482
Electroencephalography and Clinical Neurophysiology, 1 9 7 9 , 4 6 : 4 8 2 - - 4 8 5 © E l s e v i e r / N o r t h - H o l l a n d Scientific Publishers, Ltd.
Laboratory note A TECHNIQUE FOR SEPARATING ENDOGENOUS FROM EXOGENOUS HUMAN CORTICAL POTENTIALS E.J. H A M M O N D 1, D.A. S I L V A , A.J. K L E I N a n d D.C. T E A S
Institute for Advanced Study of the Communication Processes, University of Florida, Gainesville, Fla. 32610 (U.S.A.) ( A c c e p t e d for p u b l i c a t i o n : O c t o b e r 30, 1 9 7 8 )
Since t h e initial r e p o r t s o n t h e P 3 0 0 or P3 comP o n e n t of the h u m a n e v o k e d p o t e n t i a l (Davis 1 9 6 4 ; S u t t o n et al. 1 9 6 5 ) , t h e sensory a n d p e r c e p t u a l determ i n a n t s o f t h e a m p l i t u d e a n d l a t e n c y of this p o t e n t i a l have r e m a i n e d i n c o m p l e t e l y u n d e f i n e d . T h a t t h e P3 is evoked by rarely occurring or novel stimuli (e.g., R o t h 1 9 7 3 ; C o u r c h e s n e et al. 1 9 7 5 ) suggests it is a neural correlate of t h e o r i e n t i n g response, b u t it can also be elicited b y f r e q u e n t l y a p p e a r i n g stimuli u n d e r certain c i r c u m s t a n c e s ( R i t t e r a n d V a u g h a n 1 9 6 9 ; F r i e d m a n et al. 1975). While several investigations have related P3 amplit u d e to p e r c e p t u a l responses associated w i t h s t i m u l u s d e t e c t i o n (Hillyard et al. 1 9 7 1 ; K.C. Squires et al. 1 9 7 3 ; N. Squires et al. 1975), m o r e r e c e n t r e p o r t s indicate t h a t a m p l i t u d e s of averaged e v o k e d r e s p o n s e s are also d e p e n d e n t o n s e q u e n t i a l a n d a priori p r o b abilities of the stimuli (K.C. Squires et al. 1976, 1977). In c o n t r a s t to s h o r t l a t e n c y sensory e v o k e d p o t e n tials r e p o r t s of the P3 l a t e n c y f r o m d i f f e r e n t laboratories have varied over s o m e 200 msec ( R i t t e r a n d Vaughan 1969; Roth 1973); and intrasubject latency can vary over a similar range ( R i t t e r e t al. 1972). F r i e d m a n et al. ( 1 9 7 5 ) , h a v i n g f o u n d longer visual P3 latencies in r e s p o n s e to w o r d s w h e n t h e y delivered s y n t a c t i c i n f o r m a t i o n as c o m p a r e d t o w h e n t h e y did n o t , suggested P3 i n v o l v e m e n t in a language processing s y s t e m ; similarly, slightly longer s p e e c h - e v o k e d a u d i t o r y P3 latencies have b e e n a t t r i b u t e d t o m o r e c o m p l e x neural processing involved in s p e e c h perception ( G a l a m b o s e t al. 1975). However, a m o r e parsim o n i o u s p s y c h o a c o u s t i c a l d e t e r m i n a n t of t h e amplit u d e a n d l a t e n c y f l u c t u a t i o n s of these c o m p o n e n t s can be s h o w n .
I Address c o r r e s p o n d e n c e to: E d w a r d J. H a m m o n d , Ph.D., N e u r o l o g y Service, V e t e r a n s A d m i n i s t r a t i o n Hospital, Gainesville, Fla. 3 2 6 0 2 , U.S.A.
In our p a r a d i g m p r e s e n t e d here t h e s t i m u l u s d i m e n s i o n to be d i s c r i m i n a t e d is a d u r a t i o n c h a n g e r a t h e r t h a n a p i t c h or i n t e n s i t y c h a n g e ; t h e m o m e n t of decision a n d also P3 l a t e n c y can be m a n i p u l a t e d by c h a n g i n g t h e d u r a t i o n of t h e target stimulus.
Methods Ag-AgCl electrodes were applied t o scalp l o c a t i o n s Fz, Cz, a n d Pz a n d r e f e r e n c e d t o t h e left m a s t o i d . T h e t w o subjects were highly e x p e r i e n c e d in P3 experim e n t s . T h e y were i n s t r u c t e d to sit quietly a n d avoid eye m o v e m e n t s . Each E E G c h a n n e l was amplified ( × 2 0 , 0 0 0 , half a m p l i t u d e frequencies: 0.1 a n d 100 Hz), m o n i t o r e d b y an oscilloscope a n d r e c o r d e d o n an FM tape recorder. E v o k e d p o t e n t i a l s were digitized and p l o t t e d o n an X-Y p l o t t e r . T h e s u b j e c t ' s task was to d i s c r i m i n a t e ( c o u n t ) t h e n u m b e r of s h o r t t o n e bursts (target signals) e m b e d d e d in trains of longer t o n e bursts. E v o k e d responses were collected (in 4 separate r u n s ) in w h i c h the target stimuli were e i t h e r 50, 100, 200 or 4 0 0 msec in d u r a t i o n . T h e d u r a t i o n of the signal was m e a s u r e d with a digital timer. All t o n e b u r s t s ( 8 0 0 Hz) were of e q u a l i n t e n s i t y (50 dB SPL in a b a c k g r o u n d noise of 25 dB SPL) a n d were p r e s e n t e d to t h e right ear t h r o u g h T D H - 3 9 headp h o n e s . All t o n e bursts h a d 3 msec rise-fall times. T h e i n t e r - s t i m u l u s interval was 3 s e c ; t h e target signal was p r e s e n t e d r a n d o m l y w i t h a p r o b a b i l i t y of a b o u t 0.15. T h e s u b j e c t s sat in a reclining chair in a s o u n d a t t e n u ating r o o m . T h e d i s c r i m i n a t i o n task r e q u i r e d m o d e r ate b u t n o t i n t e n s e a t t e n t i o n to p e r f o r m . T h e subjects' i n s t r u c t i o n s were to i n f o r m t h e e x p e r i m e n t e r after 10 target signals h a d b e e n p r e s e n t e d , a n d t o avoid eye and finger m o v e m e n t s . It is difficult t o a c c u r a t e l y c o u n t t h e target stimuli w i t h o u t e m p l o y ing a c o u n t i n g strategy such as s u b - a u d i b l e vocalization or finger m o v e m e n t s , b u t it is a simple m a t t e r for the p r o p e r l y i n s t r u c t e d s u b j e c t to w i t h h o l d such a c t i o n for 2 or 3 see a f t e r t h e decision is m a d e , t h u s
SEPARATION OF ENDOGENOUS AND EXOGENOUS POTENTIALS avoiding any interactions.
483 time-locked electroencephalographic
Results
Pz 50msm tone ~ i
Fz Pz ,,00m.B
With this paradigm a psychoacoustical determinant of the latency of the decision-related components can be demonstrated by varying the duration of the target stimulus (Fig. 1). The increase in duration of the target stimulus necessarily forces an increase in the latency or m o m e n t of decision. Recorded from Pz in the highly experienced subject, the negative and positive peak latencies are about 220 msee and 340 msec, respectively, after the offset of the stimulus. By forcing subject's decision beyond 300 msec after stimulus onset, the negative potential can be temporally separated from the N2 of the on-response (Davis and Zerlin 1966). The late positive co m p o n en t has two components which resemble the P3a and P3b (N. Squires et al. 1975) both in latency and scalp topography; the earlier component is relatively larger at Fz and the later component is relatively larger at Pz (see Fig. 1). Control runs in which potentials were averaged in response to trains of target stimuli showed no offresponses (Schweitzer 1977). In this paradigm no motor response is required; the late negative and positive components therefore might represent different stages of orienting and stimulus evaluation uncontaminated by response selection processes. This demonstration that the latency of these waves varies directly with the m o m e n t of decision implies that this mechanism, which results in a latency jitter across many responses, is also a major determinant of the amplitude of averaged responses.
Discussion
200 m s total
This technique provides a method for measuring neural events, which reflect stimulus evaluation time, and avoids the temporal variability and electroen-
Fig. 1. Superimposed tracings of individual evoked responses from one subject. Five evoked responses are superimposed for each stimulus condition; responses were recorded simultaneously from Fz and Pz. These responses demonstrate time-locking to the m o m e n t of decision but nevertheless display latency jitter. The responses demonstrate a more frontal distribution for a positive co m p o n en t at about 300 msec after the offset of the tone and a more posterior distribution Tor a positive component at 340 msec. The preceding negativity at about 220 msec does not show a distinctive scalp topography. Calibrations: 10 pV and 100 msec.
484
E.J. HAMMOND ET AL.
cephalographic contamination (Vaughn et al. 1968; Arezzo and Vaughn 1975) due to efferent processes associated with motor response generation. Without reaction time measures, however, independent evidence that these potentials actually reflect the moment of decision is lacking. Independent of the stimulus duration, they are time-locked to the offset of the tone burst at which time the decision is presumably made by the attentive subject. The latency variability of reaction time measures and reaction timeP3 latency covariation is in fact greater because reaction time is a measure of stimulus evaluation time, response selection and generation time. The task-contingent negative potential occurring at about 220 msec has received little attention in previous experiments (e.g.K.C. Squires et al. 1976, 1977) perhaps because it occurs concurrently with the P2 of the motor potential (Vaughn et al. 1968; Arezzo and Vaughn 1975), and the P2 of the auditory off-response occurs in the same latency range.
Summary In a temporal discrimination paradigm, if signal duration is the stimulus dimension to be discriminated, the moment of subject's decision can be manipulated by varying the duration of the target stimulus. As stimulus duration is lengthened, taskcontingent potentials related to the decision process can be separated from those evoked by stimulus onset. Analysis of individual evoked responses indicates that 3 task-contingent components, occurring at 220, 300 and 340 msec are precisely time-locked to the moment of decision. R6sum6
Technique de separation des potentiels corticaux exogdnes et endogdnes chez l'homme Dans un paradigme de discrimination temporelle, si la dur4e du signal est ta dimension du stimulus qui dolt ~tre discrimin~e, le m o m e n t de la d4cision du sujet peut ~tre manipul6 en modifiant la dur~e du stimulus-cible. Au fur et ~ mesure que la dur~e du stimulus est allongde, les potentiels d~pendant de la t~che et lids au processus de ddcision peuvent ~tre s~par~s de ceux qui sont 6voqu~s par le d~but du stimulus. L'analyse des r6ponses ~voqu6es individueUes indique que trois eomposantes dSpendant de la t~che, survenant ~ 220, 300 et 340 msec, sont temporeltement li6es de fagon tr~s pr6cise au m o m e n t de la d6cision. Supported 05475.
by NIH Grants NS-06459 and NS-
References Arezzo, J. and Vaughn, Jr., H.G. Cortical potentials associated with voluntary movements in the monkey. Brain Res., 1975, 88: 99--104. Courchesne, E., Hitlyard, S.A. and Galambos, R. Stimulus novelty, task relevance and the visual evoked potential in man. Electroenceph. clin. Neurophysiol., 1975, 39: 131--143. Davis, H, Enhancement of evoked cortical potentials in humans related to a task requiring a decision. Science, 1964, 145: 182--183. Davis, H. and Zerlin, S. Acoustic relations of the human vertex potential. J. acoust. Soc. Amer., 1966, 39: 109--116. Friedman, D., Simson, R., Ritter, W. and Rapin, I. The late positive component (P300) and information processing in sentences. Electroenceph. clin. Neurophysiol., 1975, 38: 255--262. Galambos, R., Benson, P., S m i t h , T,, SchulmanGalambos, C. and Osier, H. On hemispheric differences in evoked potentials to speech stimuli. Electroenceph, clin. Neurophysiol., 1975, 39: 279-283. Hillyard, S.A., Squires, K.C., Bauer, J.W. and Lindsay, P.H. Evoked potential correlates of auditory signal detection. Science, 1971, 172: 1357--1360. Ritter, W. and Vaughan, H.G. Averaged evoked responses in vigilance and discrimination: a reassessment. Science, 1969, 164: 326--328. Ritter, W., Simson, R. and Vaughan, H.G. Association cortex potentials and reaction time in auditory discrimination. Electroenceph. clin. Neurophysiol., 1972, 33: 547--555. Roth, W. Auditory evoked responses to unpredictable stimuli. Psychophysiology, 1973, 10: 125--137. Schweitzer, P.K. Auditory evoked brain responses: comparison of ON and O F F responses at long and short durations. Percept. Psychophys., 1977, 22: 87--94. Squires, K.C., Hiilyard, S.A. and Lindsay, P.L. Vertex potentials evoked during auditory signal detection: relation to decision criteria. Percept. Psychophys., 1973, 14: 265--272. Squires, K.C., Wickens, C., Squires, N.K. and Donchin, E. The effect of stimulus sequence on the waveform of the cortical event-related potential. Science, 1976, 193: 1142--1146. Squires, K.C., Donchin, E., Herning, R.I. and McCarthy, G. On the influence of task relevance and stimulus probability on event-related potential components. Electroenceph. clin. Neurophysiol., 1977, 42: 1--14. Squires, N., Squires, K. and Hillyard, S.A. Two varieties of long-latenCy positive waves evoked by unpredictable auditory stimuli in man. Electroenceph. clin. Neurophysiol., 1975, 38: 387--401.
SEPARATION
OF ENDOGENOUS
AND EXOGENOUS
Sutton, S., Braren, M. and Zubin, J. Evoked potential correlates of stimulus uncertainty. Science, 1965, 150: 1187--1188. Sutton, S., Braren, M., Zubin, J. and John, E.R. Information delivery and the sensory evoked
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potential. Science, 1967, 155: 1436--1439. Vaughn, H., Costa, L. and Ritter, W. Topography of the human motor potential. Electroenceph. clin. Neurophysiol., 1968, 25: 1--10.
Electroencephalography and Clinical Neurophysiology, 1 9 7 9 , 4 6 : 4 8 2 - - 4 8 5 © E l s e v i e r / N o r t h - H o l l a n d Scientific Publishers, Ltd.
Laboratory note A TECHNIQUE FOR SEPARATING ENDOGENOUS FROM EXOGENOUS HUMAN CORTICAL POTENTIALS E.J. H A M M O N D 1, D.A. S I L V A , A.J. K L E I N a n d D.C. T E A S
Institute for Advanced Study of the Communication Processes, University of Florida, Gainesville, Fla. 32610 (U.S.A.) ( A c c e p t e d for p u b l i c a t i o n : O c t o b e r 30, 1 9 7 8 )
Since t h e initial r e p o r t s o n t h e P 3 0 0 or P3 comP o n e n t of the h u m a n e v o k e d p o t e n t i a l (Davis 1 9 6 4 ; S u t t o n et al. 1 9 6 5 ) , t h e sensory a n d p e r c e p t u a l determ i n a n t s o f t h e a m p l i t u d e a n d l a t e n c y of this p o t e n t i a l have r e m a i n e d i n c o m p l e t e l y u n d e f i n e d . T h a t t h e P3 is evoked by rarely occurring or novel stimuli (e.g., R o t h 1 9 7 3 ; C o u r c h e s n e et al. 1 9 7 5 ) suggests it is a neural correlate of t h e o r i e n t i n g response, b u t it can also be elicited b y f r e q u e n t l y a p p e a r i n g stimuli u n d e r certain c i r c u m s t a n c e s ( R i t t e r a n d V a u g h a n 1 9 6 9 ; F r i e d m a n et al. 1975). While several investigations have related P3 amplit u d e to p e r c e p t u a l responses associated w i t h s t i m u l u s d e t e c t i o n (Hillyard et al. 1 9 7 1 ; K.C. Squires et al. 1 9 7 3 ; N. Squires et al. 1975), m o r e r e c e n t r e p o r t s indicate t h a t a m p l i t u d e s of averaged e v o k e d r e s p o n s e s are also d e p e n d e n t o n s e q u e n t i a l a n d a priori p r o b abilities of the stimuli (K.C. Squires et al. 1976, 1977). In c o n t r a s t to s h o r t l a t e n c y sensory e v o k e d p o t e n tials r e p o r t s of the P3 l a t e n c y f r o m d i f f e r e n t laboratories have varied over s o m e 200 msec ( R i t t e r a n d Vaughan 1969; Roth 1973); and intrasubject latency can vary over a similar range ( R i t t e r e t al. 1972). F r i e d m a n et al. ( 1 9 7 5 ) , h a v i n g f o u n d longer visual P3 latencies in r e s p o n s e to w o r d s w h e n t h e y delivered s y n t a c t i c i n f o r m a t i o n as c o m p a r e d t o w h e n t h e y did n o t , suggested P3 i n v o l v e m e n t in a language processing s y s t e m ; similarly, slightly longer s p e e c h - e v o k e d a u d i t o r y P3 latencies have b e e n a t t r i b u t e d t o m o r e c o m p l e x neural processing involved in s p e e c h perception ( G a l a m b o s e t al. 1975). However, a m o r e parsim o n i o u s p s y c h o a c o u s t i c a l d e t e r m i n a n t of t h e amplit u d e a n d l a t e n c y f l u c t u a t i o n s of these c o m p o n e n t s can be s h o w n .
I Address c o r r e s p o n d e n c e to: E d w a r d J. H a m m o n d , Ph.D., N e u r o l o g y Service, V e t e r a n s A d m i n i s t r a t i o n Hospital, Gainesville, Fla. 3 2 6 0 2 , U.S.A.
In our p a r a d i g m p r e s e n t e d here t h e s t i m u l u s d i m e n s i o n to be d i s c r i m i n a t e d is a d u r a t i o n c h a n g e r a t h e r t h a n a p i t c h or i n t e n s i t y c h a n g e ; t h e m o m e n t of decision a n d also P3 l a t e n c y can be m a n i p u l a t e d by c h a n g i n g t h e d u r a t i o n of t h e target stimulus.
Methods Ag-AgCl electrodes were applied t o scalp l o c a t i o n s Fz, Cz, a n d Pz a n d r e f e r e n c e d t o t h e left m a s t o i d . T h e t w o subjects were highly e x p e r i e n c e d in P3 experim e n t s . T h e y were i n s t r u c t e d to sit quietly a n d avoid eye m o v e m e n t s . Each E E G c h a n n e l was amplified ( × 2 0 , 0 0 0 , half a m p l i t u d e frequencies: 0.1 a n d 100 Hz), m o n i t o r e d b y an oscilloscope a n d r e c o r d e d o n an FM tape recorder. E v o k e d p o t e n t i a l s were digitized and p l o t t e d o n an X-Y p l o t t e r . T h e s u b j e c t ' s task was to d i s c r i m i n a t e ( c o u n t ) t h e n u m b e r of s h o r t t o n e bursts (target signals) e m b e d d e d in trains of longer t o n e bursts. E v o k e d responses were collected (in 4 separate r u n s ) in w h i c h the target stimuli were e i t h e r 50, 100, 200 or 4 0 0 msec in d u r a t i o n . T h e d u r a t i o n of the signal was m e a s u r e d with a digital timer. All t o n e b u r s t s ( 8 0 0 Hz) were of e q u a l i n t e n s i t y (50 dB SPL in a b a c k g r o u n d noise of 25 dB SPL) a n d were p r e s e n t e d to t h e right ear t h r o u g h T D H - 3 9 headp h o n e s . All t o n e bursts h a d 3 msec rise-fall times. T h e i n t e r - s t i m u l u s interval was 3 s e c ; t h e target signal was p r e s e n t e d r a n d o m l y w i t h a p r o b a b i l i t y of a b o u t 0.15. T h e s u b j e c t s sat in a reclining chair in a s o u n d a t t e n u ating r o o m . T h e d i s c r i m i n a t i o n task r e q u i r e d m o d e r ate b u t n o t i n t e n s e a t t e n t i o n to p e r f o r m . T h e subjects' i n s t r u c t i o n s were to i n f o r m t h e e x p e r i m e n t e r after 10 target signals h a d b e e n p r e s e n t e d , a n d t o avoid eye and finger m o v e m e n t s . It is difficult t o a c c u r a t e l y c o u n t t h e target stimuli w i t h o u t e m p l o y ing a c o u n t i n g strategy such as s u b - a u d i b l e vocalization or finger m o v e m e n t s , b u t it is a simple m a t t e r for the p r o p e r l y i n s t r u c t e d s u b j e c t to w i t h h o l d such a c t i o n for 2 or 3 see a f t e r t h e decision is m a d e , t h u s
SEPARATION OF ENDOGENOUS AND EXOGENOUS POTENTIALS avoiding any interactions.
483 time-locked electroencephalographic
Results
Pz 50msm tone ~ i
Fz Pz ,,00m.B
With this paradigm a psychoacoustical determinant of the latency of the decision-related components can be demonstrated by varying the duration of the target stimulus (Fig. 1). The increase in duration of the target stimulus necessarily forces an increase in the latency or m o m e n t of decision. Recorded from Pz in the highly experienced subject, the negative and positive peak latencies are about 220 msee and 340 msec, respectively, after the offset of the stimulus. By forcing subject's decision beyond 300 msec after stimulus onset, the negative potential can be temporally separated from the N2 of the on-response (Davis and Zerlin 1966). The late positive co m p o n en t has two components which resemble the P3a and P3b (N. Squires et al. 1975) both in latency and scalp topography; the earlier component is relatively larger at Fz and the later component is relatively larger at Pz (see Fig. 1). Control runs in which potentials were averaged in response to trains of target stimuli showed no offresponses (Schweitzer 1977). In this paradigm no motor response is required; the late negative and positive components therefore might represent different stages of orienting and stimulus evaluation uncontaminated by response selection processes. This demonstration that the latency of these waves varies directly with the m o m e n t of decision implies that this mechanism, which results in a latency jitter across many responses, is also a major determinant of the amplitude of averaged responses.
Discussion
200 m s total
This technique provides a method for measuring neural events, which reflect stimulus evaluation time, and avoids the temporal variability and electroen-
Fig. 1. Superimposed tracings of individual evoked responses from one subject. Five evoked responses are superimposed for each stimulus condition; responses were recorded simultaneously from Fz and Pz. These responses demonstrate time-locking to the m o m e n t of decision but nevertheless display latency jitter. The responses demonstrate a more frontal distribution for a positive co m p o n en t at about 300 msec after the offset of the tone and a more posterior distribution Tor a positive component at 340 msec. The preceding negativity at about 220 msec does not show a distinctive scalp topography. Calibrations: 10 pV and 100 msec.
484
E.J. HAMMOND ET AL.
cephalographic contamination (Vaughn et al. 1968; Arezzo and Vaughn 1975) due to efferent processes associated with motor response generation. Without reaction time measures, however, independent evidence that these potentials actually reflect the moment of decision is lacking. Independent of the stimulus duration, they are time-locked to the offset of the tone burst at which time the decision is presumably made by the attentive subject. The latency variability of reaction time measures and reaction timeP3 latency covariation is in fact greater because reaction time is a measure of stimulus evaluation time, response selection and generation time. The task-contingent negative potential occurring at about 220 msec has received little attention in previous experiments (e.g.K.C. Squires et al. 1976, 1977) perhaps because it occurs concurrently with the P2 of the motor potential (Vaughn et al. 1968; Arezzo and Vaughn 1975), and the P2 of the auditory off-response occurs in the same latency range.
Summary In a temporal discrimination paradigm, if signal duration is the stimulus dimension to be discriminated, the moment of subject's decision can be manipulated by varying the duration of the target stimulus. As stimulus duration is lengthened, taskcontingent potentials related to the decision process can be separated from those evoked by stimulus onset. Analysis of individual evoked responses indicates that 3 task-contingent components, occurring at 220, 300 and 340 msec are precisely time-locked to the moment of decision. R6sum6
Technique de separation des potentiels corticaux exogdnes et endogdnes chez l'homme Dans un paradigme de discrimination temporelle, si la dur4e du signal est ta dimension du stimulus qui dolt ~tre discrimin~e, le m o m e n t de la d4cision du sujet peut ~tre manipul6 en modifiant la dur~e du stimulus-cible. Au fur et ~ mesure que la dur~e du stimulus est allongde, les potentiels d~pendant de la t~che et lids au processus de ddcision peuvent ~tre s~par~s de ceux qui sont 6voqu~s par le d~but du stimulus. L'analyse des r6ponses ~voqu6es individueUes indique que trois eomposantes dSpendant de la t~che, survenant ~ 220, 300 et 340 msec, sont temporeltement li6es de fagon tr~s pr6cise au m o m e n t de la d6cision. Supported 05475.
by NIH Grants NS-06459 and NS-
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