Evidence for visual memory in the averaged and single evoked potentials of human infants

INFANT B1EHAVlORAND DEVELOPMENT ,4, 401-421 (1981)

This article was originally published in issue 2 of this volume. It is being reprinted here due to numerous typographical errors. The publisher regrets these errors.

Evidence for Visual Memory in the Averaged and Single Evoked Potentials of Human Infants* BY M A R T I N J. H O F M A N N P H I L I P SALAPATEK M I C H A E L KUSKOWSKI

University of Minnesota Institute of Child Development 51 East River Road Minneapolis, Minnesota 55455

When an infrequent or unexpected stimulus is presented to the adult, a characteristic enhancement of the late positive component (LPC) of the averaged evoked cortical potential is observed. To test whether this effect obtains near birth, we presented low and high probability visual stimuli to 29 3-month-old infants in two studies. In Study 1, electrical ~ t ~ . ' a l s were recorded from occipital and parietal scalp sites (Oz a.nd Opz), and in Study 2 also from a frontal lead (Fz). A clear LPC effect was observed over the posterior regions between 300-600 msec fallowing the onset of the infrequent stimulus. This is the first demonstration of an LPC effect in infants, reflecting cognitive processing involving memory. A linear discriminant analysis was used to analyze the nature of the LPC effect on single trials. A local probability index (LPI) was calculated to examine the effects of prior presentations of familiar events on the infants' responses to specific occurrences of unfamiliar events. The LPC occurred more frequently for Ihose unfamiliar trials preceded by. a sequence~ three or .mor.e familiar stimuli than by only one or two. Thus, single trial analysis of infant evoked potentials provides an opportunity to assesstrial-to-trial variation in the brain's responses to rapidly dlanging events. *This resesmn was s u p p o ~ by: HD-07317 to P. 5alapatek, HD-01136 to ~ Institute of Child Development and NS~P'2BI3S9 to the Center for Research in H,tmAn L e m ~ , University of Minnesota. Reprint mauesls should be directed to the g-cnnd author. W e thank Josnne. ~ a n for invaluable asshtance mall phases of the research, and Dan Ashmesd, Martin BankR, Leg Cohen and GIannnr for crilic~l comments on a late version of this manuscript.

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The averaged evoked potential (AEP) is an event-related brain response obtained by averaging the scalp electrical potentials elicited by repeated presentations of the same event. Complex mental processing involving ~nemory, attention and categorization can be efficiently indexed by changes in the evoked potential (Callaway, Tueting, & Koslow, 1978; Desmedt, 1979; John & Schwartz, 1978; Thatcher! & John, 1977).. An enhancement of the late positive components of the AEP with respect to a suitable baseline con~ol (LPC effect) has been shown to occu/in adults when they are engaged in tasks involving selective attention, stimulus novelty and uncertainty, and match-mismatch operations (Courchesne, Hillyard, & Galambos, 1975; Sutton, Braren, Zubin, & John, 1965; Thatcher, 1977). For visual tasks the LPC effect typically occurs betwee.n 300 and 600 msec following stimulus onset.~ However, LPC effects in infants have received little attention, although even young infants have provided ample evidence of visual discrimination in behavioral studies of memory and habituation (Cohen & Gelber, 1975; Fagan, 1970; Milewski & Siqueland, 1975). Courchesne, Ganz, Norcia, and Courchesne (1977) tested 4-, 5-, 6-, and 7-month-old infants using familiar and unfamiliar faces. They recorded AEPs from frontal regions in the infants and did not detect an LPC effect. However, they did report the appearance of a negative component, Nc, in the AEP occurring around 700 msec, which did reflect distinct processing of unfamiliar faces. Courchesne et al. (1977) suggest that this long latency negative component reflects the processing of discrepancy during infancy and is eventually superseded by shorter latency positive potentials, i.e., LPCs, during mature, adult processing. It must be noted that there are some features of the Courchesne et al. (1977) study that leave open the possibility of an LPC effect in young infants. First, they did not report data from posterior visual regions of the scalp. In young infants these regions are, in general, more mature than frontal regions (Conel, 1947; Huttenlocher, 1979; Yakovlev & Le Cours, 1967), and strong LPC effects have been obtained over posterior regions even in adults (Courcbesne et al., 1975; Picton & Hillyard, 1974, Squires, Squ~es, & Hillyard, 1975). Second, the Courchesne et al. (1977) sample was very small (two subjects in each of the four age groups). Third, the number of trials in their AEPs was small (5-20 responses to a discrepant event). Finally, no data regarding the presence of an LPC in the first 500 msec was reported, despite the fact that this is the interval during which the LPC occurs in adults. Taken together, these considerations preclude any strong conclusions concerning the absence of an LPC effect in young infants. The experiments we now report present the first clear evidence for an LPC effect in infants. We chose a paradigm which would maximize the infant's attention and would permit the infant to develop an expectancy for a stimulus event. This paradigm was a variant of the design originally employed by Sutton ~Distinct, peaked positive waves, sometimes termed P3 or P300 waves, are often used as an index of the LPC effect in adults. In this paper, we do not imply that LPC effects observed a r e identical in all respects to the P3 or P300 waveforms reported in adult studies.

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et al. (1965) to establish stimulus expectancies in adults. Sutton et al. verbally instructed subjects to notice infre.quently-oceurritag events in a stimulus sequence. An LPC effect occurred in posterior regions of the brain in response to these low probability events. The magnitude of the LPC effect in adults is inversely related to the probability of stimulus occurrence when the adult is instructed to detect low probability events (Duncan-Johnson & Donchin, 1977). Since we could not verbally instruct infants to notice the occurrence of a low probability stimulus, we first presented trials of one stimulus alone, hoping thereby to develop a strong stimulus expectancy. After this familiarization phase, we randomly interspersed presentations of a low probability stimulus with the familiarized one, expecting to see an LPC on the low probability trials. It should be noted that the paradigm we chose is a blend of traditional adult LPC and infant, habituation paradigms. STUDY 1

METHOD

Subjects Subjects were 13 full-term 3-month-old infants (mean age = 94.0 days; range = 87-103 days; 8 males, 5 females) who provided sufficient data in all phases of the experiment. An additional 14 infants failed to complete all conditions due to extreme fussiness or crying. Another 24 infants completed the session, but were dropped from the study due to excessive eye and/or head movement artifacts in the record or due to equipment difficulties. Infants for both Studies 1 and 2 were obtained on a volunteer basis through telephone solicitation based on newspaper birth announcements.

Stimuli The stimuli were two high-contrast vertical square wave gratings with frequencies of 0.4 cy/deg and 1.0 cy/deg, chosen because they were well within the 3-month-old infant's contrast sensitivity (Banks & Salapatek, 1978). The gratings were individually flashed onto a rear projection screen. Field size was 30 ° x 30 °, space-averaged luminance was 16.1 ftL and contrast was approximately 75%. Background luminance with no slide displayed was negligible, only 0.025 ftl_,. Stimulus duration was 500 msec with an interstimulus interval of 500 msec.

Design One of the gratings was presented at least 40 consecutive times during an initial familiarization phase (100% probability of occurrence). An attempt was made to

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counterbalance order of presentation of each grating across successive subjects tested. Five ofthe 13 infants in Study 1 viewed the 0.4 cy/deg grating first, and 8 of the 16 infants in Study 2 viewed the vertical grating.first. A test phase followed, to determine whether the introduction of an in~fr~iuent event Would yield an LPC effect. During this test phase, the familiarized grating was presented with a high probability of occurrence (80%). The second grating was randomly introduced into the stimulus sequence with a low probabili~ of occurrence (20%), with the restriction that this stimulus never occurred twiee in succeSsion. There were at least 120 trials in this test phase. Following completion of this test phase, we presented a second familiarization phase and a second test phase, identical to the f'n'st, but with the fLrst and second gratings exchanged. A typical experimental session lasted approximately 10 minutes.

Procedeure Grass silver-silver chloride electrode cups filled with non-adhesive conducting cream w.ere placed on the scalp With adhesive foam padding and held securely with a headband. The electroencephalogram (EBG) was recorded from midline visual and parietal locations (monopolar leads Oz and Opz--located half the distance between Oz and Pz--referenced to the left mastoid). Eye movements were recorded from two transverse eye leads to assist in detecting artifacts. The infant sat in the patent's lap $0 cm from the screen. An Observer sat with the infant and parent to monitor the infant's fixation. Stibjects received extra trials based on the observer's judgments about the infant's coOperative behavioral state. If the infant became fussy, an attempt was made to calrd him during a short break. If it seemed feasible to resume the experiment, a familiarization phase alWays preceded the test phase, Prolonged fussiness or crying resulted in termination of the experiment. The signals were passed through a Gilson five-channel polygraph with EEG amplifiers (upper half-amplitude = 90 cy/sec, time constant = 0.6 sec). Vetter FM recording adaptors multiplexed the signals onto stereo tape. A Tracor signal averager was used to monitor responses on-llne. Averaged evoked potentials were computed off-line on a PDP-12 computer. The sampling rate was one point per 3 msec with an epoch length of 600 msec. Individual trials were edited off-line and rejected if they contained eye and/or head movement artifacts. After editing~ those infants who gave at least 32 artifact-free trials for the low probability stimuli (20% from the combined test phases were includrd in the study. The fact that test phases had to be combined to provide a stable number of trials for statistical tests precluded examination of the effects of particular square wave gratings. For all comparisons, an equal number of artifact:free trials from each condition (100%, 80%, and 20%) were included in the averaged EP for each subject. The scorer was unaware of condition during test phase trials and used both the eye movement channel and EEG leads for rejecting artifacts,

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Figure 1. Data from Oz are plotted for the 13 infants in Study 1. The AEP from the famiJiarization phases (100%) and the AEP from the low probability trials (20%) are superimposed with respect to initial baseline. The darker trace is the AEp from the low probability trials (20%). Vertical shading indicates greater positive voltage in the 20% AEP relative to the 100% AEP. Horizontal shading indicates greater negative voltage. Area shading extends from 300 msec to 600 msec. NS refers to the number of s w ~ s (trials) used in the average. Each AEP plot consists of crt least 32 trials. Far each infant, an equal number of trials is included in both the 100% and the 20% traces. Posith,ity is up.

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RESULTS Figure 1 shows AEP responses for the Oz lead from each infant. The lighter curves were obtained by averaging trials from the two'familiarization phases combined (100% condition). The darker curves represent the AEPs obtained for the 20% condition, that is, for the low probability stimuli presented during the two test phases combined. To allow comparison of late potentials, the two curves were superimposed with respect to an initial baseline referent (IB referent). The IB referent was the mean voltage during the first 30 msec following stimulus onset. Comparisons D u r i n g the 300-600 msec Interval Inspection of the infant AEPs suggested a late positive enhancement in the low probability condition (20%) for most infants. Areas under the AEP curve from 300--600 msec were computed for Oz and for Opz with respect to the IB referent

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for each infant for the 100% condition (the two 100% phases combined), the 80% condition (the two 80% phases combined), and the 20% condition (the two 20% phases combined). These areas were subtracted from each other to provide area ~fference scores in integrated microvolts to assess late potential effects. The upper portion of Figure 2 provides the distribution of area difference scores for the 20% minus 100% comparison at both Oz and Opz. For both Oz and Opz, area difference scores were significantly greater than an area difference score of zero (Oz: Wilcoxon matched-pairs, signed-ranks test,2 T = 15, N = 13, p < .025; Opz: T = 16, N = 13, p < .025), indicating an LPC effect. To further ensure that the foregoing differences were not due to chance fluctuations in late potentials, we examined area difference scores for the 80% minus 100% comparison, where a strong LPC effect based on stimulus novelty was not expected. As can be seen in the middle portion of Figure 2, the distribution of area difference scores for the 80% minus 100% comparison did not differ significantly from zero (Oz: T = 5 0 . 5 , N = 13, n.s.; Opz: T = 4 0 , N = 13, n.s.), with no evidence of the positive skew seen in the 20% minus 100% comparison. A significant LPC effect was expected for the 20% minus 80% comparison of area difference scores, since there was a considerable difference in novelty between the two conditions. For the Oz lead it may be seen in the lower portion of Figure 2 that the distribution of area difference scores for the 20% minus 80% comparison did not differ significantly from zero (T=27, N = 13, n.s.). However, some evidence of a 20% minus 80% LPC effect is provided by the significant difference between difference scores and zero for the Opz lead ( T = 19, N = 13, p < .05). Interval Analysis Separate comparisons were made for the four successive 150 msec intervals between 0 and 600 msec, to establish more precisely the temporal location of the LPC. Table 1 provides the significance values for Wilcoxon Ts between the 20% minus 100%, 80% minus 100%, and 20% minus 80% comparisons in Study 1.3 2Between-su'bject variability in AEP amplitude and number of trials across subjects did not allow us to assume an absolute parametric scale for changes in electrical potentials. Therefore, we used a nonparametric statistic for the comparison of areas under the AEP curve. All T values reported in this paper refer to the Wilcoxon matched-pairs, signed-ranks test. 3In this study and in Study 2, a very large number of specific comparisons are made using the Wilcoxon T. As indicated in Footnote 3, we assumed only ordinal scaling for our difference scores. Thus, a comprehensive analysis of variance was not feasible. One would expect a number of significant comparisons to be spurious, given the number of comparisons made. We doubt that this is true in either Study 1 or Study 2, since only one significant difference score for 52 difference score comparisons emerged for comparisons involving the 80% minus 100% groups. These groups were not expected to differ. Secondly, all significant differences reported for the 20% minus 100% comparison and for the 20% minus 80% comparison were in favor of the 20% stimulus, as expected. These two results probably indicate that, if anything, our significance test, Wilcoxon T, was conservative.

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Three baseline referents, IB, N1, and the mean of N1 and P2 (N1 is here defined as the most negative deflection before 100 msec, and P2 is defined as the most positive deflection between 100 and 200 msec following stimulus onset), were used to ensure that the results obtained were not due to chance fluctuations in the period prior to the emergence of the LPC.

Late Intervals. In the adult, the LPC effect is often restricted to a narrow region between 300 and 600 msec following stimulus onset. To investigate the temporal extent of the LPC effect in infants, we examined the magnitude of the effect within two separate intervals beyond 300 msec: 300--450 msec and 450600 msec. We did not analyze beyond 600 msec, since stimulus duration was 500 msec and thus any effect beyond 600 msec would have been influenced by stimulus offset as well as by LPC responses. The LPC effect was significant for both intervals tested. Area difference scores with respect to the IB referent were significantly greater than zero for the 20% minus 100% comparison at Oz between 300-450 msec (T= 14, N = 13, p < .01) and between 450-600 msec (.T= 15, N = 13, p < .01). A similar trend was found for the Opz comoarisons (see-Table 1). No 80% minus 100% comparisons were significant. However, there was some evidence of an LPC effect for the 20% minus 80% comparison during both late intervals (see Table 1), supporting the results provided above for the 20% minus 100% comparisons. There were no significant differences in area difference scores between the two late intervals (Oz: T=29.5, N = 1 3 , n.s.: Opz: T = 3 1 , N = 1 3 , n.s.). This suggests that the LPC effect occurred throughout the 300-600 msec interval. Late interval analyses on the other two referents, N1 and mean N1-P2, produced similar results. Early Intervals. We did not expect early components of AEPs to vary strongly by condition in our task. Since we used these early components as referents for assessing the LPC effect, we examined whether chance factors may have produced an enhancement similar to an LPC effect, even before 300 msec. No significant differences were found for area difference scores during the 0-150 msec or the 150--300 msec intervals for the 20% minus 100% comparison (see Table 1). Thus, it appears that the early components of the AEP were not strongly implicated in the LPC effects observed. The early components were also not involved in the LPC effect for the 20% minus 80% comparison during the 0-150 msec interval (see Table 1). A1.~ough there was evidence for the emergence of positive area difference scores during the 150-300 msec interval at Oz, this was not true for a comparable comparison at Opz. Overall, the LPC effect was strongest in the interval beyond 300 msec for the 20% minus 100% comparison. The other baseline referents, NI and mean N1-P2, provided compelling support for the interval effects described using the IB referent for the 20% minus 100% comparison and for the 80% minus 100% comparison.

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STUDY 2 Study I provided the first evidence for an infant LPC effect. Study 2 'was designed to confirm these results and to increase the lilCelihood that some involvement of the cortex contributed to the effect. The stimuli used in Study 1 varied only in spatial frequency. Therefore, local retinal or subcortical adaptation effects alone might have produced sensory adaptation, rather than an LPC involving cortical habituation. In Study 2, spatial frequency was held constant and orientation was varied. Visual cells specific for orientation have been routinely found in the cortex, but not as yet at subcortical levels (Coltheart, 1971). In addition, an electrode placement was added at Fz to test the possibility that this area, although relatively immature, might exhibit an LPC effect.

METHOD Subj~ts Subjects were 16 full-term 3-month-old infants (mean age = 93.6 days; range = 86-104 days; 11 males, 5 females) who provided sufficient data in all phases of the experiment. An additional 10 infants failed to complete all conditions due to extreme fussiness or crying. Another 14.infanis completed the session but were dropped from the study due to excessive eye and/or head movement artifacts in the record or due to equipment difficulties.

Procedure The stimuli were a 0.4 cy/deg vertical square wave grating (used in Study 1),.and a 0.4 cy/deg horizontal square wave grating. The design and procedure were the same as in Study 1.

RESULTS Figure 3 shows the AEPs at Oz for the 20% minus 100% comparison. A majority of subjects appeared to show an LPC effect. Figure 4 provides the distribution of area difference scores at Oz, Opz and Fz for all three comparisons during the 300-600 msec interval. The pa~ern of results at Oz and Opz closely paralleled that obtained in Study 1. Area difference scores for the 20% minus 100% comparison were signifi-

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cantly greater than zero at Oz, indicating a strong LPC effect ( T = 7 , N = 14, p < .005) 4 . No significant difference was found for the 80% minus 100% comparison ( T = 2 8 , N = 14, n.s.), nor for the 20% minus 80% comparison ( T = 4 1 , N = 14, n.s.). Similar results, indicating an LPC effect, were obtained at Opz for the 20% minus 100% comparison ( T = 2 0 , N = 16, p < .01L Again, no significant dif4Degrees of freedom in Study 2 were normally 16. AEPs from Oz for two of the 16 infants were lost due to equipment failure. One subject's AEP at Fz was also lost.

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ference was found for the 80% minus 100% comparison (T=54, N = 16, n.s.), nor for the 20% minus 80% comparison (T=37, N = 16, n.s.). When the AEPs were computed at Fz, a clear N1-P2 component was not generally present. Therefore, in the analysis of the Fz data only the IB referent was used. For all comparisons in the interval 300-600 msec for Fz there were no significant area differences (20% minus 100% comparison: T=34, N=15, n.s.; 80% minus 100% comparison: T=50, N=15, n.s.; 20% minus 80% compa0"son: T=49, N = 15, n.s.). Table 2 is a summary of the three area difference score comparisons obtained for the three electrode locations and t h r ~ baseline referents. Significance values are provided for each comparison in 150 msec intervals from 0 to 600 msec. The basic pattern of results is the same as that found in Study 1. In Study 2, as in Study 1, interval analyses using NI and mean N1-P2 referents were in general agreement with the analyses using the IB referent.

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SINGLE TRIAL ANALYSIS In the foregoing analysis we provided evidence for an LPC effect in the AEP of 3-month-old infants. In most adult LPC or P3 studies, the characteristic increase in positivity is determined on the basis of averaging responses across a large number of trials, with a few notable exceptions (e.g., Cooper, McCaUum, Newton, Papakostopoulos, Pocock, & Warren, 1977; Squires & Donchin, 1976). Averaging is typically employed with the adult because the signal-to-noise ratio of single evoked potentials (EPs) to background EEG is low. However, we noted that the corresponding signal-to-noise ratio for the infant appeared very large. A clear P2 component was often present in the raw EEG record from visual scalp regions for our visual EP tasks. This fact offered the possibility of examining the LPC effect trial by trial within subjects in the data already described. Each infant in Study 1 provided data for the single trial analysis whether or not he showed a likely LPC effect (a positive area difference score in the 20% minus 100% comparison in the previous analyses). A linear discriminant function analysis was used to maximize and to test the distinctiveness of the EP waveforms from the 20% and 100% conditions. The raw single-tfial EP waveform served as the basic datum. Trials were selected from those artifact-free trials used in the previously described AEP analysis. Criterion for selection was a strong P2 from the Oz location in the 20% and 100% conditions, suggesting a higher signal-to-noise ratio on those trials. P2 was defined as a distinctive positive deflection occurring in the interval 100 to 200 msec post-stimulus with an amplitude of at least 10 ~V. Examples of such waveforms can be seen in Figure 5. Within each subject, we obtained an equal number of trials from the 20% condition and the 100% condition. In the discfiminant analysis for all the subjects in Study 1 there was a total of 534 trials, composed of 267 trials from the 20% condition and 267 trials from the 100% condition. Individual subjects provided a minimum of 11 and a maximum of 34 trials (median= 16) from each condition. Since the purpose of the discriminant analysis was to maximize the distinc-' tiveness of the 20% and 100% conditions, we chose a set of five predictor variables from the interval where the LPC effect based on averaged evoked responses was strongest. This interval, from 300-to 600 msec, was divided into five equal intervals (300-360, 360--420, 420-480, 480-540, 540-600), and the mean EP voltages in each interval served as a predictor variable in the discriminant analysis. Five intervals were used to capture any systematic variations in the nature o f the LPC response within smaller temporal intervals on single trials. In the left portion of Figure 5 are plotted examples of HITS and MISSES from the 100% condition for an infant in Study 1. In this case, HITS were 100% trials correctly identified as 100% trials, and MISSES were 100% trials incorrectly identified as 20% trials. Note that among the HITS in the 100% condition there is not one with an extended late positive elevation. In the fight portion of

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Figure 5 are plotted examples of HITS and MISSES from the 20% condition for that same infant. Note that among the HITS in the 20% condition there is an enhancement of the late positive component as compared to misclassified trials. In the preceding analysis, where averaging of evoked potentials was used, a number of infants did not show a strong LPC effect. It is possible that some of these infants did not process the discrepancy between the stimuli. However, it is also possible that some infants did process the discrepancy on a limited number of trials but not on other trials. We tested this hypothesis using the single trial method of analysis. In Figure 6 are plotted the predictions obtained from the discriminant for single trials from the 20% condition for a subject not showing a strong LPC effect when the AEP analysis was used. After editing for a strong P2, 15 trials were available from the f'ursttest phase and 12 trials from the second test phase. During the first test phase, the discriminant could not distinguish between low probability trials (20%) and trials from the familiarization phase (100%). However, during the second test phase, the discriminant distinguished very well between the low probability trials and trials from the familiarization phase, indicating the

HOFMANN, 5ALAPATEK AND KUSKOWSKI

416

Discriminant Prediction of Single Trials in the 2 0 % Condition for One Infant 2 0 % Trials from First Test Phase in Order of Presentation

HIT MISS

X X

X X

X X

X

X X

X

X

20% Trials from Second Test Phase in Order of Presentation HIT X X X X X X X X MISS X.

X

X

X

X X

X

X

Figure 6. Artifoct-free single trials from the 20% condifion._.areprovided for a subject who did not show a strong LPC effect in the averaged evoked potential analysis. Trials are ordered in the sequence in which they occurred. Note the pattern of consecutive HITS in the second test phase as compared to the first.

appearance of an LPC effect in this second test phase. The appearance of an LPC effect did not occur until exposure to the secOnd-familiarized stimulus. Thus, single trial analysis provides access to certain aspects of the LPC effect which ndght not be uncovered by averaging procedures. Local Probability Analysis An analysis of the single trial EPs was designed to test whether low probability El~s were distinguishable from each other based on the number of familiar or high probability stimufi that immediately preceded them. Since the 20% trials occurred in the test phases where the sequencing was randomly ordered, we arranged the 20% EPs in groups according to the number of 80% ~ a l s which immediately preceded each occurrence of the 20% trials. The number of such trials, which we called the Local Probability Index (LPI), ranged from a minimum of one to a maximum of 15. ~ypotheses were tested using the LPI. For example, it seemed reasonable that, if the LPI was small (LPI ~ 2), then the 20% trial should have been perceived by the infant as occurring with a high probability. Thus, the infant should not have shown a strong LPC on such trials. On the other hand, if the LPI was high (LPI ~ 3), indicating many presentations of 80% tri.als directly preceding a 20% trial, the 20% trial should have been perceived as occurring, in the local sense, with a low probability. Therefore, high LPI trials should have e xhi'_bited a stronger LPC than low LPI trials. These hypotheses were tested using the results of the single trial" group discriminant analysis from the 50% condition. Two groups of EPs were formed: LOW INDEX (LPI ~ 2) and HIGH INDEX (LPI ~ 3). This particular division of EPs into a LOW INDEX and a HIGH INDEX group roughly equalized the number of EPs in each group.

417

INFANT MEMORY IN EVOKED POTENTIALS

Each trial was classified as either a HIT or a MISS by the group discriminant. Table 3 shows the results of the discriminant afialysis based on the LPI. The discriminant's prediction accuracy was considerably better for the HIGH INDEX group for Study 1 (~=5.57, d . f . = l , p < .025). This analysis suggests that whether an LPC. was obtained for a given 20% trial depends on the number of preceding 80% trials, as referenced in the LPI. A second group of trials was analyzed from three infants in Study 2 to cross-validate the findings from Study I. In the discriminant analysis for Study 2, the three subjects respectively provided 20, 24, and 27 trials from each condition. Once again, the discriminant's accuracy was higher for the HIGH INDEX group ( ~ = 6 . 9 6 , d.f. = 1, p < .01). TABLE 3 Results from the Discriminant Analysis for Single Trials from the 20% Condition as Determined by LPI

STUDY I

STUDY 2

HIT

MISS

HIT

MISS

LPI ~< 2

78 (51%)

74 (49%)

21 (58%)

15 (42%)

LPI ~> 3

75 (65%)

40 (35%)

30 (86%)

5 (14%)

Analysis of Single Trials by Temporal Location in 20% and 1 ~

Conditions

Results of the group discriminant of single trials were also analyzed according to when they occurred during a particular phase. The HITS and MISSES for the 20% trial EPs are shown in Table 4. For Study 1, temporal sequence had no effect on the discriminant's power of prediction (X2=0.17, d.f. =2, n.s.). HowTABLE 4 Results from the Discriminant Analysis for Single Trials from the 20% Condition According to Trial Position in the Test Sequence

1-40 41-80 >

80

Total

HIT

MISS

TOTAL

57 40 55 152

41 33 41 115

98 73 96 267

18 17 16 51

13 7

31 24 16 71

Study 2 1-40 41-80 ~, 80 Total

0

20

"

418

HOFMANN, SALAPATEK AND KUSKOWSKI

ever, in Study 2, the power of prediction increased as a function of the temporal location of the 20% trials in the test phase (X2=7.83, d.f.=2, p <.025). Trials occurring late in the phase were more likely to be classified correctly as 20% trials than those appearing early. In other words, the LPC effect in Study 2 was stronger for presentations later in the temporal sequencing. In Table 5, the number of HITS and MISSES are shown for the 100% condition. There was no consistent change in the discriminant's power of prediction for Study 1 (X2=3.97, d.f.=3, n.s.), nor for Study 2 (X2=0.52, d.f.=3, n.s.), as a function of temporal location. TABLE 5 Results from the Discriminant Analysis for Single Trials from the 1 0 0 0 Condition According to Trial Position in the Familiarization Sequence

Study 1 1-10 11-20 21-30 > 30 Total Study2 1-10 11-20 21-30 > 30 Total

HIT

MISS

TOTAL

51 41 22 34 148

29 36 25 29 119

80 77 47 63 267

12 11 12 11 46

7 6 8 4 25

19 17 20 15 71

DISCUSSION The foregoing studies provide clear evidence of an LPC effect in 3-month-old infants, obtained using each of three baseline referents. It should be noted that the LPC effect was obtained under conditions designed to maximize the chance of observing this effect in infants. First, visual stimuli were presented a large number of times in all conditions to enhance the opportunity for memory on the part of the infants. Second, recording was from the posterior pole as well as the frontal region, since the posterior pole is probably more mature at this age. Third, AEPs were calculated from a large number of trials to improve the signal-to-noise ratio. Finally, the data from only those infants who were alert throughout the experiment were included in the analysis. The paradigm we developed lies somewhere between a typical infant behavioral habituation task and a typical adult LPC task. Infants in a habituation task have not been instructed to watch for a particular event, but they do dishabituate to a novel stimulus. Adults in an LPC task usually receive verbal

INFANT MEMORY IN EVOKEDPOTENTIALS

419

instructions to search for an infrequent stimulus.. When this stimulus is perceived, an enhancement of the LPC is observed. Thus, in many adult studies of the LPC effect, both search and novelty components are present (Donchin, Ritter, & McCallum, 1978). If adults are instructed to search for a familiar event, a somewhat diminished LPC effect occurs (Duncan-Johnson & Donchin, 1977). Adults also show an exaggerated LPC effect to very novel stimuli introduced unexpectedly into the regular sequence of a search task (Courchesne, 1977). In our studies, infants probably demonstrated an LPC effect, not because they were looking for a particular event, but only because they perceived that a novel event had occurred. The current studies indicated that the strongest LPC effect was obtained from posterior pole leads rather than from a frontal lead. This may have been because our task was visual, or because the posterior pole is relatively more mature than the frontal cortex at this age (Conel, 1947; Huttenlocher, 1979; Yakovlev & Le Cours, 1967). It may also have been because we did not analyze beyond 600 msec. Courchesne et al. (1977) have reported a negative component in the infants' AEP from the frontal lead beyond 600 msec. The relationship between this late negative component and the LPC effect is not clear. We were a little surprised to see the LPC emerge with approximately the same latency as it does in adult paradigms. Since landmarks such as N1 and P2 appeared with typical infant latencies in our studies and these latencies were later than in the adult, one might have expected the LPC to emerge, later in infants. Our task may have allowed unusually rapid detection of the novel stimulus on at least some trials. Studies using more complicated stimuli should reveal the extent to which the latency of the infant LPC is task-depe.ndent. The stimuli we used in our studies were repeatedly presented, but each presentation was only 500 msec in duration. In most behavioral habituation studies, the stimuli are presented for several seconds or longer, often under the control of the infant himself. Although no masking stimuli were employed in the present studies, 3-month-olds clearly demonstrated encoding and recognition of stimuli presented much m o ~ briefly than in typical habituation studies. This rapid analysis of stimuli was also found in the only other infant report using brief stimulus durations (Harris & Bassett, 1977). Applying a discrirmnant function based on a single trial analysis, we were able to predict with reasonable accuracy whether or not a given EP was associated with the occurrence of a particular stimulus event (lilT). In generat, the accuracy of prediction, i.e., HIT and MISS rate of the discriminant function, was fairly constant across all phases of the experiment. The presence of LPCs early as well as late in the test phases suggests that the preceding 100% condition resulted in some familiarization to the fn'st stimulus. The local probability index (LP1) provided a means for examining the temporal course of cognitive processing. Analysis based on the LPI indicated that the amplitude of the LPC on any given 20% trial was a function of the number of

420

HOFMANN, SALAPATEKAND KUSKOWSKI

80% trials directly prec .ed_ling that trial. There was very compelling evidence for an LPC effect on low probability trials preceded by three or more 80% trials. More specifically, the discriminant could correctly identify HIT and MISS trials with an accuracy o f approximately 75% for LPI > 3 across Study 1 and Study 2. To our knowledge, there have been no infant habituation studies that have mixed a high probability and low probability stimulus. Our results suggest that the LPC effect to the low probability stimulus is not one that will occur as an average over many trials, but rather only in those sequences where the actual local probability matches the average probability. Young infants appear to keep track of reasonably long strings of similar events, but they probably have difficulty in judging the relative frequencies o f more equally probable events. We now' have strong electrophysiological as weli as behavioral evidence that infants remember past events and react to novel ones. The LPI data in this study suggest that the LPC effect in the infant can even be observed on a single trial when a novel stimulus is presented. Since the LPC behaves in this fashion, and since EPs may be collected rapidly without any behavioral responses, the nature of infant memory may be very efficiently examined using the electrophysiologieal techniques described.

REFERENCES Banks, M. S., & Salapatek, P. Acuity and contrast sensitivity in 1-, 2-, and 3-month-old human infants. Investigative Ophthalmology & Visual Science, 1978, 17,361-365. Callaway, E., Tueting, P., & Koslow, S. H. (Eds.), Event-related brain potentials in man. New York: Academic Press, 1978. Cohen, L. B., & Gelber, E. R. Infant visual memory. In L. B. Cohen and P. Salapatek (Eds.), Infant perception:from sensation to cognition. Vol. I: Basic visual processes. New York: Academic Press, 1975. Coltheart, M. Vistha.I feature analyzers and aftereffects of tilt and curvature. Psychological Review, 1971, 18(2), 114--121. Conel, J. L, The postnatal development of the human cerebral cortex, Vol. 3: The cortex of the three-month infant. Cambridge, Massachusetts: Harvard University Press, 1947. Cooper, R., McCallum, W. C., Newton, R., Papakostopoulos,D., Poeock, P. V., & Warren, W. J. Cortical potentials associated with the detection of visual events. Science, 1977,/96, 74--77. Courchasne, E. Event-related brain potentials: a comparison between children and adults. Science, 1977, 197, 589-592. Courchesne, E., Ganz, L., Norcia, A, M., & C0urchesne, R. Y. Long-latencyevent-related potentials in infants: probability dependent waves. Society for Neuroscience: Abstracts, 1977, 3, 103. Courchasne, E., Hillyard, S. A., & Galambos,R. Stimulus novelty, task relevanceand the visual evoked potential in man. Electroencephalography and Clinical Neurophysiology, 1975, 39,

131-143. Desmedt, J, E. Progress in clinical neurophysiology. I/ol. 6: Cognitive components in event-related cerebral potentials. New York: Karger, 1979. Donchin, E., Ritter, W., & McCailum, W. C. Cognitivepsychophysiology:the endogenous components of the ERP. In E. Callaway, P. "Fueling& S. H. Koslow (Eds.), Event-related brain potentials in man. New York: Academic Press, 1978.

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Duncan-Johnson, C. C., & Donchin, E. On quantifying surprise: the variation of event-related potentials with subjective probability. Psychophysiology, 1977, 14,456--467. Fagan, J. F. Memory in the infant. Journal of Experimental Child Psychology, 1970, 9,217-226. Harris, P. L., & Bassett, E. Discrimination by young infants of stimuli presented discontinuously. Perception, 1977, 6, 685-690. Huttenlocher, P. R. Synaptic density in human frontal cortex----developmental changes and effects of aging. Brain Research, 1979, 163, 195-205. John, E. R., & Schwartz, E. L. The neurophysiology of information processing and cognition. Annual Review of Psychology, 1978, 29, 1-29. Milewski, A., & Siqualand, E. Discrimination of color and pattern novelty in one-month human infants. Journal of Experimental Child Psychology, 1975, 19, 122-136. Picton, T. W., & Hiilyard, S. A. Human auditory evoked potentials. II. Effects of attention. Electroencephalography and Clinical Neurophysiology , 1974, 36, 191-199. Squires, K., & Donchin, E. Beyond averaging: the use of discriminant functions to recognize event related potentials elicited by single auditory stimuli. Electroencephalography and Clinical Neurophysiology , 1976, 41,449-459. Squires, N. K., Squires, K. C., & Hillyard, S. A. Two varieties of long-latency positive waves evoked by unpredictable auditory stimuli in man. Electroencephalography and Clinical Neurophysiology , 1975, 38,387-401. Sutton, S., Braren, M., Zubin, J., & John. E. R. Evoked potential correlates of stimulus uncertainty. Science, 1965, 150, 1187-I 188. Thatcher, R. W. Evoked potential correlates of delayed letter-matching. Behavioral Biology, 1977, 19, 1-23. Thatcher, R. W., & John, E. R. Functional neuroscience. Vol. 1: Foundations of cognitive processes. New Jersey: Eflbaum Associates, 1977. Yakovlev, P. E., & Le Cours, A. The myelogenedc cycles of regional maturation of the brain. In A. Minkowski (Ed.), Regional development of the brain in early life. Philadelphia: F. A. Davis Co., 1967.