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Slow brain potentials in the CNV-paradigm
Acta Psychologica 44 (1980) 1 4 7 - 1 6 3 © North-Holland Publishing C o m p a n y
SLOW BRAIN POTENTIALS IN THE CNV-PARADIGM*
A. W. K. GAILLARD and J. PERDOK Institute for Perception, Soesterberg, The Netherlands Received May 1979
Slow brain potentials were recorded during the foreperiod of a reaction time task, and the effects of instructions governing the trade-off between speed and accuracy were investigated. One brain potential, a slow negative shift preceding S 2 , was largely attenuated under accuracy instructions. It is suggested that this shift is dependent on a motor response and that its amplitude reflects the level of m o t o r preparation. Two other brain potentials, a slow positive and a slow negative wave, seem to depend on the psychological properties of S~. Enhanced amplitudes were found, w h e n S 1 provides information, besides its warning function.
Introduction
Slow potential shifts (SPSs) develop in the human brain between a warning signal ($1) and an imperative signal ($2), which requires a response (overt or covert). The most familiar SPS is the contingent negative variation (CNV), which was first recorded by Walter et al. (1964). Recent studies which used interstimulus intervals (ISis) o f longer duration (>/ 3 sec) have shown that the CNV is not a unitary phenomenon, but consists of several SPSs: at least two negative SPSs were found which differ not only in their functional significance, but also in their topographical distribution (Loveless and Sanford 1974; Rohrbaugh et al. 1976; Gaillard 1976, 1977). The first potential has the form of an inverted U-curve, which reaches its peak within one sec after S~; the second potential gradually develops during the interstimulus interval (ISI), and reaches its maximum towards the end of this interval. Because of its occurrence towards the end of the ISI and its contingency to $2 (or to the response to $2) the second potential seems to resemble more * Address requests for reprints to: A. W. K. Gaillard, ~ s t i t u t e for Perception TNO, Postbox 23, / 3769 ZG Soesterberg, T h e Netherlands.
148
A. W. K. Gaillard, ,L Perdok / Slow brain potentials
closely the 'classical CNV' obtained with short ISis; therefore, it will be called t e r m i n a l C N V . The term CNV will be used in its original meaning, to denote the negative shift preceding S~ in studies with short ISis, which generally did not discriminate between the two negative potentials described here. The first potential will be called s l o w negative wave (SNW) because of its similarity to the 'slow wave' observed in selective listening tasks (e.g., Squires et al. 1977). The SNW appears to be dependent on the characteristics of $1 in much the same way as the P3. Therefore, both seem to belong to a family of slow potential shifts following $1. Similar to the SNW the P3 will be called s l o w positive wave (SPW). This notation is preferred because often a clear peak cannot be detected in these slow waves following $1. Thus, in the present study three slow potential shifts are distinguished: SPW, SNW and terminal CNV. The terminal CNV appears to be determined mainly by the level of m o t o r preparation, since it is affected by task variables such as time uncertainty (i.e. the duration or irregularity of the ISI), muscle effort and instructions governing the trade-off between speed and accuracy, which are known to influence m o t o r preparation. This notion is supported by the remarkable similarity with regard to both morphology and topography between this terminal CNV and the readiness potential (RP) which precedes any voluntary movement (Gaillard 1978; Rohrbaugh et al. 1976). In this view the terminal CNV and the RP reflect largely the same neurophysiological phenomenon, which, however, is obtained in different experimental situations and by different averaging methods. In the present study this issue is further investigated by comparing the usual averages time locked to $2, with movement potentials, i.e. with averages time locked to button presses given in the context of the S~-$2 paradigm. This is done for several task requirements. The trade-off between speed-accuracy is varied by using a deadline procedure (see Ollman 1977). The subject was instructed to respond to $2 before a deadline set by the experimenter. If the subject did not respond before the deadline, a noise burst was given through earphones. In this way the subject received immediate feedback about his reaction performance. This procedure was used to ensure that subjects would follow the instructions throughout the block of trials. Especially under speed instruction subjects tend to slow down their performance during the experimental sessions or even during one block of trials.
A. 14/.K. Gaillard, J. Perdok / Slow brain potentials
149
Since the RP is usually obtained when the subject produces voluntary movements without external stimulation, an experimental condition was added in which the subject had to synchronize the response with the arrival of $2. This time-estimation task resembles the $1 - $ 2 paradigm and the voluntary-movement situation. It is generally assumed that the estimation of the arrival o f $2 is an important determinant of the preparatory processes for the response to S:. The increase of RT with longer ISI is generally explained as resulting from increased difficulty to predict the m o m e n t $2 will arrive (e.g. N~i~it~inen and Merisalo 1977). A second aim of the present study was to investigate whether the SNW was affected by the information revealed by $1. It has been shown that the SNW is influenced by the physical characteristics of S~, such as modality, probability, intensity, and duration o f this stimulus (Gaillard 1976; Kok 1978; Loveless and Sanford 1975; Klorman and Bentsen 1975). It is also conceivable that this wave may be affected by the psychological significance of S~. Indeed, in earlier studies (Gaillard 1977; Gaillard and Perdok 1979) the SNW was enhanced, when S~ had more relevance for the subject. In the present study the psychological significance of $1 was varied by making the signal informative. A larger SNW was expected when S~ was informative as compared to the situation where $1 only served as a warning that $2 would soon arrive. $1 provided information with regard to the instructions to be followed at $2 ; this was done in two ways: in one situation the speed and accuracy were varied from trial-to-trial within one block of trials and in the other situation S~ indicated either a speed or a detection instruction; under the latter instruction the subject was required to make a discrimination at $2, but to delay the response with one sec. The e x p e r i m e n t Me th o d S u b j e c t s a n d apparatus T h e Ss were 10 male University s t u d e n t s , w h o were paid for t h e i r p a r t i c i p a t i o n . T h e data o f o n e o t h e r S were discarded for having t o o m a n y e y e - m o v e m e n t artifacts. T h e Ss were seated in a c o m f o r t a b l e chair, w h i c h also p r o v i d e d a neck- and arm-rest. $1 was a 7 0 dB t o n e (delivered via h e a d p h o n e s ) w i t h a f r e q u e n c y o f e i t h e r 5 0 0 or 2 0 0 0 Hz. $2 consisted o f slides w h i c h were p r o j e c t e d ( K o d a k - C a r o u s e l ) t h r o u g h t h e w i n d o w of t h e cubicle, o n a screen at a d i s t a n c e o f ca. 1 m e t e r f r o m t h e
150
A. W. K. Gaillard, J. Perdok ~Slow brain potentials
S's eyes. $2 was always c o m p o s e d of t w o vertical bars ( w i d t h 5.5 cm), w h i c h differed in l e n g t h (37.5 versus 5.5 cm). Ag-AgC1 disk e l e c t r o d e s were a t t a c h e d to t h r e e scalp sites (Fz, Cz, Pz) and to t h e earlobes, w h i c h were linked for reference. B e c k m a n m i n i a t u r e e l e c t r o d e s were t a p e d to t h e right supra- and i n f r a - o r b i t a l ridge to r e c o r d t h e vertical EOG. A n e l e c t r o d e j u s t a b o v e t h e n o s e b r i d g e served as a g r o u n d . A f t e r a m p l i f i c a t i o n (timec o n s t a n t 6 sec) t h e E E G and EOG signals were r e c o r d e d o n a m a g n e t i c tape. Procedure
A trial s t a r t e d w i t h t h e o n s e t o f $1, w h i c h was f o l l o w e d 4 sec later b y t h e slide r e m a i n i n g o n for 1 sec. T h e i n t e r t r i a l interval ( $ 2 - S 1) was varied irregularly b e t w e e n 1 1 - 1 4 sec. U n d e r all e x p e r i m e n t a l c o n d i t i o n s , e x c e p t for t h e s y n c h r o n i z a t i o n task, t h e S was asked to d i s c r i m i n a t e w h i c h o f t h e t w o bars was longer. He h a d to r e s p o n d w i t h his right h a n d w h e n t h e right bar was longer and w i t h his left h a n d w h e n t h e left o n e was longer. To i n d u c e changes in t h e s p e e d - a c c u r a c y t r a d e - o f f t w o d e a d l i n e s were used b e f o r e w h i c h t h e S had to r e s p o n d ; if he did n o t r e s p o n d b e f o r e t h e d e a d l i n e 60 dB noise was given via e a r p h o n e s ( d u r a t i o n 5 0 0 msec). U n d e r a c c u r a c y i n s t r u c t i o n t h e d e a d l i n e interval was t w i c e as l o n g as u n d e r speed i n s t r u c t i o n . T h e S was told t h a t o n l y t h e c o r r e c t r e a c t i o n s o c c u r r i n g b e f o r e t h e deadline w o u l d c o u n t . Especially u n d e r speed i n s t r u c t i o n , t h e i m p o r t a n c e o f b e a t i n g t h e d e a d l i n e was stressed, since Ss were f o u n d to t e n d t o p r e f e r receiving noise to m a k i n g errors. With t h e accuracy i n s t r u c t i o n t h e a v o i d a n c e o f m a k i n g errors was e m p h a s i z e d . T h e d u r a t i o n of t h e d e a d l i n e interval was d e t e r m i n e d in a separate t r a i n i n g session, o n e week b e f o r e t h e e x p e r i m e n t a l Session. T h e d e a d l i n e u n d e r speed i n s t r u c t i o n was c h o s e n such t h a t the S m a d e 2 0 - 3 0 % errors; in fact, the variability b e t w e e n Ss was so low t h a t all Ss c o u l d get t h e same interval o f 2 5 0 msec u n d e r speed, and 5 0 0 msec u n d e r a c c u r a c y i n s t r u c t i o n . In a d d i t i o n t h e S received a s y n c h r o n i z a t i o n task in w h i c h he had to s y n c h r o n i z e his r e s p o n s e w i t h t h e arrival o f $2. T h e S was i n s t r u c t e d to react w i t h his right h a n d o n h a l f o f t h e trials and w i t h his left h a n d in t h e o t h e r half. (Since no d i f f e r e n c e s were f o u n d b e t w e e n t h e s e c o n d i t i o n s , t h e d a t a for right a n d left h a n d r e s p o n d i n g were c o m b i n e d . ) T h e speed and a c c u r a c y i n s t r u c t i o n s were varied e i t h e r b e t w e e n or w i t h i n a b l o c k o f trials, In the l a t t e r s i t u a t i o n SI i n d i c a t e d the i n s t r u c t i o n t h e S had t o follow. F o r h a l f o f t h e Ss t h e high t o n e ( 2 0 0 0 Hz) i n d i c a t e d a speed i n s t r u c t i o n and t h e low t o n e ( 5 0 0 Hz) t h e a c c u r a c y i n s t r u c t i o n ; this was reversed for t h e o t h e r half. F i n a l l y , t h e speed i n s t r u c t i o n was also given w i t h i n a b l o c k of trials, t o g e t h e r w i t h a d e t e c t i o n i n s t r u c t i o n ; u n d e r this i n s t r u c t i o n t h e S was asked to i n d i c a t e w h i c h bar was longer at t h e off-set of t h e slide (1 sec a f t e r $2). Again $1 i n d i c a t e d w h i c h i n s t r u c t i o n h a d to be f o l l o w e d ; t h e t o n e w h i c h h a d i n d i c a t e d t h e a c c u r a c y instruction now indicated the detection instruction. Half o f t h e Ss received t h e 7 e x p e r i m e n t a l c o n d i t i o n s in t h e f o l l o w i n g order: s y n c h r o n i z a t i o n / s p e e d / a c c u r a c y ( b e t w e e n ) , s p e e d / a c c u r a c y , s p e e d / d e t e c t i o n ; and t h e o t h e r half o f t h e Ss received t h e m in t h e reversed order. T h e r e were 50 trials in each e x p e r i m e n t a l c o n d i t i o n .
A. W. K. Gaillard, J. Perdok / Slow brain potentials
151
Data analysis T h e t h r e e E E G c h a n n e l s , and EOG c h a n n e l were digitized at a rate of 25 samples per sec. T h e analysis p e r i o d started 1 sec b e f o r e $1 and e n d e d at t h e o n s e t of $2. F o r each trial t h e n u m b e r of data p o i n t s was r e d u c e d to 25 b y t a k i n g t h e averages o f successive g r o u p s of 5 p o i n t s ; this yielded o n e average data p o i n t per 2 0 0 msec. T h i r t e e n o f t h e s e data p o i n t s were r e t a i n e d for f u r t h e r analysis (see also fig. 1). T h e s e d a t a p o i n t s were averaged across 4 0 trials, leaving 10 trials for a r t i f a c t rejection, and t h e following m e a s u r e s were o b t a i n e d , separately for each d e r i v a t i o n : SPW, SNW and t e r m i n a l CNV, i.e. t h e average E E G in t h e period 2 0 0 4 0 0 msec and 6 0 0 - 8 0 0 msec after S1 and 2 0 0 msec before $2, referred to a baseline (average E E G in t h e period 2 0 0 msec b e f o r e $1). O n these m e a s u r e s and t h e m e a n RT, s t a n d a r d d e v i a t i o n and % o f errors t w o t y p e s o f A N O V A s were carried out: a t w o - w a y A N O V A , w h i c h i n c l u d e d instruct i o n s (speed vs. a c c u r a c y ) and t h e b e t w e e n / w i t h i n b l o c k s variable. In t h e second A N O V A t h e t h r e e speed i n s t r u c t i o n s were c o m p a r e d : speed ( b e t w e e n ) , speed ( a c c u r a c y ) , speed ( d e t e c t i o n ) . In a d d i t i o n , t h e EEG was averaged twice (sample interval 14 msec): o n c e t i m e l o c k e d to Sz and o n c e to t h e response. Results R T-performance T a b l e 1 p r e s e n t s t h e R T - d a t a separately for t h e e x p e r i m e n t a l c o n d i t i o n s . F o r b o t h t h e b e t w e e n a n d w i t h i n c o n d i t i o n t h e m e a n R T was m u c h s h o r t e r u n d e r speed t h a n u n d e r a c c u r a c y i n s t r u c t i o n ( F ( 1 / 9 ) = 1 10; p < 0.001). O f course, t h e r e were m a n y m o r e errors u n d e r speed t h a n u n d e r a c c u r a c y i n s t r u c t i o n s ( F ( 1 / 9 ) = 4 0 . 7 ; p < 0.01), while t h e s t a n d a r d d e v i a t i o n was m u c h smaller ( F ( 1 / 9 ) = 4 2 . 4 ; p < 0.01). A l t h o u g h t h e d i f f e r e n c e in R T and t h e s t a n d a r d d e v i a t i o n b e t w e e n t h e speed
Table 1 The mean RT, standard deviation (in msec) and the % of errors, separately for the experimental conditions. Standard deviation
% of errors
- 5
252
-
224 375
48 80
29 2
223 362
46 83
29 2
231 1220
55 77
27 2
Mean RT Synchronization Between Speed Accuracy Within Speed Accuracy Within Speed Detection
152
A. 14I.K. Gaillard, J. Perdok / S l o w brain potentials
and a c c u r a c y i n s t r u c t i o n s were smaller in t h e w i t h i n - t h a n in t h e b e t w e e n - c o n d i t i o n , this i n t e r a c t i o n was n o t significant ( F ( 1 / 9 ) < 1). Also t h e m a i n effect of b e t w e e n versus w i t h i n b l o c k s was n o t significant for e i t h e r m e a n RT, % errors or s t a n d a r d deviation. Ss were q u i t e able to s y n c h r o n i z e t h e i r r e s p o n s e s w i t h $2; five o f t h e m reacted o n average w i t h i n 50 msec b e f o r e or a f t e r $2, while all reacted w i t h i n -+260 msec. Nevertheless, the s t a n d a r d d e v i a t i o n was very large, five t i m e s m o r e t h a n u n d e r speed i n s t r u c t i o n . S l o w p o t e n t i a l shifts Fig. 1 shows t h e c o m p o s i t e m e a n s o f the v e r t e x slow p o t e n t i a l , u n d e r t h e s y n c h r o n i z a t i o n , speed and a c c u r a c y i n s t r u c t i o n s (all varied b e t w e e n blocks). U n d e r each i n s t r u c t i o n a SNW w i t h a l a t e n c y of ca. 6 0 0 msec was f o l l o w e d b y a t e r m i n a l CNV. In a t w o - w a y A N O V A o n t h e t e r m i n a l C N V t h e s p e e d - a c c u r a c y effect was significant ( F ( 1 / 9 ) = 7 . 8 4 ; p < 0.02), while t h e v a r i a t i o n o f i n s t r u c t i o n b e t w e e n or w i t h i n a b l o c k o f trials h a d n o i n f l u e n c e ( F ( 1 / 9 ) < 1). A l t h o u g h t h e effect o f i n s t r u c t i o n s was larger in t h e b e t w e e n - c o n d i t i o n (6.4 # V ) t h a n in the w i t h i n - c o n d i t i o n (2 # V ) , this i n t e r a c t i o n did n o t r e a c h statistical significance ( F / 1 / 9 ) = 3.54; p < 0.09). No d i f f e r e n c e s were f o u n d b e t w e e n t h e speed and t h e s y n c h r o n i z a t i o n i n s t r u c t i o n , e x c e p t t h a t in t h e latter c o n d i t i o n the t e r m i n a l C N V reaches its m a x i m u m o n e data p o i n t earlier t h a n u n d e r speed i n s t r u c t i o n , w h i c h c o r r e s p o n d s w i t h t h e earlier response. In c o n t r a s t to t h e t e r m i n a l CNV, t h e SNW a m p l i t u d e was n o t significantly affected b y i n s t r u c t i o n s (see fig. 1). However, t h e f r o n t a l SNW was i n f l u e n c e d b y t h e b e t w e e n / w i t h i n b l o c k s variable ( F ( 1 / 9 ) = 11.9 ;p < 0.01). T h e SNW was e n h a n c e d w h e n $I i n d i c a t e d the i n s t r u c t i o n s to be f o l l o w e d at $2. T h e parietal SPW was a f f e c t e d b y b o t h factors. Larger a m p l i t u d e s were f o u n d u n d e r speed i n s t r u c t i o n
]
-12
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i
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-2
..:
/ //0 / / / °~"
z,
0 I
I
Sl
1
I
I
J
2
3
S
time (see) Fig. 1. The composite means of the vertex slow potential under the synchronization, speed and accuracy instructions, all varied between blocks.
A. W. K. Gaillard, J. Perdok / Slow brain potentials
[ -12
•~ , x~--x
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i speed speed speed
153
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j
(between) (accuracy) (detection)
t" / /~o Cz
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-2
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'
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Fig. 2. The composite means of the vertex slow potential under the speed instruction conditions; this instruction was either held constant over a block of trials (between) or it was varied from trial-to-trial in combination with accuracy or with the detection instruction. ( F ( 1 / 9 ) = 4.8; p < 0 . 0 5 ) a n d u n d e r t h e w i t h i n c o n d i t i o n ( F ( 1 / 9 ) = 5.9; p < 0 . 0 4 ) ; t h e i n t e r a c t i o n b e t w e e n t h e s e f a c t o r s w a s n o t s i g n i f i c a n t ( F ( 1 / 9 ) < 1). Fig. 2 p r e s e n t s t h e c o m p o s i t e v e r t e x d a t a u n d e r t h e s p e e d i n s t r u c t i o n , f o r t h e t h r e e e x p e r i m e n t a l c o n d i t i o n s : t h i s i n s t r u c t i o n w a s e i t h e r held c o n s t a n t o v e r a b l o c k o f trials ( b e t w e e n ) o r varied f r o m trial-to-trial in c o m b i n a t i o n w i t h a c c u r a c y Table 2 The EEG and the corresponding EOG amplitudes (in/~V) of the SPW, SNW and the terminal CNV, separately for the experimental conditions. Between Synchronization SPW Fz
- 0.6
cz
Pz EOG SNW
+0.3
+ 2.6 - 2.0
Within
Within
Speed
Accuracy
Speed
Accuracy
Speed
Detection
+ 0.6 +0.8 + 3.2 0.0
4.6 4.8 +1.0 0.0
0.0 0.0 +4.8 0.0
-0.6 4.6 +3.0 0.0
-0.4 0.0 + 4.0 -2.0
-1.2 -1.4 +4.6 0.0
4.8
-2.4
-2.6
-7.4
4.4
-9.8
4.0
Cz
- 4.9
-3.0
-3.8
-6.2
-8.0
-2.6
Pz
+ 1.3
-2.2
+2.2
- 5.0
-2.2 4.0
+1.2
EOG CNV Fz Cz Pz EOG
0.0 - 6.0
4.2 +0.8
-
-2.0
-
8.0
+2.0
- 6.3 -10.4 - 6.5 - 6.0
-4.2 -12.0 - 7.2 0.0
-1.6 -5.6 4.0 +4.0
- 2.4 -9.4 -6.6 +12.0
-5.2 -7.4 4.6 -8.0
-4.0 -10.0 -7.2 +8.0
+1.0 -1.4 -1.0 +8.0
rz
-
2.0
154
A. W. K. Gaillard, J. Perdok / S l o w brain potentials
or w i t h d e t e c t i o n . O n e - w a y A N O V A s revealed t h a t t h e t e r m i n a l CNV and t h e SPW were u n a f f e c t e d b y t h e s e c o n d i t i o n s , b u t t h e f r o n t a l SNW was i n f l u e n c e d ( F ( 2 / 1 8 ) = 5.77; p ~ 0.01). T h e a m p l i t u d e o f t h e SNW was e n h a n c e d w h e n S1 i n d i c a t e d t h e speed i n s t r u c t i o n ; t h i s e f f e c t was m o s t p r o m i n e n t w h e n speed and d e t e c t i o n instruct i o n s were varied w i t h i n o n e b l o c k o f trials (see fig. 2 and table 2). To illustrate t h e r e l a t i o n s h i p b e t w e e n t h e SNW and SPW t h e f r o n t a l a n d parietal slow p o t e n t i a l s in t h e 1.5 sec a f t e r S~ are d e p i c t e d in fig. 3. It can be seen t h a t b o t h t h e SNW and t h e SPW are e n h a n c e d w h e n $1 provides i n f o r m a t i o n , besides its w a r n i n g f u n c t i o n (first vs. s e c o n d panel). A f u r t h e r e n h a n c e m e n t o c c u r s w h e n $1 i n d i c a t e s t h e speed i n s t r u c t i o n where speed and d e t e c t i o n i n s t r u c t i o n s are varied f r o m trial-to-trial. This e n h a n c e m e n t is n o t seen w h e n Sx i n d i c a t e s the d e t e c t i o n i n s t r u c t i o n , whereas also t h e SPW does n o t differ b e t w e e n t h e two t y p e s of withinc o n d i t i o n s ( s e c o n d vs. t h i r d panel). T h e s e effects are also s h o w n in fig, 4 w h e r e the E E G is averaged w i t h a sample interval of 14 msec. B o t h t h e SNW and t h e SPW ( a n d also P3) are increased w h e n Sa is m a d e i n f o r m a t i v e . However, for t h e SNW this e n h a n c e m e n t is larger u n d e r the speed t h a n u n d e r t h e d e t e c t i o n i n s t r u c t i o n , w h e r e a s for t h e SPW t h e r e is n o d i f f e r e n c e b e t w e e n t h e i n s t r u c t i o n s in this respect. [
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Figs. 5 and 6 p r e s e n t t h e v e r t e x p o t e n t i a l s t i m e - l o c k e d t o e i t h e r $2 (S-locked) or to t h e r e s p o n s e (R-locked). In all S-locked averages ( e x c e p t u n d e r d e t e c t i o n ) the t e r m i n a l CNV is f o l l o w e d b y an N1 peak ( l a t e n c y 175 msec) a n d a large P3 ( l a t e n c y 3 2 5 - 3 7 5 msec). T h e P2 c o m p o n e n t is o n l y observed in t h o s e c o n d i t i o n s ( a c c u r a c y and d e t e c t i o n ) w h e r e P3 is r e d u c e d and delayed. T h e l a t e n c y of the P3 c o m p o n e n t was marginally i n f l u e n c e d b y e x p e r i m e n t a l c o n d i t i o n s . T h e l a t e n c y was 325 msec u n d e r s y n c h r o n i z a t i o n , 3 5 0 msec u n d e r b o t h speed c o n d i t i o n s a n d 375 msec u n d e r
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a c c u r a c y and d e t e c t i o n i n s t r u c t i o n s . These differences in l a t e n c y are small comp a r e d t o t h e d i f f e r e n c e s in RT. F o r e x a m p l e , the d i f f e r e n c e in R T b e t w e e n speed and a c c u r a c y i n s t r u c t i o n s was 151 msec, while t h e difference in P3 l a t e n c y was only 25 msec. T h e evoked p o t e n t i a l s to $2 were largest in t h e t w o speed c o n d i t i o n s ; the N1 peak was smallest u n d e r s y n c h r o n i z a t i o n , w h i c h illustrates t h e relative u n i m p o r t a n c e of $2 in this c o n d i t i o n . As can be seen in figs. 5 and 6, t h e R-locked averages show a r e m a r k a b l e similarity in f o r m and t i m e course to p o t e n t i a l s s y n c h r o n i z e d to v o l u n t a r y m o v e m e n t s . As w i t h t h e p o t e n t i a l s t i m e l o c k e d to v o l u n t a r y m o v e m e n t s , t h e r e s p o n s e is p r e c e d e d b y a slow negative shift, w h i c h could be d e n o t e d as readiness p o t e n t i a l (RP). This gradual negative shift t u r n s in to a sharp negative i n f l e c t i o n , w h i c h is f o l l o w e d b y a slow positive change. U n d e r a c c u r a c y i n s t r u c t i o n s this positivity already starts 150 msec b e f o r e t h e closure of t h e b u t t o n switch. This p h e n o m e n o n , w h i c h was p r e s e n t in 9 o u t of 10 Ss, l o o k s very similar to t h e p r e - m o t i o n - p o s i t i v i t y o b s e r v e d by Deecke et al. ( 1 9 7 6 ) . To c o m p a r e t h e slow negative shifts in t h e S- and R-locked averages, t h e a m p l i t u d e of t h e t e r m i n a l C N V (average E E G 100 msec b e f o r e $2) was c o m p u t e d in b o t h averages. T h e s e m e a s u r e s were c o m p a r e d to t h e a m p l i t u d e of t h e RP (average E E G in the p e r i o d 2 5 0 - 1 5 0 msec b e f o r e t h e response) (see Deecke et al. 1976). All
156
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t h r e e m e a s u r e s were referred to t h e same baseline (average E E G 2 0 0 0 - 1 8 0 0 msec b e f o r e t h e response). Largest a m p l i t u d e s were o b t a i n e d at C2 for all m e a s u r e s a n d e x p e r i m e n t a l c o n d i t i o n s . T h e r e were virtually n o d i f f e r e n c e s (less t h a n 1 /~V) b e t w e e n t h e t h r e e v e r t e x m e a s u r e s u n d e r speed i n s t r u c t i o n . This is n o t so surprising since b o t h t h e m e a n a n d t h e variability of t h e RT in this c o n d i t i o n were small, and c o n s e q u e n t l y t h e d i s c r e p a n c y b e t w e e n t h e t w o ways of averaging will also be small. In c o n t r a s t , u n d e r s y n c h r o n i z a t i o n i n s t r u c t i o n t h e r e was a d i f f e r e n c e in v e r t e x term i n a l C N V o f 2.4 /~V b e t w e e n t h e t w o t y p e s o f averages. Since t h e variability in r e s p o n s e t i m e was large, t h e e n h a n c e d a m p l i t u d e in t h e R-locked average suggests t h a t t h e t e r m i n a l C N V is c o n t i n g e n t o n t h e r e s p o n s e a n d n o t o n t h e s t i m u l u s (Sz). H o w e v e r , u n d e r a c c u r a c y i n s t r u c t i o n t h e a m p l i t u d e s o b t a i n e d in t h e R-locked averages were n o t larger. This m a y be caused b y t h e larger t i m e lag b e t w e e n $2 and t h e r e s p o n s e in this c o n d i t i o n ( 3 7 5 msec). As can b e seen in fig. 5, t h e negativity t i m e - l o c k e d t o t h e r e s p o n s e still increases a f t e r $2 u n t i l 150 msec b e f o r e $2. There-
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fore, the RP measure taken at this moment is much larger than the terminal CNV in both the S-locked and in R-locked averages. Although small in amplitude, the RP preceding the delayed response under the detection shows the same midline distributions as the terminal CNV and the RP under the other instructions.
Discussion T h e p r e s e n t e f f e c t s o f t h e s p e e d - a c c u r a c y i n s t r u c t i o n s w e r e larger f o r b o t h the terminal CNV and RT than in earlier e x p e r i m e n t s (Gaillard
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1977; Loveless and Sanford 1974). This might be due to the deadline procedure, which induced stable and fast RT performance throughout a block of trials. Although not significant, the speed-accuracy effect was somewhat smaller when S~ indicated the instruction to be followed (within-condition). Some subjects may have found it difficult to change their strategy from trial to trial. The enhanced terminal CNV under speed instruction may be explained by assuming that under this instruction the level of m o t o r preparation is increased, as compared to the accuracy instruction. Also the absence of a terminal CNV under detection instruction found in this and in the previous study, supports the idea that the terminal CNV is responserelated. The potentials time-locked to the b u t t o n press to $2 (figs. 5 and 6) are similar in form to movement potentials preceding voluntary finger presses (see Deecke et al. 1976; Rohrbaugh et al. 1976). Similar to the RP preceding voluntary movements, the negative shift preceding responses in the S~-$2 paradigm is also most prominent at the m o t o r region (Cz). This was also the case when responses were synchronized to $2 or were delayed by one sec after $2 (detection instruction). Syndulko and Lindsley (1977) found the same midline distribution under both the RT- and detection instruction; in addition, they showed that this negative shift had a lateral asymmetry contralateral to the responding hand. The present results obtained under the synchronization instruction deviate from Ruchkin et al. (1977), who used a similar task but with an ISI of 900 msec. Their data (op. cit., fig. 1) show an inverted U-curve with a peak ca. 700 msec after S~. Therefore, their data suggest tllat a gradual rise similar to the RP was not found because the negative shift preceding the response was disrupted by the SNW to S~ (a click). However, gradual negative shifts were obtained when the EEG was timelocked to the finger presses (op. cit., fig. 2). Moreover, these negative shifts in the synchronization task were quite similar to RPs preceding voluntary movements, obtained from the same subjects. The above findings suggest that the negativity preceding the responses in the $1-$2 paradigm is the same as the RP preceding voluntary movements. In this view the terminal CNV obtained from stimulus locked averages largely consists of the increasing limb o f the movement related negative shift. Consequently, the terminal CNV could be regarded as that part o f the RP which precedes $2. It is conceivable that with shorter RTs (for example, induced by deadlines or speed instructions)
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the RP intrudes more into the S 1 - - S 2 interval and in this way enhances the amplitude of the terminal CNV. This view is supported by the larger negativity in the averages time-locked to the response as compared to the negative shifts in the stimulus-locked averages. There are, however, also some differences in the negative shifts observed in the present experimental conditions and the RP: the onset of negative shift under speed, accuracy and synchronization instructions was about one sec earlier and this shift reached an amplitude which was about two times larger than the RP reported by Deecke et al. (1976). It could be argued that in the synchronization task, as well as in the RTtasks, not only motor but also perceptual and decision related processes are involved. However, the terminal CNV seems to be related neither to perceptual nor to decision processes (see Gaillard 1978). Therefore, a more likely explanation would be that the present experimental conditions are more i n t e r e s t i n g than monotonously repeating a large number of voluntary movements. It has been shown that the CNV (Irwin e t al. 1966; Waszak and Obrist 1969) and also the RP (McAdam and Seales 1969) are influenced by motivation. In the latter study RP-amplitudes preceding voluntary finger presses, increased from 5 /JV to 12/aV, when subjects were told that they would receive 10 cents for each response, if given 'at the right time'. In fact, rewards for these 'correct' responses were given in a random fashion. Thus, it could be that the increased RP-values reflect a higher level o f m o t o r preparation, caused by an enhanced interest of the subject in the task. Both above-mentioned explanations assume that terminal CNV and RP are largely produced by the same neurophysiological generator, but according to the first both can be influenced also by perceptual and decision processes, while in the second view they are only determined by the level of m o t o r preparation. Recent models for the speed-accuracy trade-off assume that speed instructions primarily increase the level of m o t o r preparation, whereas the quality of information processing or the rate at which information builds up is not enhanced (Pachella 1974; Ollman 1977; Posner et al. 1973; Thomas 1974). Moreover, the preparation for and the execution of a m o t o r response are regarded as processes which run parallel to the processing o f stimulus information. Another common property of some of these models is the idea that at least a proportion o f the responses are guesses, which are given independently of the ongoing evaluation of the stimulus information and which are correct only by chance. For
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example, the deadline model (Ollman 1977) supposes that there is a time limit (deadline), which is set by the subject or by the experimenter. A response is made when processing is terminated or when the deadline expires, whichever comes first. If processing finishes before the deadline, the response time is representative o f the time needed to process the stimulus information. If the deadline elapses before processing is complete, the response is a guess and less likely to be accurate. N~fitfinen and Merisalo (1977) regard m o t o r preparation as a gradually increasing process, which reaches an optimal level just before $2 arrives. In order to produce quick RTs it is necessary that when $2 arrives the level o f m o t o r preparation is in the vicinity o f the 'motor-action limit', the level at which a response is initiated. The distance kept between the level of m o t o r preparation and the m o t o r action limit, largely depends on the a m o u n t of training and the instructions given by the experimenter. During training subjects will learn to what extent they can increase their m o t o r preparation without making too many errors. Under normal RT instructions subjects will time their m o t o r preparation such that it is close to the motor-action limit at the m o m e n t they expect information processing to be terminated. When speed is stressed the m o t o r action limit will be approached very closely, which increases the possibility for premature response, but shortens the RT. Under accuracy instructions m o t o r preparation will never come close to the motor-action limit before information processing is terminated; so that, on the one hand, no errors will be made, on the other hand, it will cost more time to reach the m o t o r action limit, once information processing is terminated. The terminal CNV seems to be a good candidate for the m o t o r preparation, as defined in the above view, whereas the latency of the P3 of the evoked potential to S~ indicates that information processing is terminated. On the basis o f a trial-to-trial analysis Kutas et al. ( 1 9 7 7 ) c o m p a r e d the RT with the latency o f the P3 on the same trial. They found the correlation between P3 latency and RT was larger under accuracy than under speed instruction. Moreover, under speed instruction the incorrect RTs mostly preceded the P3. According to the authors these data suggest that under accuracy instruction the m o t o r processes of selection and execution o f the response are tightly coupled with the evaluation of the stimulus. However, under speed instruction stimulus evaluation, as indexed by the P3 component, is only loosely related with the
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execution o f the motor response: responses may be generated b e f o r e the stimulus has been fully evaluated. Also in the present study the mean RT (224 msec) preceded the P3 (350 msec) under speed instructions, while these latencies coincided under accuracy instructions (375 msec). In the present study the frontal SNW was strongly affected by the information content o f S1 (see also fig. 2). The amplitude o f this wave was enhanced when $1 indicated either that a speed or an accuracy instruction had to be followed at $2. This effect was even larger when speed and detection instructions were varied within one block of trials. The SPW was also influenced by the present conditions, but in a different way, a result also found by Squires e t al. (1977). Thus, it was not only enhanced when S~ contained information, but this wave was also larger under speed than under accuracy instructions. Moreover, in the condition where speed and detection were varied within one block, the amplitude of the parietal SPW did not differ between speed (4/aV) and detection instructions (4.6 uV), while for the frontal SNW a difference of 5.8 uV was found (see also table 2). Thus, the SPW and SNW seem to be affected by some conditions in the same way and differently in other instances, which suggests that these waves reflect different, but related mechanisms. That different processes are involved is also suggested by the differences in latency ( 3 0 0 - 4 0 0 msec vs. 6 0 0 - 7 0 0 msec after $1) and midline distribution (parietal-frontal). An explanation o f the close inter-relationship between the SNW and the SPW might be found in the fact that both waves are most prominent in non-specific parts of the cortex: the SPW in the association cortex and the SNW in the frontal cortex. These observations can be described rather well in terms of Luria's model of the functional organization of the brain. According to Luria (1973) the function of the frontal lobes is to organize behavior and to regulate arousal systems in accordance with the immediate task demands. This view is supported by the observation that on the one hand largest effects on the SNW were found when $1 indicated task demands which considerably diverged (speed vs. detection). On the other hand, when task demands are constant (between) or similar (speed vs. accuracy) over a block of trials, the preparation for the execution of the task is very likely to take place before the arrival of S~.
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References Deecke, L., B. Gr~Szinger and H. H. Kornhuber, 1976. Voluntary finger movement in man: cerebral potentials and theory. Biological Cybernetics 23, 99 119. Galliard, A. W. K., 1976. Effects of warning-signal modality on the contingent negative variation (CNV). Biological Psychology 4, 139-- 154. Gaillard, A. W. K., 1977. The late CNV wave: preparation versus expectancy, Psycho physiology 14,563-568. Gaillard, A. W. K., 1978. Slow brain potentials preceding task performance. (Doctoral dissertation.) Soesterberg, Institute for Perception. Gaillard, A. W. K. and J. Perdok, 1979. Slow cortical and heart rate correlates of discrimination performance. Acta Psychologica 43,185 198. lrwin, P. A., J. R. Knott, D. W. McAdam and C. S. Robert, 1966. Motivational determinants of the 'contingent negative variation'. Electroencephalography and Clinical Neurophysiology 21,538 543. Klorman, R. and E. Bentsen, 1975. Effects of warning-signal duration on the early and late components of the contingent negative variation. Biological Psychology 3,263 275. Kok, A., 1978. The effect of warning stimulus novelty on the P300 and components of the contingent negative variation. Biological Psychology 6, 219-233. Kutas, M., G. McCarthy and E. Donchin, 1977. Augmenting mental chronometry: the P3o0 as a measure of stimulus evaluation time. Science 197,792-795. Loveless, N. E. and A. J. Sanford, 1974. Slow potential correlates of preparatory set. Biological Psychology 1,303-314. Loveless, N. E. and A. J. Sanford, 1975. The impact of warning-signal intensity on reaction time and components of the contingent negative variation. Biological Psychology 2, 217-226. Luria, A. R., 1973. The working brain. London: Penguin Books, Ltd. McAdam, D. W. and D. M. Scales, 1969. Bereitschaftspotential enhancement with increased level of motivation. Electroencephalography and Clinical Neurophysiology 27, 73-75. NS/it/inen, R. and A. Merisalo, 1977. Expectancy and preparation in simple reaction time. in: S. Dornic (ed.), Attention and Performance VII. Hillsdale: Erlbaum. pp. 115-138. Olhnan, R., 1977. Choice reaction time and the problem of distinguishing task effects from strategy effects. In: S. Dornic (ed.), Attention and Performance VI. Hillsdale: Erlbaum. pp. 99-114. Pachella, R. G., 1974. The interpretation of reaction time in information processing research. In: Kantowitz (ed.), Human information processing: tutorials in performance and cognition. Potomac, Md.: Erlbaum Associates. pp. 41-82. Posner, M. I., R. Klein, J. Summers and S. Buggie, 1973. On the selection of signals. Memory and Cognition 1, 2 12. Rohrbaugh, J. W., K. Syndulko and D. B. Lindsley, 1976. Brain wave components of the contingent negative variation in humans. Science 191, 1055 1057. Ruchkin, R. S., M. G. McCalley and E. M. Glaser, 1977. Event related potentials and time estimation. Psychophysiology 14, 451-455. Squires, K. C., E. Donchin, R. I. Herning and G. McCarthy, 1977. On the influence of task relevance and stimulus probability on event related potential components. Electroencephalography and Clinical Neurophysiology 42, 1-14. Syndulko, K. and D. B. Lindsley, 1977. Motor and sensory determinants of cortical slow potential shifts in man. In: J. E. Desmedt (ed.), Attention, voluntary contraction and eventrelated cerebral potentials. Basel: Karger. pp. 97 131.
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Thomas, E. A. C., 1974. The selectivity of preparation. Psychological Review 8 1 , 4 4 2 - 4 6 4 , Waiter, W. G., R. Cooper, V. J. Aldridge, W. C. McCaUum and A. L. Winter, 1964. Contingent negative variation: an electrical sign of sensory-motor association and expectancy in the human brain. Nature (London) 2 0 3 , 3 8 0 - 3 8 4 . Waszak, M. and W. D. Obrist, 1969. Relationship of slow potential changes to response speed and motivation in man. Electroencephalography and Clinical Neurophysiology 27, 113-120.
SLOW BRAIN POTENTIALS IN THE CNV-PARADIGM*
A. W. K. GAILLARD and J. PERDOK Institute for Perception, Soesterberg, The Netherlands Received May 1979
Slow brain potentials were recorded during the foreperiod of a reaction time task, and the effects of instructions governing the trade-off between speed and accuracy were investigated. One brain potential, a slow negative shift preceding S 2 , was largely attenuated under accuracy instructions. It is suggested that this shift is dependent on a motor response and that its amplitude reflects the level of m o t o r preparation. Two other brain potentials, a slow positive and a slow negative wave, seem to depend on the psychological properties of S~. Enhanced amplitudes were found, w h e n S 1 provides information, besides its warning function.
Introduction
Slow potential shifts (SPSs) develop in the human brain between a warning signal ($1) and an imperative signal ($2), which requires a response (overt or covert). The most familiar SPS is the contingent negative variation (CNV), which was first recorded by Walter et al. (1964). Recent studies which used interstimulus intervals (ISis) o f longer duration (>/ 3 sec) have shown that the CNV is not a unitary phenomenon, but consists of several SPSs: at least two negative SPSs were found which differ not only in their functional significance, but also in their topographical distribution (Loveless and Sanford 1974; Rohrbaugh et al. 1976; Gaillard 1976, 1977). The first potential has the form of an inverted U-curve, which reaches its peak within one sec after S~; the second potential gradually develops during the interstimulus interval (ISI), and reaches its maximum towards the end of this interval. Because of its occurrence towards the end of the ISI and its contingency to $2 (or to the response to $2) the second potential seems to resemble more * Address requests for reprints to: A. W. K. Gaillard, ~ s t i t u t e for Perception TNO, Postbox 23, / 3769 ZG Soesterberg, T h e Netherlands.
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closely the 'classical CNV' obtained with short ISis; therefore, it will be called t e r m i n a l C N V . The term CNV will be used in its original meaning, to denote the negative shift preceding S~ in studies with short ISis, which generally did not discriminate between the two negative potentials described here. The first potential will be called s l o w negative wave (SNW) because of its similarity to the 'slow wave' observed in selective listening tasks (e.g., Squires et al. 1977). The SNW appears to be dependent on the characteristics of $1 in much the same way as the P3. Therefore, both seem to belong to a family of slow potential shifts following $1. Similar to the SNW the P3 will be called s l o w positive wave (SPW). This notation is preferred because often a clear peak cannot be detected in these slow waves following $1. Thus, in the present study three slow potential shifts are distinguished: SPW, SNW and terminal CNV. The terminal CNV appears to be determined mainly by the level of m o t o r preparation, since it is affected by task variables such as time uncertainty (i.e. the duration or irregularity of the ISI), muscle effort and instructions governing the trade-off between speed and accuracy, which are known to influence m o t o r preparation. This notion is supported by the remarkable similarity with regard to both morphology and topography between this terminal CNV and the readiness potential (RP) which precedes any voluntary movement (Gaillard 1978; Rohrbaugh et al. 1976). In this view the terminal CNV and the RP reflect largely the same neurophysiological phenomenon, which, however, is obtained in different experimental situations and by different averaging methods. In the present study this issue is further investigated by comparing the usual averages time locked to $2, with movement potentials, i.e. with averages time locked to button presses given in the context of the S~-$2 paradigm. This is done for several task requirements. The trade-off between speed-accuracy is varied by using a deadline procedure (see Ollman 1977). The subject was instructed to respond to $2 before a deadline set by the experimenter. If the subject did not respond before the deadline, a noise burst was given through earphones. In this way the subject received immediate feedback about his reaction performance. This procedure was used to ensure that subjects would follow the instructions throughout the block of trials. Especially under speed instruction subjects tend to slow down their performance during the experimental sessions or even during one block of trials.
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149
Since the RP is usually obtained when the subject produces voluntary movements without external stimulation, an experimental condition was added in which the subject had to synchronize the response with the arrival of $2. This time-estimation task resembles the $1 - $ 2 paradigm and the voluntary-movement situation. It is generally assumed that the estimation of the arrival o f $2 is an important determinant of the preparatory processes for the response to S:. The increase of RT with longer ISI is generally explained as resulting from increased difficulty to predict the m o m e n t $2 will arrive (e.g. N~i~it~inen and Merisalo 1977). A second aim of the present study was to investigate whether the SNW was affected by the information revealed by $1. It has been shown that the SNW is influenced by the physical characteristics of S~, such as modality, probability, intensity, and duration o f this stimulus (Gaillard 1976; Kok 1978; Loveless and Sanford 1975; Klorman and Bentsen 1975). It is also conceivable that this wave may be affected by the psychological significance of S~. Indeed, in earlier studies (Gaillard 1977; Gaillard and Perdok 1979) the SNW was enhanced, when S~ had more relevance for the subject. In the present study the psychological significance of $1 was varied by making the signal informative. A larger SNW was expected when S~ was informative as compared to the situation where $1 only served as a warning that $2 would soon arrive. $1 provided information with regard to the instructions to be followed at $2 ; this was done in two ways: in one situation the speed and accuracy were varied from trial-to-trial within one block of trials and in the other situation S~ indicated either a speed or a detection instruction; under the latter instruction the subject was required to make a discrimination at $2, but to delay the response with one sec. The e x p e r i m e n t Me th o d S u b j e c t s a n d apparatus T h e Ss were 10 male University s t u d e n t s , w h o were paid for t h e i r p a r t i c i p a t i o n . T h e data o f o n e o t h e r S were discarded for having t o o m a n y e y e - m o v e m e n t artifacts. T h e Ss were seated in a c o m f o r t a b l e chair, w h i c h also p r o v i d e d a neck- and arm-rest. $1 was a 7 0 dB t o n e (delivered via h e a d p h o n e s ) w i t h a f r e q u e n c y o f e i t h e r 5 0 0 or 2 0 0 0 Hz. $2 consisted o f slides w h i c h were p r o j e c t e d ( K o d a k - C a r o u s e l ) t h r o u g h t h e w i n d o w of t h e cubicle, o n a screen at a d i s t a n c e o f ca. 1 m e t e r f r o m t h e
150
A. W. K. Gaillard, J. Perdok ~Slow brain potentials
S's eyes. $2 was always c o m p o s e d of t w o vertical bars ( w i d t h 5.5 cm), w h i c h differed in l e n g t h (37.5 versus 5.5 cm). Ag-AgC1 disk e l e c t r o d e s were a t t a c h e d to t h r e e scalp sites (Fz, Cz, Pz) and to t h e earlobes, w h i c h were linked for reference. B e c k m a n m i n i a t u r e e l e c t r o d e s were t a p e d to t h e right supra- and i n f r a - o r b i t a l ridge to r e c o r d t h e vertical EOG. A n e l e c t r o d e j u s t a b o v e t h e n o s e b r i d g e served as a g r o u n d . A f t e r a m p l i f i c a t i o n (timec o n s t a n t 6 sec) t h e E E G and EOG signals were r e c o r d e d o n a m a g n e t i c tape. Procedure
A trial s t a r t e d w i t h t h e o n s e t o f $1, w h i c h was f o l l o w e d 4 sec later b y t h e slide r e m a i n i n g o n for 1 sec. T h e i n t e r t r i a l interval ( $ 2 - S 1) was varied irregularly b e t w e e n 1 1 - 1 4 sec. U n d e r all e x p e r i m e n t a l c o n d i t i o n s , e x c e p t for t h e s y n c h r o n i z a t i o n task, t h e S was asked to d i s c r i m i n a t e w h i c h o f t h e t w o bars was longer. He h a d to r e s p o n d w i t h his right h a n d w h e n t h e right bar was longer and w i t h his left h a n d w h e n t h e left o n e was longer. To i n d u c e changes in t h e s p e e d - a c c u r a c y t r a d e - o f f t w o d e a d l i n e s were used b e f o r e w h i c h t h e S had to r e s p o n d ; if he did n o t r e s p o n d b e f o r e t h e d e a d l i n e 60 dB noise was given via e a r p h o n e s ( d u r a t i o n 5 0 0 msec). U n d e r a c c u r a c y i n s t r u c t i o n t h e d e a d l i n e interval was t w i c e as l o n g as u n d e r speed i n s t r u c t i o n . T h e S was told t h a t o n l y t h e c o r r e c t r e a c t i o n s o c c u r r i n g b e f o r e t h e deadline w o u l d c o u n t . Especially u n d e r speed i n s t r u c t i o n , t h e i m p o r t a n c e o f b e a t i n g t h e d e a d l i n e was stressed, since Ss were f o u n d to t e n d t o p r e f e r receiving noise to m a k i n g errors. With t h e accuracy i n s t r u c t i o n t h e a v o i d a n c e o f m a k i n g errors was e m p h a s i z e d . T h e d u r a t i o n of t h e d e a d l i n e interval was d e t e r m i n e d in a separate t r a i n i n g session, o n e week b e f o r e t h e e x p e r i m e n t a l Session. T h e d e a d l i n e u n d e r speed i n s t r u c t i o n was c h o s e n such t h a t the S m a d e 2 0 - 3 0 % errors; in fact, the variability b e t w e e n Ss was so low t h a t all Ss c o u l d get t h e same interval o f 2 5 0 msec u n d e r speed, and 5 0 0 msec u n d e r a c c u r a c y i n s t r u c t i o n . In a d d i t i o n t h e S received a s y n c h r o n i z a t i o n task in w h i c h he had to s y n c h r o n i z e his r e s p o n s e w i t h t h e arrival o f $2. T h e S was i n s t r u c t e d to react w i t h his right h a n d o n h a l f o f t h e trials and w i t h his left h a n d in t h e o t h e r half. (Since no d i f f e r e n c e s were f o u n d b e t w e e n t h e s e c o n d i t i o n s , t h e d a t a for right a n d left h a n d r e s p o n d i n g were c o m b i n e d . ) T h e speed and a c c u r a c y i n s t r u c t i o n s were varied e i t h e r b e t w e e n or w i t h i n a b l o c k o f trials, In the l a t t e r s i t u a t i o n SI i n d i c a t e d the i n s t r u c t i o n t h e S had t o follow. F o r h a l f o f t h e Ss t h e high t o n e ( 2 0 0 0 Hz) i n d i c a t e d a speed i n s t r u c t i o n and t h e low t o n e ( 5 0 0 Hz) t h e a c c u r a c y i n s t r u c t i o n ; this was reversed for t h e o t h e r half. F i n a l l y , t h e speed i n s t r u c t i o n was also given w i t h i n a b l o c k of trials, t o g e t h e r w i t h a d e t e c t i o n i n s t r u c t i o n ; u n d e r this i n s t r u c t i o n t h e S was asked to i n d i c a t e w h i c h bar was longer at t h e off-set of t h e slide (1 sec a f t e r $2). Again $1 i n d i c a t e d w h i c h i n s t r u c t i o n h a d to be f o l l o w e d ; t h e t o n e w h i c h h a d i n d i c a t e d t h e a c c u r a c y instruction now indicated the detection instruction. Half o f t h e Ss received t h e 7 e x p e r i m e n t a l c o n d i t i o n s in t h e f o l l o w i n g order: s y n c h r o n i z a t i o n / s p e e d / a c c u r a c y ( b e t w e e n ) , s p e e d / a c c u r a c y , s p e e d / d e t e c t i o n ; and t h e o t h e r half o f t h e Ss received t h e m in t h e reversed order. T h e r e were 50 trials in each e x p e r i m e n t a l c o n d i t i o n .
A. W. K. Gaillard, J. Perdok / Slow brain potentials
151
Data analysis T h e t h r e e E E G c h a n n e l s , and EOG c h a n n e l were digitized at a rate of 25 samples per sec. T h e analysis p e r i o d started 1 sec b e f o r e $1 and e n d e d at t h e o n s e t of $2. F o r each trial t h e n u m b e r of data p o i n t s was r e d u c e d to 25 b y t a k i n g t h e averages o f successive g r o u p s of 5 p o i n t s ; this yielded o n e average data p o i n t per 2 0 0 msec. T h i r t e e n o f t h e s e data p o i n t s were r e t a i n e d for f u r t h e r analysis (see also fig. 1). T h e s e d a t a p o i n t s were averaged across 4 0 trials, leaving 10 trials for a r t i f a c t rejection, and t h e following m e a s u r e s were o b t a i n e d , separately for each d e r i v a t i o n : SPW, SNW and t e r m i n a l CNV, i.e. t h e average E E G in t h e period 2 0 0 4 0 0 msec and 6 0 0 - 8 0 0 msec after S1 and 2 0 0 msec before $2, referred to a baseline (average E E G in t h e period 2 0 0 msec b e f o r e $1). O n these m e a s u r e s and t h e m e a n RT, s t a n d a r d d e v i a t i o n and % o f errors t w o t y p e s o f A N O V A s were carried out: a t w o - w a y A N O V A , w h i c h i n c l u d e d instruct i o n s (speed vs. a c c u r a c y ) and t h e b e t w e e n / w i t h i n b l o c k s variable. In t h e second A N O V A t h e t h r e e speed i n s t r u c t i o n s were c o m p a r e d : speed ( b e t w e e n ) , speed ( a c c u r a c y ) , speed ( d e t e c t i o n ) . In a d d i t i o n , t h e EEG was averaged twice (sample interval 14 msec): o n c e t i m e l o c k e d to Sz and o n c e to t h e response. Results R T-performance T a b l e 1 p r e s e n t s t h e R T - d a t a separately for t h e e x p e r i m e n t a l c o n d i t i o n s . F o r b o t h t h e b e t w e e n a n d w i t h i n c o n d i t i o n t h e m e a n R T was m u c h s h o r t e r u n d e r speed t h a n u n d e r a c c u r a c y i n s t r u c t i o n ( F ( 1 / 9 ) = 1 10; p < 0.001). O f course, t h e r e were m a n y m o r e errors u n d e r speed t h a n u n d e r a c c u r a c y i n s t r u c t i o n s ( F ( 1 / 9 ) = 4 0 . 7 ; p < 0.01), while t h e s t a n d a r d d e v i a t i o n was m u c h smaller ( F ( 1 / 9 ) = 4 2 . 4 ; p < 0.01). A l t h o u g h t h e d i f f e r e n c e in R T and t h e s t a n d a r d d e v i a t i o n b e t w e e n t h e speed
Table 1 The mean RT, standard deviation (in msec) and the % of errors, separately for the experimental conditions. Standard deviation
% of errors
- 5
252
-
224 375
48 80
29 2
223 362
46 83
29 2
231 1220
55 77
27 2
Mean RT Synchronization Between Speed Accuracy Within Speed Accuracy Within Speed Detection
152
A. 14I.K. Gaillard, J. Perdok / S l o w brain potentials
and a c c u r a c y i n s t r u c t i o n s were smaller in t h e w i t h i n - t h a n in t h e b e t w e e n - c o n d i t i o n , this i n t e r a c t i o n was n o t significant ( F ( 1 / 9 ) < 1). Also t h e m a i n effect of b e t w e e n versus w i t h i n b l o c k s was n o t significant for e i t h e r m e a n RT, % errors or s t a n d a r d deviation. Ss were q u i t e able to s y n c h r o n i z e t h e i r r e s p o n s e s w i t h $2; five o f t h e m reacted o n average w i t h i n 50 msec b e f o r e or a f t e r $2, while all reacted w i t h i n -+260 msec. Nevertheless, the s t a n d a r d d e v i a t i o n was very large, five t i m e s m o r e t h a n u n d e r speed i n s t r u c t i o n . S l o w p o t e n t i a l shifts Fig. 1 shows t h e c o m p o s i t e m e a n s o f the v e r t e x slow p o t e n t i a l , u n d e r t h e s y n c h r o n i z a t i o n , speed and a c c u r a c y i n s t r u c t i o n s (all varied b e t w e e n blocks). U n d e r each i n s t r u c t i o n a SNW w i t h a l a t e n c y of ca. 6 0 0 msec was f o l l o w e d b y a t e r m i n a l CNV. In a t w o - w a y A N O V A o n t h e t e r m i n a l C N V t h e s p e e d - a c c u r a c y effect was significant ( F ( 1 / 9 ) = 7 . 8 4 ; p < 0.02), while t h e v a r i a t i o n o f i n s t r u c t i o n b e t w e e n or w i t h i n a b l o c k o f trials h a d n o i n f l u e n c e ( F ( 1 / 9 ) < 1). A l t h o u g h t h e effect o f i n s t r u c t i o n s was larger in t h e b e t w e e n - c o n d i t i o n (6.4 # V ) t h a n in the w i t h i n - c o n d i t i o n (2 # V ) , this i n t e r a c t i o n did n o t r e a c h statistical significance ( F / 1 / 9 ) = 3.54; p < 0.09). No d i f f e r e n c e s were f o u n d b e t w e e n t h e speed and t h e s y n c h r o n i z a t i o n i n s t r u c t i o n , e x c e p t t h a t in t h e latter c o n d i t i o n the t e r m i n a l C N V reaches its m a x i m u m o n e data p o i n t earlier t h a n u n d e r speed i n s t r u c t i o n , w h i c h c o r r e s p o n d s w i t h t h e earlier response. In c o n t r a s t to t h e t e r m i n a l CNV, t h e SNW a m p l i t u d e was n o t significantly affected b y i n s t r u c t i o n s (see fig. 1). However, t h e f r o n t a l SNW was i n f l u e n c e d b y t h e b e t w e e n / w i t h i n b l o c k s variable ( F ( 1 / 9 ) = 11.9 ;p < 0.01). T h e SNW was e n h a n c e d w h e n $I i n d i c a t e d the i n s t r u c t i o n s to be f o l l o w e d at $2. T h e parietal SPW was a f f e c t e d b y b o t h factors. Larger a m p l i t u d e s were f o u n d u n d e r speed i n s t r u c t i o n
]
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time (see) Fig. 1. The composite means of the vertex slow potential under the synchronization, speed and accuracy instructions, all varied between blocks.
A. W. K. Gaillard, J. Perdok / Slow brain potentials
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Fig. 2. The composite means of the vertex slow potential under the speed instruction conditions; this instruction was either held constant over a block of trials (between) or it was varied from trial-to-trial in combination with accuracy or with the detection instruction. ( F ( 1 / 9 ) = 4.8; p < 0 . 0 5 ) a n d u n d e r t h e w i t h i n c o n d i t i o n ( F ( 1 / 9 ) = 5.9; p < 0 . 0 4 ) ; t h e i n t e r a c t i o n b e t w e e n t h e s e f a c t o r s w a s n o t s i g n i f i c a n t ( F ( 1 / 9 ) < 1). Fig. 2 p r e s e n t s t h e c o m p o s i t e v e r t e x d a t a u n d e r t h e s p e e d i n s t r u c t i o n , f o r t h e t h r e e e x p e r i m e n t a l c o n d i t i o n s : t h i s i n s t r u c t i o n w a s e i t h e r held c o n s t a n t o v e r a b l o c k o f trials ( b e t w e e n ) o r varied f r o m trial-to-trial in c o m b i n a t i o n w i t h a c c u r a c y Table 2 The EEG and the corresponding EOG amplitudes (in/~V) of the SPW, SNW and the terminal CNV, separately for the experimental conditions. Between Synchronization SPW Fz
- 0.6
cz
Pz EOG SNW
+0.3
+ 2.6 - 2.0
Within
Within
Speed
Accuracy
Speed
Accuracy
Speed
Detection
+ 0.6 +0.8 + 3.2 0.0
4.6 4.8 +1.0 0.0
0.0 0.0 +4.8 0.0
-0.6 4.6 +3.0 0.0
-0.4 0.0 + 4.0 -2.0
-1.2 -1.4 +4.6 0.0
4.8
-2.4
-2.6
-7.4
4.4
-9.8
4.0
Cz
- 4.9
-3.0
-3.8
-6.2
-8.0
-2.6
Pz
+ 1.3
-2.2
+2.2
- 5.0
-2.2 4.0
+1.2
EOG CNV Fz Cz Pz EOG
0.0 - 6.0
4.2 +0.8
-
-2.0
-
8.0
+2.0
- 6.3 -10.4 - 6.5 - 6.0
-4.2 -12.0 - 7.2 0.0
-1.6 -5.6 4.0 +4.0
- 2.4 -9.4 -6.6 +12.0
-5.2 -7.4 4.6 -8.0
-4.0 -10.0 -7.2 +8.0
+1.0 -1.4 -1.0 +8.0
rz
-
2.0
154
A. W. K. Gaillard, J. Perdok / S l o w brain potentials
or w i t h d e t e c t i o n . O n e - w a y A N O V A s revealed t h a t t h e t e r m i n a l CNV and t h e SPW were u n a f f e c t e d b y t h e s e c o n d i t i o n s , b u t t h e f r o n t a l SNW was i n f l u e n c e d ( F ( 2 / 1 8 ) = 5.77; p ~ 0.01). T h e a m p l i t u d e o f t h e SNW was e n h a n c e d w h e n S1 i n d i c a t e d t h e speed i n s t r u c t i o n ; t h i s e f f e c t was m o s t p r o m i n e n t w h e n speed and d e t e c t i o n instruct i o n s were varied w i t h i n o n e b l o c k o f trials (see fig. 2 and table 2). To illustrate t h e r e l a t i o n s h i p b e t w e e n t h e SNW and SPW t h e f r o n t a l a n d parietal slow p o t e n t i a l s in t h e 1.5 sec a f t e r S~ are d e p i c t e d in fig. 3. It can be seen t h a t b o t h t h e SNW and t h e SPW are e n h a n c e d w h e n $1 provides i n f o r m a t i o n , besides its w a r n i n g f u n c t i o n (first vs. s e c o n d panel). A f u r t h e r e n h a n c e m e n t o c c u r s w h e n $1 i n d i c a t e s t h e speed i n s t r u c t i o n where speed and d e t e c t i o n i n s t r u c t i o n s are varied f r o m trial-to-trial. This e n h a n c e m e n t is n o t seen w h e n Sx i n d i c a t e s the d e t e c t i o n i n s t r u c t i o n , whereas also t h e SPW does n o t differ b e t w e e n t h e two t y p e s of withinc o n d i t i o n s ( s e c o n d vs. t h i r d panel). T h e s e effects are also s h o w n in fig, 4 w h e r e the E E G is averaged w i t h a sample interval of 14 msec. B o t h t h e SNW and t h e SPW ( a n d also P3) are increased w h e n Sa is m a d e i n f o r m a t i v e . However, for t h e SNW this e n h a n c e m e n t is larger u n d e r the speed t h a n u n d e r t h e d e t e c t i o n i n s t r u c t i o n , w h e r e a s for t h e SPW t h e r e is n o d i f f e r e n c e b e t w e e n t h e i n s t r u c t i o n s in this respect. [
-10 -8
,
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|
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Fig. 3. The frontal (F z) and parietal (Pz) slow potentials in the 1.5 sec after S 1 , in the following conditions: speed and accuracy instructions varied either within or between blocks of trials; and the speed and detection instructions varied within one block of trials. S - l o c k e d vs. R - l o c k e d p o t e n t i a l s
Figs. 5 and 6 p r e s e n t t h e v e r t e x p o t e n t i a l s t i m e - l o c k e d t o e i t h e r $2 (S-locked) or to t h e r e s p o n s e (R-locked). In all S-locked averages ( e x c e p t u n d e r d e t e c t i o n ) the t e r m i n a l CNV is f o l l o w e d b y an N1 peak ( l a t e n c y 175 msec) a n d a large P3 ( l a t e n c y 3 2 5 - 3 7 5 msec). T h e P2 c o m p o n e n t is o n l y observed in t h o s e c o n d i t i o n s ( a c c u r a c y and d e t e c t i o n ) w h e r e P3 is r e d u c e d and delayed. T h e l a t e n c y of the P3 c o m p o n e n t was marginally i n f l u e n c e d b y e x p e r i m e n t a l c o n d i t i o n s . T h e l a t e n c y was 325 msec u n d e r s y n c h r o n i z a t i o n , 3 5 0 msec u n d e r b o t h speed c o n d i t i o n s a n d 375 msec u n d e r
A. W. K. Galliard, J. Perdok ~Slow brain potentials
155 Fz Pz-
lO.uV
~
--
speed [between;
V
I
I
S1
I
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1 time (sec)
I
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Fig. 4. The composite means of the frontal and parietal EEG under the speed instructions held constant over a block of trials (between), under the speed instruction and under the detection instruction, both varied within a block of trials.
a c c u r a c y and d e t e c t i o n i n s t r u c t i o n s . These differences in l a t e n c y are small comp a r e d t o t h e d i f f e r e n c e s in RT. F o r e x a m p l e , the d i f f e r e n c e in R T b e t w e e n speed and a c c u r a c y i n s t r u c t i o n s was 151 msec, while t h e difference in P3 l a t e n c y was only 25 msec. T h e evoked p o t e n t i a l s to $2 were largest in t h e t w o speed c o n d i t i o n s ; the N1 peak was smallest u n d e r s y n c h r o n i z a t i o n , w h i c h illustrates t h e relative u n i m p o r t a n c e of $2 in this c o n d i t i o n . As can be seen in figs. 5 and 6, t h e R-locked averages show a r e m a r k a b l e similarity in f o r m and t i m e course to p o t e n t i a l s s y n c h r o n i z e d to v o l u n t a r y m o v e m e n t s . As w i t h t h e p o t e n t i a l s t i m e l o c k e d to v o l u n t a r y m o v e m e n t s , t h e r e s p o n s e is p r e c e d e d b y a slow negative shift, w h i c h could be d e n o t e d as readiness p o t e n t i a l (RP). This gradual negative shift t u r n s in to a sharp negative i n f l e c t i o n , w h i c h is f o l l o w e d b y a slow positive change. U n d e r a c c u r a c y i n s t r u c t i o n s this positivity already starts 150 msec b e f o r e t h e closure of t h e b u t t o n switch. This p h e n o m e n o n , w h i c h was p r e s e n t in 9 o u t of 10 Ss, l o o k s very similar to t h e p r e - m o t i o n - p o s i t i v i t y o b s e r v e d by Deecke et al. ( 1 9 7 6 ) . To c o m p a r e t h e slow negative shifts in t h e S- and R-locked averages, t h e a m p l i t u d e of t h e t e r m i n a l C N V (average E E G 100 msec b e f o r e $2) was c o m p u t e d in b o t h averages. T h e s e m e a s u r e s were c o m p a r e d to t h e a m p l i t u d e of t h e RP (average E E G in the p e r i o d 2 5 0 - 1 5 0 msec b e f o r e t h e response) (see Deecke et al. 1976). All
156
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t h r e e m e a s u r e s were referred to t h e same baseline (average E E G 2 0 0 0 - 1 8 0 0 msec b e f o r e t h e response). Largest a m p l i t u d e s were o b t a i n e d at C2 for all m e a s u r e s a n d e x p e r i m e n t a l c o n d i t i o n s . T h e r e were virtually n o d i f f e r e n c e s (less t h a n 1 /~V) b e t w e e n t h e t h r e e v e r t e x m e a s u r e s u n d e r speed i n s t r u c t i o n . This is n o t so surprising since b o t h t h e m e a n a n d t h e variability of t h e RT in this c o n d i t i o n were small, and c o n s e q u e n t l y t h e d i s c r e p a n c y b e t w e e n t h e t w o ways of averaging will also be small. In c o n t r a s t , u n d e r s y n c h r o n i z a t i o n i n s t r u c t i o n t h e r e was a d i f f e r e n c e in v e r t e x term i n a l C N V o f 2.4 /~V b e t w e e n t h e t w o t y p e s o f averages. Since t h e variability in r e s p o n s e t i m e was large, t h e e n h a n c e d a m p l i t u d e in t h e R-locked average suggests t h a t t h e t e r m i n a l C N V is c o n t i n g e n t o n t h e r e s p o n s e a n d n o t o n t h e s t i m u l u s (Sz). H o w e v e r , u n d e r a c c u r a c y i n s t r u c t i o n t h e a m p l i t u d e s o b t a i n e d in t h e R-locked averages were n o t larger. This m a y be caused b y t h e larger t i m e lag b e t w e e n $2 and t h e r e s p o n s e in this c o n d i t i o n ( 3 7 5 msec). As can b e seen in fig. 5, t h e negativity t i m e - l o c k e d t o t h e r e s p o n s e still increases a f t e r $2 u n t i l 150 msec b e f o r e $2. There-
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fore, the RP measure taken at this moment is much larger than the terminal CNV in both the S-locked and in R-locked averages. Although small in amplitude, the RP preceding the delayed response under the detection shows the same midline distributions as the terminal CNV and the RP under the other instructions.
Discussion T h e p r e s e n t e f f e c t s o f t h e s p e e d - a c c u r a c y i n s t r u c t i o n s w e r e larger f o r b o t h the terminal CNV and RT than in earlier e x p e r i m e n t s (Gaillard
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1977; Loveless and Sanford 1974). This might be due to the deadline procedure, which induced stable and fast RT performance throughout a block of trials. Although not significant, the speed-accuracy effect was somewhat smaller when S~ indicated the instruction to be followed (within-condition). Some subjects may have found it difficult to change their strategy from trial to trial. The enhanced terminal CNV under speed instruction may be explained by assuming that under this instruction the level of m o t o r preparation is increased, as compared to the accuracy instruction. Also the absence of a terminal CNV under detection instruction found in this and in the previous study, supports the idea that the terminal CNV is responserelated. The potentials time-locked to the b u t t o n press to $2 (figs. 5 and 6) are similar in form to movement potentials preceding voluntary finger presses (see Deecke et al. 1976; Rohrbaugh et al. 1976). Similar to the RP preceding voluntary movements, the negative shift preceding responses in the S~-$2 paradigm is also most prominent at the m o t o r region (Cz). This was also the case when responses were synchronized to $2 or were delayed by one sec after $2 (detection instruction). Syndulko and Lindsley (1977) found the same midline distribution under both the RT- and detection instruction; in addition, they showed that this negative shift had a lateral asymmetry contralateral to the responding hand. The present results obtained under the synchronization instruction deviate from Ruchkin et al. (1977), who used a similar task but with an ISI of 900 msec. Their data (op. cit., fig. 1) show an inverted U-curve with a peak ca. 700 msec after S~. Therefore, their data suggest tllat a gradual rise similar to the RP was not found because the negative shift preceding the response was disrupted by the SNW to S~ (a click). However, gradual negative shifts were obtained when the EEG was timelocked to the finger presses (op. cit., fig. 2). Moreover, these negative shifts in the synchronization task were quite similar to RPs preceding voluntary movements, obtained from the same subjects. The above findings suggest that the negativity preceding the responses in the $1-$2 paradigm is the same as the RP preceding voluntary movements. In this view the terminal CNV obtained from stimulus locked averages largely consists of the increasing limb o f the movement related negative shift. Consequently, the terminal CNV could be regarded as that part o f the RP which precedes $2. It is conceivable that with shorter RTs (for example, induced by deadlines or speed instructions)
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the RP intrudes more into the S 1 - - S 2 interval and in this way enhances the amplitude of the terminal CNV. This view is supported by the larger negativity in the averages time-locked to the response as compared to the negative shifts in the stimulus-locked averages. There are, however, also some differences in the negative shifts observed in the present experimental conditions and the RP: the onset of negative shift under speed, accuracy and synchronization instructions was about one sec earlier and this shift reached an amplitude which was about two times larger than the RP reported by Deecke et al. (1976). It could be argued that in the synchronization task, as well as in the RTtasks, not only motor but also perceptual and decision related processes are involved. However, the terminal CNV seems to be related neither to perceptual nor to decision processes (see Gaillard 1978). Therefore, a more likely explanation would be that the present experimental conditions are more i n t e r e s t i n g than monotonously repeating a large number of voluntary movements. It has been shown that the CNV (Irwin e t al. 1966; Waszak and Obrist 1969) and also the RP (McAdam and Seales 1969) are influenced by motivation. In the latter study RP-amplitudes preceding voluntary finger presses, increased from 5 /JV to 12/aV, when subjects were told that they would receive 10 cents for each response, if given 'at the right time'. In fact, rewards for these 'correct' responses were given in a random fashion. Thus, it could be that the increased RP-values reflect a higher level o f m o t o r preparation, caused by an enhanced interest of the subject in the task. Both above-mentioned explanations assume that terminal CNV and RP are largely produced by the same neurophysiological generator, but according to the first both can be influenced also by perceptual and decision processes, while in the second view they are only determined by the level of m o t o r preparation. Recent models for the speed-accuracy trade-off assume that speed instructions primarily increase the level of m o t o r preparation, whereas the quality of information processing or the rate at which information builds up is not enhanced (Pachella 1974; Ollman 1977; Posner et al. 1973; Thomas 1974). Moreover, the preparation for and the execution of a m o t o r response are regarded as processes which run parallel to the processing o f stimulus information. Another common property of some of these models is the idea that at least a proportion o f the responses are guesses, which are given independently of the ongoing evaluation of the stimulus information and which are correct only by chance. For
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example, the deadline model (Ollman 1977) supposes that there is a time limit (deadline), which is set by the subject or by the experimenter. A response is made when processing is terminated or when the deadline expires, whichever comes first. If processing finishes before the deadline, the response time is representative o f the time needed to process the stimulus information. If the deadline elapses before processing is complete, the response is a guess and less likely to be accurate. N~fitfinen and Merisalo (1977) regard m o t o r preparation as a gradually increasing process, which reaches an optimal level just before $2 arrives. In order to produce quick RTs it is necessary that when $2 arrives the level o f m o t o r preparation is in the vicinity o f the 'motor-action limit', the level at which a response is initiated. The distance kept between the level of m o t o r preparation and the m o t o r action limit, largely depends on the a m o u n t of training and the instructions given by the experimenter. During training subjects will learn to what extent they can increase their m o t o r preparation without making too many errors. Under normal RT instructions subjects will time their m o t o r preparation such that it is close to the motor-action limit at the m o m e n t they expect information processing to be terminated. When speed is stressed the m o t o r action limit will be approached very closely, which increases the possibility for premature response, but shortens the RT. Under accuracy instructions m o t o r preparation will never come close to the motor-action limit before information processing is terminated; so that, on the one hand, no errors will be made, on the other hand, it will cost more time to reach the m o t o r action limit, once information processing is terminated. The terminal CNV seems to be a good candidate for the m o t o r preparation, as defined in the above view, whereas the latency of the P3 of the evoked potential to S~ indicates that information processing is terminated. On the basis o f a trial-to-trial analysis Kutas et al. ( 1 9 7 7 ) c o m p a r e d the RT with the latency o f the P3 on the same trial. They found the correlation between P3 latency and RT was larger under accuracy than under speed instruction. Moreover, under speed instruction the incorrect RTs mostly preceded the P3. According to the authors these data suggest that under accuracy instruction the m o t o r processes of selection and execution o f the response are tightly coupled with the evaluation of the stimulus. However, under speed instruction stimulus evaluation, as indexed by the P3 component, is only loosely related with the
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execution o f the motor response: responses may be generated b e f o r e the stimulus has been fully evaluated. Also in the present study the mean RT (224 msec) preceded the P3 (350 msec) under speed instructions, while these latencies coincided under accuracy instructions (375 msec). In the present study the frontal SNW was strongly affected by the information content o f S1 (see also fig. 2). The amplitude o f this wave was enhanced when $1 indicated either that a speed or an accuracy instruction had to be followed at $2. This effect was even larger when speed and detection instructions were varied within one block of trials. The SPW was also influenced by the present conditions, but in a different way, a result also found by Squires e t al. (1977). Thus, it was not only enhanced when S~ contained information, but this wave was also larger under speed than under accuracy instructions. Moreover, in the condition where speed and detection were varied within one block, the amplitude of the parietal SPW did not differ between speed (4/aV) and detection instructions (4.6 uV), while for the frontal SNW a difference of 5.8 uV was found (see also table 2). Thus, the SPW and SNW seem to be affected by some conditions in the same way and differently in other instances, which suggests that these waves reflect different, but related mechanisms. That different processes are involved is also suggested by the differences in latency ( 3 0 0 - 4 0 0 msec vs. 6 0 0 - 7 0 0 msec after $1) and midline distribution (parietal-frontal). An explanation o f the close inter-relationship between the SNW and the SPW might be found in the fact that both waves are most prominent in non-specific parts of the cortex: the SPW in the association cortex and the SNW in the frontal cortex. These observations can be described rather well in terms of Luria's model of the functional organization of the brain. According to Luria (1973) the function of the frontal lobes is to organize behavior and to regulate arousal systems in accordance with the immediate task demands. This view is supported by the observation that on the one hand largest effects on the SNW were found when $1 indicated task demands which considerably diverged (speed vs. detection). On the other hand, when task demands are constant (between) or similar (speed vs. accuracy) over a block of trials, the preparation for the execution of the task is very likely to take place before the arrival of S~.
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