting range. A tracing effect should lead to slower proceding of the patterns i n this case.
.
EEG-State Dependent Processing
223
2. Assuming the state q1 is connected with a distortion of this traces a quicker p r 'wg of the patterns shculd follow after state 9,. With other mrds there is no o r minimal interference w i t h the preceding input.
After stirmlating the receptive fields of the retina Wurtz e t al. (1982) maswed a 50 ms delayed reaction of the cortex neurons i n the brain of monkeys. They concluded that "nearly a l l the gathering of visual information that leads t o visual perception occurs in the periods betwen saccades when the gaze i s fixated on a visual target". The saccades are the jmping nuxrernents of the eyes with the average duration of circa 50 ms. W e suppose that there is a link between the 40 - 60 ms periods i n the histogram of reaction times and the disamtinuous visual
perception.
Searching for an electronic prosthesis to give blind people enough vision to becane mbile and even read Dobelle e t al. (1974) could shuw that the pause interval of electric patterns of stimli for inplanted electrdes i n the human brain (Aria 17) should durate a t least 50 ms t o permit neurons stimulated in the preceding interval t o 'rest'.
Splittihg b according eq. (5) i n periods of circa 50 m s and circa 100 ms one is forced t o conclude that the canparison time i n experiment I takes circa 103 m s . This conclusion is amsistent with the results of Fisher (19821, who calculate 90 ms ccmparison time despite the parallel activ channels i n his rodel with Markov chain structure. The difference between the sum of both periods circa 1 5 0 m s and the calculated ascent of 174;168 ms respectively, could man that the subject prefers sanetws jmping to the next letter before pressing the button and sanetimes there is no position saccade.
224
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(1) pdler, J . D . , and S i f f t , J., Alpha EM: and Smle Reaction T i n e , Perceptual and Motor S k i l l s 52 (1981) 306. ( 2 ) Ankfft, G. and Schorradt, B., ZUdihgigkeit der Geschwindigk e i t v i s u e l l e r Informationsverarbeitung w n definierten Zust3nden des Gehirns. P r a k t i k u m s a r k i t , Sektion Psychologie, Humbldt-Universit ZU Berlin (1982, unpubl.).
(3) Burgess, J. and Spoor, D., Seven Faces i n a C r d : P a r a l l e l or Serial Infarmation Processing, Intern. J. Neuroscience 12 (1981) 21-23.
( 4 ) Creutzfeldt, O., Physiological conditions of conciousness, Proceedings of the 1 l t h World Congress of Neurology, Amsterdam, Sep. 11 - 16 (1977). (5) Dehlke, R., Entwurf und Testung schneller digitaler Verfahren zur on-line Trennung von F’requenzkanponenten des EFG Signals, Praktikumsarbeit, Sektion Psycholcgie und Sektion Elektronik, H v n n b o l d t - U n i v e r s i t zu B e r l i n (1984, unpubl. )
.
(6) Doklle, W.H. e t al., D a t a processing, LSI w i l l h e l p t o bring s i g h t to t h e blind, E l e c t r o n i c s Jan. 24 (1974).
(7) Fisher, D.L., Limited-channel Models of A u t a M t i c Detection Capacity and Scanning i n Visual Search, Psychological kw. 89 (1982) 662-692.
(8) G i e s e , B., V i s u e l l e Erkennungsleistung in Ati-Gngigkeit von der mmsntanen Grundfrequenz des Spontan-El%, Diplanarbeit, Sektion Psycholcgie, Humboldt-Universit zu B e r l i n (1983, unpubl.
.
(9) Kirchner, R., Alpha-= g e s t e u e r t e Relationserkennung, Diplanarkit, Sektion Psycholcgie, Hunbldt-Universitt Berlin (1983, unpubl.).
zu
(10) Spurrier, J . D . , Heett, J.E., M i d i , Z., Ccmparison of ’I’m E&gression L i ~ over s a Finite I n t e r v a l , B i a n e t r i c s 38 (1982) 827-836. (11) Wurtz, R.H.,
Goldberg, M.E. and Mbinsan, D.L., Brain Mechanisnus of Visual Attention, S c i e n t i f i c American, J u l y (1982).
=-STATE
21 1
DEPENDENT PNlCFSSING CF VISUAL INE'OFMATICN
Rudolf Schmitt, Eckard Schulz and E l k e van der M e e r Department of Psycholcgy Humbldt-University of Berlin Berlin GDR
The e f f e c t of daninant alpha waves i n the EM; on reaction times i n visual stimulated task w a s studied i n two on-line closed-loop experiments. The real t i m e processing of the EM; consisted i n detectirq dcminant alpha waves. Parameters of stochastic models, periods extracted fran Walsh and Fcurier special pmer are features to decide the hypotheses.
The present paper deals w i t h the controlling of the human motor action i n connection w i t h visual processing. The main attention is directed t o the so called elementary operatims which are assumed t o have a constant t h e consumption. The measurements were received by means of two on-line closed-loop experiments. The trails were f u l l y automatized and controlled by a desk top canputer HP9845t. Stochastic model building meth& are used. Features f o r further analysing were produced by time series analysis. I n p t s t o the visual system of healthy subjects were alpha-numeric s t r i n g s where the f i r s t figure should be detected and chessboard l i k e pairs of patterns which were connected by a transformation. The subject w a s inclined t o identify these transformations. Feaction times and identification e r r o r s were the outputs of the motor system. States q i are defined by the spontaneously and endogenous produced waves on the human brain c l a s s i f i e d by frequency ranges of the EEG (Electroencephalogram). The so called alpha waves q1 were chosen f r m the set Q of typical EEG patterns and used as a trigger f o r the input. Alpha waves are characterized by the frequency range f r a n 8 till 13 Hz and r e l a t i v e l y high mplitudes modulated by lower frequencies. Assuming there are measurements of the t r i p e l (x,q,y) the i q l i c a t i v e r u l e s
212
R Schmitt el aL
are the first approximation of the system under study. The indices i , j , k indicate one element of the state set Q, i n p t set X and output set Y. The state set Q contains a l l possible spectral w i n d m of the EM; (alpha, beta, ganma, delta waves) , X - t h e visual s t i r m l i and Y - p a r a m t e r s in time and frequency or sequency danain derived fran time series of the reactim times. The e f f e c t s of daninant alpha waves on the i n p t output relations are demonstrated by parameters of stochastic models f i r s t and second by s h i f t s of periods i n the histograms of reaction times f i l t e r e d by Walsh and F o x i e r analyses.
The alpha waves here mentioned i n t h e basic work of Norbert Wiener very often. H e assurd that a so called master clock produced these waves and that all infarmation processing in the human brain took place between these periods. An analogy between the discontinims work of a d i g i t a l canputer and the hypothetical work of the brain w a s discussed. N o there are m e than one res u l t showing us that a large amount of interactsubsystems w i t h the oscillating property exist in the human body. Fwthermore t h e r e s u l t s of d i f f e r e n t experbents led t o the conclusion that t k infarmation could be proceeded on a continious and discontinicus way. Ccgnitive researchers have suggested that the order of processing incaning information may be either serial or parallel. Evidence f o r both types of processing has been found (Burgess a d Spoor, 1982). T i l l naw no kind of information processing can be excluded. There are saw mare or less convincimj hypothetical assumptions c o n c e m i g the origin of the alpha waves (Cseutzfeld, 1977) hut t h e influence of the temporary daninant alpha waves i n the EEG on the i n p t o u t p t r e l a t i o n of equation (1) is not yet clear despite much publications done i n this f i e l d (Mler and S i f f t , 1981). CXU: research w a s aimed a t finding a w e l l masurable state containing the past of the system and shawing its influence on the chosen i n p t output r e l a t i o n both in time and frequency or sequency domain. The periods i n the reacticn times shauld be lengthened and more pronounced i n the 8-13 Hz range in case of using alpha waves as trigger and sequential informaticn processirag can be assumed. The folloging hypotheses m e proposed:
EEG-State Dependent Processing
213
1. Using the daninant alpha waves as trigger f o r the input presentation leads t o significant change i n the reaction times. The degree of changes depends on the type of the input (stimulus), the order of the s t i n u l u s presentation and the &ration of the alpha waves. 2. Processing incanhg sensory informatian is f o l l u e d by decisions procedures based on elementary operations. These operations take constant time intervalls and can be roeasured by periods i n the reaction times. The periods are fuzzy distributed i n the amplitudes and frequencies and they can be divided by frequency and sequency analysis. 3 . The p e r of the rhythms i n deperdence of q show higher parts i n the lcmx frequency ranges, i n the 8-13 $z range expecial-
lY.
Canputer controlled experimental equipnent (hardware) : The used hardware is described by means of Figure 1. The subject is placed i n an e l e c t r i c a l l y and acoustically shielded Faraday cage. T m terminals display the tasks and instructions f o r each trial through a window. The keyboard with five switches is connected with the central u n i t HP9845T of the autaMtic data aquisition system 3052A and serves for the on-line measurements of reaction times and errors. The on-line processing of the EM; is based on the electrode position 02 of the international 10-20 system. The linked mastoids w e used as reference electrodes. The signal is amplified t o the maxinun amplitude of 1 V and f i l t e r e d by a bandpass with the upper frequency of 11 Hz and the lcmx frequency of 8 Hz. Cne of the four counters of the real time clock receives pulses f r a n the Schmitt-Trigger i n case of the amplitude of the EM; signal is greater than a subject de@ent limit. The sum of the pulses are used by the ccmputer to recognize a production of daninant alpha waves in the EM; by the subject with eyes open and t o react according a fixed experinent a l design. The length of the EEGmeasuren-ent interval is one second. The criterion for the daninance of alpha waves i s
with qu = 8 amplitude different l i m i t and q, limit. This criterion cqmkzing of d d n a n t quick enough t o react
values greater than the interindividually = 13 q l i t u d e values greater than the can be shown t o be sufficient for the realpha waves. The procedure i s very easy and a t once. W i n g the test of the amplitude
214
R Schmitt et al.
'
II
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EEG-State Dependent Processing
215
l i m i t a canparison is made betwa?n the detected alpha waves by a pcwzx spectrum of the F a s t Fourier Transfonnation (E'F'I!) and our procedure. The incorporation of a further c a p r a t o r with connected counter made it impossible t o overlook waves in case of strong modulation by counting the zero crossings of the f i l t e r e d EEG signal. Daninant alpha waves mans in tern of spectral p r t h a t mre o r equal 70 % of the whole pmer should belong t o the alpha range. T i l l now he could successfully test a further improver k n t of the detection of daninant alpha waves. This new mthod consists i n the application of a moving Slow Fourier Transform (Dehlke, 1984). The algorithn w a s programed i n machine code and is f a s t enough f o r real t i m e processing.
string scanning (experiment I ) : The input o r s t h l u s f o r the 17 healthy subjects aged 18 up to 36 years wre a l p h a n w r i c strings consisting of 4 0 elements. Each s t r i n g w a s presented on a carpxlter controlled screen i n a horizontal line. The instruction infonred the subject to scan the string fran left t o r i g h t and t o press the button of the keyboard i n case of the f i r s t feature is detected. The sequential display of the features f r a n 0,l to 9 followed a f t e r the measurement of the reaction time by the real time clock of the canputer. The t a s k of the subject consisted i n pressing the ixtton in case of the feature of the s t r i n g and the presentation w a s identical. This procedure served the aquisition of errors. The positions of both the f i r s t feature ard the letters were randmized by an autocorrelation criterion. Exactly three i t e m s with the same position of 4 0 p o s s i b i l i t i e s were contained i n each of the 120 strings. The beginnings of presentations were controlled by the EEG pattern state. Half of the 240 strings were produced after detection of state q1 and the second half a f t e r 92, t h a t means not 91. This factor of the design w a s randanized also. The CaTIputer ccanpared the masured state w i t h the upper and lower baund according the equation (2) a f t e r the display of the sentence "please relax". In case of the equatian (2) is f u l f i l l e d the next i t e m is presented inn-ediately.
...
In a second trial the subjective estimations of the d i f f i c u l t i e s t o discriminate letters and features were scaled. Table 1 demonstrates the two f a c t o r i a l design of this experiment. The temporal order of the values of the two factors were r d a n i z e d and the values were balanced.
2 16
R. Schmitt et aL
92
Table 1
-
Design experiment I:p . p o s i t i m i ; q., waves q2 = GI ( d e s y n h m i s a t i o n )
.
- daninance of
alpha
P a t t e r n t r a n s f o m t i o n (experiment 11): The inputs m e 8 x 8 matrices of black and white squares. They here selected fran patterns w i t h horizontal (HS) or v e r t i c a l (VS) or diagonal synmtry (El. Figure 2 illustrates one typical pair of patterns used. The right pattern was produced by turning the l e f t pattern w i t h horizontal symnetry around the v e r t i c a l axes.
Figure 2
217
EEG-State Dependent Aocessmg
Three possible turnings (transformations) arourtd the diagonal axes (El'), the vertical axes (VT) and the horizontal axes (HT)
m e incorporated. Table 2 shows the three factor design w i t h 3 x 3 x 2 factors. The abbreviation HSVT is used for the pattern pair of Figure 2 for example. The possible nine canbinations were presented seven times. The states q, and q were used as trigger and the 4-3 division of t h i s third fact& was balanced on t m subjects i n each case. The subjects had t o recognize the relation be-en the two patterns and t o press the reserved button irrmediately. Transf.
IHSHT
m Hsm
... Ern
Syrrmetry
Hs
7
7
7
VS
7
7
7
Ds
7
7
7
3
4
3 4
I Table 2
mprimentaldesign: V h t 11. HS - horizontal syrrmetry, - vertical symtletry, J B - diagonal symnetry, HT - horizontal transformation, W. vertical transformation, m diagmal transformation, q, - daninance of alpha waves (synchronisation), q2 - state of desynchronisatim.
VS
-
-
Stochastic model building and t i n e series analysis: The procedures and models shown in Figure 3 were used for a suitable transformation of the data t o prepare decisions concerning the hypotheses.
The m i n orientation for experiment I was the pure s e r i a l processing. we assumed that the relatively great length of the strings, the f a c t s that more than one feature are present in the strings and small distances betvieen the l e t t e r s exist, are sufficient t o force the subject t o scan. The reaction tiroes should consist of a time constant a(qk) needed for accumdation, muscular response, etc. and a canponent f (xi,q ) depending on the position xi of the first feature and the s h e q,. That means, the reaction time yi is
R Schmitt eta,!
218
w kakh
f mnsjbrma6bur
sequtey power spednx
of v a h e s I
J
+
Figure 3 Applied algorithms and their data base derived f m experiment I.
219
EEG-State Dependent Processing
where is the normal d i s t r i b u t e t error. The sinplification
is reasonable i f the f o l l m i n g assmptions are made: ( 1 ) Each further position implicates
a further wnparison,
( 2 ) each further position implicates a further perception of the letter as a r u l e individual, (3) it is a self-terminating search. After an unsuccessful cunparison one new i t e m is perceived as a rule. It follaws
where bl (q ) is the tim f o r the perception of one new i t e m and b (q,) is $he c a q x @ o n t i m e . estimation of a ( q ) I 6 ( q ) w a s performed by the least square method. The splittingkof % dcording eq. (5) was received by time series analysis. The assuqtions 2 and 3 are statistical by nature. After creating the hypotheses on the basis of 8 subj e c t s (Ankert, Schorradt, 1982) the experiments were carried cut w i t h 17 subjects ( G i e s e , 1983). P a r m t e r estimation and residual analysis were used t o clean the data by the c r i t e r i o n
de
I 'i I
-3's
Y
>0
n- 1 i=l A where z . = yi - y . I Ayi - predicted reaction tirnes and s Y standarh d e v i a t i d of the measured values.
-
To ccmpare the f i t n e s s an approximation by polynans
... + $ ( q k ) . x y l + E (7) w i t h the normal distributed error e , k = 1,2, N = 1 , 2 , . ..4, y . = a ( qk ) 3
+
b(qk)-xi
+ k l ( q k ) * xi2 +
w a s p e r f o d . The structure of eq. ( 4 ) led to the mst favour-
220
R.Schmitt et al
able approximation measulfed by
The state dependency of the regression lines w a s tested by a mthod of Spurrier et al. (1982). With
E(y/xqi) = y
=
a(q2)+b(q2) 'x
=
a(ql)+b(q,)-x+de. (l-x)+ga.x (9)
To split b(q) according to eq. (6) a histogram (H(At,q) was canputed with the time interval at = 20 m s . The first 256 values of Hi( ~ t , q )can be sham to form stationary time series. A f t e r canputing the autocarrelation function r (z,q) with 7 = 128 on the first 256 values thedensity function 0,l was performed by Fourier transformation with the maXirmm frequency of 25 Hz. The obvious non-sine-consine function form of the histograms convinced us to produce the sequmcy ordered pmer spectrum p,(s,q) of the s a w interval to ccmpare the results. The equations
...
are used with fc - cal coefficients and fS
-
sal coefficients.
22 1
EEG-State Dependent Processing
RESULTS The follawing r e s u l t s cauld be obtained:
Experimnt I:
1. The approximation of the data by the regression lines led t o satisfactory values of the used criterion B: 16 times was B 7 0.9 , 11 times was 0.85sB 5 0.9 and 7 tires was B < 0.89. 2. The regression lines of 14 subjects m e placed over the second line or crossed each other after alpha wave daninance. The difference betwen the l i n e s of 8 subjects was significant, according t o H of quation (12). The arithmetic man m and the standaxd deviations s of t h e estimated parmter OF1 a l l subjects m e ( i n ms): P
ma (q1 )=174,19, sa1=47,78, ma(q2)= 173,21, sa2= 48,4
%(ql ) =1010,34, sbl=357 ,07 ,% (q2)= where m a ( q l ) '17
1
l7
845,26,
sb2= 352,61
A
ai(ql) and sal is the standard i=l deviation t h a t belongs t o ma(ql). The regression on the data of a l l subjects led to similar parameter values (in ms) :: a ( q , ) = 168,81, a(q2) = 168,11, b(ql) = 1143,72, b(q2)=933,81. 3. There were a t least one period in the range 4@60 m s ( = 5 0 1 ~ ) and one period in the range 90 - 110 ms ( e100 ms) found in b & hspectra after Walsh and Fmiertransformation. 4. Periods of greater length could be derivated f r a n circa 100 m s periods.
5. In accordance w i t h the hypothesis 3 periods i n the range 77 - 125 a= m e pronounced a f t e r alpha wave daninance. Stage q2 led t o doninant periods of 4 0 60 ms.
-
6. The sequency ordered pak~1'spectra discriminate sharper betW S e n d i f f e r e n t tyPeS Of kiStOgramS H( At,ql) and H( At,q2).
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222
Fxperiment I1 7. The effects of the symnetry of the patterns and the transformations between the two patterns on the reaction time could be sham to be significant on the 5 % level by Friedman's range analysis of variance (Kirchner, 1983).
8. In contradiction t o the r e s u l t s of experiment I the state q, led to shorter reaction times in mst cases. DISCUSSION
In conclusion it should be eqhasized that the effects of daninant alpha waves are evident despite the ccntradictive results: greater times in experiment I and 1reaction times i n experimt 11. The following statenwts on the basis of psychophysiological processes of sensory perceptions could explain the results. Sane
of them are hypothetical by nature.
1. The training of the feature discrimination f r a n the string lead to the orienting of the visual system of the subject t o the range 'string'. There are a t least two nerve f i b e r s that contrilxltes t o the visual disaimination. The main pass
leads fran the receptive f i e l d s of the retina through the lateral geneculate body t o the cortical neurons i n the Area 17. At the s a m time the unspecific axes of Thalamus are excited and pulse ccded informations reach the s a w Area 17. This bypass makes the excitation of the cortex neurons easier or possible a t a l l (tracing e f f e c t or 'Bahnungs-effect' ) A s e d information of the same orienting range can be proceeded quicker.
2. The advantage in processing an input of the sam! orienting
range (tracing effect) is destroyd i n case of alpha waves are daninant. This results i n lengthening of the reaction tinM2.5.
The reasm for shorter reaction t h s i n experiment I1 a f t e r q1 can be explained on the basis of the s a m psychophysiological llOd€?l.
1. Each pattern induced an other orienting range. A tracing effect should lead to slower proceding of the patterns i n this case.
.
EEG-State Dependent Processing
223
2. Assuming the state q1 is connected with a distortion of this traces a quicker p r 'wg of the patterns shculd follow after state 9,. With other mrds there is no o r minimal interference w i t h the preceding input.
After stirmlating the receptive fields of the retina Wurtz e t al. (1982) maswed a 50 ms delayed reaction of the cortex neurons i n the brain of monkeys. They concluded that "nearly a l l the gathering of visual information that leads t o visual perception occurs in the periods betwen saccades when the gaze i s fixated on a visual target". The saccades are the jmping nuxrernents of the eyes with the average duration of circa 50 ms. W e suppose that there is a link between the 40 - 60 ms periods i n the histogram of reaction times and the disamtinuous visual
perception.
Searching for an electronic prosthesis to give blind people enough vision to becane mbile and even read Dobelle e t al. (1974) could shuw that the pause interval of electric patterns of stimli for inplanted electrdes i n the human brain (Aria 17) should durate a t least 50 ms t o permit neurons stimulated in the preceding interval t o 'rest'.
Splittihg b according eq. (5) i n periods of circa 50 m s and circa 100 ms one is forced t o conclude that the canparison time i n experiment I takes circa 103 m s . This conclusion is amsistent with the results of Fisher (19821, who calculate 90 ms ccmparison time despite the parallel activ channels i n his rodel with Markov chain structure. The difference between the sum of both periods circa 1 5 0 m s and the calculated ascent of 174;168 ms respectively, could man that the subject prefers sanetws jmping to the next letter before pressing the button and sanetimes there is no position saccade.
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(1) pdler, J . D . , and S i f f t , J., Alpha EM: and Smle Reaction T i n e , Perceptual and Motor S k i l l s 52 (1981) 306. ( 2 ) Ankfft, G. and Schorradt, B., ZUdihgigkeit der Geschwindigk e i t v i s u e l l e r Informationsverarbeitung w n definierten Zust3nden des Gehirns. P r a k t i k u m s a r k i t , Sektion Psychologie, Humbldt-Universit ZU Berlin (1982, unpubl.).
(3) Burgess, J. and Spoor, D., Seven Faces i n a C r d : P a r a l l e l or Serial Infarmation Processing, Intern. J. Neuroscience 12 (1981) 21-23.
( 4 ) Creutzfeldt, O., Physiological conditions of conciousness, Proceedings of the 1 l t h World Congress of Neurology, Amsterdam, Sep. 11 - 16 (1977). (5) Dehlke, R., Entwurf und Testung schneller digitaler Verfahren zur on-line Trennung von F’requenzkanponenten des EFG Signals, Praktikumsarbeit, Sektion Psycholcgie und Sektion Elektronik, H v n n b o l d t - U n i v e r s i t zu B e r l i n (1984, unpubl. )
.
(6) Doklle, W.H. e t al., D a t a processing, LSI w i l l h e l p t o bring s i g h t to t h e blind, E l e c t r o n i c s Jan. 24 (1974).
(7) Fisher, D.L., Limited-channel Models of A u t a M t i c Detection Capacity and Scanning i n Visual Search, Psychological kw. 89 (1982) 662-692.
(8) G i e s e , B., V i s u e l l e Erkennungsleistung in Ati-Gngigkeit von der mmsntanen Grundfrequenz des Spontan-El%, Diplanarbeit, Sektion Psycholcgie, Humboldt-Universit zu B e r l i n (1983, unpubl.
.
(9) Kirchner, R., Alpha-= g e s t e u e r t e Relationserkennung, Diplanarkit, Sektion Psycholcgie, Hunbldt-Universitt Berlin (1983, unpubl.).
zu
(10) Spurrier, J . D . , Heett, J.E., M i d i , Z., Ccmparison of ’I’m E&gression L i ~ over s a Finite I n t e r v a l , B i a n e t r i c s 38 (1982) 827-836. (11) Wurtz, R.H.,
Goldberg, M.E. and Mbinsan, D.L., Brain Mechanisnus of Visual Attention, S c i e n t i f i c American, J u l y (1982).