- Email: [email protected]
Effects of selective attention on the late components of evoked potentials in man
Electroencephalography and Clinical Neurophysiology, 1977, 4 2 : 7 0 2 - - 7 0 4
702
© E l s e v i e r / N o r t h - H o l l a n d Scientific Publishers L t d Laboratory EFFECTS EVOKED
note OF SELECTIVE ATTENTION POTENTIALS IN MAN
ON THE
LATE
COMPONENTS
OF
C. D E M A I R E and J.-M. C O Q U E R Y
Laboratoire de Psychophysiologie, Universite de Lille I, B.P. 36, 59650 Villeneuve d'Ascq (France) ( A c c e p t e d for p u b l i c a t i o n : J u l y 15, 1 9 7 6 )
W h e n e v e r a s t i m u l u s requires p a r t i c u l a r processing, it is followed b y a l t e r a t i o n s in t h e late c o m p o n e n t s of the cortical e v o k e d p o t e n t i a l (CEP). Stimuli to w h i c h s u b j e c t s have to a t t e n d or t o react, even t h o s e w h i c h are e x p e c t e d t o o c c u r b u t d o not, i n d u c e or e n h a n c e a late positive wave f o r m in t h e CEP, t h e P3 t h e P300 ( d o n c h i n a n d C o h e n 1 9 6 7 ; Karlin et al. 1 9 7 0 ; P i c t o n a n d Hillyard 1 9 7 4 ; R u c h k i n et al. 1 9 7 5 ) . On t h e o t h e r h a n d , stimuli of n o relevance t o a n y o n g o i n g m e n t a l or m o t o r activity b u t w h i c h are rare, new or u n p r e d i c t a b l e also elicit similar late positive waves ( T u e t i n g et al. 1 9 7 0 ; R o t h 1973). O t h e r m o d i f i c a t i o n s o f t h e late c o m p o n e n t s o f t h e CEP have b e e n r e p o r t e d , n a m e l y , negative waves (N2) with a p e a k b e t w e e n 1 9 0 and 280 msec in r e s p o n s e to r e l e v a n t or irrelevant s t i m u l i f r o m a r e l e v a n t m o d a l i t y ( F o r d et al. 1 9 7 3 ) or to novel stimuli ( C o u r e h e s n e et al. 1975). T h e p r e s e n t s t u d y s h o w s t h a t in t h e a b s e n c e of a n y task t o b e p e r f o r m e d b y t h e s u b j e c t s a f t e r t h e o c c u r r e n c e o f stimuli, a N2--P3 c o m p l e x is i n d e e d t h e m o s t c o n s p i c u o u s c o m p o n e n t o f t h e CEP to be differentially a f f e c t e d b y stimuli t o e x p e c t e d a n d u n e x p e c t e d modalities.
Subjects and
procedure
Six s t u d e n t were paid for serving as subjects. T h e e x p e r i m e n t was c o n d u c t e d in a quiet, d i m l y lit r o o m . T h e r e c o r d i n g session was p r e c e d e d b y o n e or t w o t r a i n i n g sessions; it was divided i n t o 4 blocks, each a b o u t 30 m i n long, w i t h 5 rain of rest in b e t w e e n . T h e o n l y task t h e s u b j e c t h a d t o p e r f o r m was t o select, b y pressing e i t h e r o n e of t w o pedals, t h e s t i m u l u s h e w o u l d be given n e x t , e i t h e r a mild electric s h o c k or a flash. He was i n s t r u c t e d t o avoid any alternate or regular s e q u e n c e of stimuli b u t to deliver a b o u t t h e same n u m b e r of s h o c k s a n d flashes over t h e w h o l e e x p e r i m e n t a l session. He was w a r n e d a b o u t possible " e r r o r s " in t h e delivery o f stimuli due t o " u n r e l i a b i l i t y " of t h e p r o g r a m m i n g device, a n d told
to ignore t h e m . Finally, he was told to relax a n d to keep his eyes closed. Stimuli were paced b y the subject h i m s e l f ( o n e every 6 sec o n t h e average); t h e y were delivered 2 sec a f t e r t h e s u b j e c t h a d pressed o n e i t h e r pedal. Nine stimuli o u t of 10 were given in a c c o r d a n c e with t h e s u b j e c t ' s choice; 1 o u t of 10 did n o t m a t c h t h e subject's s e l e c t i o n t h e so-called " e r r o r " . T h e s e q u e n c e of e x p e c t e d a n d u n e x p e c t e d stimuli was arranged in a r a n d o m order. A t the e n d of t h e e x p e r i m e n t , the subject had t h u s received a b o u t 500 s h o c k s a n d as m a n y flashes; in each m o d a l i t y one t e n t h of t h e stimuli were given w h e n stimuli f r o m t h e o t h e r m o d a l i t y h a d asked for b y t h e subject.
Stimuli Electrical stimuli, consisting of trains of 3 pulses w i t h a f r e q u e n c y of 500/sec, were applied to t h e tip of t h e forefinger. Stimuli were delivered b y a stimul a t o r t h r o u g h a s t i m u l u s isolation u n i t a n d a c o n s t a n t c u r r e n t unit. S t i m u l u s i n t e n s i t y was set at 2.5 × t h r e s h o l d for p e r c e p t i o n , as d e t e r m i n e d at t h e beginning o f t h e e x p e r i m e n t . Flashes of 1 0 0 psec d u r a t i o n were p r o v i d e d b y a silent s t r o b o s c o p e 80 cm in f r o n t of the s u b j e c t ' s eyes. No a t t e m p t s were m a d e to c o n t r o l pupil size b u t t h e s u b j e c t s h a d to k e e p t h e i r eyes closed.
Recording and data analysis T h e e v o k e d p o t e n t i a l s were r e c o r d e d b e t w e e n each of 3 active e l e c t r o d e s a n d a r e f e r e n c e e l e c t r o d e placed o n t h e bridge of t h e nose. T h e first active e l e c t r o d e was o n t h e v e r t e x ( p o s i t i o n C z of t h e 1 0 / 2 0 system), t h e s e c o n d over t h e occipital c o r t e x ( p o s i t i o n O1), t h e t h i r d over t h e s o m a t i c s e n s o r y area, 2 c m b e h i n d a line j o i n i n g t h e a u d i t o r y m e a t u s a n d 7 cm b e l o w the sagittal midline. Eye m o v e m e n t s were m o n i t o r e d b y t w o disc electrodes placed b e l o w t h e i n n e r c a n t h u s and a b o v e t h e o u t e r c a n t h u s of the eyes. E O G S a n d CEPS were r e c o r d e d w i t h a b a n d p a s s
C. D E M A I R E , J.-M. C O Q U E R Y 0 . 3 - - 1 0 0 0 c/sec a n d s t o r e d o n FM m a g n e t i c tape ( b a n d p a s s 0 - - 1 2 5 0 c/sec). All evoked p o t e n t i a l s were averaged w i t h a m u l t i c h a n n e l analyser over a 8 0 0 msec e p o c h . Trials c o n t a m i n a t e d b y eye m o v e m e n t s were excluded. In b o t h modalities, all CEPs elicited b y u n e x p e c t e d stimuli were averaged, as were CEPs elicited b y t w o groups, each c o n t a i n i n g a n equal number of e x p e c t e d stimuli, n a m e l y t h o s e i m m e d i a t e l y p r e c e d i n g a n d t h o s e i m m e d i a t e l y following a n u n e x p e c t e d s t i m u l u s in t h e same m o d a l i t y .
703
CUTANEOUS
n =40
Ebetore
N~
Results
A. Vertex evoked potentials F o r all subjects, w h e n e v e r CEPs were elicited b y u n e x p e c t e d stimuli, t h e P2 wave was c u t s h o r t b y a negative wave N 2 ( m e a n peak l a t e n c y , 255 msec; range: 2 1 5 - - 2 9 5 msec) (Fig. 1). CEPs elicited b y e x p e c t e d s t i m u l i also s h o w e d small N2 waves in 5 o u t of 6 subjects. Paired c o m p a r i s o n s o f N2 a m p l i t u d e ( m e a s u r e d f r o m P2 t r o u g h t o N2 p e a k ) for e x p e c t e d a n d u n e x p e c t e d stimuli s h o w e d t h a t N2 was signific a n t l y greater for u n e x p e c t e d stimuli of b o t h modalities (p < 0.05, two-tailed sign test). W h e n N2 was small it o n l y briefly i n t e r r u p t e d P2, w h i c h t h e n r e t u r n e d t o baseline. As N2 grew larger, its positive-going e n d - l i m b c o u l d merge w i t h a g e n u i n e P3 wave so t h a t it was difficult to decide if t h e positivity w h i c h f o l l o w e d t h e N2 wave was t o b e c o n s i d e r e d as a P3 wave or not. In 5 o u t of 6 subjects, CEPs t o e x p e c t e d s t i m u l i p r e s e n t e d small P3 waves which, in 3 subjects, c o u l d b e d i s t i n g u i s h e d f r o m t h e end o f N2 (see for instance, in Fig. 1, t h e small positive n o t c h with a l a t e n c y o f a b o u t 3 0 0 msec, in the visual CEP to e x p e c t e d stimuli p r e c e d i n g the u n e x p e c t e d flahes: E before, visual). CEP t o u n e x p e c t e d stimuli always s h o w e d a large P3 c o m p o n e n t which, at least in 1 s u b j e c t , was d i s t i n c t f r o m the e n d - l i m b of N2. T h e a m p l i t u d e of the negative-going l i m b of N1 (P1--N1) was n o t significantly d i f f e r e n t w h e t h e r t h e s t i m u l u s was e x p e c t e d or not.
B. Specific evoked potentials T h e late c o m p o n e n t s o f t h e CEP r e c o r d e d over t h e specific s o m a t i c and visual areas s h o w e d t h e same m o d i f i c a t i o n s in r e s p o n s e t o e x p e c t e d and u n e x p e c t e d stimuli as t h o s e of t h e v e r t e x CEP. However, t h e d i f f e r e n c e s in N2 ( a n d P3) a m p l i t u d e b e t w e e n e x p e c t e d a n d u n e x p e c t e d stimuli were less t h a n in v e r t e x CEP. In t h e 9 averaged specific evoked p o t e n t i a l s , 5 visual a n d 4 c u t a n e o u s ( b e c a u s e o f a r t e f a c t s 3 records were useless), t h e N1 wave was n e v e r h i g h e r w h e n elicited b y a n e x p e c t e d s t i m u l u s : in 2 cases t h e ampli-
E'after
L
i
i
0
Q
0
800 ms VISUAL
n =39
Ebefor e
U,
Eafter
0
800 ms
Fig. 1. A v e r a g e d v e r t e x e v o k e d r e s p o n s e to e x p e c t e d a n d u n e x p e c t e d stimuli. T o p : averaged r e e p o n s e to 4 0 electrical c u l t a n e o u s shocks. B o t t o m : averaged resp o n s e to 39 flashes. E: r e s p o n s e s to e x p e c t e d stimuli i m m e d i a t e l y p r e c e d i n g (E b e f o r e ) or f o l l o w i n g (E after) u n e x p e c t e d stimuli w i t h i n t h e same m o d a l i t y . U: r e s p o n s e s to u n e x p e c t e d s t i m u l i ; n o t e the p r e s e n c e of N2 a n d P3 waves.
704 t u d e was the same for e x p e c t e d and u n e x p e c t e d stimuli, in the r e m a i n i n g 7 cases Nj was higher in response to u n e x p e c t e d stimuli.
Discussion In s u m m a r y , w h e n a s t i m u l u s does n o t m a t c h the e x p e c t a t i o n of t h e subject, it elicits: (1) large N2 a n d P3 waves in t h e vertex e v o k e d p o t e n t i a l : (2) possibly some increase of t h e N 1 wave in evoked p o t e n t i a l s r e c o r d e d over t h e specific s e n s o r y areas. Several features of t h e p r o c e d u r e used in t h e p r e s e n t experim e n t m a k e it reasonable to assume t h a t t h e i n d e p e n d e n t variable is i n d e e d the specific e x p e c t a t i o n set up b y t h e s u b j e c t ' s o w n choice. It can be safely e x c l u d e d t h a t the observed results are c o n t a m i n a t e d or caused b y t h e m o t o r activity involved in the s t i m u l u s select i o n : pressing o n t h e pedal t o o k place 2 sec b e f o r e t h e s t i m u l u s delivery and b o t h e x p e c t e d and u n e x p p e c t e d stimuli were p r e c e d e d b y such a m o v e m e n t . As n o r e s p o n s e was r e q u i r e d u p o n the s t i m u l u s occurrence, n e i t h e r c a n t h e d i f f e r e n c e s be e x p l a i n e d b y p r e p a r a t i o n for a d i f f e r e n t i a l overt m o t o r response. A n y possible effect of a c o n t i n g e n t negative v a r i a t i o n (CNV), set up b y t h e s u b j e c t ' s o w n c h o i c e of t h e stimulus, can also b e ruled out. A l t h o u g h it is likely t h a t a CNV develops during each trial, its effects are b a l a n c e d because the s u b j e c t c a n n o t p r e d i c t if the n e x t s t i m u l u s will m a t c h his c h o i c e or not. In these c o n d i t i o n s , o n l y r a n d o m variations in t h e CHVs will be reflected in t h e evoked p o t e n t i a l to e x p e c t e d or u n e x p e c t e d stimuli. O n t h e whole, it seems legitimate to c o n s i d e r t h e differential effects of e x p e c t e d a n d u n e x p e c t e d stimuli as an o u t c o m e o f selective a t t e n t i o n to a given s e n s o r y m o d a l i t y .
Summary V e r t e x a n d specific evoked p o t e n t i a l s t o visual a n d c u t a n e o u s stimuli were r e c o r d e d in 6 subjects. T h e delivery o f r o u g h l y equal n u m b e r s of flashes a n d s h o c k s was paced b y t h e s u b j e c t s themselves. T h e y also had to c h o o s e w h i c h s t i m u l u s t h e y w o u l d be given n e x t ; 9 t i m e s o u t of 10 t h e s t i m u l u s m a t c h e d t h e s u b j e c t ' s e x p e c t a t i o n a n d stimuli f r o m t h e unexp e c t e d m o d a l i t y were given r a n d o m l y 1 t i m e o u t of 10 o n t h e average. No task was r e q u i r e d a f t e r the stimulus occurrence. Cortical evoked p o t e n t i a l s (CEPs) to u n e x p e c t e d stimuli were averaged a n d c o m p a r e d with CEPs t h e m a j o r d i f f e r e n c e consisted in large N2 a n d P3 waves in response to u n e x p e c t e d s t i m u l i ; n o d i f f e r e n c e was o b served in t h e a m p l i t u d e of the Nl wave. In specific CEPs u n e x p e c t e d stimuli also elicited N 2 a n d P3 waves; in a d d i t i o n NI s h o w e d a slight t e n d e n c y to be higher in CEPs t o u n e x p e c t e d stimuli.
SELECTIVE ATTENTION AND EVOKED POTENTIALS R~sum~
Effets de l'attention sdlective sur les composantes tardives du potenliel dvoqued chez l'homrne Les r ~ p o n s e s ~voqu~es sur le c o r t e x p a r des s t i m u l u s visuels et c u t a n ~ s o n t 6t~ enregistr~es chez 6 sujets. Les sujets c o n t r S l a i e n t la c a d e n c e de distribut i o n des ~clairs e t des c h o c s et choisissaient la m o d a l i t ~ d a n s laquelle c h a q u e s t i m u l u s devait ~tre d o n n ~ : 9 lois sur 10 les s t i m u l u s dtaient c o n f o r m e s leur a t t e n t e et 1 fois sur 10 en m o y e n n e , selon u n o r d r e au hasard le s t i m u l u s ne c o r r e s p o n d a i t pas ~ la m o d a l i t d d e m a n d ~ e . Les sujets n ' a v a i e n t pas de t ~ c h e a e x ~ c u t e r e n r~ponse aux stimulus. Les p o t e n t i e l s ~voqu6s c o r t i c a u s (PEC) d(~clench~s p a r les s t i m u l u s i n a t t e n d u s o n t dt~ m o y e n n ~ s et compares a u x PEC m o y e n s d ' u n m ~ m e n o m b r e de s t i m u l u s a t t e n d u s . Au niveau d u vertex la diff6rence principale c o n s i s t e en I ' a p p a r i t i o n de g r a n d e s o n d e s N 2 et P3 dans ]es PEC d~clench~s par les s t i m u l u s i n a t t e n d u s ; l ' o n d e Nl ne p r ~ s e n t e pas de v a r i a t i o n significative d ' a m p l i t u d e . Les PEC sp~cifiques pr6s e n t e n t dgalement des o n d e s N2 et P3 ainsi q u ' u n e 14g~re a u g m e n t a t i o n d ' a m p l i t u d e de l ' o n d e Nl, e n r~sponse a u x s t i m u l u s i n a t t e n d u s .
References
Courschesne, E., Hillyard, S.A. and Galambos, R. Stimulus novelty, task relevance and the visual evoked potential in man. Electroenceph. clin. Neurophysiol., 1975, 39: 131--143. Donkin, E. and Cohen, L. Averaged evoked potentials and intramodality selective attention. Electroenceph, clin. Neurophysiol., 1967, 22: 537--546. Ford, J.M., Roth, W.T., Dirks, S.J. and KopeII, B.S. Evoked potential correlates of signal recognition between and within modalities. Science, 1973, 181: 4 6 5 - - 4 6 6 . Karlin, L., Martz, M.J. and M o r d k o f f , A.M. M o t o r p e r f o r m a n c e and s e n s o r y evoked potentials. Elect r o e n c e p h , clin. N e u r o p h y s i o l . , 1 9 7 0 , 28: 3 0 7 - 313. Picton, T.W. a n d Hillyard, S.A. H u m a n a u d i t o r y
evoked potentials. If: Effects of attention. Elctroenceph, clin.Neurophysiol., 1974, 36: 191--199. Roth, W.T. Auditory evoked responses to unpredictable stimuli. Psychophysiology, 1973, 10: 125--139. Ruehkin, D.S., Sutton, S. and Tueting, P. Emitted and evoked P300 potentials and variation in stimulus probability. Psychophysiology, 1975, 12: 591-595. Tueting, P., Sutton, S. and Zubin, J. Quantitative evoked potential correlates of the probability of events. Psyehophysiology, 1970, 7: 385--394.
702
© E l s e v i e r / N o r t h - H o l l a n d Scientific Publishers L t d Laboratory EFFECTS EVOKED
note OF SELECTIVE ATTENTION POTENTIALS IN MAN
ON THE
LATE
COMPONENTS
OF
C. D E M A I R E and J.-M. C O Q U E R Y
Laboratoire de Psychophysiologie, Universite de Lille I, B.P. 36, 59650 Villeneuve d'Ascq (France) ( A c c e p t e d for p u b l i c a t i o n : J u l y 15, 1 9 7 6 )
W h e n e v e r a s t i m u l u s requires p a r t i c u l a r processing, it is followed b y a l t e r a t i o n s in t h e late c o m p o n e n t s of the cortical e v o k e d p o t e n t i a l (CEP). Stimuli to w h i c h s u b j e c t s have to a t t e n d or t o react, even t h o s e w h i c h are e x p e c t e d t o o c c u r b u t d o not, i n d u c e or e n h a n c e a late positive wave f o r m in t h e CEP, t h e P3 t h e P300 ( d o n c h i n a n d C o h e n 1 9 6 7 ; Karlin et al. 1 9 7 0 ; P i c t o n a n d Hillyard 1 9 7 4 ; R u c h k i n et al. 1 9 7 5 ) . On t h e o t h e r h a n d , stimuli of n o relevance t o a n y o n g o i n g m e n t a l or m o t o r activity b u t w h i c h are rare, new or u n p r e d i c t a b l e also elicit similar late positive waves ( T u e t i n g et al. 1 9 7 0 ; R o t h 1973). O t h e r m o d i f i c a t i o n s o f t h e late c o m p o n e n t s o f t h e CEP have b e e n r e p o r t e d , n a m e l y , negative waves (N2) with a p e a k b e t w e e n 1 9 0 and 280 msec in r e s p o n s e to r e l e v a n t or irrelevant s t i m u l i f r o m a r e l e v a n t m o d a l i t y ( F o r d et al. 1 9 7 3 ) or to novel stimuli ( C o u r e h e s n e et al. 1975). T h e p r e s e n t s t u d y s h o w s t h a t in t h e a b s e n c e of a n y task t o b e p e r f o r m e d b y t h e s u b j e c t s a f t e r t h e o c c u r r e n c e o f stimuli, a N2--P3 c o m p l e x is i n d e e d t h e m o s t c o n s p i c u o u s c o m p o n e n t o f t h e CEP to be differentially a f f e c t e d b y stimuli t o e x p e c t e d a n d u n e x p e c t e d modalities.
Subjects and
procedure
Six s t u d e n t were paid for serving as subjects. T h e e x p e r i m e n t was c o n d u c t e d in a quiet, d i m l y lit r o o m . T h e r e c o r d i n g session was p r e c e d e d b y o n e or t w o t r a i n i n g sessions; it was divided i n t o 4 blocks, each a b o u t 30 m i n long, w i t h 5 rain of rest in b e t w e e n . T h e o n l y task t h e s u b j e c t h a d t o p e r f o r m was t o select, b y pressing e i t h e r o n e of t w o pedals, t h e s t i m u l u s h e w o u l d be given n e x t , e i t h e r a mild electric s h o c k or a flash. He was i n s t r u c t e d t o avoid any alternate or regular s e q u e n c e of stimuli b u t to deliver a b o u t t h e same n u m b e r of s h o c k s a n d flashes over t h e w h o l e e x p e r i m e n t a l session. He was w a r n e d a b o u t possible " e r r o r s " in t h e delivery o f stimuli due t o " u n r e l i a b i l i t y " of t h e p r o g r a m m i n g device, a n d told
to ignore t h e m . Finally, he was told to relax a n d to keep his eyes closed. Stimuli were paced b y the subject h i m s e l f ( o n e every 6 sec o n t h e average); t h e y were delivered 2 sec a f t e r t h e s u b j e c t h a d pressed o n e i t h e r pedal. Nine stimuli o u t of 10 were given in a c c o r d a n c e with t h e s u b j e c t ' s choice; 1 o u t of 10 did n o t m a t c h t h e subject's s e l e c t i o n t h e so-called " e r r o r " . T h e s e q u e n c e of e x p e c t e d a n d u n e x p e c t e d stimuli was arranged in a r a n d o m order. A t the e n d of t h e e x p e r i m e n t , the subject had t h u s received a b o u t 500 s h o c k s a n d as m a n y flashes; in each m o d a l i t y one t e n t h of t h e stimuli were given w h e n stimuli f r o m t h e o t h e r m o d a l i t y h a d asked for b y t h e subject.
Stimuli Electrical stimuli, consisting of trains of 3 pulses w i t h a f r e q u e n c y of 500/sec, were applied to t h e tip of t h e forefinger. Stimuli were delivered b y a stimul a t o r t h r o u g h a s t i m u l u s isolation u n i t a n d a c o n s t a n t c u r r e n t unit. S t i m u l u s i n t e n s i t y was set at 2.5 × t h r e s h o l d for p e r c e p t i o n , as d e t e r m i n e d at t h e beginning o f t h e e x p e r i m e n t . Flashes of 1 0 0 psec d u r a t i o n were p r o v i d e d b y a silent s t r o b o s c o p e 80 cm in f r o n t of the s u b j e c t ' s eyes. No a t t e m p t s were m a d e to c o n t r o l pupil size b u t t h e s u b j e c t s h a d to k e e p t h e i r eyes closed.
Recording and data analysis T h e e v o k e d p o t e n t i a l s were r e c o r d e d b e t w e e n each of 3 active e l e c t r o d e s a n d a r e f e r e n c e e l e c t r o d e placed o n t h e bridge of t h e nose. T h e first active e l e c t r o d e was o n t h e v e r t e x ( p o s i t i o n C z of t h e 1 0 / 2 0 system), t h e s e c o n d over t h e occipital c o r t e x ( p o s i t i o n O1), t h e t h i r d over t h e s o m a t i c s e n s o r y area, 2 c m b e h i n d a line j o i n i n g t h e a u d i t o r y m e a t u s a n d 7 cm b e l o w the sagittal midline. Eye m o v e m e n t s were m o n i t o r e d b y t w o disc electrodes placed b e l o w t h e i n n e r c a n t h u s and a b o v e t h e o u t e r c a n t h u s of the eyes. E O G S a n d CEPS were r e c o r d e d w i t h a b a n d p a s s
C. D E M A I R E , J.-M. C O Q U E R Y 0 . 3 - - 1 0 0 0 c/sec a n d s t o r e d o n FM m a g n e t i c tape ( b a n d p a s s 0 - - 1 2 5 0 c/sec). All evoked p o t e n t i a l s were averaged w i t h a m u l t i c h a n n e l analyser over a 8 0 0 msec e p o c h . Trials c o n t a m i n a t e d b y eye m o v e m e n t s were excluded. In b o t h modalities, all CEPs elicited b y u n e x p e c t e d stimuli were averaged, as were CEPs elicited b y t w o groups, each c o n t a i n i n g a n equal number of e x p e c t e d stimuli, n a m e l y t h o s e i m m e d i a t e l y p r e c e d i n g a n d t h o s e i m m e d i a t e l y following a n u n e x p e c t e d s t i m u l u s in t h e same m o d a l i t y .
703
CUTANEOUS
n =40
Ebetore
N~
Results
A. Vertex evoked potentials F o r all subjects, w h e n e v e r CEPs were elicited b y u n e x p e c t e d stimuli, t h e P2 wave was c u t s h o r t b y a negative wave N 2 ( m e a n peak l a t e n c y , 255 msec; range: 2 1 5 - - 2 9 5 msec) (Fig. 1). CEPs elicited b y e x p e c t e d s t i m u l i also s h o w e d small N2 waves in 5 o u t of 6 subjects. Paired c o m p a r i s o n s o f N2 a m p l i t u d e ( m e a s u r e d f r o m P2 t r o u g h t o N2 p e a k ) for e x p e c t e d a n d u n e x p e c t e d stimuli s h o w e d t h a t N2 was signific a n t l y greater for u n e x p e c t e d stimuli of b o t h modalities (p < 0.05, two-tailed sign test). W h e n N2 was small it o n l y briefly i n t e r r u p t e d P2, w h i c h t h e n r e t u r n e d t o baseline. As N2 grew larger, its positive-going e n d - l i m b c o u l d merge w i t h a g e n u i n e P3 wave so t h a t it was difficult to decide if t h e positivity w h i c h f o l l o w e d t h e N2 wave was t o b e c o n s i d e r e d as a P3 wave or not. In 5 o u t of 6 subjects, CEPs t o e x p e c t e d s t i m u l i p r e s e n t e d small P3 waves which, in 3 subjects, c o u l d b e d i s t i n g u i s h e d f r o m t h e end o f N2 (see for instance, in Fig. 1, t h e small positive n o t c h with a l a t e n c y o f a b o u t 3 0 0 msec, in the visual CEP to e x p e c t e d stimuli p r e c e d i n g the u n e x p e c t e d flahes: E before, visual). CEP t o u n e x p e c t e d stimuli always s h o w e d a large P3 c o m p o n e n t which, at least in 1 s u b j e c t , was d i s t i n c t f r o m the e n d - l i m b of N2. T h e a m p l i t u d e of the negative-going l i m b of N1 (P1--N1) was n o t significantly d i f f e r e n t w h e t h e r t h e s t i m u l u s was e x p e c t e d or not.
B. Specific evoked potentials T h e late c o m p o n e n t s o f t h e CEP r e c o r d e d over t h e specific s o m a t i c and visual areas s h o w e d t h e same m o d i f i c a t i o n s in r e s p o n s e t o e x p e c t e d and u n e x p e c t e d stimuli as t h o s e of t h e v e r t e x CEP. However, t h e d i f f e r e n c e s in N2 ( a n d P3) a m p l i t u d e b e t w e e n e x p e c t e d a n d u n e x p e c t e d stimuli were less t h a n in v e r t e x CEP. In t h e 9 averaged specific evoked p o t e n t i a l s , 5 visual a n d 4 c u t a n e o u s ( b e c a u s e o f a r t e f a c t s 3 records were useless), t h e N1 wave was n e v e r h i g h e r w h e n elicited b y a n e x p e c t e d s t i m u l u s : in 2 cases t h e ampli-
E'after
L
i
i
0
Q
0
800 ms VISUAL
n =39
Ebefor e
U,
Eafter
0
800 ms
Fig. 1. A v e r a g e d v e r t e x e v o k e d r e s p o n s e to e x p e c t e d a n d u n e x p e c t e d stimuli. T o p : averaged r e e p o n s e to 4 0 electrical c u l t a n e o u s shocks. B o t t o m : averaged resp o n s e to 39 flashes. E: r e s p o n s e s to e x p e c t e d stimuli i m m e d i a t e l y p r e c e d i n g (E b e f o r e ) or f o l l o w i n g (E after) u n e x p e c t e d stimuli w i t h i n t h e same m o d a l i t y . U: r e s p o n s e s to u n e x p e c t e d s t i m u l i ; n o t e the p r e s e n c e of N2 a n d P3 waves.
704 t u d e was the same for e x p e c t e d and u n e x p e c t e d stimuli, in the r e m a i n i n g 7 cases Nj was higher in response to u n e x p e c t e d stimuli.
Discussion In s u m m a r y , w h e n a s t i m u l u s does n o t m a t c h the e x p e c t a t i o n of t h e subject, it elicits: (1) large N2 a n d P3 waves in t h e vertex e v o k e d p o t e n t i a l : (2) possibly some increase of t h e N 1 wave in evoked p o t e n t i a l s r e c o r d e d over t h e specific s e n s o r y areas. Several features of t h e p r o c e d u r e used in t h e p r e s e n t experim e n t m a k e it reasonable to assume t h a t t h e i n d e p e n d e n t variable is i n d e e d the specific e x p e c t a t i o n set up b y t h e s u b j e c t ' s o w n choice. It can be safely e x c l u d e d t h a t the observed results are c o n t a m i n a t e d or caused b y t h e m o t o r activity involved in the s t i m u l u s select i o n : pressing o n t h e pedal t o o k place 2 sec b e f o r e t h e s t i m u l u s delivery and b o t h e x p e c t e d and u n e x p p e c t e d stimuli were p r e c e d e d b y such a m o v e m e n t . As n o r e s p o n s e was r e q u i r e d u p o n the s t i m u l u s occurrence, n e i t h e r c a n t h e d i f f e r e n c e s be e x p l a i n e d b y p r e p a r a t i o n for a d i f f e r e n t i a l overt m o t o r response. A n y possible effect of a c o n t i n g e n t negative v a r i a t i o n (CNV), set up b y t h e s u b j e c t ' s o w n c h o i c e of t h e stimulus, can also b e ruled out. A l t h o u g h it is likely t h a t a CNV develops during each trial, its effects are b a l a n c e d because the s u b j e c t c a n n o t p r e d i c t if the n e x t s t i m u l u s will m a t c h his c h o i c e or not. In these c o n d i t i o n s , o n l y r a n d o m variations in t h e CHVs will be reflected in t h e evoked p o t e n t i a l to e x p e c t e d or u n e x p e c t e d stimuli. O n t h e whole, it seems legitimate to c o n s i d e r t h e differential effects of e x p e c t e d a n d u n e x p e c t e d stimuli as an o u t c o m e o f selective a t t e n t i o n to a given s e n s o r y m o d a l i t y .
Summary V e r t e x a n d specific evoked p o t e n t i a l s t o visual a n d c u t a n e o u s stimuli were r e c o r d e d in 6 subjects. T h e delivery o f r o u g h l y equal n u m b e r s of flashes a n d s h o c k s was paced b y t h e s u b j e c t s themselves. T h e y also had to c h o o s e w h i c h s t i m u l u s t h e y w o u l d be given n e x t ; 9 t i m e s o u t of 10 t h e s t i m u l u s m a t c h e d t h e s u b j e c t ' s e x p e c t a t i o n a n d stimuli f r o m t h e unexp e c t e d m o d a l i t y were given r a n d o m l y 1 t i m e o u t of 10 o n t h e average. No task was r e q u i r e d a f t e r the stimulus occurrence. Cortical evoked p o t e n t i a l s (CEPs) to u n e x p e c t e d stimuli were averaged a n d c o m p a r e d with CEPs t h e m a j o r d i f f e r e n c e consisted in large N2 a n d P3 waves in response to u n e x p e c t e d s t i m u l i ; n o d i f f e r e n c e was o b served in t h e a m p l i t u d e of the Nl wave. In specific CEPs u n e x p e c t e d stimuli also elicited N 2 a n d P3 waves; in a d d i t i o n NI s h o w e d a slight t e n d e n c y to be higher in CEPs t o u n e x p e c t e d stimuli.
SELECTIVE ATTENTION AND EVOKED POTENTIALS R~sum~
Effets de l'attention sdlective sur les composantes tardives du potenliel dvoqued chez l'homrne Les r ~ p o n s e s ~voqu~es sur le c o r t e x p a r des s t i m u l u s visuels et c u t a n ~ s o n t 6t~ enregistr~es chez 6 sujets. Les sujets c o n t r S l a i e n t la c a d e n c e de distribut i o n des ~clairs e t des c h o c s et choisissaient la m o d a l i t ~ d a n s laquelle c h a q u e s t i m u l u s devait ~tre d o n n ~ : 9 lois sur 10 les s t i m u l u s dtaient c o n f o r m e s leur a t t e n t e et 1 fois sur 10 en m o y e n n e , selon u n o r d r e au hasard le s t i m u l u s ne c o r r e s p o n d a i t pas ~ la m o d a l i t d d e m a n d ~ e . Les sujets n ' a v a i e n t pas de t ~ c h e a e x ~ c u t e r e n r~ponse aux stimulus. Les p o t e n t i e l s ~voqu6s c o r t i c a u s (PEC) d(~clench~s p a r les s t i m u l u s i n a t t e n d u s o n t dt~ m o y e n n ~ s et compares a u x PEC m o y e n s d ' u n m ~ m e n o m b r e de s t i m u l u s a t t e n d u s . Au niveau d u vertex la diff6rence principale c o n s i s t e en I ' a p p a r i t i o n de g r a n d e s o n d e s N 2 et P3 dans ]es PEC d~clench~s par les s t i m u l u s i n a t t e n d u s ; l ' o n d e Nl ne p r ~ s e n t e pas de v a r i a t i o n significative d ' a m p l i t u d e . Les PEC sp~cifiques pr6s e n t e n t dgalement des o n d e s N2 et P3 ainsi q u ' u n e 14g~re a u g m e n t a t i o n d ' a m p l i t u d e de l ' o n d e Nl, e n r~sponse a u x s t i m u l u s i n a t t e n d u s .
References
Courschesne, E., Hillyard, S.A. and Galambos, R. Stimulus novelty, task relevance and the visual evoked potential in man. Electroenceph. clin. Neurophysiol., 1975, 39: 131--143. Donkin, E. and Cohen, L. Averaged evoked potentials and intramodality selective attention. Electroenceph, clin. Neurophysiol., 1967, 22: 537--546. Ford, J.M., Roth, W.T., Dirks, S.J. and KopeII, B.S. Evoked potential correlates of signal recognition between and within modalities. Science, 1973, 181: 4 6 5 - - 4 6 6 . Karlin, L., Martz, M.J. and M o r d k o f f , A.M. M o t o r p e r f o r m a n c e and s e n s o r y evoked potentials. Elect r o e n c e p h , clin. N e u r o p h y s i o l . , 1 9 7 0 , 28: 3 0 7 - 313. Picton, T.W. a n d Hillyard, S.A. H u m a n a u d i t o r y
evoked potentials. If: Effects of attention. Elctroenceph, clin.Neurophysiol., 1974, 36: 191--199. Roth, W.T. Auditory evoked responses to unpredictable stimuli. Psychophysiology, 1973, 10: 125--139. Ruehkin, D.S., Sutton, S. and Tueting, P. Emitted and evoked P300 potentials and variation in stimulus probability. Psychophysiology, 1975, 12: 591-595. Tueting, P., Sutton, S. and Zubin, J. Quantitative evoked potential correlates of the probability of events. Psyehophysiology, 1970, 7: 385--394.