Brain potentials associated with structural and functional visual matching

Neumpaychologia,Vol. 16. p. 571to 585. PergamonPres8Lid. 1978.kinted in Great Britain.

BRAIN POTENTIALS ASSOCIATED WITH STRUCTURAL AND FUNCTIONAL VISUAL MATCHING GREGORY

Cognitive Psychophysiology

MCCARTHYand EMANUELDONCHIN Laboratory, Department of Psychology, University of Illinois, Champaign, Illinois 61820, U.S.A. (Received 6 March 1978)

Abstract-Ten subjects were presented with a series of slides composed of three figures each. Two of the figures were structurally matched (look alikes) while two were functionally (conceptually) matched. A tone burst preceded each slide by 100 msec. In mixed mode conditions, the frequency of the tone cued which type of match was to be performed on the following slide. In fixed mode conditions, the same type of match was made for each slide; the tones conveying only temporal information. Reaction time data indicated that functional matching was a more difficult task than structural matching. Spectral analysis of the EEG indicated that less alpha power was present for functional than structural matching. Mode of matching, however, had no effect on either the performance or spectral data. Two event-related potential components were dissociated within the preparatory interval. The tirst component was sensitive to the mode of matching becoming larger parietally when the warning tone conveyed task relevant information. The second component was more pronounced centrally and asymmetric in its distribution over the two hemispheres for all experimental tasks.

INTRODUCTION THIS study is concerned with those Event-Related Potentials (ERPs) of the human brain associated with the preparation for, and subsequent performance of, a visual matching task. The experimental paradigm has been designed to allow an examination of several issues pertaining to the Contingent Negative Variation (CNV) [l]. Experiments using long (i.e. 34 set) inter-stimulus intervals (ISI) in forewarned RT paradigms have identified two independent components in the preparatory interval. The first component appears predominantly in frontal electrodes and has been attributed, by some investigators, to an orientation process activated by the warning stimulus [2-6]. The second component is largest at central electrodes and appears to resemble the CNV. Some investigators, however, have emphasized the relationship of this later component to motor preparation [7] and to the readiness potential [8]. These studies used long foreperiods assuming that with the foreperiods commonly employed in CNV paradigms (800-1500 msec), the two components overlap temporally and their separate identities are obscured. In the present experiment we have been able to dissociate the two components in a short (1OfKl msec) preparatory foreperiod. Many attempts have been made to discover relationships between the lateral or interhemispheric distributions of ERPs and the hemispheric specialization of the human brain (see [9, lo] for reviews). In an early report the CNV was described as bilaterally symmetric [l 11. Subsequent reports have indicated that the CNV can be lateralized in the performance of certain tasks, especially those involving lateral movement or speech production [7, 12-141. 571

572

GREC~RY MCCARTHYand EMANUELDONCHIN

The apparent asymmetry of the CNV in such paradigms, however, may reflect the addition of a laterally asymmetric movement-related slow potential to an otherwise symmetric CNV. Whether the CNV can be lateralized as a function of the preparation to perform cognitive tasks presumed to differentially engage the cerebral hemispheres is less certain. Studies concerned with this issue have produced some positive evidence but conflict on the direction that the lateral asymmetry takes [IS, 161. This issue is addressed in the present experiment by using visual matching tasks derived from LEVY’S [17] study of commissurotomized patients. Levy’s data indicate that for most dextral subjects the right hemisphere is prepotent for gestalt analysis-as when subjects are required to note the structural characteristics of visual stimuli. The left hemisphere, by virtue of its language development, is dominant for tasks in which conceptual relationships among items must be assessed. Our purpose, then, is to examine the anterior-posterior (A-P) and lateral distributions of ERPs (especially the negative components) preceding and following the performance of specialized matching tasks with special emphasis on the interaction of components within the preparatory interval. METHODS Subjects Ten (5 male) dextral university students (mean age: 23.7 yr) were paid for their participation in this experiment. Each was administered the Edinburgh Inventory [18] to assess handedness (mean score = +0.84; a maximum dextral score = + 1.OO).None of the subjects reported having sinistral relatives. Procedure Each subject sat in a reclining chair facing a small translucent screen onto which visual stimuli were back-projected from an adjoining room. The projector was equipped with a shutter and both were under computer control. The subjects were instructed to fixate on a small triangle of reflective tape placed in the center of the image that each slide formed when projected on the screen. The insulation between the rooms eliminated all sounds associated with the operation of either the projector or shutter. A tone burst (1000 Hz or 2000 Hz, 83 dB SPL) lasting a total of 20 msec (10 msec rise/fall) was presented through TDH-39 earphones against a constant white noise background (50 dB SPL). The tone preceded the presentation of the slide by 1000 msec. A total of 42 slides were used in this experiment. Each was composed of three figures oriented across the horizontal axis of the slide. Two of the figures were drawn so as to look alike (a structural match), while two of the figures were conceptually related (a functional match). One figure, therefore, was common to both match categories (see [9] for an example of a slide stimulus). The subject’s task was to make either a structural or functional match for each slide and report the decision by pressing one of three buttons coded for the position of the chosen slides. A response box was provided on which the subjects rested the first three fingers of their right hand throughout the experiment. The subjects were instructed to respond as quickly as possible following the presentation of the slide (each slide was exposed for 50 msec). The slides were constructed such that both kinds of matches were equally distributed among figure positions. Therefore, differences in reaction time which might have been related to responding finger were balanced across conditions. The match that the subject was asked to make formed the basis of the experimental conditions. In “tied” mode conditions, the subjects were asked to make the same type of match for every slide in a block of trials. In these conditions the tone frequency (Sl) did not vary from trial to trial and served only to warn the subject that the slide (S2) was about to be presented. In “mixed” mode conditions, the Sl stimulus indicated which match type was required on the following slide. For half of the subjects, the 1000 Hz tone signaled a structural match while the 2000 Hz tone signaled a functional match. The significance of the tones was reversed for the other subjects. As the tones varied randomly with equal probability from trial to trial, the subjects depended on Sl for task relevant information. For each tixed block, 84 slides were presented (each slide twice); in the mixed block, a total of 84 slides were presented (approx 42 of each match type). In addition, a control condition was employed in which a fixed Sl signaled a slide composed of three Xs placed in the same orientation as the line figures. The subjects were instructed to respond as quickly as they could to all of these slides using their index finger. Four experimental blocks (structural fixed, functional fixed, mixed, and RT control), therefore, were presented to each subject. With the constraint that the mixed block occur sometime after the two fixed blocks in the experimental sequence, the order of conditions was balanced across subjects. An additional block of

EEG, ERPS AND VISUAL

MATCHING

573

trials (Rest control) was run in whichno stimuliwere presentedto the subjectbut in whichsamplesof EEG and EOG were taken at the same intervals as in the other blocks.These data were collectedat the beginning of each experimentalsession with the subject’seyes open. A preliminary practice session,without EEG recording, was conducted no less than 3 days and no more than 8 days prior to the experimental session. Recording EEG was recorded from nine scalp electrodes (F3, Fz, F4, C3, Cz, C4, P3, Pz, P4-according to the 10-20 System) referenced to linked mastoids. Electrodes were also placed above and below the right eye to record the vertical electrooculogram (EOG). For six of the subjects, electrodes were also placed at F7 and F8. The symmetry of the electrode placements was checked by two experimenters. Electrode impedances beyond 10 kn were not tolerated and in most cases were below 3 kR. Care was taken to assure that homologous electrode pairs have roughly equivalent impedances. The EEG and EOG were amplified with Grass 7P 122 amplifiers with an upper cut-off of 35 Hz. (- 3 dB) and with a time constant measured at 1.5 sec. Pilot data recorded at these settings and simultaneously with a d.c. lower cut-off indicated that the time constant was adequate for faithfully representing the CNV. Data acquisition, as well as experimental control, was managed by a PDP-1 l/40 computer system [19]. Data were digitized on-line (10 msec/pt) and stored on digital tape. For each of the EEG and EOG channels, a total epoch of 2560 msec was acquired, beginning 360 msec prior to the Sl stimulus. The EOG was evaluated for eye movements by calculating its variance and comparing it to a criterion established in practice trials. Trials in which detectable eye movements occurred were not included in signal averages or in any analyses. Reaction time and response accuracy were determined for every trial. Trials with response errors were excluded from all analyses.

RESULTS Four major results were obtained: (1) functional matching proved a more demanding task than structural matching, as shown by a greater number of errors and longer RTs. (2) Spectral density in the alpha band was inversely related to task demands. (3) Two ERP components were clearly distinguishable in the foreperiod: the earlier of these components was clearly related to the mode of matching; the second and later component was laterally asymmetric for all experimental conditions. (4) Slow wave components, differentiable in scalp distribution, continued for over 1000 msec after the slide presentation. These results will be considered in detail in the following sections. Behavioral performance

The matching behavior can be described in terms of the subjects’ reaction times and in the number of correct responses made for each experimental condition. Response errors occurred whenever the subject made the inappropriate match for that trial. Reference to fig. 1 reveals that structural matching was performed faster (P < 0.001, F = 87.8, df = l/8) and with fewer errors (P < 0.001, F = 156.2, df = l/8) than functional matching. Mode of matching did not have a significant effect on subjects’ RT or matching accuracy. These data establish that our two matching tasks imposed different processing loads upon the subjects. They do not, however, give any indication of specialized hemispheric involvement. Spectral analysis The spectral composition

of the EEG at homologous scalp sites was evaluated for two reasons. There have been numerous reports that EEG power spectra reflect hemispheric specialization [9, lo]; it was therefore useful to attempt replication in this more structured paradigm. Furthermore, interpretations of ERP differences, if found, might depend on the extent to which they could be attributed to spectral differences [20]. To evaluate task

induced effects on the on-going EEG, spectral analysis was performed on each single trial for an epoch beginning at Sl and extending for 2200 msec. This epoch included 1000 msec of EEG prior to the slide presentation (presumably while subjects were preparing to make

574

JRE~ORY MCLARTHY

Rllcl hMANUEL YONCHIN

-

15001400 13001200 6 : I

lloo-

;: a

IOOO-

900 -

300 I-

-

I

1

-

SF

SM

FF

FM

RT

SF

SM

FF

FM

Fro. 1. Mean reaction time (RT) and per cent correct matches for the Structural-Fixed (SF), Structural-Mixed (SM), Functional-Fixed (FF), Functional-Mixed (FM), and RT-Control (RT) conditions.

either the structural or functional match). The epoch extended to 1200 msec after the slide presentation (when subjects were actively engaged in the visual discriminations and encoding their response). For each lateral electrode site, an autocovariance function was determined on 25 lags and transformed using the discrete cosine transform (for machine algorithm see [21]). A Tukey-Hanning window was used to smooth the spectral estimates and to minimize leakage. Estimates of spectral power were obtained every 2 Hz from 0 to 50 Hz (the folding frequency). A separate analysis of variance was performed on the mean power for each estimate from 2 to 12 Hz (i.e. 6 dependent variables) using a three factor repeated measures design (10 subjects, 2 matching modes by 2 types of match by 6 lateral electrodes). The analysis reveals that there is less power in the alpha band (i.e. 8-12 Hz) during functional than during structural matching (P < 0.04, F = 5.48, df = l/9 at 8 Hz, P < 0.02, F = 8.04, df = l/9 at 10 Hz, P < 0.02, F = 7.62, df = l/9 at 12 Hz). This effect did not interact with electrode position or matching mode. There was no consistent evidence, even in the individual subject’s data, for selective alpha suppression at any electrode or over either hemisphere related to the type of match performed. The main effect of matching mode was not significant; the matching task alpha differences were present whether the subjects performed in fixed or mixed mode. While the two matching tasks differ from each other in alpha power, both tasks are accompanied by less alpha than either the Rest or RT control conditions. Thus there is no evidence in these data of differences in the EEG power spectrum which reflect hemispheric specialization presumably associated with these matching tasks. Alpha power is systematic ally affected by the tasks but this is an electrode-independent effect probably reflecting the different processing loads imposed by the tasks.

575

El?43,ERPS AND VISUAL MATCXUUQ

ERP waveforms

Grand-averaged waveforms (over subjects with equal weighting) are shown in Fig. 2 where the ERPs for all experimental conditions have been overlapped along the 360 msec pre-Sl “baseline” record for frontal, central, and parietal midline electrode sites. This figure presents a visual summary of the central tendencies in the data. Inspection reveals segments within the waveforms where variability between experimental conditions is most notable. These segments are quite distinct and need to be examined with respect to scalp distribution as well as experimental condition. Waveform variability is not restricted to the interval between Sl and S2; rather, differences between conditions are apparent long after the presentation of the slide.

SF -

Tone

.Slidr

*....*..,... cz Pr

FIG. Wavefom for RT-Control (RT), (SF), Stmtural-Mixed Functional-Fiied (FF), Functional-Mixed (FM) shown aligned the 360 baseline for (Fz), Central and Parietal midline electrodes. tone prethe slide by 1000

Considering to S2, central

the waveform are large Both mixed

within the differences between conditions are

interval, we that, just at the and with waveforms in

576

GRJXORY MCCARTHYand EMANUELDONCHIN

amplitude than the waveforms recorded during the fixed mode conditions. These differences, however, are not present in this region of the waveform at the parietal electrode site. Striking differences in the ERP waveforms are apparent 400-600 msec after Sl . The scalp distribution of the potentials in this region is variable and interacts strongly with the matching mode. This interaction is demonstrated in Fig. 3 where the data presented in Fig. 2 are replotted with the ERPs from the midline electrode sites overlapped for each of the experimental conditions. A positive-going slow wave, prominent parietally, appears in both mixed mode averages in which Sl indicates the type of match to be made. A negative “bump” in this region appears frontally for both match modes. The somewhat rectangular shaped CNV at the central position in both fixed mode and RT control conditions becomes ramp shaped in the mixed mode conditions, reflecting modulation by the positive parietal component.

J

I-+5lJv

ad

...........

RT

___--

SF

-a-

SM FF FM

~

FIG. 3. Waveforms for the Frontal (Fz), Central (Cz), and Parietal (Pz) midline electrode sites are shown overlapped for the RT-Control (RT), Structural-Fixed (SF), Structural-Mixed (SM), Functional-Fixed (FF), and Functional-Mixed (FM) conditions. The tone preceded the slide by 1000 msec.

Pronounced differences are also present in the segment of the waveform approximately 200400 msec after S2. Examining the central and parietal waveforms of Fig. 3, it appears

EEO,

ERPS AND

VISUAL MATCHINO

577

that the CNV resolves (i.e. goes positive) most rapidly for the RT control condition. In this time region the waveforms associated with both tied mode matching conditions are more positive than either of the mixed mode waveforms. At longer latency, however, the waveforms recorded during functional matching are more negative-going than structural matching ERPs, independent of the mode of presentation. Large differences in scalp distribution occur in this post-S2 epoch as shown in Fig. 2. There is a definite trend in all experimental conditions for the parietal ERP to be more positive-going 200-400 msec after S2 than the frontal or central CNVs. At longer latency the waveforms for the three scalp positions dissociate for all matching conditions; positivegoing slow waves present frontally, negative-going slow waves present parietally. In Fig.4, ERPs from homologous lateral positions have been overlapped. Large asymmetries in the post-S2 region are evident for all experimental conditions at the central electrodes. In all cases, the left central (C3) is more negative than the right (Cl). ,Asymmetries of the same direction (albeit, of smaller magnitude) present in the Sl-S2 interval prior to the S2 stimulus. Across conditions, the magnitude of the asymmetry appears greatest at the parietals, with P3 more negative than P4. It appears that waveforms are most asymmetric in association with mixed mode matching conditions. There is no evidence in these grand averages or in the averages for individual subjects for asymmetries that reverse as a function of the type of match to be made. Principal components analysis

Inspection of the ERP waveforms revealed several distinct loci of variance related to the three independent variables under consideration. To objectify these impressions and to take into consideration inter-subject variability, the ERPs from each subject, condition, and electrode position were submitted to a principal component analysis @‘CA). Principal component analysis can be of aid by decomposing the waveforms into orthogonal regions of variance which can then be measured and assessed for experimental effects, see DONCHIN [22, 231 for a discussion of the application of PCA to ERP data and DONCHIN,TUETING, RITTER,KUTASand HEFFLEY[24], SQUIRES,DONCHIN,HERNINGand MCCARTHY[25], and MCCARTHYand DONCHIN[26] for illustrations. The PCA was computed using the time-point covariance matrix. The first six factors extracted by the PCA were further rotated using an orthogonal (Varimax) rotation. A plot of the loadings for the first four components from this analysis along with the grand-mean waveform is presented in Fig. 5. The component loading is a measure of the association between each time point and each component. Component scores were derived, measuring the magnitude of each component in each waveform. Separate ANOVAs were performed on the component scores for these components to assess their lability to the independent variables. Two ANOVA designs were implemented for the analysis of the component scores. The first design was a repeated measures model with 2 levels of match type, 2 levels of matching mode, 3 levels of anteriorposterior (A-P) electrode position (frontal, central, parietal), and 3 levels of lateral electrode position (left, right, midline) for 10 subjects. The second design was used to evaluate the RT control condition, considering the four matching conditions (structural fixed (SF), structural mixed (SM), functional &red (FF), functional mixed (FM) as separate levels of a 5 level condition factor. Additional analyses were performed on component scores derived from a PCA excluding the midline electrodes to achieve maximum sensitivity for small lateral differences.

578

GR@GORYMCCARTHYand EMANUELDONCHIN

FF

Tone

Tone

Slide

SI

Two components are present in the N-S2 interval. On the basis of their differential time course and scalp distributions, one will be referred to as the CNV, the other as the early component of the CNV epoch. The CNV component (labeled 3 in Fig. 5) extends from about 480 msec after Sl and achieves its maximum just beyond S2. For all experimental conditions, the CNV component

Sk,

ERPS AND VISUAL MATCHING

GRAND

TONE

MEAN

WAVEFORM

SLiDE

COMPONENT -I 1... . . . . 2 --..:3 -._ 4

LOADINGS :? ; \_ j \

FIG. 5. The Grand Mean Waveform (averaged across subjects, electrode positions, and experimental conditions) is shown at the top. Below are displayed the component loadings for the first four components derived from the principal Components Analysis (using the covariance matrix) of the waveform data.

is largest (i.e. most negative) centrally (P < 0.001, F = 15.2, df = 2/18). The mode of matching has an effect on the magnitude of the CNV component, but only at the frontal and central electrode sites where mixed mode is associated with a larger CNV than fixed mode (P < 0.007, F = 6.4, df = 2/18). Thus the conclusions drawn from the qualitative examination of this region of the waveform in Fig. 3 are cotirmed. The interaction of the lateral electrode distribution with matching task was significant (P < 0.04, F = 3.9, df = 2/18). Examination of the treatment means shows that the magnitude of the CNV component is essentially equivalent at the midline electrodes for both matching tasks. The source of the interaction is at the left and right lateral electrodes (this conclusion is strengthened by the PCA analysis which excluded the midline electrodes which also showed this interaction to be statistically significant (P < 0.042, F = 5.6, df = l/9). Examination of the component means reveals a somewhat larger magnitude CNV over the left hemisphere for structural matching and a larger magnitude CNV over the right hemisphere for functional matching. These differences do not represent reversals in component asymmetry as they are superimposed upon a left more negative than right

580

GRW;ORYMCCARTHYand EMANUELDONCHIN

asymmetry for both matching tasks. The small magnitude of the effect urges caution in interpreting this finding. The trend for more asymmetry in mixed conditions noted in the grand-averaged waveforms of Fig. 4 did not reach statistical significance. The early component within the Sl-S2 interval (labeled 4 on Fig. 5) which extends over a latency range of about 400-600 msec post-S1 is strongly affected by the mode of matching. For fixed mode conditions, this component is negative at both the frontals and centrals and is nearly absent parietally. In mixed mode conditions, however, a dramatic positive shift occurs in the parietals and centrals, while the frontals increase in negativity. This interaction of mode by A-P scalp distribution is clearly evident in this latency region for the grand-averaged waveforms. When Sl gives specific task relevant information (as in the mixed modes), the early component becomes positive at the parietal electrodes and nearly absent at the central electrodes. While the early component appears more negative over the left hemisphere, this trend is not statistically significant (P < 0.061, F = 4.5, df = l/9). The lateral distribution of this component did not interact with the type of match made by the subject or the mode in which the match was made. The two components in the post-S2 region of the waveform have a time course similar to the components in the Sl-S2 interval. The earlier component from this region (labeled 2 on Fig. 5) peaks at about 440 msec post-S2 and has an A-P distribution similar to that of the early component of the CNV region. It tends to be positive-going in parietal electrodes and negative-going in frontal electrodes (P < 0.001, F = 9.3, df = 2/l@. The two features noted in the qualitative examination of this waveform segment are confirmed for this component. The most parietal positivity is associated with the RT control condition (P < 0.001, F = 4.2, df = 8/72). Also, excluding the RT control data, this component is more positive parietally for the fixed than the mixed conditions (P < 0.001, F = 20.2, df = 2/l@. This component appears to be related to the resolution of the CNV. Reference to Figs. 2 and 3 reveals that the positive-going trend appears sooner after S2 in the parietal electrodes, while the negativity is more sustained in the frontal electrodes. Such differences in CNV resolution have been reported previously [27]. The second component which follows S2 (labeled 1 on Fig. 5) is ramp shaped in appearance, begining at S2 and reaching a sustained maximum about 800 msec after S2. As noted in the waveforms, there is a distinct A-P dissociation for this component; negative-going at the parietals and positive-going frontally (P < 0.001, F = 29.6, df = 2/18). A number of observations suggests that this component may be a pre-movement potential. First, there is a clear lateral asymmetry (left more negative than right) which is most pronounced at the central electrode sites (P =L0.001, F = 10.1, df = 2118). As the motor response required in all cases was a right hand response, such a contralateral enhancement at the centrals would be expected [28, 291. Second, examination of the midline electrode by condition (5 level) interaction (P < 0.001, F = 7.1, df = 8/72) and the lateral electrode by condition (P < 0.002, F = 3.4, df = 8/72) interaction reveals that the A-P magnitude and lateral asymmetry is markedly reduced for the RT control condition. This is consistent with a pre-movement potential hypothesis as the motor responses in the RT control conditions are completed as early as 200-300 msec after S2. Examination of the matching task effects suggests a trend for greater lateral asymmetry in the functional matching conditions than in the structural matching conditions (P < 0.067, F = 3.1, df = 2/18). The longer RTs found for the functional matching conditions, presumably associated with more prolonged motor preparation, could explain this trend. However, as the mean RTs (and associated

EEG, ERPS AND

VISUAL

MATCJXING

581

standard deviations) for both match types included nearly all of the post-S2 record) this trend did not emerge as statistically significant. DISCUSSION We have described an experiment designed to examine the effects of complex visual matching tasks on the magnitude and scalp distribution of event-related potentials. The performance data indicate that the two tasks, structural matching and functional matching, had profound differential effects on RT and on accuracy of matching. The mode of presentation for the matching tasks (fixed or mixed mode) did not affect either measure. Spectral analyses, performed to obtain converging evidence for hemispheric specialization, clearly discriminated structural from functional matching; the latter associated with less overall alpha power than the former. No effects of mode of presentation were noted and no evidence was found for differential hemispheric asymmetries in alpha power as a function of matching task. By contrast, an ERP component early in the Sl-S2 interval was very sensitive to the mode in which matching tasks were presented. When Sl provided specific task relevant information, this component dramatically altered its anterior-posterior distribution. Some evidence was found suggesting that match type might have a differential effect on the symmetry of the later CNV component, although this effect was superimposed on a considerably larger left greater than right asymmetry present for both match types. The slow potentials within the Sl-S2 interval proved most interesting and their relation to the experimental variables forms the crux of this report. We identify the CNV as that component with an approximate latency to the peak of 440 msec post-Sl, peaking in amplitude just at the second stimulus. For all conditions, the CNV was largest at Cz. The mode of matching had a significant effect on the magnitude of the CNV, but only at the frontal and (to a lesser extent) central scalp sites, where mixed mode conditions produced larger CNVs. The CNV was more negative at the left than at the right hemisphere at all electrode positions. As the response required of the subjects involved the right hand only, this asymmetry may be a reflection of motor preparation. The results of the principal components analysis indicated that there is some variance in this asymmetry controlled by the matching task to be performed by the subject. Larger CNVs over the left hemisphere were associated with structural matching with larger CNVs over the right hemisphere for functional matching. These differences were marginally significant and not readily apparent in the waveforms. The direction of the asymmetry noted, however, is consistent with an observation by MARSH and THOMPSON[16] that smaller CNVs were present over the hemisphere presumed dominant for the task to be performed. These differences are inconsistent, however, with the findings of BUTLERand GLASS[15] who observed larger CNVs over the left hemisphere during the performance of an arithmetic task presumed to engage the left hemisphere. The early negative component in the Sl-S2 interval displayed dramatic alterations in form as a function of the mode of matching. In many recent papers [2-71, an early component in the Sl-S2 interval has been described with a frontal, negative maximum. In these studies, the Sl-S2 interval has been extended to 34 set to dissociate this component from the later slow component. Our experiment demonstrates the power of the principal components technique for identifying this component in the more usual 1 set Sl-S2 interval and for assessing its relationship to experimental variables. Quite novel is our finding that this component becomes positive-going parietally when the Sl stimulus presents specific

582

GREGORYMCCARTHYand EMANUELDONCHIN

task relevant information. Its A-P distribution in this condition and its latency to Sl suggest its similarity to the P300-slow wave complex observed in many recent experiments [25, 26, 301. This early component has been related to the orienting response to the Sl stimulus (e.g. [3]). The data suggest that the task relevance of the Sl stimulus plays an important role in its scalp distribution. Further research is needed to explore the similarity of this component to the P300 and the “slow wave” [25, 301. One avenue of research would involve determining whether this early component is affected by the sequence of the Sl stimulus among two alternatives as is the P300 component [31]. One of the striking features of the data is the apparent dissociation between performance and electrophysiological measures. Of the two independent variables, one, match type, has a powerful effect on subjects’ performance but appears to have little effect on the ERP waveform. The other variable, presentation mode, has no effect on subjects’ performance but does affect electrocortical activity. It is precisely such dissociations which have made it so difficult to arrive at a satisfactory elucidation of the functional significance of the CNV and led to doubts about its meaning and value. It is possible, however, to interpret these data in quite a different manner. Consideration of the experimental paradigm suggests that the subjects are required to perform at their best under all of the various experimental conditions. It is plausible that as circumstances change, the subjects vary the manner in which their resources are deployed so as to optimize performance. If electrocortical activity is sensitive to variations in resource allocation and these variations yield a relatively constant performance level, the ERPs may vary widely across experimental conditions without concomitant variation in the performance measures. Thus, performance measures represent the end product of a multiplicity of interacting processes. The specific constellation of the processes may change yet the final product remains constant. Measures which reflect variance in the processes may, but need not, be correlated with measures of the final product. Interpretation of the variance in process measures depends, therefore, on an understanding of the circumstances in which the data are obtained. Our data suggest that the ERPs discussed in this paper can be viewed as reflecting variations in resource deployment. The preparation undertaken prior to a match depends more on the information available prior to the match than on the nature of the match. No amount of preparation can eliminate the difference in difficulty between structural and functional matching, thus the large difference in performance. The match type does not, however, cause any striking change in the ERP waveforms. The point in time at which the required match type is identified to the subject does have a significant effect upon the waveforms. The implications of these data were discussed above; we note only that attempts to interpret ERP variance will profit by considering the relation between the ERP and subject strategies as well as the relation between ERPs and performance measures. We are proposing a “terrain” hypothesis. As the correlation between the speed of an automobile and the depression of the accelerator depends on the terrain being traversed, so the correlation between measures of the CNV and the organism’s performance may depend upon the psychological and physiological terrain over which the organism is travelling. A consideration of this metaphorical terrain may well aid in determining the functional significance of the CNV. Acknowledgements-The authors would like to express appreciation to MARTA KUTAS, GREGORYCHESNEY and CONNIEDUNCAN-JOHNSON for their aid and comments. This research was supported by the Advanced Research Projects Agency’s Cybrenetic Technology Office

Effi, ERPs AND

VISUAL MATCHING

583

under ONR contract No. N-tXlO-14-76-C-0002 to E. DONCHIN. G. MCCARTNEY was supported by a University of Illinois fellowship. A preliminary report of these data was presented before a meeting of the Society for Psychophysiological Research in San Diego, October 1976.

REFERENCES 1. WALTER, W. G., COOPER, R., ALDIUD~E, V. J., MCCALLUM, W. C. and WINTER, A. L. Contingent negative variation: an electric sign of sensorimotor association and expectancy in the human brain. Nature, Land. 203, 380-384, 1964. 2. KL~RMAN,R. and BENTSEN,E. Effects of warning-signal intensity on the early and late components of the Contingent Negative Variation. Biol. Psychol. 3,263-275,1975. 3. LOVELESS,N. E. The effect of warning interval on signal detection and event-related slow potentials of the brain. Percept. Psycbophys. 17,565-570, 1975. 4. LOVELESS,N. E. and SANFORD, A. J. Slow potential correlates of preparatory set. BiologicalPsychology 1, 303-314,1974. 5. Lovam, N. E. and SANFORD,A. J. The impact of warning signal intensity on reaction time and components of the Contingent Negative Variation. Biol. Psych& 2,217~226,1975. 6. WEERTS,T. C. and LANO, P. J. The effects of eye fixation and stimulus and response location on the Contingent Negative Variation (CNV). Biol. Psych&. 1,l-19, 1973. 7. SYNDULKO,K. and LINDSLEY,D. B. Motor and sensory determinants of cortical slow potential shifts in man. In Attention, Voluntary Contraction and Event-Related Cerebral Potentials. Progress in Clinical Neurophysiology,J. E. DESMEDT(Editor). Vol. 1, pp. 97-131. S. Karger, Basel, 1977. 8. ROHRBAUOH,J. W., SYNDULKO,K. and LINDSLEY,D. B. Brain components of the Contingent Negative Variations in humans. Science, N.Y. 191, 1055-1057, 1976. 9. DONCIUN, E., KV~AS, M. and MCCARMY, G. Electrocortical indices of hemispheric utilization. In Laterulizutionin the Nervous System, S. IIARNAD,R. W. Don, L. GOLDSTEIN,J. JAYNESand G. KRAUTHAMER (Editors), pp. 339-384. Academic Press, New York, 1977. 10. DONCHIN, E., MCCART~FY,G. and KUTAS, M. Electroencephalographic

_

17. 18, 19. 20. 21. 22. 23. 24.

J.. Psychobiological bilateral .asymmetry. Hemispheric Function the Human S. J. and J. BEAUMONT (Editors). DD. 121-183. Paul Elek. London. 1974. OLD&LD, R. C. The assessment and analysis of ha;;dkhness: The Edinburgh’lnventory. Neuropsychologia, 9,97-113, 1971. DONCHIN, E. and HEFFLEY,E. Minicomputers in the signal-averaging laboratory. Am. Psychol. 30 299- 312,1975. GALIN,D. and Eurs, R. R. Asymmetry in evoked potentials as an index of lateralized cognitive processes: relation to EEG alpha asymmetry. Psychophysiology13,45-50,1975. JENKINS,G. M. and W~rrs, D. G. Spectral Analysis and its Applications.Holden-Day, San Francisco, 1968. DONCHIN.E. A multivariate approach to the analysis of average evoked potentials. IEEE Trans. Biomed. Engrq B-13,131-139, 1966. DONCH~N,E. Data analysis techniques in evoked potential research. In Average Evoked Potentials, Methods, Results andEvuluation, E. D~NCHIN and D. B. LINDSLEY(Editors), PP. __ 199-236. U.S. Govemment Printing Oflice, NASA-191, Washington, D.C., 1969. DONCHIN.E.. TOEING. P.. ROTTER.W.. KUTAS. M. and HE~FLEY.E. On the indenendence of the CNV and P3OO*components of the hu& averaged kvoked response. Mectroenceph. c/in. Neurophysiol. 38, 449-461,1975.

584

GREGORYMCCARTHYand EMANUELDONCHIN

25. SQUIRES,K. C., DONCHIN,E., HERNING,R. I. and MCCART~+Y, G. On the intluence of task relevance and stimulus probability on event-related-potential components. Electroenceph. din. Neurophysiol. 42, 1-14, 1977. 26. MCCARTHY, G. and DONCHIN, E. The effects of temporal and event uncertainty in determining the waveforms of the auditory event related potential (ERP). Psychophysiology 13,581-590,1976. 27. DONCHIN, E., KUTAS, M. and MCCARTZIY,G. Comparison of the hemispheric asymmetries of the readiness potential and CNV. Paper read before the Psychonomic Society 15th Annual Meeting,-. Boston. Mass., 1974. 28. KUTAS, M. and DONCHIN, E. Studies of squeezing: Handedness, responding hand, response force, and asymmetry of readiness potential. Science, N.Y. 186.545-548. 1974. 29. K&AS, M. and DONCHIN,E. The effect of handedness, of responding hand, and of response force on the contralateral dominance of the readiness potential. In Attention, Voluntary Contraction and EventRelated Cerebral Potentials. Progress in Clinical Neurophysiology, J. E. DESMEDT(Editor). Vol. 1, pp. 189-210. Sl Karger, Basel, 1977. 30. SQUIRES,N., SQUIRES,K. and HILLYARD,S. Two varieties of long-latency positive waves evoked by unpredictable auditory stimuli in man. Electroenceph. din. Neurophysiol. 38,387-401,1975. 31. SQUIRES, K. C., WICKENS,C., SQUIRES,N. C. and DONCHIN,E. The effect of stimulus sequence on the waveform of the cortical event-related potential. Science, N. Y. 193, 1142-l 146,1976.

On a prEsent6 L 10 sujets une serie de diapositives tant chacune 3 images. 2 des imacjes Btaient structurellement

comporappariees

(meme aspect) tandis que 2 Btaient appariees fonctionnellement ceptuellement).

un eclat sonore precedait chaque diapositive

(con-

de

mixte, la frequence du son 1000 msec. Dans les conditions de modalit indiquait quel type d'appariement devait ttre accompli SW la diapositive suivante. Dans les conditions de modalit pariement

etait fait pour chaque diapositive:

qu'une information

fixe, le miZme type d'aples sons n'apportant

temporelle.

Les donn6es de temps de reaction indiquaient que l'appariement fonctionnel L'analyse

Btait une tkhe

plus difficile que l'appariement

sance d'alpha pour l'appariement

fonctionnel que pour l'appariement

structure1.

Le mode d'appariement

performance

ni sur les donnees spectrales.

Deux composants de potentiels

cependant n'avait d'effet ni sur la relies au

ci6s dans l'intervalle preparatoire. sensible B la modalit

6venements

Btaient disso-

Le premier de ces composants Btait

d'appariement.

quand le son avertisseur t&he.

structurel.

spectrale de 1'E.E.G. indiquait qu'il y await mains de puis-

11 devenait plus grand en parietal

apportait une information pertinente pour la

Le second composant etait plus prononce au niveau central et

etait asymetrique

dans sa distribution

toutes les t&zhes experimentales.

SW

les 2 hemisphC?res pour

DeutschsprachigeZusammenfassungt 10 Probandenwurde eine Serie von Dias mit jeweils 3 Figuren angeboten.Zwei der Figuren'paNen strukturellzueinander (gleichartigaussehend),wshrend 2 funktionell(konzeptuell) Ubereinstimmten.Ein pl&tzlicherTonging jedem Dia um 1000 msec voraus. Bei gemisc,hten Daybietungsbedingungen stellte die Tonfrequenzden.Hinweisdar, welche Art von Entsprechung auf den ntichstfolgenden Dia zu finden war. Bei festen Darbietungsbedingungen war der gleiche Typus van ubereinstimmung fiirjedes einzelneDia zu finden,und die Tane besassen nur zeitlichenInformationswert. Die Reaktionszeitdatendeutetendarauf hin, daR.funktionelles Zuordnen eine schwierigereAufgabe als strukturellesZusammenfiigenwar. Aus einer Spektralanalysedes EEGs ergab sich eine geringereAlpha-Aktivitgtftirdas funktionellegegeniiber dem strukturellenEntsprechungen-Finden. Der Darbietungsmodus beim Zuordnenhatte hingegen,wedereine Auswirkungauf'die Leistung noch auf die KEG-Frequenz, Zwei Ereignis-bezogenePotentialkomponentenlieBen,sich innerhalb des 'Vo~rbereitungsintervalls voneinandertrennen.Die erste Komponente,empfindlichgegeniiber dem &uR'eren Modus des Gemeinsamkeitenfindens? war parietalausgeprzgter,wenn der Aufforderungstonaufgabenrelevante Informationenenthielt. Die zweite Komponentewar zentral ausgeprligter und asymmetrisch in ihrer Verteilungiiberdie beiden.HemisphWenbei allen experimentellenAufgaben.