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A method for analyzing variations in evoked responses
407
ELECTP,OENCEPHALOGRAPHY AND CLINICAL NEUROPHYSIOLOGY
A METHOD FOR ANALYZING VARIATIONS IN EVOKED
RESPONSES 1
S. K . BURNS AND R . MELZACK
Center for Communications Sciences, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Mass. (U.S.A.); Department of Psychology, McGill University, Montreal, Que. (Canada) (Accepted for publication: October 18, 1965)
The method of averaging a number of evoked responses (Rosenblith 1962) provides a powerful tool for the detection of electrophysiologieal signals in a high "noise" background and permits observation of evoked potentials in awake animals including man. But averaged evoked responses do not tell whether or not there are changes in the amplitude and wave shape of the response which accompany changes in behavioral state during the accumulation of the average. In fact, the basic assumption underlying the averaging of evoked responses is that the probability distribution of the data does not change during the time needed to obtain an average s. Data accumulated while subjects traverse several different behavioral states reflect composite characteristics from these states; it is not possible to distinguish in the average the individual contributions made by a particular state. Since both the behavior and EEG records of awake, freely moving dogs raised in restriction cages are characterized by rapid shifts in attention and alertness (see Melzack and Burns 1963, 1965), it became essential to find a method that would permit the assessment of variation in successive evoked responses, in order to observe the possible effect of continually changing alertness and attention on the evoked response. A method was therefore developed that is similar to the well-known cumulative records of lever pressing (Skinner 1938). Instead of recording cumulative presses, the cumulative sum of amplitude of a partitular deflection in successive evoked responses is presented. Using a previously computed average, a point of interest (e.g., the peak of the surface positive componen0 in the evoked response is selected and its latency determined. To determine the cumulative evoked response from the This work was supported in part by the Joint Services Electronics Program under contract DA36-039AMC-03200(E), National Science Foundation (Grant GP-2495), the National Institutes of Health (Grant MH04737-05); in part by the National Aeronautics and Space Administration (Grant NsG-496); the National Institutes of Health (Grant MH-04235-03); and by the Advanced Research Projects Agency of the U.S. Department of Defense (Contract SD-193). More precisely: It is assumed that the probability distribution of the set of data sampled at a specific latency does not change throughout the entire experimental period.
data, the amplitude of the first evoked response is sampled at the selected latency. The value of this sample, vl, is 0
NUMBER OF RESPONSES 256 512
W
(a)
Z 0 Q. tll n,, I
!
a W 0 > W
,, ,.,
.J
0
(c)
Fig. 1 Cumulative evoked responses based on a known input signal. Three different inputs occur during each computation, thus the trace is divided into three distinct segments. Beneath each segment is the averaged evoked response based on the same data that produced the segment above it. Trace a starts with a fixed amplitude input as can be seen from the uniform build-up of the cumulative evoked response. During the second segment the input was reduced to one-half its initial value resulting again in a uniform trace building up at half the initial rate. The input was turned off during the third segment; hence the trace remains constant, neither increasing nor decreasing. Trace b shows the effect of starting with a fixed amplitude input, turning it off and then turning it back on but reversed. Trace c illustrates the effect of variations of the amplitude of the input. The first segment is formed from a fixed input amplitude. The amplitude was varied during the second segment and this variation is reflected by the varying slope which increases as the amplitude of the input was increased. During the third segment, the amplitude was reduced to zero. Notice that the averages for the first and second segments are identical even though the character of the two differ considerably.
Electroenceph. clin. Neurophysiol., 1966, 20:407-409
408
S. K. BURNS AND R. MELZACK
stored in the first o f a set o f n registers. D e n o t i n g the contents o f the registers ya, y2, . . . . , y,, t h e n y~ == v~. After the second response, a n o t h e r sample, v2, is t a k e n at the selected latency a n d the s u m o f the first sample a n d this, the second, sample is stored in the second register y2 =- y~ + v2 == Vl + ve. T h e analysis proceeds, storing t h e s u m o f the ith sample, v~, a n d the contents o f the register i ..... 1, yi z, in register i. yi = yi 1 + vi. After n samples the contents o f the n registers, yl, y~. . . . , y~ c o n t a i n vl, Vl + v2, V1 .+' V2 -{ V3 . . . . VI 4- V2 -~ V3 + ... -~ Vn, respectively. T h e contents of the registers are t h e n displayed o n a n oscilloscope in order y~, y2. . . . , y, to f o r m a g r a p h o f the cumulative value o f the deflections at the selected latency as this point builds u p to its final value. T h e expected value of this statistic is a straight line if t h e m e a n o f the d a t a does n o t change. If, in the course of the records, the m e a n changes, this c h a n g e will be reflected in a c h a n g e in slope in the cumulative a m p l i t u d e display. If there are several changes in the course of a c o m p u t a t i o n , t h e display will be m a d e up of straight line segments with a change in slope occurring for each c h a n g e in the m e a n statistics. M o r e important, there is detailed information presented a b o u t the time w h e n the c h a n g e occurred. If the animal falls asleep after the 37th s t i m u l u s presentation, one would expect a c h a n g e in slope s o m e w h e r e near the 37th point o f the display. Several examples o f the display are included in Fig. I. This display h a s been obtained using the T X - 0 c o m p u t e r at M.I.T. T h e c o m p u t e r has the capability o f perf o r m i n g this c o m p u t a t i o n at different values of latency simultaneously. U p to 512 responses m a y be taken for each individual c o m p u t a t i o n . I n addition, the display can be segmented into three portions. E a c h portion is m a r k e d o n the cumulative display and a separate 250 point en-
NUMBER OF RESPONSES 256
~,2
7~?_.......... ! o ~
< Ixl
•
.
..... ,28o
7 -.v.
8 ~
9
i0 J
O
Fig. 2 C u m u l a t i v e evoked response display f r o m data recorded f r o m a d o g ' s auditory cortex d u r i n g c h a n g e s in t h e d o g ' s e n v i r o n m e n t . Samples t a k e n simultaneously with t h e instant o f s t i m u l u s presentation (upper trace) s h o w m u c h smaller variation f r o m a straight line t h a n do samples t a k e n at the peak o f the evoked response (lower trace). A small negative c o n s t a n t h a s been a d d e d to the data. T h e s a m p l i n g time is indicated by a dot b e n e a t h the averaged evoked response wave form.
NUMBER OF RESPONSES 512
1024
W if) Z
O Q. if) W ILl V O > W
• 2
;024
w > ._1
Fig. 3 C u m u l a t i v e evoked response to clicks f r o m data recorded f r o m the scalp of a h u m a n subject w h o is falling asleep. T h e lower trace is f r o m the s a m e subject who h a s been awakened a n d is a g a i n falling asleep. T h e sampling time is indicated by the dot b e n e a t h the average evoked response. T h e stage o f sleep is indicated above the cumulative evoked response record by n u m b e r s based o n a modification of the m e t h o d used by D e m e n t ( G u i l b a u d et al. 1965) to categorize raw E E G tracings. Stage l a refers to a pred o m i n a n t l y waking alpha r h y t h m while stage 1b describes the drowsy or interrupted a l p h a stage.
semble average is c o m p u t e d for each segment. At the s a m e time, a c o m b i n e d average for the entire record is also formed. Fig. 2 s u m m a r i z e s data recorded f r o m the auditory cortex of a dog. Clicks were presented once each 2 sec t h r o u g h a speaker in the a n i m a l ' s cage. T h e recording session was divided into l I periods. Just prior to the first period, the door of the a n i m a l ' s cage was opened, allowing it to see a n o t h e r cage. T h e d o o r was closed immediately before second period, o p e n e d before third, closed before the fourth, etc. Averages o f the first 100 responses in each period were c o m p u t e d in a n effort to determine the effect o f this " c h a n g i n g e n v i r o n m e n t " o n the evoked responses. These averages are displayed just below each period in Fig. 2. A reasonable initial interpretation was to a s s u m e that lack o f change in the average was being caused by a c h a n g e in the environment. This view is confirmed by the behavior o f t h e cumulative evoked response (which c h a n ges as the door is opened just prior to period 1). W i t h o u t the cumulative evoked response display, it w o u l d also seem reasonable to attribute t h e change in t h e average o f period 8 to the closing of the cage door i m m e diately prior to the eighth period. However, the slope o f the cumulative evoked response does n o t change with this e n v i r o n m e n t a l change, suggesting that there is n o causal relationship b u t simply on-going changes in the a n i m a l ' s state which are apparently n o t related to the experimental manipulation.
Electroenceph. clin. Neurophysiol., 1966, 20:407~09
VARIATIONS IN EVOKED RESPONSES The upper trace in Fig. 2 is a control trace; it is derived from the same data as the lower one but is computed for a latency of zero; i.e., a time interval which supposedly is too small to show any effect of the stimulus which is being delivered at this instant. Superimposed on these data is a small, negative constant. The upper trace has a small negative slope. Notice that variations from a straight line are considerably smaller than variations occurring in the lower trace. Whereas this might be explained by the greater over-all variance of the lower trace compared with upper trace, studies (Barlow 1961) have shown that variance estimated from a small sample of responses may be smaller during the larger deflection of an evoked response than it is before or after these portions of the response. This suggests that the mean is changing. The mean itself, when computed for small (25-50) sequences of responses, is a function of time. Thus the changes we see in the cumulative record are caused by changes in the mechanism producing the response rather than by greater but unsynchronized activity near peaks in the evoked response. The upper trace in Fig. 3 comes from data recorded from the scalp of a h u m a n subject who is falling asleep. Clicks are presented to the subject once every 3 sec. The lower trace is from the same subject after he has been awakened and is again falling asleep. It is quite apparent that the statistics of the data are changing considerably during this averaging operation: initially the mean is positive; it then becomes quite negative and finally approaches a less negative, but steady, value. The cumulative evoked response seems to provide a way of characterizing the temporal characteristics of a person falling asleep in a rather nice, compact form and may often be as useful as an average. It, at least, provides information about the time when a meaningful average can be determined. SUMMARY This study illustrates two examples in which the usual
409
assumption of an unchanging probability distribution made to justify averaging is not true. In both the behaving animal and the sleeping human subject large changes in the amplitude and wave shape of the evoked response occur in the time needed to obtain an average with a reasonably large signal-to-noise ratio. This result points out the risk in inferring a causal relationship between a behavioral manipulation or observation and a change from one average to another in the amplitude or wave shape of the averaged evoked response. We acknowledge gratefully the encouragement and advice given to us by Professor W, A. Rosenblith and sincerely thank D. Langbein for his help with the TX-0 computer. REFERENCES
BARLOW,J. S. A versatile analog computer for neurophysiological research. Digest of the 1961 International Conference on Medical Electronics, New York, N.Y., 1961 : 27. GUILBAUD, G., ROSENBLITH, W., BURNS, S. et ALBE-FESSARD, D. t~volution chez l'homme au cours des diff6rents stades du sommeil des r6ponses 61ectrocorticales 6voqu6es au vertex par deux modes de stimulation. C. R. Aead. Sci. (Paris), 1965, 260: 5366-5369. MELZACK,R. and BURNs, S. K. Neuropsychological effect of early sensory restriction. Bol. Inst. Estud. m~d. biol. (Mdx.), 1963, 21 : 407-425. MELZACK,R. and BURNS, S. K. Neurophysiological effects of early sensory restriction. Exp. Neurol., 1965, (in Press). ROSENBLITH, W. A. (Ed.). Processing neuroeleetric data. M.I.T. Press, Cambridge, 1962,127 p. SKINNER,B. F. The behavior of organisms. Appleton-Century-Crofts, New York, 1938, 457 p.
Reference: BURNS, S. K. and MELZACK,R. A method for analyzing variations in evoked responses. Electroenceph. clin. Neurophysiol., 1966, 20: 407-409.'
ELECTP,OENCEPHALOGRAPHY AND CLINICAL NEUROPHYSIOLOGY
A METHOD FOR ANALYZING VARIATIONS IN EVOKED
RESPONSES 1
S. K . BURNS AND R . MELZACK
Center for Communications Sciences, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Mass. (U.S.A.); Department of Psychology, McGill University, Montreal, Que. (Canada) (Accepted for publication: October 18, 1965)
The method of averaging a number of evoked responses (Rosenblith 1962) provides a powerful tool for the detection of electrophysiologieal signals in a high "noise" background and permits observation of evoked potentials in awake animals including man. But averaged evoked responses do not tell whether or not there are changes in the amplitude and wave shape of the response which accompany changes in behavioral state during the accumulation of the average. In fact, the basic assumption underlying the averaging of evoked responses is that the probability distribution of the data does not change during the time needed to obtain an average s. Data accumulated while subjects traverse several different behavioral states reflect composite characteristics from these states; it is not possible to distinguish in the average the individual contributions made by a particular state. Since both the behavior and EEG records of awake, freely moving dogs raised in restriction cages are characterized by rapid shifts in attention and alertness (see Melzack and Burns 1963, 1965), it became essential to find a method that would permit the assessment of variation in successive evoked responses, in order to observe the possible effect of continually changing alertness and attention on the evoked response. A method was therefore developed that is similar to the well-known cumulative records of lever pressing (Skinner 1938). Instead of recording cumulative presses, the cumulative sum of amplitude of a partitular deflection in successive evoked responses is presented. Using a previously computed average, a point of interest (e.g., the peak of the surface positive componen0 in the evoked response is selected and its latency determined. To determine the cumulative evoked response from the This work was supported in part by the Joint Services Electronics Program under contract DA36-039AMC-03200(E), National Science Foundation (Grant GP-2495), the National Institutes of Health (Grant MH04737-05); in part by the National Aeronautics and Space Administration (Grant NsG-496); the National Institutes of Health (Grant MH-04235-03); and by the Advanced Research Projects Agency of the U.S. Department of Defense (Contract SD-193). More precisely: It is assumed that the probability distribution of the set of data sampled at a specific latency does not change throughout the entire experimental period.
data, the amplitude of the first evoked response is sampled at the selected latency. The value of this sample, vl, is 0
NUMBER OF RESPONSES 256 512
W
(a)
Z 0 Q. tll n,, I
!
a W 0 > W
,, ,.,
.J
0
(c)
Fig. 1 Cumulative evoked responses based on a known input signal. Three different inputs occur during each computation, thus the trace is divided into three distinct segments. Beneath each segment is the averaged evoked response based on the same data that produced the segment above it. Trace a starts with a fixed amplitude input as can be seen from the uniform build-up of the cumulative evoked response. During the second segment the input was reduced to one-half its initial value resulting again in a uniform trace building up at half the initial rate. The input was turned off during the third segment; hence the trace remains constant, neither increasing nor decreasing. Trace b shows the effect of starting with a fixed amplitude input, turning it off and then turning it back on but reversed. Trace c illustrates the effect of variations of the amplitude of the input. The first segment is formed from a fixed input amplitude. The amplitude was varied during the second segment and this variation is reflected by the varying slope which increases as the amplitude of the input was increased. During the third segment, the amplitude was reduced to zero. Notice that the averages for the first and second segments are identical even though the character of the two differ considerably.
Electroenceph. clin. Neurophysiol., 1966, 20:407-409
408
S. K. BURNS AND R. MELZACK
stored in the first o f a set o f n registers. D e n o t i n g the contents o f the registers ya, y2, . . . . , y,, t h e n y~ == v~. After the second response, a n o t h e r sample, v2, is t a k e n at the selected latency a n d the s u m o f the first sample a n d this, the second, sample is stored in the second register y2 =- y~ + v2 == Vl + ve. T h e analysis proceeds, storing t h e s u m o f the ith sample, v~, a n d the contents o f the register i ..... 1, yi z, in register i. yi = yi 1 + vi. After n samples the contents o f the n registers, yl, y~. . . . , y~ c o n t a i n vl, Vl + v2, V1 .+' V2 -{ V3 . . . . VI 4- V2 -~ V3 + ... -~ Vn, respectively. T h e contents of the registers are t h e n displayed o n a n oscilloscope in order y~, y2. . . . , y, to f o r m a g r a p h o f the cumulative value o f the deflections at the selected latency as this point builds u p to its final value. T h e expected value of this statistic is a straight line if t h e m e a n o f the d a t a does n o t change. If, in the course of the records, the m e a n changes, this c h a n g e will be reflected in a c h a n g e in slope in the cumulative a m p l i t u d e display. If there are several changes in the course of a c o m p u t a t i o n , t h e display will be m a d e up of straight line segments with a change in slope occurring for each c h a n g e in the m e a n statistics. M o r e important, there is detailed information presented a b o u t the time w h e n the c h a n g e occurred. If the animal falls asleep after the 37th s t i m u l u s presentation, one would expect a c h a n g e in slope s o m e w h e r e near the 37th point o f the display. Several examples o f the display are included in Fig. I. This display h a s been obtained using the T X - 0 c o m p u t e r at M.I.T. T h e c o m p u t e r has the capability o f perf o r m i n g this c o m p u t a t i o n at different values of latency simultaneously. U p to 512 responses m a y be taken for each individual c o m p u t a t i o n . I n addition, the display can be segmented into three portions. E a c h portion is m a r k e d o n the cumulative display and a separate 250 point en-
NUMBER OF RESPONSES 256
~,2
7~?_.......... ! o ~
< Ixl
•
.
..... ,28o
7 -.v.
8 ~
9
i0 J
O
Fig. 2 C u m u l a t i v e evoked response display f r o m data recorded f r o m a d o g ' s auditory cortex d u r i n g c h a n g e s in t h e d o g ' s e n v i r o n m e n t . Samples t a k e n simultaneously with t h e instant o f s t i m u l u s presentation (upper trace) s h o w m u c h smaller variation f r o m a straight line t h a n do samples t a k e n at the peak o f the evoked response (lower trace). A small negative c o n s t a n t h a s been a d d e d to the data. T h e s a m p l i n g time is indicated by a dot b e n e a t h the averaged evoked response wave form.
NUMBER OF RESPONSES 512
1024
W if) Z
O Q. if) W ILl V O > W
• 2
;024
w > ._1
Fig. 3 C u m u l a t i v e evoked response to clicks f r o m data recorded f r o m the scalp of a h u m a n subject w h o is falling asleep. T h e lower trace is f r o m the s a m e subject who h a s been awakened a n d is a g a i n falling asleep. T h e sampling time is indicated by the dot b e n e a t h the average evoked response. T h e stage o f sleep is indicated above the cumulative evoked response record by n u m b e r s based o n a modification of the m e t h o d used by D e m e n t ( G u i l b a u d et al. 1965) to categorize raw E E G tracings. Stage l a refers to a pred o m i n a n t l y waking alpha r h y t h m while stage 1b describes the drowsy or interrupted a l p h a stage.
semble average is c o m p u t e d for each segment. At the s a m e time, a c o m b i n e d average for the entire record is also formed. Fig. 2 s u m m a r i z e s data recorded f r o m the auditory cortex of a dog. Clicks were presented once each 2 sec t h r o u g h a speaker in the a n i m a l ' s cage. T h e recording session was divided into l I periods. Just prior to the first period, the door of the a n i m a l ' s cage was opened, allowing it to see a n o t h e r cage. T h e d o o r was closed immediately before second period, o p e n e d before third, closed before the fourth, etc. Averages o f the first 100 responses in each period were c o m p u t e d in a n effort to determine the effect o f this " c h a n g i n g e n v i r o n m e n t " o n the evoked responses. These averages are displayed just below each period in Fig. 2. A reasonable initial interpretation was to a s s u m e that lack o f change in the average was being caused by a c h a n g e in the environment. This view is confirmed by the behavior o f t h e cumulative evoked response (which c h a n ges as the door is opened just prior to period 1). W i t h o u t the cumulative evoked response display, it w o u l d also seem reasonable to attribute t h e change in t h e average o f period 8 to the closing of the cage door i m m e diately prior to the eighth period. However, the slope o f the cumulative evoked response does n o t change with this e n v i r o n m e n t a l change, suggesting that there is n o causal relationship b u t simply on-going changes in the a n i m a l ' s state which are apparently n o t related to the experimental manipulation.
Electroenceph. clin. Neurophysiol., 1966, 20:407~09
VARIATIONS IN EVOKED RESPONSES The upper trace in Fig. 2 is a control trace; it is derived from the same data as the lower one but is computed for a latency of zero; i.e., a time interval which supposedly is too small to show any effect of the stimulus which is being delivered at this instant. Superimposed on these data is a small, negative constant. The upper trace has a small negative slope. Notice that variations from a straight line are considerably smaller than variations occurring in the lower trace. Whereas this might be explained by the greater over-all variance of the lower trace compared with upper trace, studies (Barlow 1961) have shown that variance estimated from a small sample of responses may be smaller during the larger deflection of an evoked response than it is before or after these portions of the response. This suggests that the mean is changing. The mean itself, when computed for small (25-50) sequences of responses, is a function of time. Thus the changes we see in the cumulative record are caused by changes in the mechanism producing the response rather than by greater but unsynchronized activity near peaks in the evoked response. The upper trace in Fig. 3 comes from data recorded from the scalp of a h u m a n subject who is falling asleep. Clicks are presented to the subject once every 3 sec. The lower trace is from the same subject after he has been awakened and is again falling asleep. It is quite apparent that the statistics of the data are changing considerably during this averaging operation: initially the mean is positive; it then becomes quite negative and finally approaches a less negative, but steady, value. The cumulative evoked response seems to provide a way of characterizing the temporal characteristics of a person falling asleep in a rather nice, compact form and may often be as useful as an average. It, at least, provides information about the time when a meaningful average can be determined. SUMMARY This study illustrates two examples in which the usual
409
assumption of an unchanging probability distribution made to justify averaging is not true. In both the behaving animal and the sleeping human subject large changes in the amplitude and wave shape of the evoked response occur in the time needed to obtain an average with a reasonably large signal-to-noise ratio. This result points out the risk in inferring a causal relationship between a behavioral manipulation or observation and a change from one average to another in the amplitude or wave shape of the averaged evoked response. We acknowledge gratefully the encouragement and advice given to us by Professor W, A. Rosenblith and sincerely thank D. Langbein for his help with the TX-0 computer. REFERENCES
BARLOW,J. S. A versatile analog computer for neurophysiological research. Digest of the 1961 International Conference on Medical Electronics, New York, N.Y., 1961 : 27. GUILBAUD, G., ROSENBLITH, W., BURNS, S. et ALBE-FESSARD, D. t~volution chez l'homme au cours des diff6rents stades du sommeil des r6ponses 61ectrocorticales 6voqu6es au vertex par deux modes de stimulation. C. R. Aead. Sci. (Paris), 1965, 260: 5366-5369. MELZACK,R. and BURNs, S. K. Neuropsychological effect of early sensory restriction. Bol. Inst. Estud. m~d. biol. (Mdx.), 1963, 21 : 407-425. MELZACK,R. and BURNS, S. K. Neurophysiological effects of early sensory restriction. Exp. Neurol., 1965, (in Press). ROSENBLITH, W. A. (Ed.). Processing neuroeleetric data. M.I.T. Press, Cambridge, 1962,127 p. SKINNER,B. F. The behavior of organisms. Appleton-Century-Crofts, New York, 1938, 457 p.
Reference: BURNS, S. K. and MELZACK,R. A method for analyzing variations in evoked responses. Electroenceph. clin. Neurophysiol., 1966, 20: 407-409.'