Amplitude asymmetry of hemifield pattern reversal VEPs in healthy subjects

Electroencephalography and clinical Neurophysiology, 1990, 77:81-85

81

Elsevier Scientific Publishers Ireland, Ltd. EVOPOT 89063

Amplitude asymmetry of hemifield pattern reversal VEPs in healthy subjects Y. A b e * a n d Y. K u r o i w a

**

• Department of Neurology, lwate Medical University, 19-1, Morioka 020 (Japan), and * * Department of Neurology, Toranomon Hospital, 2-2-2, Toranomon, Minatoku, Tokyo 105 (Japan)

(Accepted for publication: 12 June 1989)

Summary The amplitudes of transient and steady-state visual evoked potentials (VEPs) were measured during hemifield stimulation of the left eye in 10 healthy adults. Pattern reversal of a checkerboard was produced at 4 stimulation frequencies: 1, 5, 10 and 15 Hz. The amplitudes of pattern VEPs were evaluated using the paired t test to determine significant differences between right and left hemifields. The transient VEP amplitudes from midoccipital, midparietal, ipsilateral occipital and contralateral occipital electrodes were significantly greater with right hemifield stimulation. The steady-state VEP amplitudes from the midoccipital electrode during 15 Hz stimulation were significantly greater with right hemifield stimulation. Our neurophysiological data may be compatible with neuroanatomical asymmetries Of the occipital lobes in humans. Key words: Visual evoked potential; Pattern reversal; Hemifield stimulation; Visual cortex

The a m p l i t u d e of the transient VEP d u r i n g hemifield p a t t e r n s t i m u l a t i o n has been claimed to differ between right a n d left s t i m u l a t i o n ( K u r o i w a et al. 1987). The p u r p o s e of our present study is to evaluate whether or n o t the same rule applies to steady-state VEPs. We a t t e m p t e d to answer the q u e s t i o n by m a k i n g q u a n t i t a t i v e m e a s u r e m e n t s of p a t t e r n V E P a m p l i t u d e s in healthy subjects, using 4 different s t i m u l a t i o n frequencies: 1, 5, 10 a n d 15 Hz.

Method and material Subjects

Studies were c o n d u c t e d o n 10 adult" v o l u n t e e r s (3 women, 7 men) who were neurologically a n d o p h t h a l m o l o g i c a l l y n o r m a l . The left side was arbitrarily selected as the s t i m u l a t e d eye. N i n e sub-

Correspondence to: Dr. Y. Kuroiwa, Department of Neurology, Toranomon Hospital, 2-2-2, Toranomon, Minatoku, Tokyo 105 (Japan).

jects were right-handed; one was left-handed. The subjects in the present p a p e r were u n r e l a t e d to those reported previously ( K u r o i w a et al. 1987).

Stimuli

Each subject was seated o n a chair 1 m from the TV screen o n which a c h e c k e r b o a r d p a t t e r n appeared. Hemifield s t i m u l a t i o n was p e r f o r m e d with a square field s u b t e n d i n g 1 4 x 14°; each square of the checkerboard s u b t e n d e d 38 m i n of arc at the subject's eye. The c o n t r a s t b e t w e e n the black a n d white checks was 85%. Hemifield lumin a n c e was 17.5 lux. The b a c k g r o u n d l u m i n a n c e was 15 lux. The subject was i n s t r u c t e d to look at a red dot, 5 m m in diameter, placed 1 ° laterally from the stimulated hemifield. M o n o c u l a r stimulation of the left eye was d o n e with a n a t u r a l pupil in all subjects. T r a n s i e n t VEPs were o b t a i n e d at a s t i m u l a t i o n frequency of one complete p a t t e r n reversal every 1000 msec. Steady-state VEPs were p r o d u c e d by p a t t e r n reversal at 3 frequencies: every 200 msec (5 Hz), every 100 msec (10 Hz) a n d every 67 msec (15 Hz). The order of stimulus p r e s e n t a t i o n be-

0168-5597/90/$03.50 © 1990 Elsevier Scientific Publishers Ireland, Ltd.

Y. ABE, Y. KUROIWA

82 tween right and left hemifields was selected at random in each stimulus condition.

Recording VEPs were recorded from silver-silver chloride electrodes applied to the scalp with collodion, with impedances below 5 kI2. A midoccipital (MO) and a midparietal (MP) electrode were placed 5 and 10 cm above the inion, respectively. Three electrodes called Oip~i(O i), Yipsi(T i) and Pipsi(Pi) were placed on the scalp ipsilateral to the hemifield stimulated. Conversely, 3 electrodes called Oco,tra(O~), Tcomra(Tc) and Pcoot,JPc) were placed contralateral to the hemifield stimulated. O i and Oc were placed 5 cm from the MO. T i and Tc were placed 10 cm from the MO. Pi and Pc were placed 5 cm from the MP. These 8 electrodes were referred to a common midfrontal reference electrode 12 cm above the nasion. The bandwidth of the preamplifiers ranged from 0.8 to 1500 Hz. Simultaneous averaging of the E E G in 8 channels was done; each run was the result of the summation of 200 responses. Our previous study showed high reproducibility of VEPs on repeated trials of the same eye, although the amplitudes varied considerably from one subject to another (Kuroiwa et al. 1987). The amplitude data in the present paper were obtained from a single trial for each stimulus condition. The averaging time was 716.8 msec for T-VEP and 1024 msec for S-VEP during 5 Hz stimulation and 512 msec during 10 and 15 Hz stimulation.

Quantitative analysis of VEP ampfitude The method of measuring amplitudes of T-VEP followed previous papers (Celesia and Daly 1977; Kuroiwa et al. 1987). The amplitude of T-VEP was defined as the distance in /~V between the peak of the first major negative wave (N1) and the peak of the first major positive wave (P1). The amplitude of S-VEP was defined as the mean value in ttV of the amplitude at the peak of the first 10 consecutive half waves measured peak to peak. The amplitudes of T-VEP and S-VEP were evaluated using the paired t test to determine if the values differed significantly between right and left hemifield stimulation.

Results

The distribution of potentials to transient pattern reversal stimulation showed that P1 waves had larger amplitude responses during right hemifield stimulation. Fig. 1 shows mean and standard deviation of T-VEP amplitude from each scalp electrode. The T-VEP amplitudes from MO, MP, O i and Oc were significantly greater with right hemifield stimulation than with left hemifield stimulation (MO, t = 3.03, df= 9, P < 0.02; MP, t--3.34, df=9, P < 0 . 0 1 ; O~, t = 3 . 0 6 , df=9, P < 0 . 0 2 ; Oc, t = 2 . 3 8 , dr=9, P < 0 . 0 5 ) . The TVEP amplitudes from T i, To, Pi and Pc were not different between right and left hemifield stimuli. Figs. 2, 3 and 4 show mean and standard deviation of S-VEP amplitudes. The S-VEP amplitudes after stimulation at 5 and 10 Hz revealed no significant difference between right and left hemifield stimulation (Figs. 2 and 3). We found that the S-VEP amplitude from MO during 15 Hz stimulation was significantly greater with right than with left hemifield stimulation (t = 4.10, df = 9, P < 0.01 (Fig. 4). The S-VEP amplitude to 15 Hz stimuli, recorded from MP, T i, T~ and Pi, tended to be greater with right hemifield stimula-

P a t t e r n reversal stimulation at 1Hz

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0 RH LH RH RH r,ght hem,fiel0 st,mubat,on '~" P<0.05

NS

0

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RH

not s,gn,ficant (palred t test )

Fig. 1. Mean and standard deviation of T-VEP amplitudes from 8 scalp electrodes. The amplitudes were compared by a paired t test between right hemifield VEP(RH) and left hemifield VEP(LH). Note that RH was significantly greater than LH, over MO, MP, Oipsi and O¢ontra.

H E M I F I E L D PATTERN REVERSAL VEP

83

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LH lett hemmf,eldst,mulatlon RH r,ght hemlf,eld stimulation NS not sk~;nlflcant (Pa,red t-test)

Fig. 2. Mean and standard deviation of S-VEP amplitudes at 5 Hz stimulation. The amplitudes were compared by a paired t test between right hemifield VEP(RH) and left hemifield VEP (LH). No significant difference was found.

Fig. 3. Mean and standard deviation of S-VEP amplitudes at 10 Hz stimulation. The amplitudes were compared by a paired t test between right hemifield VEP(RH) and left hemifield VEP(LH). No significant difference was found.

tion, although the difference was not significant (0.05 < P < 0.10). Table I shows the results of individual analysis on amplitude differences between right and left hemifield stimulation.

jects. They found that the T-VEP amplitudes from MO and O i were significantly greater with right than with left hemifield stimulation. Their results were c_on_sidered to be compatible with neuroanatomical asymmetries of the human striate cortex. Although the present investigations represent an extension of the previously reported work (Kuroiwa et al. 1987), the methods of our present paper were different from those in the former study in the following ways: (1) the size of the

Discussion Kuroiwa et al. (1987) measured T-VEP amplitudes after pattern reversal hemifield stimulation every 600 msec, in 14 healthy right-handed sub-

TABLE I Amplitude comparison between right hemifield VEP (RH) and left hemifield VEP (LH). RH > LH: number of eyes in which RH was greater than LH; RH = LH: number of eyes in which RH was equal to LH; RH < LH: number of eyes in which RH was smaller than LH. 1 Hz stimulation

MO MP Oipsi O~om~~ Tipsi Tcontra Pipsi Pcontra

5 Hz stimulation

RH > LH

RH = LH

RH < LH

RH > LH

RH = LH

10 7 9 8 7 7 6 8

0 0 0 0 0 0 1 0

0 3 1 2 3 3 3 2

5 4 6 6 5 5 3 4

1 0 0 0 1 1 0 0

10 Hz stimulation RH < LH 4 6 4 4 4 4 7 6

15 HZ stimulation

RH > LH

RH = LH

RH < LH

RH > LH

RH = LH

RH < LH

8 6 5 4 4 3 5 4

0 0 2 0 1 0 1 0

2 4 3 6 5 7 4 6

6 9 6 7 7 8 8 7

1 0 1 0 0 0 0 0

3 1 3 3 3 2 2 3

84

Y. ABE, Y. KUROIWA P a t t e r n reversal stimulation at 15Hz MO

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Fig. 4. Mean and standard deviation of S-VEP amplitudes at

15 Hz stimulation. The amplitudes were compared by a paired t test between right hemifield VEP(RH) and left hemifield VEP(LH). Note that RH was significantly greater than LH, over the MO electrode.

Kido 1978) also f o u n d a strong t e n d e n c y for the left striate cortex to extend further laterally on the occipital lobe t h a n does the right. W e i n b e r g e r et al. (1982) m e a s u r e d the v o l u m e of 40 serially sectioned whole b r a i n s a n d showed that the v o l u m e of the left occipital gray m a t t e r was significantly larger by a paired t test t h a n that of the right. Such n e u r o a n a t o m i c a l asymmetries m a y be compatible with our neurophysiological results, disclosing d o m i n a n c e of VEPs generated over the left occipital cortex after right hemifield stimulation. However, we c a n n o t exclude the possibility that our results were i n f l u e n c e d by i n t e r h e m i s p h e r i c transfer of visual i n f o r m a t i o n (Cusick et al. 1984, 1985; Spatz and K u n z 1984) a n d b y the effect of the m i d f r o n t a l electrode which m a y n o t be completely inactive.

References s t i m u l a t e d hemifield in the present study (14 ° ) was larger t h a n that in the former ( 9 ° ) ; (2) a variety of s t i m u l a t i o n frequencies was employed, i n c l u d i n g 1, 5, 10 a n d 15 Hz; (3) m u l t i - c h a n n e l recording from 8 scalp electrodes was performed, a d d i n g 3 parietal electrodes (MP, Pi a n d Pc) to the 5 in the ' Q u e e n Square System' ( B l u m h a r d t et al. 1977). A c c o r d i n g to o u r results, a paired t test revealed significant a m p l i t u d e difference between right a n d left hemifields, at the s t i m u l a t i o n frequencies of 1 a n d 15 Hz. O u r present p a p e r has c o n f i r m e d a greater a m p l i t u d e of right hemifield VEPs in b o t h T - V E P a n d S-VEP. As discussed extensively in the former report ( K u r o i w a et al. 1987), a larger mass of occipital lobe o n the left has been suggested (Inglessis 1925; Hadziselimovic a n d A n d e l i c 1963; Hadziselimovic a n d Cus 1966; G u n d a r a a n d Z i v a n o v i c 1968; M c R a e et al. 1968; Strauss a n d Fitz 1980). Stensaas et al. (1974) studied the topographic a n a t o m y of the p r i m a r y visual cortex in h u m a n s a n d d e m o n s t r a t e d that the left hemisphere had more striate cortex a n d more mesial exposed cortex in 15 a m o n g 23 cases. Eliot-Smith (1907), B r o d m a n n (1918) a n d subseq u e n t researchers ( L e M a y 1976, 1977; L e M a y a n d

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