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Feedback Evoked Potentials during an Auditory Concept Formation Task
Feedback Evoked Potentials during an Auditory Concept Formation Task D. T. STUSS, A. TOGA, J. HUTCHISON and T. W. PICTON Department of Medicine and School of Psychology, University of Ottawa, Ottawa (Canada); Department of Neurology, Boston University School of Medicine, Boston, Mass.; and Psychology Research Department, Boston V A Hospital, Boston, Mass. (U.S.A.)
INTRODUCTION In a previous study (Stuss and Picton, 1978), subjects used auditory feedback to learn by trial and error the correct sorting criterion for a set of complex visual stimuli. Two prominent late positive waves - P3 (355 msec) and P4 (647 msec) - were noted in the evoked potential to the feedback stimuli. These components varied similarly with the different stages of concept formation, but were distinct in their scalp distribution, the P4 being more posteriorly located. They were considered to represent a feedbackfeedforward system (Teuber, 1960; Pribram, 1971), P3 indexing the evaluation of feedback information, and the P4 utilization of that information to adjust perceptual hypotheses. The posterior scalp distribution of the P4 wave could relate to a specifically visual adjustment in the occipital region, or to a more general strategic adjustment in the parietal area. In order to evaluate these possibilities further, feedback evoked potentials were studied during auditory concept formation. METHODOLOGY Paradigm (Fig. 1)
A brief yellow light occurring 500 msec after the initiation of the trial warned the subject that a complex auditory stimulus would occur 700msec later. This auditory stimulus could be classified according to 4 possible criteria: the number of discrete tones occurring, their frequency, their intensity, or the timing of the last tone in the sequence. Each criterion had 3 possible variations. Following the auditory stimuli, there was a 1.3 sec response time wherein the subject pressed one of 3 buttons with the fingers of his right hand according to his hypothesized classification criterion. A feedback light occurred 1.8 sec after the termination of the auditory stimulus, provided that the response had been made within the required time-window. A red light signalled that the response had been incorrect, a green light that the response had been correct. Subject groups
Twenty right-handed male subjects, age range 20-35, participated in the experiment, 10 as an experimental and 10 as a control group. For the experimental subjects, the classification criterion for the auditory stimuli in a block of trials was chosen by the
404
experimenter without the subject’s knowledge. On each trial, the subject attempted to discover the criterion by responding according to one of the criteria (i.e., his hypothesis) and then being informed whether his response was correct or incorrect. Using this feedback, the subjects soon discovered the correct criterion - usually within 1-4 trials. After 6-10 consecutive correct responses, the criterion was changed without warning, and the trial-and-error concept formation process began again. The control group of subjects was used to evaluate attentional and intentional involvement in the task independent of the concept formation process. The control subjects responded to the same stimuli as the experimental subjects, but were informed of the criterion for each block of trials just prior to its change. The red and green feedback lights were given arbitrarily in a sequence approximating that obtained by the subjects in the experimental group. For the control subjects the feedback light merely informed the subject that his response had been within the correct time-window. Stages of’concept formation
The trials were divided for averaging, across criterion blocks, into 5 stages. This was based upon the ordering of the feedback light sequence: (1) pre-insight (PRE) -all trials with red lights except the trial in which the criterion was changed; (2) insight (INS) -the first 2 of 4 consecutive green light trials following the pre-insight trials; (3) confirm (CON) - the second pair of consecutive green light trials following pre-insight; (4) overlearn (OVE) -all correct trials after confirm and preceding the change of criterion; (5) change of criterion (COC) - the trial in which the subject was informed that the criterion had been changed by the presentation of a red light even though the response had been correct according to the previous criterion. Evoked potential recordings
Electroencephalographic activity was recorded from F,, C,, P,, P, and 0, scalp locations using Ag/AgCl electrodes referred to a linked mastoid reference. An electro-oculogram was recorded between diagonally placed supra- and infra-orbital electrodes. The electrophysiological signals were amplified on Grass 7P1 DC amplifiers modified to have a time constant of 12 sec and with a high frequency cut-off of 75 Hz. The data were taperecorded and averaging was performed off-line on a Nicolet 1072 signal analyzer according to the defined stages of concept formation, using a 5.12 sec sweep. Trials contaminated by ocular artifact were omitted from the averaging. Each final average was derived from 12 trials or multiples thereof. Analysis
Each measurement was taken relative to a baseline drawn between the mean values of the event-related potential waveform in the first 300 msec and the final 300 msec of the averaging sweep. Only the following four responses to the feedback stimuli were analyzed for this paper: (1) N 1 - the amplitude of the maximum negative peak between 100 and 170 msec after the feedback light onset; (2) P2 -the amplitude of the maximum positive peak between 170 and 250 msec after feedback onset; (3) P3 - the maximum positive peak occurring between 275 and 500 msec after the onset of the feedback light; (4) ’P4 - the peak amplitude of the maximum positive deflection between 500 and 800 msec after feedback onset.
405 PARADIGM
STAGES Criterion
Feedbock
Stoqe
+ + - - + T T V T T
OVE CM;
- + + +
PRE
INS
+
CON
+ - - - + + + + + ~ r - r - 7 -
OMCOCF‘RE INS
CON
Fig. 1. Experimental methodology. The upper portion of the figure illustrates the stimulus timing and depicts sample recordings from the vertex and occiput for an experimental pre-insight stage. Each recording represents the average of 12 trials (subject D.S.). Although all dependent measurements are identified in the C, tracing, only the last 4 (feedback NI, P2, P3, P4) were analyzed for this paper. The central section of the figure illustrates 4 possible examples of the complex auditory stimulus. The lower section schematically illustrates how individual trials were classified, based upon the feedback stimuli, into stages for averaging: PRE, “preinsight”; INS, “insight”; CON, “confirmation”; OVE, “overlearning”; COC, “change of criterion”. Trials between the arrows were blocked according to one sorting criterion. Plus and minus signs denote positive (green light, “correct”) and negative (red light, “incorrect”) feedback. The blank space between CON and OVE illustrates that no feedback was given because the response was not made within the required response time limits.
A two-factor analysis of variance with repeated measurements on the second factor was performed for each measurement for “group x electrode” and again for “group x stage” effects. Post hoc testing was performed using Tukey’s HSD statistic. A significance level of 0.01 was used.
RESULTS Illustrative data are shown in Figs. 2 and 3. Fig. 2 compares the recordings between typical experimental and control subjects in the “pre-insight” stage. Fig. 3 compares the feedback-evoked potentials at the different stages of concept formation for an experimental subject. The results of the statistical analyses for each of the feedback-evoked potential components are detailed in the following paragraphs.
406 PREINSIGHT
EX PER I MENTAL
CONTROL
-10 pv
n
r
I
n
n
i
I
n
542 a
Fig. 2. Scalp distribution and comparison of the event-related potentials recorded from experimental and control subjects at the “pre-insight” stage. Recordings from all cortical areas are referred to linked mastoids. Each tracing is an average of 12 trials. The feedback components for the experimental subject are larger than for the control subject. Of note in this figure is the absence of any P3 or P4 components in the control condition and the scalp distribution of the 3 late positive waves within the experimental condition. Subjects M.C. (experimental) and D.D. (control).
( 1 ) NI (mean latency 138msec)
This component was recorded maximally in the parieto-occipital regions. The scalp distribution expressed as a percentage of the amplitude at P, (where it was maximum) was 9% at F,, 80% at C,, 96% at P,, and 87% at 0,. This wave was larger in amplitude for the experimental than for the control group, but was unaffected by the stage of concept formation. ( 2 ) P2 (mean latency 211 msec)
This component was maximally recorded in the fronto-central regions. The scalp distribution of the component expressed as a percentage of the amplitude at F, was 80% at C,, 36% at Pi, 38% at P,, and 28% at 0,.This component was larger in the experimental subjects, but only during the “change of criterion”, “pre-insight”, and “insight” stages. ( 3 ) P 3 (mean latency 354 msec)
This large late positive wave was maximally recorded from the vertex. It was approximately equally recorded in frontal and parietal areas at 86% (F,), 73% (P3) and
407 PRE
INS
CON
OVE
COC
Fig. 3. Sample recordings for the experimentalcondition at each stage (PRE, pre-insight;INS, insight; CON, confirmation;OVE, overlearning; COC, change of criterion) for each of the 5 cortical areas. Each tracing represents the average of 12 event-relatedpotentials. Replications were not obtained for COC since there was an insufficient number of artifact-freetrials. The 3 late positive waves are largest and most distinct for the “preinsight” and “change of criterion” stages. Subject B.B.
79% (P,) of the vertex amplitude (overall measurements from the experimental group). It was quite small in the occipital region - 43% of the vertex amplitude. This scalp distribution was significantly more posterior than that of the P2 component. The amplitude of the P3 wave was significantlylarger for the experimental group than for the control group at every stage of the task. Within the experimental group the P3 wave was larger for the “change of concept” and the “pre-insight” stages than for the “confirm” or “overlearn” stages, with the P3 in the “insight” stage having an intermediate amplitude. ( 4 ) P4 (mean latency 590 msec)
This wave was only recognizable as a distinct visual peak in approximately one-half of the subjects. In the other subjects the component was arbitrarily measured at the maximum positivity within the defined latency range. Like the P3, the P4 wave was also maximally recorded at the vertex. The P4 component, however, spreads significantly less frontally and more posteriorly than the P3 component. The amplitude in the experimental group at F, was 64% of the vertex amplitude, at P, SO%, at P, 86%, and at 0,60%. The P4 component was similar to the P3 in the group x stage analysis. It was larger for the experimental group, and larger in the “change of criterion” and “pre-insight” stages.
408 DISCUSSION The N1 component appears to reflect the subject’s attention to the informative feedback, being larger in amplitude for the experimental subjects to whom the feedback was relevant, than for the control subjects to whom the feedback was relatively meaningless (Hillyard et al., 1978). It occurs in the parieto-occipital regions which is appropriate to the visual modality of the feedback stimulus (Simson et al., 1977). There are 3 late positive waves that then follow in the evoked potential to the feedback stimulus. They are all similar in their relationship to the experimental manipulations being of greater amplitude in the experimental than in the control group, and being of greater amplitude in those stages of the task that occurred prior to the discovery of the criterion. They therefore probably all reflect the processing of relevant feedback information. Their distinct scalp distributions suggest, however, that they are each related to different aspects of such information processing. These components are similar to those reported previously in the evoked potentials to feedback about performance on a visual concept formation task (Picton et al., 1978; Stuss and Picton, 1978). They therefore probably reflect modality-independent aspects of information processing. The 3 distinct late positive components of the feedback-evoked potential may represent the sequential recognition, evaluation and utilization of significant information. The precise physiological and psychological nature of these components, however, remains to be determined. Several late positive waves in the evoked potential to task-relevant stimuli have been reported by other researchers. Their relationship to the components recorded in this experiment is discussed more fully in the review paper by Picton and Stuss (this volume). SUMMARY In a previous study of visual concept formation two prominent late positive waves were observed in response to informative auditory feedback: a centro-parietal P3 (355 msec) and a parieto-occipital P4 (647 msec). These two evoked potential components were considered as possibly representing a feedback-feedforward system, with the P3 wave reflecting the appreciation of feedback information, and a P4 wave its subsequent utilization in the modification of visual perceptual processes. The present study was designed to examine these hypotheses in an auditory concept formation task with visual feedback. Complex auditory stimuli were used which varied along 4 dimensions. After each stimulus, the subject indicated his hypothesized sorting criterion by a button press, and feedback was subsequently given as to whether this hypothesis was correct or not. Using such feedback the subject was able to discover the correct sorting criterion for the auditory stimuli within a few trials. As in the earlier study, the two late positive waves P3 (354 msec) and P4 (590 msec) were observed. These waves were significantlylarger when feedback was most informative, i.e., in those trials prior to the final confirmation of a correct response strategy. The scalp distributions of these two components were quite similar to the previous experiment with the P3 being centroparietal and the P4 more parieto-occipital. These results suggest that both of these components represent general perceptual processes occurring during concept formation, independently of the sensory modality in which the concepts are formed.
409
ACKNOWLEDGEMENTS This research was supported in part by NIH Grant No. NS 06209, the National Research Council of Canada, the Medical Research Council of Canada, and the Ontario Mental Health Foundation. The authors gratefully acknowledge the assistance of Amy Zieper in running the experiment, and Mary Hyde for computer statistical analyses. REFERENCES Hillyard. S. A,, Picton, T. W. and Regan, D. (1978) Sensation, perception and attention: analysis using ERPs. In Event-Related Brain Potentials in Man, E. Callaway, P. Tueting and S. H. Koslow (Eds.), Academic Press, New York, pp. 223-321. Picton, T. W. and Stuss, D. T. (1980) The component structure of human event-related potentials. This volume. Picton, T. W., Woods, D. L., Stuss, D. T. and Campbell, K. B. (1978) Methodology and meaning of human evoked potential scalp distribution studies. In Multidisciplinary Perspectives in Event-Related Brain Potential Research, D. Otto (Ed.), EPA-600/9-77-043, U.S. Environmental Protection Agency, Washington, D.C., pp. 515-522. Pribram, K. H. (1971) Languages of the Brain: Experimental Paradoxes and Principles in Neuropsychology. Prentice-Hall, Englewood Cliffs, N.J. Simson, R., Vaughan, Jr., H. G. and Ritter, W. (1977) The scalp topography ofpotentials in auditory and visual discrimination tasks. Electroenceph. clin. Neurophysiol., 4 2 528-535. Stuss, D. T. and Picton, T. W. (1978) Neurophysiological correlates ofhuman concept formation. Behav. Eiol., 23: 135-162. Teuber, H.-L. (1960) Perception. In Handbook of Physiology. Section I : Neurophysiology, Vol. III, J. Field, H. W. Magoun and V. E. Hall (Eds.), American Physiological Society, Washington, D.C., pp. 1595-1668.
INTRODUCTION In a previous study (Stuss and Picton, 1978), subjects used auditory feedback to learn by trial and error the correct sorting criterion for a set of complex visual stimuli. Two prominent late positive waves - P3 (355 msec) and P4 (647 msec) - were noted in the evoked potential to the feedback stimuli. These components varied similarly with the different stages of concept formation, but were distinct in their scalp distribution, the P4 being more posteriorly located. They were considered to represent a feedbackfeedforward system (Teuber, 1960; Pribram, 1971), P3 indexing the evaluation of feedback information, and the P4 utilization of that information to adjust perceptual hypotheses. The posterior scalp distribution of the P4 wave could relate to a specifically visual adjustment in the occipital region, or to a more general strategic adjustment in the parietal area. In order to evaluate these possibilities further, feedback evoked potentials were studied during auditory concept formation. METHODOLOGY Paradigm (Fig. 1)
A brief yellow light occurring 500 msec after the initiation of the trial warned the subject that a complex auditory stimulus would occur 700msec later. This auditory stimulus could be classified according to 4 possible criteria: the number of discrete tones occurring, their frequency, their intensity, or the timing of the last tone in the sequence. Each criterion had 3 possible variations. Following the auditory stimuli, there was a 1.3 sec response time wherein the subject pressed one of 3 buttons with the fingers of his right hand according to his hypothesized classification criterion. A feedback light occurred 1.8 sec after the termination of the auditory stimulus, provided that the response had been made within the required time-window. A red light signalled that the response had been incorrect, a green light that the response had been correct. Subject groups
Twenty right-handed male subjects, age range 20-35, participated in the experiment, 10 as an experimental and 10 as a control group. For the experimental subjects, the classification criterion for the auditory stimuli in a block of trials was chosen by the
404
experimenter without the subject’s knowledge. On each trial, the subject attempted to discover the criterion by responding according to one of the criteria (i.e., his hypothesis) and then being informed whether his response was correct or incorrect. Using this feedback, the subjects soon discovered the correct criterion - usually within 1-4 trials. After 6-10 consecutive correct responses, the criterion was changed without warning, and the trial-and-error concept formation process began again. The control group of subjects was used to evaluate attentional and intentional involvement in the task independent of the concept formation process. The control subjects responded to the same stimuli as the experimental subjects, but were informed of the criterion for each block of trials just prior to its change. The red and green feedback lights were given arbitrarily in a sequence approximating that obtained by the subjects in the experimental group. For the control subjects the feedback light merely informed the subject that his response had been within the correct time-window. Stages of’concept formation
The trials were divided for averaging, across criterion blocks, into 5 stages. This was based upon the ordering of the feedback light sequence: (1) pre-insight (PRE) -all trials with red lights except the trial in which the criterion was changed; (2) insight (INS) -the first 2 of 4 consecutive green light trials following the pre-insight trials; (3) confirm (CON) - the second pair of consecutive green light trials following pre-insight; (4) overlearn (OVE) -all correct trials after confirm and preceding the change of criterion; (5) change of criterion (COC) - the trial in which the subject was informed that the criterion had been changed by the presentation of a red light even though the response had been correct according to the previous criterion. Evoked potential recordings
Electroencephalographic activity was recorded from F,, C,, P,, P, and 0, scalp locations using Ag/AgCl electrodes referred to a linked mastoid reference. An electro-oculogram was recorded between diagonally placed supra- and infra-orbital electrodes. The electrophysiological signals were amplified on Grass 7P1 DC amplifiers modified to have a time constant of 12 sec and with a high frequency cut-off of 75 Hz. The data were taperecorded and averaging was performed off-line on a Nicolet 1072 signal analyzer according to the defined stages of concept formation, using a 5.12 sec sweep. Trials contaminated by ocular artifact were omitted from the averaging. Each final average was derived from 12 trials or multiples thereof. Analysis
Each measurement was taken relative to a baseline drawn between the mean values of the event-related potential waveform in the first 300 msec and the final 300 msec of the averaging sweep. Only the following four responses to the feedback stimuli were analyzed for this paper: (1) N 1 - the amplitude of the maximum negative peak between 100 and 170 msec after the feedback light onset; (2) P2 -the amplitude of the maximum positive peak between 170 and 250 msec after feedback onset; (3) P3 - the maximum positive peak occurring between 275 and 500 msec after the onset of the feedback light; (4) ’P4 - the peak amplitude of the maximum positive deflection between 500 and 800 msec after feedback onset.
405 PARADIGM
STAGES Criterion
Feedbock
Stoqe
+ + - - + T T V T T
OVE CM;
- + + +
PRE
INS
+
CON
+ - - - + + + + + ~ r - r - 7 -
OMCOCF‘RE INS
CON
Fig. 1. Experimental methodology. The upper portion of the figure illustrates the stimulus timing and depicts sample recordings from the vertex and occiput for an experimental pre-insight stage. Each recording represents the average of 12 trials (subject D.S.). Although all dependent measurements are identified in the C, tracing, only the last 4 (feedback NI, P2, P3, P4) were analyzed for this paper. The central section of the figure illustrates 4 possible examples of the complex auditory stimulus. The lower section schematically illustrates how individual trials were classified, based upon the feedback stimuli, into stages for averaging: PRE, “preinsight”; INS, “insight”; CON, “confirmation”; OVE, “overlearning”; COC, “change of criterion”. Trials between the arrows were blocked according to one sorting criterion. Plus and minus signs denote positive (green light, “correct”) and negative (red light, “incorrect”) feedback. The blank space between CON and OVE illustrates that no feedback was given because the response was not made within the required response time limits.
A two-factor analysis of variance with repeated measurements on the second factor was performed for each measurement for “group x electrode” and again for “group x stage” effects. Post hoc testing was performed using Tukey’s HSD statistic. A significance level of 0.01 was used.
RESULTS Illustrative data are shown in Figs. 2 and 3. Fig. 2 compares the recordings between typical experimental and control subjects in the “pre-insight” stage. Fig. 3 compares the feedback-evoked potentials at the different stages of concept formation for an experimental subject. The results of the statistical analyses for each of the feedback-evoked potential components are detailed in the following paragraphs.
406 PREINSIGHT
EX PER I MENTAL
CONTROL
-10 pv
n
r
I
n
n
i
I
n
542 a
Fig. 2. Scalp distribution and comparison of the event-related potentials recorded from experimental and control subjects at the “pre-insight” stage. Recordings from all cortical areas are referred to linked mastoids. Each tracing is an average of 12 trials. The feedback components for the experimental subject are larger than for the control subject. Of note in this figure is the absence of any P3 or P4 components in the control condition and the scalp distribution of the 3 late positive waves within the experimental condition. Subjects M.C. (experimental) and D.D. (control).
( 1 ) NI (mean latency 138msec)
This component was recorded maximally in the parieto-occipital regions. The scalp distribution expressed as a percentage of the amplitude at P, (where it was maximum) was 9% at F,, 80% at C,, 96% at P,, and 87% at 0,. This wave was larger in amplitude for the experimental than for the control group, but was unaffected by the stage of concept formation. ( 2 ) P2 (mean latency 211 msec)
This component was maximally recorded in the fronto-central regions. The scalp distribution of the component expressed as a percentage of the amplitude at F, was 80% at C,, 36% at Pi, 38% at P,, and 28% at 0,.This component was larger in the experimental subjects, but only during the “change of criterion”, “pre-insight”, and “insight” stages. ( 3 ) P 3 (mean latency 354 msec)
This large late positive wave was maximally recorded from the vertex. It was approximately equally recorded in frontal and parietal areas at 86% (F,), 73% (P3) and
407 PRE
INS
CON
OVE
COC
Fig. 3. Sample recordings for the experimentalcondition at each stage (PRE, pre-insight;INS, insight; CON, confirmation;OVE, overlearning; COC, change of criterion) for each of the 5 cortical areas. Each tracing represents the average of 12 event-relatedpotentials. Replications were not obtained for COC since there was an insufficient number of artifact-freetrials. The 3 late positive waves are largest and most distinct for the “preinsight” and “change of criterion” stages. Subject B.B.
79% (P,) of the vertex amplitude (overall measurements from the experimental group). It was quite small in the occipital region - 43% of the vertex amplitude. This scalp distribution was significantly more posterior than that of the P2 component. The amplitude of the P3 wave was significantlylarger for the experimental group than for the control group at every stage of the task. Within the experimental group the P3 wave was larger for the “change of concept” and the “pre-insight” stages than for the “confirm” or “overlearn” stages, with the P3 in the “insight” stage having an intermediate amplitude. ( 4 ) P4 (mean latency 590 msec)
This wave was only recognizable as a distinct visual peak in approximately one-half of the subjects. In the other subjects the component was arbitrarily measured at the maximum positivity within the defined latency range. Like the P3, the P4 wave was also maximally recorded at the vertex. The P4 component, however, spreads significantly less frontally and more posteriorly than the P3 component. The amplitude in the experimental group at F, was 64% of the vertex amplitude, at P, SO%, at P, 86%, and at 0,60%. The P4 component was similar to the P3 in the group x stage analysis. It was larger for the experimental group, and larger in the “change of criterion” and “pre-insight” stages.
408 DISCUSSION The N1 component appears to reflect the subject’s attention to the informative feedback, being larger in amplitude for the experimental subjects to whom the feedback was relevant, than for the control subjects to whom the feedback was relatively meaningless (Hillyard et al., 1978). It occurs in the parieto-occipital regions which is appropriate to the visual modality of the feedback stimulus (Simson et al., 1977). There are 3 late positive waves that then follow in the evoked potential to the feedback stimulus. They are all similar in their relationship to the experimental manipulations being of greater amplitude in the experimental than in the control group, and being of greater amplitude in those stages of the task that occurred prior to the discovery of the criterion. They therefore probably all reflect the processing of relevant feedback information. Their distinct scalp distributions suggest, however, that they are each related to different aspects of such information processing. These components are similar to those reported previously in the evoked potentials to feedback about performance on a visual concept formation task (Picton et al., 1978; Stuss and Picton, 1978). They therefore probably reflect modality-independent aspects of information processing. The 3 distinct late positive components of the feedback-evoked potential may represent the sequential recognition, evaluation and utilization of significant information. The precise physiological and psychological nature of these components, however, remains to be determined. Several late positive waves in the evoked potential to task-relevant stimuli have been reported by other researchers. Their relationship to the components recorded in this experiment is discussed more fully in the review paper by Picton and Stuss (this volume). SUMMARY In a previous study of visual concept formation two prominent late positive waves were observed in response to informative auditory feedback: a centro-parietal P3 (355 msec) and a parieto-occipital P4 (647 msec). These two evoked potential components were considered as possibly representing a feedback-feedforward system, with the P3 wave reflecting the appreciation of feedback information, and a P4 wave its subsequent utilization in the modification of visual perceptual processes. The present study was designed to examine these hypotheses in an auditory concept formation task with visual feedback. Complex auditory stimuli were used which varied along 4 dimensions. After each stimulus, the subject indicated his hypothesized sorting criterion by a button press, and feedback was subsequently given as to whether this hypothesis was correct or not. Using such feedback the subject was able to discover the correct sorting criterion for the auditory stimuli within a few trials. As in the earlier study, the two late positive waves P3 (354 msec) and P4 (590 msec) were observed. These waves were significantlylarger when feedback was most informative, i.e., in those trials prior to the final confirmation of a correct response strategy. The scalp distributions of these two components were quite similar to the previous experiment with the P3 being centroparietal and the P4 more parieto-occipital. These results suggest that both of these components represent general perceptual processes occurring during concept formation, independently of the sensory modality in which the concepts are formed.
409
ACKNOWLEDGEMENTS This research was supported in part by NIH Grant No. NS 06209, the National Research Council of Canada, the Medical Research Council of Canada, and the Ontario Mental Health Foundation. The authors gratefully acknowledge the assistance of Amy Zieper in running the experiment, and Mary Hyde for computer statistical analyses. REFERENCES Hillyard. S. A,, Picton, T. W. and Regan, D. (1978) Sensation, perception and attention: analysis using ERPs. In Event-Related Brain Potentials in Man, E. Callaway, P. Tueting and S. H. Koslow (Eds.), Academic Press, New York, pp. 223-321. Picton, T. W. and Stuss, D. T. (1980) The component structure of human event-related potentials. This volume. Picton, T. W., Woods, D. L., Stuss, D. T. and Campbell, K. B. (1978) Methodology and meaning of human evoked potential scalp distribution studies. In Multidisciplinary Perspectives in Event-Related Brain Potential Research, D. Otto (Ed.), EPA-600/9-77-043, U.S. Environmental Protection Agency, Washington, D.C., pp. 515-522. Pribram, K. H. (1971) Languages of the Brain: Experimental Paradoxes and Principles in Neuropsychology. Prentice-Hall, Englewood Cliffs, N.J. Simson, R., Vaughan, Jr., H. G. and Ritter, W. (1977) The scalp topography ofpotentials in auditory and visual discrimination tasks. Electroenceph. clin. Neurophysiol., 4 2 528-535. Stuss, D. T. and Picton, T. W. (1978) Neurophysiological correlates ofhuman concept formation. Behav. Eiol., 23: 135-162. Teuber, H.-L. (1960) Perception. In Handbook of Physiology. Section I : Neurophysiology, Vol. III, J. Field, H. W. Magoun and V. E. Hall (Eds.), American Physiological Society, Washington, D.C., pp. 1595-1668.