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The late positive components of the human EEG in a signal detection task
Neurolysychologla, 1974, Vol. 12, pp. 385 to 387. Pergamon Press. Printed in England.
NOTE THE LA T E POSITIVE COMPONENTS OF T H E HUMAN EEG IN A SIGNAL DETECTION TASK* WILLIAM W . CAEL,~" ALLAN NASH a n d JAY J. SINGER Department of Psychology, Florida Atlantic University, Boca Raton, Florida 33432, U.S.A. (Received 18 October 1973)
Abstract In 1971, Hillyard, Squires, Bauer and Lindsay using human subjects in a signal detection task, obtained a prominent late positive component (LPC) of the EEG only in the Hit category of responses. They concluded that the LPC reflects the subject's degree of certainty that a signal has occurred. In contrast the present study observed the LPC in the Hit, Miss, and Correct Rejection categories of responses, supporting the hypothesis that the LPC is a correlate of uncertainty reduction and does not directly depend on the physical presence of the signal. INTRODUCTION TH~ P-300 WAVE, or late positive component (LPC) of the averaged evoked potential, has been related in humans to the resolution of uncertainty and does not appear to be determined solely by the physical eliciting stimulus (StrrroN et aL [1]). According to this interpretation, a signal detection task which requires the observer to report on each trial whether a signal was detected, should produce LPC.s in all decision categories (Hit, Miss, Correct Rejection, False Alarm) since uncertainty is resolved in each instance, independent of the physical presence of the signal or the correctness of the observer's response. This expectation would be particularly strong when the signal is relatively easy to detect, thereby reducing the ambiguity between signal and non-signal trials. HILLYARDet aL [2], however, found the LPC to be prominent only in the Hit category over a wide range of signal intensities. They concluded that the LPC relates to certainty that a signal has occurred and may be triggered by a match between an expected, but not wholly predictable, sensory event and a neural template of the stimulus. Their subjects were given extensive training in the basic signal detection task prior to the recording of the average evoked potentials. In light of the earlier work of Sutton et aL [1] in which an LPC also occurred when the absence of a signal provided information relevant to the resolution of uncertainty, this study was designed to repeat the work of Hillyard et al. [2] using relatively untrained subjects to determine whether the LPC would appear in other than the Hit category.
METHOD Eight volunteers (4 males, 4 females), between 21 and 24 years old, served as subjects (Ss). All were students of Florida Atlantic University. For all Ss the present experiment was the first of a series of experiments whose later phases (not reported here) were designed to test for the effects of stress on the LPC and signal detection performance. The S was seated in a recliner chair inside a sound attenuated chamber. Nonchlorided Grass silver electrodes were placed on shaved portions of the scalp and skin with Grass Electrode paste. A recording cup electrode was fixed to the vertex and reference and common mode disc, electrodes were clipped to the left ear-lobe with Grass nylon earclips. A separate ground plate electrode was placed on the left forearm. A Mousseau EEG Amplifier, Model SA-4, amplified the signals and fed them directly to an on-line, Digital Equipment Corporation PDP-8/L computer for averaging. The EEG amplifier was set at a gain of 40,000 for all As. Band-pass filters were set a 1 Hz lower limit and 50 Hz upper limit. Interelectrode resistance was kept below 10,000 I). Electronic switching circuitry permitted the separate averaging of the four types of experimental trials (Hits, Misses, Correct Rejections, and *Supported by grant AFOSR 69-1823. Conducted by the first author in partial fulfilment of the requirements for the M.A. degree. "['Now at University of Texas in Arlington. 385
386
NOTE
False Alarms). EEG epochs of one second duration, with zero delay from trial onset, were averaged for each of the four response categories. Each average consisted of 200 data points and the analysis of the final waveforms was performed on plotted copies produced by a Mosely-135 X - Y plotter. Pilot investigations were run to determine if any artifacts associated with eye movements or the switching circuitry were present in the final averages. These data indicated that the electrical equipment caused a 3-5 rnsec spike at approximately 90 msec after trial initiation, but this deflection was so narrow and definable that it did not confound the analysis of the data. No evidence of eye movement artifact was found. The basic paradigm consisted of placing the S in the sound attenuated chamber which was illuminated by a 75 W lamp aimed at the ceiling, and presenting continuous white noise of 80 dB (SPL) through a Superex headset. A cue-light in S's hand-held response console was illuminated for 0.75 sec and the offset of the cuelight indicated initiation of each trial. Signals consisting of a I000 Hz frequency tone at 81 dB (SPL) for 90 msec duration were presented binaurally, at random, on 50 per cent of the trials, a procedure which produced approximately 75 per cent correct responses. Trials were spaced 12 sec apart. All Ss knew that the signal, if present, would occur in the 90 msec interval following the offset of the cue-light. Ss were instructed to respond either yes (signal present) or no (signal absent). A pair of panel lights informed the S as to the correctness of his decision 3.5 sec after the trial began. Each S was permitted a few practice trials to familiarize himself with the sequence of events on the trials but in all other aspects the Ss were regarded as untrained and none was familiar with the purpose of the experiment. A total of 300 experimental trials were run with each S. RESULTS The most positive (base-to-peak) deflection in the interval from 250 to 450 msec following trial initiation was recorded from the X - Y plotter output for each S under each of the Hit, Miss, Correct rejection, and False Alarm categories. All Ss showed a strong response bias against responding "yes" to noise-only trials, a finding that has been reported elsewhere for naive Ss in signal detection tasks (CLARK, 1966 [3]). AS a result, there were relatively few trials in the False Alarm category and the noisy character of these profiles did not reveal any prominent peaks. Analysis of the data was therefore restricted to the mean amplitudes and iatencies of LPC peaks in the Hit, Miss, and Correct rejection categories which are summarized in Table 1. Table 1. Amplitude and latency of the LPC in Hit, Miss and Correct rejection categories Hit Miss Correct Rejection
Amplitude (IxV) Latency (msec) 15.5 305 7.5
307
11-7
361
An analysis of variance was performed on the mean LPC amplitudes and significant differences were observed among the three response categories (F = 16.235, d f = 2/30. P <0.01). A Neuman-Keuls analysis of the individual response categories (WINER, 1971 [4]) showed mean Hit LPC amplitude (15'5 laV) to be significantly higher than either Correct rejection (11.7 aV) or Miss (7.5 IxV) amplitudes at the 0'05 level of significance. Correct rejection amplitude was also found to be significantly higher than Miss amplitude at the 0.01 level of significance. An analysis of variance performed on the mean latencies of the LPC's revealed no significant differences among the three response categories (F = 0.008, d f = 1/12, P > 0.25). All of the four female Ss displayed an unexpected large positive deflection with a mean latency of 701 msec in the Hit category. This second peak appeared in addition to the earlier peak of 305 msec. Male Ss did not show the second peak. The appearance of the second positive peak in the EEG profiles of all 4 female Ss and its absence in all 4 male profiles was tested by the Fisher Exact Probability test and found significant at the 0.05 level. DISCUSSION Contrary to the findings of HILLYARD et al. [2] who reported little or no evidence for an LPC in any response category other than Hits, the present study observed significant LPCs in Hit, Miss, and Correct rejection categories. Hillyard et aL [2] used experienced subjects and assumed that their detection strategy was to match the sensory input against a template for the signal. According to this theory the LPC should not appear in the
NOTE
387
non-signal categories. The pre,sent study used subjects with minimal experience in the test situation who perhaps used a different detection strategy or no specific strategy at all. Another explanation of the differences between the two studies may be that the Ss of the present study were also used in a later experiment involving stress and they may have anticipated this stress even though it was not intended to be a factor in the present study, thereby elevating their general level of arousal with respect to the signal detection task. The results of this study are consistent with the work of Sutton et al. [1] (1967) in which an LPC was obtained when the absence of the stimulus was a relevant event and support their conclusion that the LPC is related to resolution of uncertainty. The results are also consistent with KARLIN'S [5] general hypothesis that the LPC represents a "reactive change of state" which follows task completion. The unexpected finding of a second positive LPC in the female Ss' Hit profile will be investigated further in our laboratory. At present it does not appear to be attributable to artifacts of signal phase-locking, myogenic or galvanic skin response contamination. REFERENCES 1. SUTTON,S,, TUETING, P., ZUBIN, J. and JOHN,E. Information delivery and the sensory evoked potential. Science, N.Y. 155) 1436-1439, 1967. 2. HILLYARD,S., SQUIRES,K., BAUER, J. and LINDSAY,P. Evoked potential correlates of auditory signal detection. Science, N. Y. 172, 1357-1360, 1971. 3. CLARK,W. C. The psyche in psychophysics: a sensory-decision theory analysis of the effect of instructions on flicker sensitivity and response bias. Psychol. Bull. 65, 358-366, 1966. 4. WlNER, B. J. Statistical Principles in Experimental Design (2nd Edn). McGraw-Hill, New York, 1971. 5. KARLIN, L. Cognition, preparation and sensory-evoked potentials. Psychol. Bull. 73, 122-136, 1970. RI~tlm~---HILLYARD et coll,, 1971, dans une 6preuve de d6tection du signal chez des sujets humains ont obtenu une composante positive tardive (CPT) pr6dominante de rEEG, seulement dans la cat6gode exacte (signal pr6sent) des r6ponses. Ils en concluent que la CPT traduit un degr6 de certitude du sujet de l'apparition du signal. A l'oppos6 dans r6tude pr6sente, on a observ6 la CPT dans les cat6gories exacte (signal pr6sent), omission et rejet correct des r6ponses ce qui est en faveur de l'hypoth6se que la CPT est un corr6tat de la r6duction de I'incertitude et ne d6pend pas directement de la pr6sence physique du signal. Zmammenfassuag--Hillyard, Squires, Bauer und kindsay (1971), die menschliche Versuchjspersonen fiJr ein¢ Signalerkennungsaufgabe verwendeten, erhielten nut bei der Hit-Kategorie der Antworten eine herausragende sprite positive Komponente (LPC) in EEG. Sie schlossen daraus, dab die PLC den Grad der Sicherbeit des Patienten dariJber widerspiegelt, dab ein Signal gegeben wurde. Im Gegensatz dazu verfolgte die jetzige Studie die LPC bei Hit-, Miss- und Correct-Rejection-Kategorien der Antworten, so dab die Hypothese gestfitzt wird, dab die LPC ein Korrelat der Unsicherheitsverminderung ist und nicht direkt von der physikalischen Pr~.senz der Signale abhangt.
NOTE THE LA T E POSITIVE COMPONENTS OF T H E HUMAN EEG IN A SIGNAL DETECTION TASK* WILLIAM W . CAEL,~" ALLAN NASH a n d JAY J. SINGER Department of Psychology, Florida Atlantic University, Boca Raton, Florida 33432, U.S.A. (Received 18 October 1973)
Abstract In 1971, Hillyard, Squires, Bauer and Lindsay using human subjects in a signal detection task, obtained a prominent late positive component (LPC) of the EEG only in the Hit category of responses. They concluded that the LPC reflects the subject's degree of certainty that a signal has occurred. In contrast the present study observed the LPC in the Hit, Miss, and Correct Rejection categories of responses, supporting the hypothesis that the LPC is a correlate of uncertainty reduction and does not directly depend on the physical presence of the signal. INTRODUCTION TH~ P-300 WAVE, or late positive component (LPC) of the averaged evoked potential, has been related in humans to the resolution of uncertainty and does not appear to be determined solely by the physical eliciting stimulus (StrrroN et aL [1]). According to this interpretation, a signal detection task which requires the observer to report on each trial whether a signal was detected, should produce LPC.s in all decision categories (Hit, Miss, Correct Rejection, False Alarm) since uncertainty is resolved in each instance, independent of the physical presence of the signal or the correctness of the observer's response. This expectation would be particularly strong when the signal is relatively easy to detect, thereby reducing the ambiguity between signal and non-signal trials. HILLYARDet aL [2], however, found the LPC to be prominent only in the Hit category over a wide range of signal intensities. They concluded that the LPC relates to certainty that a signal has occurred and may be triggered by a match between an expected, but not wholly predictable, sensory event and a neural template of the stimulus. Their subjects were given extensive training in the basic signal detection task prior to the recording of the average evoked potentials. In light of the earlier work of Sutton et aL [1] in which an LPC also occurred when the absence of a signal provided information relevant to the resolution of uncertainty, this study was designed to repeat the work of Hillyard et al. [2] using relatively untrained subjects to determine whether the LPC would appear in other than the Hit category.
METHOD Eight volunteers (4 males, 4 females), between 21 and 24 years old, served as subjects (Ss). All were students of Florida Atlantic University. For all Ss the present experiment was the first of a series of experiments whose later phases (not reported here) were designed to test for the effects of stress on the LPC and signal detection performance. The S was seated in a recliner chair inside a sound attenuated chamber. Nonchlorided Grass silver electrodes were placed on shaved portions of the scalp and skin with Grass Electrode paste. A recording cup electrode was fixed to the vertex and reference and common mode disc, electrodes were clipped to the left ear-lobe with Grass nylon earclips. A separate ground plate electrode was placed on the left forearm. A Mousseau EEG Amplifier, Model SA-4, amplified the signals and fed them directly to an on-line, Digital Equipment Corporation PDP-8/L computer for averaging. The EEG amplifier was set at a gain of 40,000 for all As. Band-pass filters were set a 1 Hz lower limit and 50 Hz upper limit. Interelectrode resistance was kept below 10,000 I). Electronic switching circuitry permitted the separate averaging of the four types of experimental trials (Hits, Misses, Correct Rejections, and *Supported by grant AFOSR 69-1823. Conducted by the first author in partial fulfilment of the requirements for the M.A. degree. "['Now at University of Texas in Arlington. 385
386
NOTE
False Alarms). EEG epochs of one second duration, with zero delay from trial onset, were averaged for each of the four response categories. Each average consisted of 200 data points and the analysis of the final waveforms was performed on plotted copies produced by a Mosely-135 X - Y plotter. Pilot investigations were run to determine if any artifacts associated with eye movements or the switching circuitry were present in the final averages. These data indicated that the electrical equipment caused a 3-5 rnsec spike at approximately 90 msec after trial initiation, but this deflection was so narrow and definable that it did not confound the analysis of the data. No evidence of eye movement artifact was found. The basic paradigm consisted of placing the S in the sound attenuated chamber which was illuminated by a 75 W lamp aimed at the ceiling, and presenting continuous white noise of 80 dB (SPL) through a Superex headset. A cue-light in S's hand-held response console was illuminated for 0.75 sec and the offset of the cuelight indicated initiation of each trial. Signals consisting of a I000 Hz frequency tone at 81 dB (SPL) for 90 msec duration were presented binaurally, at random, on 50 per cent of the trials, a procedure which produced approximately 75 per cent correct responses. Trials were spaced 12 sec apart. All Ss knew that the signal, if present, would occur in the 90 msec interval following the offset of the cue-light. Ss were instructed to respond either yes (signal present) or no (signal absent). A pair of panel lights informed the S as to the correctness of his decision 3.5 sec after the trial began. Each S was permitted a few practice trials to familiarize himself with the sequence of events on the trials but in all other aspects the Ss were regarded as untrained and none was familiar with the purpose of the experiment. A total of 300 experimental trials were run with each S. RESULTS The most positive (base-to-peak) deflection in the interval from 250 to 450 msec following trial initiation was recorded from the X - Y plotter output for each S under each of the Hit, Miss, Correct rejection, and False Alarm categories. All Ss showed a strong response bias against responding "yes" to noise-only trials, a finding that has been reported elsewhere for naive Ss in signal detection tasks (CLARK, 1966 [3]). AS a result, there were relatively few trials in the False Alarm category and the noisy character of these profiles did not reveal any prominent peaks. Analysis of the data was therefore restricted to the mean amplitudes and iatencies of LPC peaks in the Hit, Miss, and Correct rejection categories which are summarized in Table 1. Table 1. Amplitude and latency of the LPC in Hit, Miss and Correct rejection categories Hit Miss Correct Rejection
Amplitude (IxV) Latency (msec) 15.5 305 7.5
307
11-7
361
An analysis of variance was performed on the mean LPC amplitudes and significant differences were observed among the three response categories (F = 16.235, d f = 2/30. P <0.01). A Neuman-Keuls analysis of the individual response categories (WINER, 1971 [4]) showed mean Hit LPC amplitude (15'5 laV) to be significantly higher than either Correct rejection (11.7 aV) or Miss (7.5 IxV) amplitudes at the 0'05 level of significance. Correct rejection amplitude was also found to be significantly higher than Miss amplitude at the 0.01 level of significance. An analysis of variance performed on the mean latencies of the LPC's revealed no significant differences among the three response categories (F = 0.008, d f = 1/12, P > 0.25). All of the four female Ss displayed an unexpected large positive deflection with a mean latency of 701 msec in the Hit category. This second peak appeared in addition to the earlier peak of 305 msec. Male Ss did not show the second peak. The appearance of the second positive peak in the EEG profiles of all 4 female Ss and its absence in all 4 male profiles was tested by the Fisher Exact Probability test and found significant at the 0.05 level. DISCUSSION Contrary to the findings of HILLYARD et al. [2] who reported little or no evidence for an LPC in any response category other than Hits, the present study observed significant LPCs in Hit, Miss, and Correct rejection categories. Hillyard et aL [2] used experienced subjects and assumed that their detection strategy was to match the sensory input against a template for the signal. According to this theory the LPC should not appear in the
NOTE
387
non-signal categories. The pre,sent study used subjects with minimal experience in the test situation who perhaps used a different detection strategy or no specific strategy at all. Another explanation of the differences between the two studies may be that the Ss of the present study were also used in a later experiment involving stress and they may have anticipated this stress even though it was not intended to be a factor in the present study, thereby elevating their general level of arousal with respect to the signal detection task. The results of this study are consistent with the work of Sutton et al. [1] (1967) in which an LPC was obtained when the absence of the stimulus was a relevant event and support their conclusion that the LPC is related to resolution of uncertainty. The results are also consistent with KARLIN'S [5] general hypothesis that the LPC represents a "reactive change of state" which follows task completion. The unexpected finding of a second positive LPC in the female Ss' Hit profile will be investigated further in our laboratory. At present it does not appear to be attributable to artifacts of signal phase-locking, myogenic or galvanic skin response contamination. REFERENCES 1. SUTTON,S,, TUETING, P., ZUBIN, J. and JOHN,E. Information delivery and the sensory evoked potential. Science, N.Y. 155) 1436-1439, 1967. 2. HILLYARD,S., SQUIRES,K., BAUER, J. and LINDSAY,P. Evoked potential correlates of auditory signal detection. Science, N. Y. 172, 1357-1360, 1971. 3. CLARK,W. C. The psyche in psychophysics: a sensory-decision theory analysis of the effect of instructions on flicker sensitivity and response bias. Psychol. Bull. 65, 358-366, 1966. 4. WlNER, B. J. Statistical Principles in Experimental Design (2nd Edn). McGraw-Hill, New York, 1971. 5. KARLIN, L. Cognition, preparation and sensory-evoked potentials. Psychol. Bull. 73, 122-136, 1970. RI~tlm~---HILLYARD et coll,, 1971, dans une 6preuve de d6tection du signal chez des sujets humains ont obtenu une composante positive tardive (CPT) pr6dominante de rEEG, seulement dans la cat6gode exacte (signal pr6sent) des r6ponses. Ils en concluent que la CPT traduit un degr6 de certitude du sujet de l'apparition du signal. A l'oppos6 dans r6tude pr6sente, on a observ6 la CPT dans les cat6gories exacte (signal pr6sent), omission et rejet correct des r6ponses ce qui est en faveur de l'hypoth6se que la CPT est un corr6tat de la r6duction de I'incertitude et ne d6pend pas directement de la pr6sence physique du signal. Zmammenfassuag--Hillyard, Squires, Bauer und kindsay (1971), die menschliche Versuchjspersonen fiJr ein¢ Signalerkennungsaufgabe verwendeten, erhielten nut bei der Hit-Kategorie der Antworten eine herausragende sprite positive Komponente (LPC) in EEG. Sie schlossen daraus, dab die PLC den Grad der Sicherbeit des Patienten dariJber widerspiegelt, dab ein Signal gegeben wurde. Im Gegensatz dazu verfolgte die jetzige Studie die LPC bei Hit-, Miss- und Correct-Rejection-Kategorien der Antworten, so dab die Hypothese gestfitzt wird, dab die LPC ein Korrelat der Unsicherheitsverminderung ist und nicht direkt von der physikalischen Pr~.senz der Signale abhangt.