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Power spectral analysis of flash and click evoked responses
287
Electroencephalography and Clinical Neurophysiology, 1973, 35:287-291 Elsevier Scientific Publishing Company, Amsterdam Printed in The Netherlands
POWER
SPECTRAL
RESPONSES
ANALYSIS
OF
FLASH
AND
CLICK
EVOKED
1
ALAN E. DAVIS Kins men Laboratory, Division o¢"Neurological Sciences, University oJ British Columbia, Vancouver 8, B. C. (Canada) (Accepted for publication : February 21, 1973)
Power spectral analysis is a method of examining the frequency composition of complex wave forms, independent of the component phase relationships. This phase removal allows simple comparisons of frequency distributions between different signals. Power spectral analysis has often been applied to the analysis of continuous EEG recordings (Walter 1963; Hord et al. 1965; Johnson et al. 1969; Wennberg and Zetterburg 1971), but there are many problems concerning selection of samples, sample length, and time varying statistical properties of the EEG. Much less work has been done on the pQwer spectral and Fourier analysis of evoked responses (Shipley et al. 1968; Emrich and Michael 1970). Evoked responses are in some ways ideal wave forms for this method since they have a precisely known starting point and are fairly stable and repeatable. However, short records such as evoked responses also have several disadvantages. The spectral estimates are somewhat inaccurate and the original data may not be closely gaussian or normal. These two problems can be overcome by pooling the results of several trials for each subject and by using non-parametric statistics. The purpose of this study was to investigate the properties and inter-relationships of various frequency components within the visual and auditory systems using power spectral analysis of flash and click evoked responses. This work was supported by Medical Research Council of Canada Grant MA-4781, and U.S. National Institute of Health Grant NSO-2812.
METHOD
Ten normal, right-handed human subjects, four female and six male, ages 19-29, were examined. The subjects sat upright in an 8 ft. x 8 ft. x 8 ft. soundproofed room, illuminated from above by a diffuse 40 W ceiling lamp. The flash stimulus (Grass PS1 photostimulator, 10 #sec duration flash, intensity 8) was presented through a 6 in. circular aperture 24 in. from the subject's nasion. The click stimulus (Grass $4C click stimulator, 0.1 msec duration, 4 V amplitude, comfortable listening level of approximately 60 dB) was presented from an overhead loudspeaker. Flash and click trigger pulses were randomly mixed (average inter-stimulus interval of 3 sec, minimum of 2 sec) on a previously prepared control tape which was used for all subjects. Four groups of 40 flash and 40 click randomly mixed were presented with a 1 rain rest between groups. The subjects were instructed to keep their eyes open and fixated on a small " × " in the middle of the flash aperture, to remain quiet, to avoid eye, tongue and jaw movements, and to remain passively aware of the stimuli. Monopolar recordings were made from left and right occipital areas (01 and 02) and left and right auditory areas (midway between T3 and C3, and midway between T4 and C4), using linked right and left ears as a reference, and Grass E5GH electrodes. The signals were amplified with Grass P511 amplifiers (frequency response 0.3-100 c/sec) and recorded on a Technical Measurement Corporation Model 1400 FM tape recorder. These were later played back and averaged in groups of 40 stimuli,
288
A.E. DAVIS
analysis time 1 sec, using a T M C - C A T 400B transient averager. The digital averaged evoked responses (100 samples/sec) were recorded on a Precision Instrument PI-1167 digital tape recorder for later computer analysis. The digitized evoked responses were spectrum analyzed according to the methods of Blackman and Tukey (1958), except that the fast Fourier transform was used for the spectral estimates. Power was expressed in j l V Z / c / s e c (Walter 1968), with a resolution of 1 c/sec.
BC
AD CLICK
CLICK I0 SEC
OI
01
~
~
1
.
0
SEC
02 FLASH A
U
D
I
FLASH
~
POWER SPECTRA
POWER SPECTRA
RESULTS AND ANALYSIS
1. The evoked response power spectra The power spectra of the flash and click evoked responses of two subjects are shown in Fig. 1. The responses in the primary projection area of the stimulated modality consisted of two major frequency components: group I, 0-5 c/sec; and group II, 6-12 c/sec. These two groups were present in the power spectra of all four areas in all ten subjects, and constituted at least 95 % of the total power in each response. BC
AD FLASH t.O SEC
OI ~
CLICK
02
01 . ~
1,0 SEC
CLICK AUDI
POWER SPECTRA
POWER SPECTRA
01 .
AUOI/ ~ _
5
.
~
AUDI ~ ' ~ .
AUD2.~ 0
~
IO 15 20 25
FREQUENCY(HZ)
AUD 2 0
5 ~0 15 20 25 FREQUENCY (HZ)
Fig. 1. Primary projection area responses and power spectra of two subjects--average of 160 stimuli. Note the two major frequency groups (I and II) in the power spectra. Power spectra are all scaled equally to show relationships between groups. Monopolar recordings, reference to linked right and left ears.
AUD I , ~ ~
0
.
S 10 15 20 25 FREQUENCY (HZ)
0
5 IO 15 20 2'5 FREQUENCY (RZ)
Fig. 2. Non-primary projection area responses and power spectra of same subjects as Fig. 1. Note the more variable group II as compared to the primary area responses of Fig. 1.
Fig. 2 shows the corresponding power spectra of evoked responses in areas non-primary to the stimulus modality, for example, the occipital response to click. Both group I and 1I frequencies are still present, but the group l l, 6 12 c/sec component is much more variable than in the primary areas and usually appears to represent a smaller proportion of the total evoked response power. In some subjects, the group 11 non-primary area response almost disappeared entirely. 2. Statistical analysis To quantify the observed differences in the frequency groups, the pooled results of all the subjects were analyzed. The power spectral estimates are approximately independent and additive (Blackman and Tukey 1959), therefore analysis at each frequency and summing across individuals is valid. The short records raised some question of the gaussian or normal nature of the data, therefore non-parametric statistics were used (Snedecor and Cochran 1967). For each of the four recording areas, four sets of averaged responses were available for
POWER SPECTRAL ANALYSIS OF EVOKED RESPONSES
289
each subject. Forty flash and forty click averaged responses for each area were therefore available for analysis. In order to minimize error due to inter-individual differences, each subject was used as his own control in the Wilcoxson nonparametric, paired sample test. It was used to determine if the median primary projection area response spectra were significantly different than the spectra of non-primary area responses to the same stimulus. The test was done at each frequency to determine exactly where changes were occurring. A two-tailed test of 40 pairs was used. Fig. 3 shows the median, paired differences and associated probabilities between the primary projection area response spectra and the response spectra of a non-primary area in the same hemisphere. Two major frequency groups of differences, in the same ranges as the individual response spectra are seen. The low frequency, 0 5 c/sec group I differences (median occipital minus auditory response) are always significantly negative ( P < 0.01) for both flash and click. This
shows that the auditory area group I response is always greater than the occipital area group I response for both flash and click. In contrast, the group II responses are significantly greater ( P < 0.01) in the primary projection area of the stimulus modality for all responses and areas, except the right hemisphere flash response (P > 0.05).
LEFT
HEMISPHERE
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;
O3
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3. The time wave Jorms of the .J?equemy groups Using the evoked responses of Fig. 1, the 0-5 and 6-12 c/sec frequency groups were digitally filtered and are shown in Fig. 4. Group I frequencies consist of a slow, biphasic wave which is very similar for both flash and click in the first 300 msec and corresponds to peaks IV and VII. Group II frequencies first appear at about 100 msec, are dissimilar for flash and click, and correspond to peaks IV VII. Peaks I, II, and III are missing from these wave forms since they are in the range of 12 20 c/sec. DISCUSSION
The power spectra of the flash and click evoked responses demonstrated two major frequency groups: group I, 0~5 c/sec and group II, 6-12 c/sec. The two frequency groups were remarkably constant between trials and between subjects.
~
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Fig. 3. Paired, median, occipital area response spectra minus auditory area response spectra, and associated probabilities for ten subjects (Wilcoxson matched pairs rank t e s t ~ pairs). Note the two major frequency groups of differences and probability levels. Group I responses are greater in the auditory areas for both flash and click, while the group II responses are greater in the primary projection area of the stimulus modality.
GROUPI [
6-12 HZ
01 - - ~
GROUR[I 6-t2 HZ
.....
A
U
0
2
~
~
Fig. 4. Digitally filtered group I, 0 , 5 c/sec and group I1, 6 12 c/sec components of the evoked responses of same subjects as Fig. 1.
290 This indicates that much of the observed intraand inter-individual variation of evoked responses could be a result of changing amplitude and phase relationships between two frequency components, which themselves are very similar and constant between individuals. The group II, 6-12 c/sec median response spectrum was significantly greater (P<0.01) in the primary projection area of the stimulus modality, except for the right hemisphere flash response. The group I, 0~5 c/sec response spectrum was significantly greater ( P < 0.01) in the auditory area than in the occipital area for both flash and click. This indicated that the group I responses retain a set pattern regardless of stimulus modality, while the group II response shifts and becomes greatest in the primary projection area of the stimulated modality. The generalized group i, 0-5 c/sec appearance, and maximum amplitude in the auditory areas with click agrees with the findings of Vaughn and Ritter (1970), who showed that long latency (100-300 msec) components of the auditory evoked response were strongest at the vertex and decreased towards the occiput. Gastaut et al. (1967) found that late components of the visual, auditory and somato-sensory evoked responses were strongest at the vertex and in most subjects decreased towards the occiput. Filtering of the frequency groups of the evoked responses showed that the group I, 0-5 c/sec frequencies consisted of a slow, biphasic wave with peak latencies at about 100 and 300 msec. The group II responses first appeared at about 100 msec, which is probably too late to be carrying primary, visual or auditory information. Peaks I, II, and III of the Cigfinek (1961) system, usually referred to as being stimulus and area specific (Perry and Childers 1969), were not present in either group I or II, since they are above 12 c/sec. The similar, 0-300 msec form of the group I responses for flash and click is striking, and is further support for the hypothesis that the low frequencies are non-specific to either flash or click. The non-significant group II differences in the right hemisphere flash response may be related to cerebral dominance. Most workers in this field have found that any response differences, such as CNV or evoked response
A.E. DAVIS amplitudes, have occurred on the left, dominant side. Morrell and Salamy (1971 ) found maximum evoked response component amplitude to natural speech stimuli in the left hemisphere. McAdam and Whittaker (t971) found slow, negative potentials generated when the subjects spoke and which were maximum on the left side. Buchsbaum and Fedio (1969, 1970) showed that occipital responses to tachistoscopic presentation of words and similarly patterned visual were more different on the left, dominant side than on the right. The physiological role of these frequency components is unanswered by these data. It is probable that the group t, 0-5 c/sec frequencies are related to activities that are not directly concerned with which sense modality is being stimulated. Subject variables such as attentive set (Donchin and Cohen 1967), and the CNV (Walter 1967), which are known to cause changes in the long latency components of the evoked response would fall into this category. Horizontal eye movements would not interfere because of the linked ear reference, and vertical eye movements would be minimized because of the subject's fixated vision. The group II, 6-12 c/sec component appears mainly in the primary projection area of the stimulated modality. It is therefore probably related in some way to the "message" being sent to the cortex from lower centers. Its long latency (100 msec) makes it unlikely that actual sensory information is carried, in fact it may represent a recovery of alpha activity after inhibition by the stimulus. It is also possible that sensory information could be carried in the phase relationships of higher frequency components ( > 12 c/sec) to the group II wave form. SUMMARY
Power spectral and digital filter analysis of flash and click evoked responses of ten, normal, right-handed subjects was carried out. Two major frequency groups were seen in the right and left auditory and occipital cortex responses of all subjects. G r o u p I, 0-5 c/sec, was of long latency, occurred simultaneously and in a similar form in both the visual and auditory areas for both flash and click, and was always greater ( P < 0.01) in the auditory areas. Group
POWER SPECTRAL ANALYSIS OF EVOKED RESPONSES
291
II, 6-12 c/sec, was of shorter latency, more variable than group I, and was greatest (P < 0.01) in the primary projection area of the stimulus modality. This occurred for right and left hemisphere click responses and left hemisphere flash responses, but not for right hemisphere flash responses. Some of the relationships of this data to past work on time domain analysis of evoked responses, CNV and hemispheric assymetry were discussed. Possible physiological roles were speculated upon.
power spectra. Dover Publications, New York, 1959,
RESUMt ANALYSE SPECTRALE DE FREQUENCES DES COMPOSANTES SPECIFIQUES ET NON SPECIFIQUES DES REPONSES EVOQUEES AUX ECLAIRS ET AUX CLICS
L'analyse spectrale de puissance et le filtrage digital des r6ponses 6voqu6es aux 6clairs et aux clics ont 6t6 r6alis6es chez 10 sujets normaux droitiers. Les fr6quences des r6ponses recueillies au niveau des cortex occipital et auditif droit et gauche de tous les sujets s'organisent suivant deux groupes principaux: le groupe I, de 0-5 c/sec, concerne des r6ponses de longue latence, qui surviennent en m~me temps et sous une forme semblable dans les aires visuelle et auditive, tant pour les 6clairs que pour les clics, mais avec une amplitude toujours plus grande ( P < 0 . 0 1 ) au niveau des aires auditives. Le groupe II, de 6-12 c/sec, concerne des r6ponses de latence plus courte, plus variables que celles du groupe I et qui atteignent leur maximum ( P < 0.01) darts l'aire de projection primaire de la modalit6 du stimulus. Ceci s'observe pour les r6ponses aux clics au niveau de l'h6misph6re droit et de l'h6misph6re gauche, et pour les r6ponses aux 6clairs au niveau de l'h6misph6re gauche mais pas au niveau de l'h6misph6re droit. Certaines relations entre ces donn6es et un travail ant6rieur sur l'analyse temporelle des r6ponses 6voqu6es, de la VCN et de l'asym6trie h6misph6rique sont discut6es. L'auteur 6met quelques hypoth6ses sur les m6canismes physiologiques possibles. I would tike to express my appreciation to Dr. Juhn A. Wada, without whose encouragement and support this work would not have been possible, and to Mr. Ed Jung and Miss Anne H a m m for their technical assistance. REFERENCES BI A('KMAN. R. B. and TUKEY, J. W. The measurement o /
190 pp. BUCHSBAUM, M. and FEDIO, P. Visual information and evoked responses from the left and right hemispheres. Electroenceph. olin. Neurophysio/., 1969, 2 6 : 2 6 6 272. BUCHSnAUM, M. and FEDIO, P. Hemispheric differences in evoked potentials to verbal and non-verbal stimuli in the left and right visual fields. Physiol. Behav.. 1970, 5:207 210. CIG..;,NEW, L. The EEG response (evoked potential) to light stimulus in man. Electroenceph. c/in. Neurophysiol., 1961, i3:165 172. DONClnN, E. and COHEN. L. Average evoked potentials and intra-modality selective attention. Eleetroenceph. olin. Neurophysiol., 1967, 22:537 546. EMRICtt, H. and MICHAEL, D. Fourier analysis of on- and off-effects of evoked responses. Electroenceph. din. Neuroph)'siol., 1970, 29 : 217. GASTAUT. H., RI~GIS, H., LYAGOUBI, S., MAN(), T. and S1MON, L. Comparison of the potentials recorded from the occipital, temporal and central regions of the h u m a n scalp, evoked by visual, auditory and sensor?. stimuli. Electroenceph. olin. Neurophysiol., 1967, 26. Suppl. 26: 19 28. HORD, D. J., JOHNSON, L. C., LUBIN, A. and AUSTIN, M. T. Resolution and stability in the autospectra of EEG. Electroenceph. olin. Neurophysiol., 1965, 19:305-308. JOHNSON, L. C., LUBIN, A., NAITOLI, P., NUTE, C. and AUSTIN. M. Spectral analysis of the EEG of dominant and nondominant alpha subjects during waking and sleeping. Electroenceph. clin. Neurophysiol., 1969, 26:361 370. MCADAM, D. W. and WttlTTAKER. H. A. Language production: electroencephalographic localization in the normal h u m a n brain. Science, 1971, 172:499 502. MORRELL,L. K. and SALAMY,J. G. Hemispheric assymetry of electrocortical responses to speech stimuli. Science, 1971, 174: 164-166. PERRY, N. W. and CH1LDERS, D. G. The human visual evoked response. Thomas, Springfield, llh, 1969, 187 pp. SHIPLEY, T., JOANS, R. W. and FRY, A. Spectral analysis of the visually evoked occipitogram in man. Vision Reds., 1968, 8 : 4 0 9 431. SNEDECOR,G. W. and COCHRAN, W. G. Statistical methods. Iowa State Univ. Press, Ames, Iowa, 1967, 593 pp. VAUGHN, H. G. and RITTER, W. The sources of auditory evoked responses recorded from the h u m a n scalp. Electroenceph. clin. Neurophysiol., 1970, 28:360 367. WALTER, D, O. Spectral analysis for electroencephalograms: Mathematical determination of neurophysiological relationships from records of limited duration. Exp. Neurol., 1963, 8: 155--181. WALTER, D. O. On units and dimensions for reporting spectral intensities. Electroenceph. clin. Neurophysiol., 1968, 24: 486 487. WALTER, W. G. Slow potential changes in the h u m a n brain associated with expectancy, decision and intention. Eleetroenceph. clin. Neurophysiol., 1967, 26, Suppl. 26: 123 130. WeNNBERG, A. and ZETTERBURG, L. H. Application of a computer-based model for EEG analysis. Electroenceph. clin. Neurophysiol., 1971, 31 : 457 468.
Electroencephalography and Clinical Neurophysiology, 1973, 35:287-291 Elsevier Scientific Publishing Company, Amsterdam Printed in The Netherlands
POWER
SPECTRAL
RESPONSES
ANALYSIS
OF
FLASH
AND
CLICK
EVOKED
1
ALAN E. DAVIS Kins men Laboratory, Division o¢"Neurological Sciences, University oJ British Columbia, Vancouver 8, B. C. (Canada) (Accepted for publication : February 21, 1973)
Power spectral analysis is a method of examining the frequency composition of complex wave forms, independent of the component phase relationships. This phase removal allows simple comparisons of frequency distributions between different signals. Power spectral analysis has often been applied to the analysis of continuous EEG recordings (Walter 1963; Hord et al. 1965; Johnson et al. 1969; Wennberg and Zetterburg 1971), but there are many problems concerning selection of samples, sample length, and time varying statistical properties of the EEG. Much less work has been done on the pQwer spectral and Fourier analysis of evoked responses (Shipley et al. 1968; Emrich and Michael 1970). Evoked responses are in some ways ideal wave forms for this method since they have a precisely known starting point and are fairly stable and repeatable. However, short records such as evoked responses also have several disadvantages. The spectral estimates are somewhat inaccurate and the original data may not be closely gaussian or normal. These two problems can be overcome by pooling the results of several trials for each subject and by using non-parametric statistics. The purpose of this study was to investigate the properties and inter-relationships of various frequency components within the visual and auditory systems using power spectral analysis of flash and click evoked responses. This work was supported by Medical Research Council of Canada Grant MA-4781, and U.S. National Institute of Health Grant NSO-2812.
METHOD
Ten normal, right-handed human subjects, four female and six male, ages 19-29, were examined. The subjects sat upright in an 8 ft. x 8 ft. x 8 ft. soundproofed room, illuminated from above by a diffuse 40 W ceiling lamp. The flash stimulus (Grass PS1 photostimulator, 10 #sec duration flash, intensity 8) was presented through a 6 in. circular aperture 24 in. from the subject's nasion. The click stimulus (Grass $4C click stimulator, 0.1 msec duration, 4 V amplitude, comfortable listening level of approximately 60 dB) was presented from an overhead loudspeaker. Flash and click trigger pulses were randomly mixed (average inter-stimulus interval of 3 sec, minimum of 2 sec) on a previously prepared control tape which was used for all subjects. Four groups of 40 flash and 40 click randomly mixed were presented with a 1 rain rest between groups. The subjects were instructed to keep their eyes open and fixated on a small " × " in the middle of the flash aperture, to remain quiet, to avoid eye, tongue and jaw movements, and to remain passively aware of the stimuli. Monopolar recordings were made from left and right occipital areas (01 and 02) and left and right auditory areas (midway between T3 and C3, and midway between T4 and C4), using linked right and left ears as a reference, and Grass E5GH electrodes. The signals were amplified with Grass P511 amplifiers (frequency response 0.3-100 c/sec) and recorded on a Technical Measurement Corporation Model 1400 FM tape recorder. These were later played back and averaged in groups of 40 stimuli,
288
A.E. DAVIS
analysis time 1 sec, using a T M C - C A T 400B transient averager. The digital averaged evoked responses (100 samples/sec) were recorded on a Precision Instrument PI-1167 digital tape recorder for later computer analysis. The digitized evoked responses were spectrum analyzed according to the methods of Blackman and Tukey (1958), except that the fast Fourier transform was used for the spectral estimates. Power was expressed in j l V Z / c / s e c (Walter 1968), with a resolution of 1 c/sec.
BC
AD CLICK
CLICK I0 SEC
OI
01
~
~
1
.
0
SEC
02 FLASH A
U
D
I
FLASH
~
POWER SPECTRA
POWER SPECTRA
RESULTS AND ANALYSIS
1. The evoked response power spectra The power spectra of the flash and click evoked responses of two subjects are shown in Fig. 1. The responses in the primary projection area of the stimulated modality consisted of two major frequency components: group I, 0-5 c/sec; and group II, 6-12 c/sec. These two groups were present in the power spectra of all four areas in all ten subjects, and constituted at least 95 % of the total power in each response. BC
AD FLASH t.O SEC
OI ~
CLICK
02
01 . ~
1,0 SEC
CLICK AUDI
POWER SPECTRA
POWER SPECTRA
01 .
AUOI/ ~ _
5
.
~
AUDI ~ ' ~ .
AUD2.~ 0
~
IO 15 20 25
FREQUENCY(HZ)
AUD 2 0
5 ~0 15 20 25 FREQUENCY (HZ)
Fig. 1. Primary projection area responses and power spectra of two subjects--average of 160 stimuli. Note the two major frequency groups (I and II) in the power spectra. Power spectra are all scaled equally to show relationships between groups. Monopolar recordings, reference to linked right and left ears.
AUD I , ~ ~
0
.
S 10 15 20 25 FREQUENCY (HZ)
0
5 IO 15 20 2'5 FREQUENCY (RZ)
Fig. 2. Non-primary projection area responses and power spectra of same subjects as Fig. 1. Note the more variable group II as compared to the primary area responses of Fig. 1.
Fig. 2 shows the corresponding power spectra of evoked responses in areas non-primary to the stimulus modality, for example, the occipital response to click. Both group I and 1I frequencies are still present, but the group l l, 6 12 c/sec component is much more variable than in the primary areas and usually appears to represent a smaller proportion of the total evoked response power. In some subjects, the group 11 non-primary area response almost disappeared entirely. 2. Statistical analysis To quantify the observed differences in the frequency groups, the pooled results of all the subjects were analyzed. The power spectral estimates are approximately independent and additive (Blackman and Tukey 1959), therefore analysis at each frequency and summing across individuals is valid. The short records raised some question of the gaussian or normal nature of the data, therefore non-parametric statistics were used (Snedecor and Cochran 1967). For each of the four recording areas, four sets of averaged responses were available for
POWER SPECTRAL ANALYSIS OF EVOKED RESPONSES
289
each subject. Forty flash and forty click averaged responses for each area were therefore available for analysis. In order to minimize error due to inter-individual differences, each subject was used as his own control in the Wilcoxson nonparametric, paired sample test. It was used to determine if the median primary projection area response spectra were significantly different than the spectra of non-primary area responses to the same stimulus. The test was done at each frequency to determine exactly where changes were occurring. A two-tailed test of 40 pairs was used. Fig. 3 shows the median, paired differences and associated probabilities between the primary projection area response spectra and the response spectra of a non-primary area in the same hemisphere. Two major frequency groups of differences, in the same ranges as the individual response spectra are seen. The low frequency, 0 5 c/sec group I differences (median occipital minus auditory response) are always significantly negative ( P < 0.01) for both flash and click. This
shows that the auditory area group I response is always greater than the occipital area group I response for both flash and click. In contrast, the group II responses are significantly greater ( P < 0.01) in the primary projection area of the stimulus modality for all responses and areas, except the right hemisphere flash response (P > 0.05).
LEFT
HEMISPHERE
2.O
FLASH (OI-AUDI)-PROB " r, k
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RIGHT
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.
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",
[o, ~_
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,
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02
;
O3
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~
3. The time wave Jorms of the .J?equemy groups Using the evoked responses of Fig. 1, the 0-5 and 6-12 c/sec frequency groups were digitally filtered and are shown in Fig. 4. Group I frequencies consist of a slow, biphasic wave which is very similar for both flash and click in the first 300 msec and corresponds to peaks IV and VII. Group II frequencies first appear at about 100 msec, are dissimilar for flash and click, and correspond to peaks IV VII. Peaks I, II, and III are missing from these wave forms since they are in the range of 12 20 c/sec. DISCUSSION
The power spectra of the flash and click evoked responses demonstrated two major frequency groups: group I, 0~5 c/sec and group II, 6-12 c/sec. The two frequency groups were remarkably constant between trials and between subjects.
~
04
iI FIO
AD
BC
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15
RED (HZ)
TOT~*L EVOKED RESPONSE
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I.O SEC
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GROUP I 0-5
0-5 HZ
HZ
oz~-',~
w
~ i
0" o
o
2p
- 2p
~
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Fig. 3. Paired, median, occipital area response spectra minus auditory area response spectra, and associated probabilities for ten subjects (Wilcoxson matched pairs rank t e s t ~ pairs). Note the two major frequency groups of differences and probability levels. Group I responses are greater in the auditory areas for both flash and click, while the group II responses are greater in the primary projection area of the stimulus modality.
GROUPI [
6-12 HZ
01 - - ~
GROUR[I 6-t2 HZ
.....
A
U
0
2
~
~
Fig. 4. Digitally filtered group I, 0 , 5 c/sec and group I1, 6 12 c/sec components of the evoked responses of same subjects as Fig. 1.
290 This indicates that much of the observed intraand inter-individual variation of evoked responses could be a result of changing amplitude and phase relationships between two frequency components, which themselves are very similar and constant between individuals. The group II, 6-12 c/sec median response spectrum was significantly greater (P<0.01) in the primary projection area of the stimulus modality, except for the right hemisphere flash response. The group I, 0~5 c/sec response spectrum was significantly greater ( P < 0.01) in the auditory area than in the occipital area for both flash and click. This indicated that the group I responses retain a set pattern regardless of stimulus modality, while the group II response shifts and becomes greatest in the primary projection area of the stimulated modality. The generalized group i, 0-5 c/sec appearance, and maximum amplitude in the auditory areas with click agrees with the findings of Vaughn and Ritter (1970), who showed that long latency (100-300 msec) components of the auditory evoked response were strongest at the vertex and decreased towards the occiput. Gastaut et al. (1967) found that late components of the visual, auditory and somato-sensory evoked responses were strongest at the vertex and in most subjects decreased towards the occiput. Filtering of the frequency groups of the evoked responses showed that the group I, 0-5 c/sec frequencies consisted of a slow, biphasic wave with peak latencies at about 100 and 300 msec. The group II responses first appeared at about 100 msec, which is probably too late to be carrying primary, visual or auditory information. Peaks I, II, and III of the Cigfinek (1961) system, usually referred to as being stimulus and area specific (Perry and Childers 1969), were not present in either group I or II, since they are above 12 c/sec. The similar, 0-300 msec form of the group I responses for flash and click is striking, and is further support for the hypothesis that the low frequencies are non-specific to either flash or click. The non-significant group II differences in the right hemisphere flash response may be related to cerebral dominance. Most workers in this field have found that any response differences, such as CNV or evoked response
A.E. DAVIS amplitudes, have occurred on the left, dominant side. Morrell and Salamy (1971 ) found maximum evoked response component amplitude to natural speech stimuli in the left hemisphere. McAdam and Whittaker (t971) found slow, negative potentials generated when the subjects spoke and which were maximum on the left side. Buchsbaum and Fedio (1969, 1970) showed that occipital responses to tachistoscopic presentation of words and similarly patterned visual were more different on the left, dominant side than on the right. The physiological role of these frequency components is unanswered by these data. It is probable that the group t, 0-5 c/sec frequencies are related to activities that are not directly concerned with which sense modality is being stimulated. Subject variables such as attentive set (Donchin and Cohen 1967), and the CNV (Walter 1967), which are known to cause changes in the long latency components of the evoked response would fall into this category. Horizontal eye movements would not interfere because of the linked ear reference, and vertical eye movements would be minimized because of the subject's fixated vision. The group II, 6-12 c/sec component appears mainly in the primary projection area of the stimulated modality. It is therefore probably related in some way to the "message" being sent to the cortex from lower centers. Its long latency (100 msec) makes it unlikely that actual sensory information is carried, in fact it may represent a recovery of alpha activity after inhibition by the stimulus. It is also possible that sensory information could be carried in the phase relationships of higher frequency components ( > 12 c/sec) to the group II wave form. SUMMARY
Power spectral and digital filter analysis of flash and click evoked responses of ten, normal, right-handed subjects was carried out. Two major frequency groups were seen in the right and left auditory and occipital cortex responses of all subjects. G r o u p I, 0-5 c/sec, was of long latency, occurred simultaneously and in a similar form in both the visual and auditory areas for both flash and click, and was always greater ( P < 0.01) in the auditory areas. Group
POWER SPECTRAL ANALYSIS OF EVOKED RESPONSES
291
II, 6-12 c/sec, was of shorter latency, more variable than group I, and was greatest (P < 0.01) in the primary projection area of the stimulus modality. This occurred for right and left hemisphere click responses and left hemisphere flash responses, but not for right hemisphere flash responses. Some of the relationships of this data to past work on time domain analysis of evoked responses, CNV and hemispheric assymetry were discussed. Possible physiological roles were speculated upon.
power spectra. Dover Publications, New York, 1959,
RESUMt ANALYSE SPECTRALE DE FREQUENCES DES COMPOSANTES SPECIFIQUES ET NON SPECIFIQUES DES REPONSES EVOQUEES AUX ECLAIRS ET AUX CLICS
L'analyse spectrale de puissance et le filtrage digital des r6ponses 6voqu6es aux 6clairs et aux clics ont 6t6 r6alis6es chez 10 sujets normaux droitiers. Les fr6quences des r6ponses recueillies au niveau des cortex occipital et auditif droit et gauche de tous les sujets s'organisent suivant deux groupes principaux: le groupe I, de 0-5 c/sec, concerne des r6ponses de longue latence, qui surviennent en m~me temps et sous une forme semblable dans les aires visuelle et auditive, tant pour les 6clairs que pour les clics, mais avec une amplitude toujours plus grande ( P < 0 . 0 1 ) au niveau des aires auditives. Le groupe II, de 6-12 c/sec, concerne des r6ponses de latence plus courte, plus variables que celles du groupe I et qui atteignent leur maximum ( P < 0.01) darts l'aire de projection primaire de la modalit6 du stimulus. Ceci s'observe pour les r6ponses aux clics au niveau de l'h6misph6re droit et de l'h6misph6re gauche, et pour les r6ponses aux 6clairs au niveau de l'h6misph6re gauche mais pas au niveau de l'h6misph6re droit. Certaines relations entre ces donn6es et un travail ant6rieur sur l'analyse temporelle des r6ponses 6voqu6es, de la VCN et de l'asym6trie h6misph6rique sont discut6es. L'auteur 6met quelques hypoth6ses sur les m6canismes physiologiques possibles. I would tike to express my appreciation to Dr. Juhn A. Wada, without whose encouragement and support this work would not have been possible, and to Mr. Ed Jung and Miss Anne H a m m for their technical assistance. REFERENCES BI A('KMAN. R. B. and TUKEY, J. W. The measurement o /
190 pp. BUCHSBAUM, M. and FEDIO, P. Visual information and evoked responses from the left and right hemispheres. Electroenceph. olin. Neurophysio/., 1969, 2 6 : 2 6 6 272. BUCHSnAUM, M. and FEDIO, P. Hemispheric differences in evoked potentials to verbal and non-verbal stimuli in the left and right visual fields. Physiol. Behav.. 1970, 5:207 210. CIG..;,NEW, L. The EEG response (evoked potential) to light stimulus in man. Electroenceph. c/in. Neurophysiol., 1961, i3:165 172. DONClnN, E. and COHEN. L. Average evoked potentials and intra-modality selective attention. Eleetroenceph. olin. Neurophysiol., 1967, 22:537 546. EMRICtt, H. and MICHAEL, D. Fourier analysis of on- and off-effects of evoked responses. Electroenceph. din. Neuroph)'siol., 1970, 29 : 217. GASTAUT. H., RI~GIS, H., LYAGOUBI, S., MAN(), T. and S1MON, L. Comparison of the potentials recorded from the occipital, temporal and central regions of the h u m a n scalp, evoked by visual, auditory and sensor?. stimuli. Electroenceph. olin. Neurophysiol., 1967, 26. Suppl. 26: 19 28. HORD, D. J., JOHNSON, L. C., LUBIN, A. and AUSTIN, M. T. Resolution and stability in the autospectra of EEG. Electroenceph. olin. Neurophysiol., 1965, 19:305-308. JOHNSON, L. C., LUBIN, A., NAITOLI, P., NUTE, C. and AUSTIN. M. Spectral analysis of the EEG of dominant and nondominant alpha subjects during waking and sleeping. Electroenceph. clin. Neurophysiol., 1969, 26:361 370. MCADAM, D. W. and WttlTTAKER. H. A. Language production: electroencephalographic localization in the normal h u m a n brain. Science, 1971, 172:499 502. MORRELL,L. K. and SALAMY,J. G. Hemispheric assymetry of electrocortical responses to speech stimuli. Science, 1971, 174: 164-166. PERRY, N. W. and CH1LDERS, D. G. The human visual evoked response. Thomas, Springfield, llh, 1969, 187 pp. SHIPLEY, T., JOANS, R. W. and FRY, A. Spectral analysis of the visually evoked occipitogram in man. Vision Reds., 1968, 8 : 4 0 9 431. SNEDECOR,G. W. and COCHRAN, W. G. Statistical methods. Iowa State Univ. Press, Ames, Iowa, 1967, 593 pp. VAUGHN, H. G. and RITTER, W. The sources of auditory evoked responses recorded from the h u m a n scalp. Electroenceph. clin. Neurophysiol., 1970, 28:360 367. WALTER, D, O. Spectral analysis for electroencephalograms: Mathematical determination of neurophysiological relationships from records of limited duration. Exp. Neurol., 1963, 8: 155--181. WALTER, D. O. On units and dimensions for reporting spectral intensities. Electroenceph. clin. Neurophysiol., 1968, 24: 486 487. WALTER, W. G. Slow potential changes in the h u m a n brain associated with expectancy, decision and intention. Eleetroenceph. clin. Neurophysiol., 1967, 26, Suppl. 26: 123 130. WeNNBERG, A. and ZETTERBURG, L. H. Application of a computer-based model for EEG analysis. Electroenceph. clin. Neurophysiol., 1971, 31 : 457 468.