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Multiple Positive ERP Components in Visual Discrimination Tasks
Multiple Positive ERP Components in Visual Discrimination Tasks DAVID FRIEDMAN', HERBERT VAUGHAN, Jr.2 and L. ERLENMEYER-KIMLING3 New York State Psychiatric I n ~ t i t u t e ' ,New ~ . York, N . Y . 10032 and Albert Einstein College of Medicinel. Bronx, N . Y . 10461 (U.S.A.)
The evidence that the event-related potentials (ERPs) elicited during task-oriented paradigms consist of several late positive components has steadily accumulated (e.g., N. Squires et al., 1975; K. Squires et al., 1977; Friedman et al., 1978; Stuss and Picton, 1978). These components have been reported to occur within the latency range of from 200 to 600 msec. Psychological constructs for these components have varied from frequency sensitivity (N. Squires et al., 1975) for the earlier of these peaks, to utilization of feedback information (Stuss and Picton, 1978) and response selection (Friedman et al., 1978) for the longest latency positivities reported. The most oft-used criterion for judging the uniqueness of a given peak has been scalp distribution (see review by Picton et al., 1979). However, the visual identification and measurement of distinct peaks leads to problems, since components interact both spatially and temporally at the scalp (viz., the distinction between P300 and slow wave; K. Squires et al., 1977). The factor analytic technique jcf., Donchin and Hemey, 1979) has been used in certain cases to disentangle these overlapping components, with the time courses of the resultant loading functions allowing the investigator to associate factors with visually identified ERP components (e.g., K. Squires et al., 1977; Friedman et al., 1978). We report here the results of a study of ERPs elicited during tasks known to differ in their processing requirements (Friedman et al., 1978). Principal components analysis (PCA) was used to objectively differentiate multiple positive components visually identified in the grand mean and individual data, and the factor scores were used to determine the relationship of these measures of ERP activity to the experimental variables. METHODS Subjects and tasks Subjects were 18 male and 12 female adolescents ranging in age from 11 to 18 years (mean= 14.5, S.D. = 1.8). The tasks used have been described in detail elsewhere (Friedman et al., 1978), and only a brief description will be given here. Subjects were instructed to respond to targets with a finger lift which activated a reaction time (RT) key of local design. In task A, the signal (SIG) was the number 08 (15 times/block)
118
and the non-signals (NSIG) were any other number (45 times/block), while in task B the SIG was the repetition of any immediately preceding number (16 times/block) and the NSIG were any number which did not repeat (48 times/block). Eight blocks of each task were given (alternated two at a time), with the SIG/NSIG ratio maintained at 1:4. The 50msec duration stimuli were presented via a video monitor and were flashed at moderate intensity with an inter-stimulus interval of 1 sec. Recording and data collection
EEG was recorded from a midline montage at Fz, Cz, Pz, and Oz, and vertical EOG was recorded from an electrode located above the right eye, with a reference electrode on the right earlobe. These data were amplified on a Beckman Dynograph Type RM with Type 481B pre-amplifiers and Type 482M8 amplifiers at a gain of 20,000, time constant of 1 sec and high frequency cut-off at 30 Hz. The experiments and data acquisition were controlled by a PDP 11/10 computer which digitized the EEG at 4 msec intervals and recorded it, along with RT, on digital tape. Averages of SIG and NSIG ERP were computed across blocks only for hits and correct rejections, after removal of eye and other movement artifacts. RESULTS ERP waveforms
The grand mean ERPs are presented in Fig. 1, where it can be seen that both SIG and NSIG ERPs are of complex morphology comprised mainly of a series of positive peaks with differing topographies. In order of increasing latency, these peaks are: - P240, larger to SIG than NSIG, with a fronto-central topography; - P350, clearly visible in the NSIG of both tasks, with a parietal amplitude gradient; signal
T 1000 rns
non-signal
-TASK
A
----TASK
B
L
Fig. 1. Grand mean ERPs averaged across 30 subjects for each task and stimulus. Vertical bars mark mean reaction times, and arrows mark stimulus onset.
119 - the most prominent positive peak, P450, with a distinct parietal focus, is larger to SIG than NSIG; - P550, clearly present in the NSIG waveforms, has a parieto-occipital distribution; - a slowly increasing negativity that precedes the stimulus and culminates in Nl50 is seen at the vertex electrode; - activity in the last 300 msec of the epoch remains above baseline in the task B waveforms relative to those of task A.
Principal components analysis ( P C A )
In order to objectively confirm the components we had visually identified in the grand mean and individual data, PCA with varimax rotation (BMDP4M; Dixon, 1975) was performed on the averaged ERPs. Each subject's ERP was reduced to 83 points with each point representing the mean of 3 adjacent points in the original digitized data (i.e., 12 msec of activity). PCA was performed separately on the SIG and NSIG ERPs from each task. The data base was a set of 120 waveforms (30 subjects by 4 electrodes) by 83 time points. We used the covariance matrix, since it has the advantage of producing factors that correspond to the variance around the grand mean, and maintains the original metric. Six factors were extracted from each analysis using only eigenvalues greater than 1. These factors, superimposed according to shape and latency, are presented in Fig. 2. The degree of similarity across tasks and stimuli is striking. Factor 1, which peaks at 450 msec for SIG and 400 msec for NSIG, is associated with P450 in the original waveforms. Factor 2 occupies the late portion of the epoch when P450 is returning to baseline and cannot be identified with a specific peak. Factor 3 has high loadings at 240 msec and corresponds to P240. Factor 4 peaks at 600 msec for SIG and 550 msec for NSIG and is identified with P550. Factor 5 is most highly correlated with time points in the 300-350msec portion of the response and is identified with P350. Factor 6, which peaks at 150 msec
1
2
3
4
5
6
--TASK
a
Fig. 2. Rotated factor loadings from separate PCA of the SIC and NSIG of each task superimposedaccording to shape and latency. Arrows mark stimulus onset.
120 and whose shape duplicates that of the pre-stimulus negativity, is associated with N150. Using these rotated loadings, one can examine the relationships of these factors to task and stimulus variables: for example, P450 factor is larger to SIG than NSIG, with SIG of task A producing larger loadings than those of task B, with the opposite the case for NSIG. Factor score analyses
In order to further assess the association between the factors and the experimental variables, PCA was performed on the covariance matrix computed on the entire data base of 480 waveforms (30 subjects x 4 electrodes x 2 tasks x 2 stimuli) and 83 time points. The resultant factor scores were used as the "base-to-peak" measures of ERP activity (see K. Squires et al., 1977). These scores were subjected to repeated measures ANOVA (BMDP2V) with task (A versus B), stimulus (SIG versus NSIG) and electrode site (Fz, Cz, Pz, Oz) as the main variables. The general trends reported below were all supported by highly significant effects of locus for each factor (P< 0.0001) as well as main and/or interaction effects (Px0.05 or less). Fig. 3 presents the plots of these factor scores. P450 shows a distinct parietal focus for SIG and the NSIG of task B, while for the NSIG of task A its topography is shifted centrally. It is clearly larger to SIG than NSIG and is larger for task A than task B SIG, but this relationship reverses for NSIG. Factor 2 shows a centro-parietal amplitude gradient, is larger in the task B waveforms, as well as in the NSIG relative to SIG. P240 demonstrates a fronto-central topography, is larger to SIG than NSIG, and appears to be of greater amplitude in the task B waveforms. P550 presents a complex picture: overall, it appears larger to the task B stimuli, but shifts its amplitude gradient parieto-occipitally for NSIG, shows a central 20, F l :P450
8
t
2
' z
=
+
13
F4:P550
lF2 1F3:p240
,
F5:P350
1
F6:N150
-TASK
A
--TASK
B
..
signal
*
- non- signal
0.5
00
-
05
1.5
Fig. 3. Factor scores plotted as a function of task, electrode and stimulus. These scores were obtained from PCA of the entire data set of 480 waveforms.
121 focus for taskA SIG and a centro-parietal gradient for task B SIG. Overall, P350 appears larger in the task B waveforms, with SIG of task B showing a more distinct parietal focus, while the other stimuli produce a more uniform parieto-occipital gradient. N150 is clearly larger to SIG than NSIG and is of greater amplitude in the task B responses. Since the N 150 factor represents both prestimulus negativity and peak N 150, and both are affected by attentional requirements, amplitude differences appear to reflect differences in sustained attention between tasks.
DISCUSSION We have provided further evidence that the ERPs elicited during visual discrimination tasks are comprised of multiple positive components within the latency range of the classical P300 component. While it is clear that the P450 component is associated with the traditional P300 wave (both in its topographic distribution and its relationship to task variables), it is not clear to what extent these other positivities share common psychological and physiological sources with other positive waves recently reported. Adam and Collins (1978) and Chapman et al. (1978) have recorded P240 components (the factor of Chapman et al. peaked at 250 msec) during tasks requiring visual memory search, while Stuss and Picton (1978) and Jenness (1972) have recorded P4 components with latency and topography similar to our P550. P350 seems most similar to the “P3b” component first reported by N. Squires et al. (1975), and the P350 component elicited by visual stimuli recorded by Adam and Collins (1978). Factor 2, which could not be associated with any visually identified ERP peak, appears most similar in its time course to the SW factor identified by K. Squires et al. (1977) in an auditory discrimination task. However, differences in the stimulus modality and task requirements of the two studies make it unwise to equate these long latency factors. Although it is tempting to identify each of the late components with a specific role in the sequence of information processing, it is clear that we are far from establishing such relationships.
REFERENCES Adam, N. and Collins, G. I. (1978) Late components of the visual evoked potential to search in short-term memory. Electroenceph. elin. Neurophysiol., 44: 147-156. Chapman, R. M., McCrary, J. W. and Chapman, J. A. (1978) Short-term memory: the “storage” component of the human brain response predicts recall. Science, 202: 121 1-1214. Dixon, W. D. (1975) The BMDP Biomedical Computer Programs. University of California Press, Los Angeles, Calif. Donchin, E. and Heffley, E. F. (1978) Multivariate analysis of ERP data: a tutorial review. In New Perspectives in Event-rehted Potential Research, D. Otto (Ed.), U.S. Government Printing Office, Washington, D.C., pp. 555-572. Friedman, D., Vaughan, Jr., H. G. and Erlenmeyer-Kimling, L. (1978) Stimulus and response related components of the late positive complex in visual discrimination tasks. Electroenceph. clin. Neurophysiol., 45: 31%330. Jenness, D. (1972) Auditory evoked-responsedifferentiationwith discrimination learning in humans. J . romp. physiol. Psychol., 80: 75-90. Picton, T. W., Woods, D. L., Stuss, D. T. and Campbell, K. B. (1978) Methodology and meaning of human
122 evoked potential scalp distribution studies. In New Perspectives in Event-related Potential Research, D. Otto (Ed.), U.S. Government Printing Office, Washington, D.C., pp. 515-522. Squires, K. C., Donchia, E., Herning, R.I. and McCarthy, G. (1977) On the influence of task relevance and stimulus probability on event-related potential components. Electroenceph. elin. Neurophysiol., 4 2 1-14. Squires, N., Squires, K. C. and Hillyard, S. A. (1975) Two varieties of long-latencypositive waves evoked by unpredictable auditory stimuli in man. Electroenceph. d i n . Neurophysiol., 38: 387401. Stuss, D. T. and Picton, T. W. (1978) Neurophysiologicalcorrelates of human concept formation. Behav. Bid., 23: 135162.
The evidence that the event-related potentials (ERPs) elicited during task-oriented paradigms consist of several late positive components has steadily accumulated (e.g., N. Squires et al., 1975; K. Squires et al., 1977; Friedman et al., 1978; Stuss and Picton, 1978). These components have been reported to occur within the latency range of from 200 to 600 msec. Psychological constructs for these components have varied from frequency sensitivity (N. Squires et al., 1975) for the earlier of these peaks, to utilization of feedback information (Stuss and Picton, 1978) and response selection (Friedman et al., 1978) for the longest latency positivities reported. The most oft-used criterion for judging the uniqueness of a given peak has been scalp distribution (see review by Picton et al., 1979). However, the visual identification and measurement of distinct peaks leads to problems, since components interact both spatially and temporally at the scalp (viz., the distinction between P300 and slow wave; K. Squires et al., 1977). The factor analytic technique jcf., Donchin and Hemey, 1979) has been used in certain cases to disentangle these overlapping components, with the time courses of the resultant loading functions allowing the investigator to associate factors with visually identified ERP components (e.g., K. Squires et al., 1977; Friedman et al., 1978). We report here the results of a study of ERPs elicited during tasks known to differ in their processing requirements (Friedman et al., 1978). Principal components analysis (PCA) was used to objectively differentiate multiple positive components visually identified in the grand mean and individual data, and the factor scores were used to determine the relationship of these measures of ERP activity to the experimental variables. METHODS Subjects and tasks Subjects were 18 male and 12 female adolescents ranging in age from 11 to 18 years (mean= 14.5, S.D. = 1.8). The tasks used have been described in detail elsewhere (Friedman et al., 1978), and only a brief description will be given here. Subjects were instructed to respond to targets with a finger lift which activated a reaction time (RT) key of local design. In task A, the signal (SIG) was the number 08 (15 times/block)
118
and the non-signals (NSIG) were any other number (45 times/block), while in task B the SIG was the repetition of any immediately preceding number (16 times/block) and the NSIG were any number which did not repeat (48 times/block). Eight blocks of each task were given (alternated two at a time), with the SIG/NSIG ratio maintained at 1:4. The 50msec duration stimuli were presented via a video monitor and were flashed at moderate intensity with an inter-stimulus interval of 1 sec. Recording and data collection
EEG was recorded from a midline montage at Fz, Cz, Pz, and Oz, and vertical EOG was recorded from an electrode located above the right eye, with a reference electrode on the right earlobe. These data were amplified on a Beckman Dynograph Type RM with Type 481B pre-amplifiers and Type 482M8 amplifiers at a gain of 20,000, time constant of 1 sec and high frequency cut-off at 30 Hz. The experiments and data acquisition were controlled by a PDP 11/10 computer which digitized the EEG at 4 msec intervals and recorded it, along with RT, on digital tape. Averages of SIG and NSIG ERP were computed across blocks only for hits and correct rejections, after removal of eye and other movement artifacts. RESULTS ERP waveforms
The grand mean ERPs are presented in Fig. 1, where it can be seen that both SIG and NSIG ERPs are of complex morphology comprised mainly of a series of positive peaks with differing topographies. In order of increasing latency, these peaks are: - P240, larger to SIG than NSIG, with a fronto-central topography; - P350, clearly visible in the NSIG of both tasks, with a parietal amplitude gradient; signal
T 1000 rns
non-signal
-TASK
A
----TASK
B
L
Fig. 1. Grand mean ERPs averaged across 30 subjects for each task and stimulus. Vertical bars mark mean reaction times, and arrows mark stimulus onset.
119 - the most prominent positive peak, P450, with a distinct parietal focus, is larger to SIG than NSIG; - P550, clearly present in the NSIG waveforms, has a parieto-occipital distribution; - a slowly increasing negativity that precedes the stimulus and culminates in Nl50 is seen at the vertex electrode; - activity in the last 300 msec of the epoch remains above baseline in the task B waveforms relative to those of task A.
Principal components analysis ( P C A )
In order to objectively confirm the components we had visually identified in the grand mean and individual data, PCA with varimax rotation (BMDP4M; Dixon, 1975) was performed on the averaged ERPs. Each subject's ERP was reduced to 83 points with each point representing the mean of 3 adjacent points in the original digitized data (i.e., 12 msec of activity). PCA was performed separately on the SIG and NSIG ERPs from each task. The data base was a set of 120 waveforms (30 subjects by 4 electrodes) by 83 time points. We used the covariance matrix, since it has the advantage of producing factors that correspond to the variance around the grand mean, and maintains the original metric. Six factors were extracted from each analysis using only eigenvalues greater than 1. These factors, superimposed according to shape and latency, are presented in Fig. 2. The degree of similarity across tasks and stimuli is striking. Factor 1, which peaks at 450 msec for SIG and 400 msec for NSIG, is associated with P450 in the original waveforms. Factor 2 occupies the late portion of the epoch when P450 is returning to baseline and cannot be identified with a specific peak. Factor 3 has high loadings at 240 msec and corresponds to P240. Factor 4 peaks at 600 msec for SIG and 550 msec for NSIG and is identified with P550. Factor 5 is most highly correlated with time points in the 300-350msec portion of the response and is identified with P350. Factor 6, which peaks at 150 msec
1
2
3
4
5
6
--TASK
a
Fig. 2. Rotated factor loadings from separate PCA of the SIC and NSIG of each task superimposedaccording to shape and latency. Arrows mark stimulus onset.
120 and whose shape duplicates that of the pre-stimulus negativity, is associated with N150. Using these rotated loadings, one can examine the relationships of these factors to task and stimulus variables: for example, P450 factor is larger to SIG than NSIG, with SIG of task A producing larger loadings than those of task B, with the opposite the case for NSIG. Factor score analyses
In order to further assess the association between the factors and the experimental variables, PCA was performed on the covariance matrix computed on the entire data base of 480 waveforms (30 subjects x 4 electrodes x 2 tasks x 2 stimuli) and 83 time points. The resultant factor scores were used as the "base-to-peak" measures of ERP activity (see K. Squires et al., 1977). These scores were subjected to repeated measures ANOVA (BMDP2V) with task (A versus B), stimulus (SIG versus NSIG) and electrode site (Fz, Cz, Pz, Oz) as the main variables. The general trends reported below were all supported by highly significant effects of locus for each factor (P< 0.0001) as well as main and/or interaction effects (Px0.05 or less). Fig. 3 presents the plots of these factor scores. P450 shows a distinct parietal focus for SIG and the NSIG of task B, while for the NSIG of task A its topography is shifted centrally. It is clearly larger to SIG than NSIG and is larger for task A than task B SIG, but this relationship reverses for NSIG. Factor 2 shows a centro-parietal amplitude gradient, is larger in the task B waveforms, as well as in the NSIG relative to SIG. P240 demonstrates a fronto-central topography, is larger to SIG than NSIG, and appears to be of greater amplitude in the task B waveforms. P550 presents a complex picture: overall, it appears larger to the task B stimuli, but shifts its amplitude gradient parieto-occipitally for NSIG, shows a central 20, F l :P450
8
t
2
' z
=
+
13
F4:P550
lF2 1F3:p240
,
F5:P350
1
F6:N150
-TASK
A
--TASK
B
..
signal
*
- non- signal
0.5
00
-
05
1.5
Fig. 3. Factor scores plotted as a function of task, electrode and stimulus. These scores were obtained from PCA of the entire data set of 480 waveforms.
121 focus for taskA SIG and a centro-parietal gradient for task B SIG. Overall, P350 appears larger in the task B waveforms, with SIG of task B showing a more distinct parietal focus, while the other stimuli produce a more uniform parieto-occipital gradient. N150 is clearly larger to SIG than NSIG and is of greater amplitude in the task B responses. Since the N 150 factor represents both prestimulus negativity and peak N 150, and both are affected by attentional requirements, amplitude differences appear to reflect differences in sustained attention between tasks.
DISCUSSION We have provided further evidence that the ERPs elicited during visual discrimination tasks are comprised of multiple positive components within the latency range of the classical P300 component. While it is clear that the P450 component is associated with the traditional P300 wave (both in its topographic distribution and its relationship to task variables), it is not clear to what extent these other positivities share common psychological and physiological sources with other positive waves recently reported. Adam and Collins (1978) and Chapman et al. (1978) have recorded P240 components (the factor of Chapman et al. peaked at 250 msec) during tasks requiring visual memory search, while Stuss and Picton (1978) and Jenness (1972) have recorded P4 components with latency and topography similar to our P550. P350 seems most similar to the “P3b” component first reported by N. Squires et al. (1975), and the P350 component elicited by visual stimuli recorded by Adam and Collins (1978). Factor 2, which could not be associated with any visually identified ERP peak, appears most similar in its time course to the SW factor identified by K. Squires et al. (1977) in an auditory discrimination task. However, differences in the stimulus modality and task requirements of the two studies make it unwise to equate these long latency factors. Although it is tempting to identify each of the late components with a specific role in the sequence of information processing, it is clear that we are far from establishing such relationships.
REFERENCES Adam, N. and Collins, G. I. (1978) Late components of the visual evoked potential to search in short-term memory. Electroenceph. elin. Neurophysiol., 44: 147-156. Chapman, R. M., McCrary, J. W. and Chapman, J. A. (1978) Short-term memory: the “storage” component of the human brain response predicts recall. Science, 202: 121 1-1214. Dixon, W. D. (1975) The BMDP Biomedical Computer Programs. University of California Press, Los Angeles, Calif. Donchin, E. and Heffley, E. F. (1978) Multivariate analysis of ERP data: a tutorial review. In New Perspectives in Event-rehted Potential Research, D. Otto (Ed.), U.S. Government Printing Office, Washington, D.C., pp. 555-572. Friedman, D., Vaughan, Jr., H. G. and Erlenmeyer-Kimling, L. (1978) Stimulus and response related components of the late positive complex in visual discrimination tasks. Electroenceph. clin. Neurophysiol., 45: 31%330. Jenness, D. (1972) Auditory evoked-responsedifferentiationwith discrimination learning in humans. J . romp. physiol. Psychol., 80: 75-90. Picton, T. W., Woods, D. L., Stuss, D. T. and Campbell, K. B. (1978) Methodology and meaning of human
122 evoked potential scalp distribution studies. In New Perspectives in Event-related Potential Research, D. Otto (Ed.), U.S. Government Printing Office, Washington, D.C., pp. 515-522. Squires, K. C., Donchia, E., Herning, R.I. and McCarthy, G. (1977) On the influence of task relevance and stimulus probability on event-related potential components. Electroenceph. elin. Neurophysiol., 4 2 1-14. Squires, N., Squires, K. C. and Hillyard, S. A. (1975) Two varieties of long-latencypositive waves evoked by unpredictable auditory stimuli in man. Electroenceph. d i n . Neurophysiol., 38: 387401. Stuss, D. T. and Picton, T. W. (1978) Neurophysiologicalcorrelates of human concept formation. Behav. Bid., 23: 135162.