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The contingent negative variation and the late positive wave of the average evoked potential
Electroencephalography and Clinical Neurophysiology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
CLINICAL
AND
LABORATORY
201
NOTES
T H E C O N T I N G E N T N E G A T I V E V A R I A T I O N A N D T H E LATE POSITIVE W A V E OF T H E A V E R A G E E V O K E D P O T E N T I A L 1 E. DONCHIN 2 AND D . B. D . SMITH 3
N A S A Ames Research Center, Moffett Field, Calif 94035 (U.S.A.) (Accepted for publication: December 10, 1969)
Sutton et al. (1965, 1967) have reported that a stimulus that delivers information to the subject elicits an average evoked potential (AEP) having a prominent positive component with a latency to the peak of 200-300 msec (P300). Other investigators (Chapman and Bragdon 1964; Donchin and Cohen 1967; Ritter et aL 1968) have reported that such variables as selective attention, alertness and meaningfulness have a strong control over the amplitude of this late component of the AEP. About the time the reports concerning P300 became available, Walter et al. (1964) reported their findings on the contingent negative variation (CNV). The CNV is a prolonged negative wave that precedes a stimulus requiring some action by the subject. There is general consensus (Cohen 1969) that when a contingency is established between two stimuli, such that the presentation of one informs the subject that a second stimulus requiring some response by him will follow, the CNV will develop between the stimuli. Furthermore, on presentation of the second stimulus, the slow negative wave is replaced by an abrupt positive shift. The latency with which this positive shift reaches a peak is approximately 300 msec. Thus, the "resolution" of the CNV coincides in time with P300. This is interesting because in practically all CNV paradigms, the second stimulus is "task relevant", and one would expect it to elicit a P300 component. Also, in many of the studies designed to investigate the effect on the AEP of variables such as task relevance, meaningfulness and selective attention, the cortical stimulus is preceded by a warning stimulus so that a CNV might be expected to develop just before the stimulus. It is thus possible that the CNV and P300 are two aspects of the same process and that discussion 1 This report was published in preliminary form in the Society Proceedings of the American E E G Society (This Journal, 1970, 28: 91-92). Present address: Department of Psychology, University of Illinois, Urbana, Ill. 61801, U.S.A. a Present address: Institute of Aerospace Safety and Management, University of Southern California, Los Angeles, Calif. 90007, U.S.A.
of them as separate entities results from our tendency to concentrate on pre- or post-stimulus events, as if time were truly divided by the stimulus into two separate compartments. To evaluate this suggestion, we replicated a study by Donchin and Cohen (1967) which concentrated on analyzing the EEG with an interest in post-stimulus events. In the earlier study subjects were instructed to look into a tachistoscope in which two figures alternated randomly with an average rate of 1 every 3 sec. A square flash of light was superimposed on the alternating figures at the rate of 1 every 2 sec. The subject performed one of two tasks. In one condition (FC) he was to respond to the flashes by pressing a switch as fast as he could. In the other condition (RC) he was instructed to press the switch each time the two figures alternated. Thus, different aspects of the same display became task relevant at different times. 'The results suggested that the aspect of the display which was relevant to the task the subject was performing elicited an AEP with an enhanced P300 component. The same aspect of the display, when the stimulus was irrelevant to the task, elicited an A E P in which P300 was small. When the flash was relevant, every flash served not only as an indicator to respond but as an indicator that another flash was to be expected within a certain time period. We might, therefore, expect a CNV to develop between the two flashes. By the same token, when the flash was not task relevant, no CNV should develop between the flashes. The same argument, of course, holds for the development of CNVs between the alternating figures. It was not possible to test this prediction with data collected in the earlier study for two reasons. The inter-stimulus interval was random, and therefore the CNV could not be observed. Furthermore, the bandpass characteristics of the amplifiers did not allow the recording of the slow potentials of the CNV. METHODS The Donchin and Cohen study (1967) was replicated using ten subjects. The procedure was varied in two details. First, the inter-stimulus interval was made constant, and second, the frequency response curve of
Electroenceph. clin. Neurophysiol., 1970, 29:201-203
202
E. D O N C H I N
A N D D. B, D. SMfI-H
SUBJECT: JD TRIG: B A C K G R O U N D FC , ,, ;....
P
,
RC
~'
J .... ~
BI
~. ~ K " ' ~ ~ ~
, ' ......
RC ~
the Ampex DAS-100 amplifiers was set to be flat between 0.1 c s e c and 50 c/sec. (The roll-off slope was 12 d B octave.) Again subjects looked into a tachistoscope where two figures alternated every 2 sec, and 50 msec flashes were presented every 2.25 sec. Except for these two changes, this study was the same as the earlier one, and the reader is referred to the t967 report for technical details. It should be noted that as the figures alternated every 2 sec, the flashes were presented every 2.25 sec" thus, flashes were presented at 250, 500, 750 ... msec following the figure changes. In this schedule, it was very difficult for the subject to predict the temporal interval between any specific flash and adjacent background changes.
TRIG: F L A S H
,.,
I T'
'
O
i IOftV
2000 4000 TIME ( m sec)
0
2000 4000 TIME (m sec)
SUBJECT: FJ TRIG: BACKGROUND
FC~.
,~{, s ¢,
TRIG: FLASH ,f
•
' I-
A
F C "j
,,#'
i
RC~
i)
B' i,
,""'~
~'..o.'.~'~"
<.¢
ilOfty 0
20100 4000 TIME (m sec)
2000 4000 TIME (m sec)
SUBJECT: RO TRIG: BACKGROUND
FO i- " • " " ~ . , ' " ," " "
TRIG: FLASH
RESUL IS The results obtained from the vertex electrode in three subjects are shown in Fig. 1. For each subject, average evoked potentials are shown which were elicited by the flashes (right column) and others that were elicited by the changes in background stimuli (left column). In each column there are AEPs that were elicited by the stimulus when it was and when it was not relevant. (A stimulus is defined here as relevant when the subject has to report its occurrence.) In nine subjects we found that when the eliciting stimulus is task relevant, a slow negative shift develops between two successive stimuli with. a resolution approximately 300 msec after stimulus onset. On the other hand, whenever the stimulus is not relevant, this cycle of a negative shift is not observed. This statement is based primarily on data recorded from the vertex electrode. However, the data recorded from the occipital electrode also showed a negative shift of a similar form between the relevant stimuli, although of a somewhat reduced amplitude. No detailed comparisons were made by us of the occipital and vertex electrodes. The electrooculogram was monitored closely during the experiment, and data contaminated by eye movements were discarded.
FC~ DISCUSSION
RC' ~ ............~
....
B" ~.
"~'(" ...........
0
2000 4000 TIME (m sec)
,
RC e'i A-~ ~ "2" ,~/ / ,
""
, ]lofty 0
2000 4000 TIME (m sec)
Fig. 1 Average evoked potentials recorded from the vertex of three subjects (to a linked ear reference). Negativity of the vertex electrode is represented by an upward deflection. Averaging was performed by an IBM 1800 computer. Each average is based on 250 time points spanning 4000 msec. 50 stimulations were used for each average. The column heading indicates which of the two stimuli (flashes
The results confirm that at least in this experimental paradigm, the conditions required to elicit a CNV-like phenomenon are identical with the conditions required for eliciting the P300 component in the AEP. It might be argued that as the inter-stimulus intervals in the original study (Donchin and Cohen 1967) were random, no CNV should develop. However, studies conducted on the effects of random and fixed inter-stimulus intervals on the CNV by ourselves I Fehmi et al., in preparation) and or background stimuli) was used to trigger the computer. FC, the subject was instructed to report the flash; RC, the subject was instructed to report the changes in background stimuli ; F, only the flash was presented, subject instructed to respond; B1, only background stimuli presented, subject instructed to respond; B2, same asB1, subjectcounting stimuli; Sync, subject instructed to synchronize his responses with the stimulus.
Electroenceph. c/in, Neurophysiol., 1970, 29:201-203
THE CNV AND P300 by others (McAdam et al. 1969) suggest that this is not the case. The CNV is clearly elicited in a series in which the interstimulus is randomized. Similar results were reported by Nfi~tanen (1967), who showed that a slow negative potential develops between rhythmically presented stimuli. N/i/itanen does not identify these potentials with the CNV. He suggests that they are an index of cortical activation. It seems, however, that until the physiological mechanisms of the CNV and P300 are elucidated, judgment should be reserved on this issue. (See also N/i/itanen 1969 and Donchin and Cohen 1969, as well as Chapman 1969.) These results point to the importance of considering the experimental situation in an A E P experiment not as a series of isolated stimuli, the response to which is to be determined, but as a continuous, on-going process in which the subject's response to each stimulus is conditioned by his expectancies with regard to future stimuli and his experiences with past stimuli. Thus, until the relationship between the A E P and such phenomena as the CNV are clearly elucidated, neither should be ignored when the other is studied.
203
sent appropri6es, la CNV et la P300 peuvent 6tre obtenues dans la m6me situation exp6rimentale et que leur relation devrait 6tre 61ucid6e ult6rieurement. REFERENCES
CHAPMAN,R. Discussion in E. DONCHIN and D. LINDSLEY (Eds.), Average evoked potentials: methods, results, and evaluations. NASA SP-191, Govt. Printing
RI~SUMI~
Office, Washington, D.C., 1969: 262-281. CHAPMAN, R. M. and BRA~DON, H. R. Evoked responses to numerical and nonnumerical visual stimuli while problem solving. Nature (Lend.), 1964, 203:11551157. COHEN, J. Very slow brain potentials relating to expectancy: the CNV. In E. DONCHIN and D. LINDSLEY (Eds.), Average evoked potentials: methods, results, and evaluations. NASA SP-191, Govt. Printing Office, Washington, D.C., 1969: 143-198. DONCHIN, E. and COHEN, L. Average evoked potentials and intra-modality selective attention. Electroenceph. clin. NeurophysioL, 1967, 22: 537-546. DONCHIN, E. and COHEN, L. Anticipation of relevant stimuli and evoked potentials: a reply to N~ifitanen. Percept. Motor Skills, 1969, 29: 115-117. MCADAM, D. W., KNOTT, J. R. and REBERT, C. S. Cortical slow potential changes in man related to interstimulus interval and to pre-trial prediction of interstimulus interval. Psychophysiology, 1969, 5: 349-358. N.~,~TANEN,R. Selective attention and evoked potentials. Ann. Acad. Sci.fenn. B, 1967, 151: 1-226. N.~.X.TANEN,R. Anticipation of relevant stimuli and evoked potentials: a comment on Donchin's and Cohen's "Average evoked potentials and intramodality selective attention". Percept. Motor Skills, 1969, 28: 639-646. RITTER, W., VAUGHAN JR., H. G. and COSTA, L. D. Orienting and habituation to auditory stimuli: a study of short term changes in average evoked responses. Electroenceph. clin. NeurophysioL , 1968, 25:
VARIATION CONTINGENTE NI~GATIVE ET DERNIf/RE ONDE POSITIVE DU POTENTIEL I~VOQUi~ MOYEN
550-556. SUTTON, S., BRAREN, M. and ZUmN, J. Evoked potential correlates of stimulus uncertainty. Science, 1965, 150:
Des stimuli en rapport avec une thche provoquent un potentiel 6voqu6 moyen qui montre une composante positive augment6e (P300) avec une latence de pic entre 200 et 300 msec. La variation contingente n6gative (CNV) se termine habituellement par une onde positive d'une latence d'approximativement 300 msec. La possibilit6 que ces deux ph6nom6nes soient en relation a 6t6 explor6e. Les donn6es obtenues indiquent que quand les conditions
1187-1188. SUTTON, S., TUETING, P., ZUBIN, J. and JOHN, E. R. Information delivery and the sensory evoked potential. Science, 1967, 155: 1436-1439. WALTER,W. G., COOPER, R., ALDRIDGE,V. J., MCCALLUM, W. C. and WINTER, A. L. Contingent negative variation: an electrical sign of sensorimotor association and expectancy in the human brain. Nature (Lend.J, 1964, 203: 380-384.
SUMMARY Stimuli that are task relevant elicit an averaged evoked potential (AEP) that shows an enhanced positive component (P300) with a peak latency between 200 and 300 msec. The contingent negative variation (CNV) usually terminates with a positive wave with a latency of approximately 300 msec. The possibility that these two phenomena are related was investigated. The data indicate that when conditions are appropriate, both the CNV and P300 can be obtained in the same experimental situation, and their relationship should be further elucidated.
Reference: DONCHIN,E. and SMITH,D. B. D. The contingent negative variation and the late positive wave of the average evoked potential. Electroenceph. clin. Neurophysiol., 1970, 29: 201-203.
CLINICAL
AND
LABORATORY
201
NOTES
T H E C O N T I N G E N T N E G A T I V E V A R I A T I O N A N D T H E LATE POSITIVE W A V E OF T H E A V E R A G E E V O K E D P O T E N T I A L 1 E. DONCHIN 2 AND D . B. D . SMITH 3
N A S A Ames Research Center, Moffett Field, Calif 94035 (U.S.A.) (Accepted for publication: December 10, 1969)
Sutton et al. (1965, 1967) have reported that a stimulus that delivers information to the subject elicits an average evoked potential (AEP) having a prominent positive component with a latency to the peak of 200-300 msec (P300). Other investigators (Chapman and Bragdon 1964; Donchin and Cohen 1967; Ritter et aL 1968) have reported that such variables as selective attention, alertness and meaningfulness have a strong control over the amplitude of this late component of the AEP. About the time the reports concerning P300 became available, Walter et al. (1964) reported their findings on the contingent negative variation (CNV). The CNV is a prolonged negative wave that precedes a stimulus requiring some action by the subject. There is general consensus (Cohen 1969) that when a contingency is established between two stimuli, such that the presentation of one informs the subject that a second stimulus requiring some response by him will follow, the CNV will develop between the stimuli. Furthermore, on presentation of the second stimulus, the slow negative wave is replaced by an abrupt positive shift. The latency with which this positive shift reaches a peak is approximately 300 msec. Thus, the "resolution" of the CNV coincides in time with P300. This is interesting because in practically all CNV paradigms, the second stimulus is "task relevant", and one would expect it to elicit a P300 component. Also, in many of the studies designed to investigate the effect on the AEP of variables such as task relevance, meaningfulness and selective attention, the cortical stimulus is preceded by a warning stimulus so that a CNV might be expected to develop just before the stimulus. It is thus possible that the CNV and P300 are two aspects of the same process and that discussion 1 This report was published in preliminary form in the Society Proceedings of the American E E G Society (This Journal, 1970, 28: 91-92). Present address: Department of Psychology, University of Illinois, Urbana, Ill. 61801, U.S.A. a Present address: Institute of Aerospace Safety and Management, University of Southern California, Los Angeles, Calif. 90007, U.S.A.
of them as separate entities results from our tendency to concentrate on pre- or post-stimulus events, as if time were truly divided by the stimulus into two separate compartments. To evaluate this suggestion, we replicated a study by Donchin and Cohen (1967) which concentrated on analyzing the EEG with an interest in post-stimulus events. In the earlier study subjects were instructed to look into a tachistoscope in which two figures alternated randomly with an average rate of 1 every 3 sec. A square flash of light was superimposed on the alternating figures at the rate of 1 every 2 sec. The subject performed one of two tasks. In one condition (FC) he was to respond to the flashes by pressing a switch as fast as he could. In the other condition (RC) he was instructed to press the switch each time the two figures alternated. Thus, different aspects of the same display became task relevant at different times. 'The results suggested that the aspect of the display which was relevant to the task the subject was performing elicited an AEP with an enhanced P300 component. The same aspect of the display, when the stimulus was irrelevant to the task, elicited an A E P in which P300 was small. When the flash was relevant, every flash served not only as an indicator to respond but as an indicator that another flash was to be expected within a certain time period. We might, therefore, expect a CNV to develop between the two flashes. By the same token, when the flash was not task relevant, no CNV should develop between the flashes. The same argument, of course, holds for the development of CNVs between the alternating figures. It was not possible to test this prediction with data collected in the earlier study for two reasons. The inter-stimulus interval was random, and therefore the CNV could not be observed. Furthermore, the bandpass characteristics of the amplifiers did not allow the recording of the slow potentials of the CNV. METHODS The Donchin and Cohen study (1967) was replicated using ten subjects. The procedure was varied in two details. First, the inter-stimulus interval was made constant, and second, the frequency response curve of
Electroenceph. clin. Neurophysiol., 1970, 29:201-203
202
E. D O N C H I N
A N D D. B, D. SMfI-H
SUBJECT: JD TRIG: B A C K G R O U N D FC , ,, ;....
P
,
RC
~'
J .... ~
BI
~. ~ K " ' ~ ~ ~
, ' ......
RC ~
the Ampex DAS-100 amplifiers was set to be flat between 0.1 c s e c and 50 c/sec. (The roll-off slope was 12 d B octave.) Again subjects looked into a tachistoscope where two figures alternated every 2 sec, and 50 msec flashes were presented every 2.25 sec. Except for these two changes, this study was the same as the earlier one, and the reader is referred to the t967 report for technical details. It should be noted that as the figures alternated every 2 sec, the flashes were presented every 2.25 sec" thus, flashes were presented at 250, 500, 750 ... msec following the figure changes. In this schedule, it was very difficult for the subject to predict the temporal interval between any specific flash and adjacent background changes.
TRIG: F L A S H
,.,
I T'
'
O
i IOftV
2000 4000 TIME ( m sec)
0
2000 4000 TIME (m sec)
SUBJECT: FJ TRIG: BACKGROUND
FC~.
,~{, s ¢,
TRIG: FLASH ,f
•
' I-
A
F C "j
,,#'
i
RC~
i)
B' i,
,""'~
~'..o.'.~'~"
<.¢
ilOfty 0
20100 4000 TIME (m sec)
2000 4000 TIME (m sec)
SUBJECT: RO TRIG: BACKGROUND
FO i- " • " " ~ . , ' " ," " "
TRIG: FLASH
RESUL IS The results obtained from the vertex electrode in three subjects are shown in Fig. 1. For each subject, average evoked potentials are shown which were elicited by the flashes (right column) and others that were elicited by the changes in background stimuli (left column). In each column there are AEPs that were elicited by the stimulus when it was and when it was not relevant. (A stimulus is defined here as relevant when the subject has to report its occurrence.) In nine subjects we found that when the eliciting stimulus is task relevant, a slow negative shift develops between two successive stimuli with. a resolution approximately 300 msec after stimulus onset. On the other hand, whenever the stimulus is not relevant, this cycle of a negative shift is not observed. This statement is based primarily on data recorded from the vertex electrode. However, the data recorded from the occipital electrode also showed a negative shift of a similar form between the relevant stimuli, although of a somewhat reduced amplitude. No detailed comparisons were made by us of the occipital and vertex electrodes. The electrooculogram was monitored closely during the experiment, and data contaminated by eye movements were discarded.
FC~ DISCUSSION
RC' ~ ............~
....
B" ~.
"~'(" ...........
0
2000 4000 TIME (m sec)
,
RC e'i A-~ ~ "2" ,~/ / ,
""
, ]lofty 0
2000 4000 TIME (m sec)
Fig. 1 Average evoked potentials recorded from the vertex of three subjects (to a linked ear reference). Negativity of the vertex electrode is represented by an upward deflection. Averaging was performed by an IBM 1800 computer. Each average is based on 250 time points spanning 4000 msec. 50 stimulations were used for each average. The column heading indicates which of the two stimuli (flashes
The results confirm that at least in this experimental paradigm, the conditions required to elicit a CNV-like phenomenon are identical with the conditions required for eliciting the P300 component in the AEP. It might be argued that as the inter-stimulus intervals in the original study (Donchin and Cohen 1967) were random, no CNV should develop. However, studies conducted on the effects of random and fixed inter-stimulus intervals on the CNV by ourselves I Fehmi et al., in preparation) and or background stimuli) was used to trigger the computer. FC, the subject was instructed to report the flash; RC, the subject was instructed to report the changes in background stimuli ; F, only the flash was presented, subject instructed to respond; B1, only background stimuli presented, subject instructed to respond; B2, same asB1, subjectcounting stimuli; Sync, subject instructed to synchronize his responses with the stimulus.
Electroenceph. c/in, Neurophysiol., 1970, 29:201-203
THE CNV AND P300 by others (McAdam et al. 1969) suggest that this is not the case. The CNV is clearly elicited in a series in which the interstimulus is randomized. Similar results were reported by Nfi~tanen (1967), who showed that a slow negative potential develops between rhythmically presented stimuli. N/i/itanen does not identify these potentials with the CNV. He suggests that they are an index of cortical activation. It seems, however, that until the physiological mechanisms of the CNV and P300 are elucidated, judgment should be reserved on this issue. (See also N/i/itanen 1969 and Donchin and Cohen 1969, as well as Chapman 1969.) These results point to the importance of considering the experimental situation in an A E P experiment not as a series of isolated stimuli, the response to which is to be determined, but as a continuous, on-going process in which the subject's response to each stimulus is conditioned by his expectancies with regard to future stimuli and his experiences with past stimuli. Thus, until the relationship between the A E P and such phenomena as the CNV are clearly elucidated, neither should be ignored when the other is studied.
203
sent appropri6es, la CNV et la P300 peuvent 6tre obtenues dans la m6me situation exp6rimentale et que leur relation devrait 6tre 61ucid6e ult6rieurement. REFERENCES
CHAPMAN,R. Discussion in E. DONCHIN and D. LINDSLEY (Eds.), Average evoked potentials: methods, results, and evaluations. NASA SP-191, Govt. Printing
RI~SUMI~
Office, Washington, D.C., 1969: 262-281. CHAPMAN, R. M. and BRA~DON, H. R. Evoked responses to numerical and nonnumerical visual stimuli while problem solving. Nature (Lend.), 1964, 203:11551157. COHEN, J. Very slow brain potentials relating to expectancy: the CNV. In E. DONCHIN and D. LINDSLEY (Eds.), Average evoked potentials: methods, results, and evaluations. NASA SP-191, Govt. Printing Office, Washington, D.C., 1969: 143-198. DONCHIN, E. and COHEN, L. Average evoked potentials and intra-modality selective attention. Electroenceph. clin. NeurophysioL, 1967, 22: 537-546. DONCHIN, E. and COHEN, L. Anticipation of relevant stimuli and evoked potentials: a reply to N~ifitanen. Percept. Motor Skills, 1969, 29: 115-117. MCADAM, D. W., KNOTT, J. R. and REBERT, C. S. Cortical slow potential changes in man related to interstimulus interval and to pre-trial prediction of interstimulus interval. Psychophysiology, 1969, 5: 349-358. N.~,~TANEN,R. Selective attention and evoked potentials. Ann. Acad. Sci.fenn. B, 1967, 151: 1-226. N.~.X.TANEN,R. Anticipation of relevant stimuli and evoked potentials: a comment on Donchin's and Cohen's "Average evoked potentials and intramodality selective attention". Percept. Motor Skills, 1969, 28: 639-646. RITTER, W., VAUGHAN JR., H. G. and COSTA, L. D. Orienting and habituation to auditory stimuli: a study of short term changes in average evoked responses. Electroenceph. clin. NeurophysioL , 1968, 25:
VARIATION CONTINGENTE NI~GATIVE ET DERNIf/RE ONDE POSITIVE DU POTENTIEL I~VOQUi~ MOYEN
550-556. SUTTON, S., BRAREN, M. and ZUmN, J. Evoked potential correlates of stimulus uncertainty. Science, 1965, 150:
Des stimuli en rapport avec une thche provoquent un potentiel 6voqu6 moyen qui montre une composante positive augment6e (P300) avec une latence de pic entre 200 et 300 msec. La variation contingente n6gative (CNV) se termine habituellement par une onde positive d'une latence d'approximativement 300 msec. La possibilit6 que ces deux ph6nom6nes soient en relation a 6t6 explor6e. Les donn6es obtenues indiquent que quand les conditions
1187-1188. SUTTON, S., TUETING, P., ZUBIN, J. and JOHN, E. R. Information delivery and the sensory evoked potential. Science, 1967, 155: 1436-1439. WALTER,W. G., COOPER, R., ALDRIDGE,V. J., MCCALLUM, W. C. and WINTER, A. L. Contingent negative variation: an electrical sign of sensorimotor association and expectancy in the human brain. Nature (Lend.J, 1964, 203: 380-384.
SUMMARY Stimuli that are task relevant elicit an averaged evoked potential (AEP) that shows an enhanced positive component (P300) with a peak latency between 200 and 300 msec. The contingent negative variation (CNV) usually terminates with a positive wave with a latency of approximately 300 msec. The possibility that these two phenomena are related was investigated. The data indicate that when conditions are appropriate, both the CNV and P300 can be obtained in the same experimental situation, and their relationship should be further elucidated.
Reference: DONCHIN,E. and SMITH,D. B. D. The contingent negative variation and the late positive wave of the average evoked potential. Electroenceph. clin. Neurophysiol., 1970, 29: 201-203.