Mental representations for visual sequences: Increased speed of central processing from 22 to 32 months

INTELLIGENCE 20, 41-63 (1995)

Mental Representations for Visual Sequences: Increased Speed of Central Processing From 22 to 32 Months PHILIP R.

ZELAZO

McGill University and Montreal Children’s Hospital RICHARD B.

KEARSLEY

Center for Behavioral Pediatrics and Infant Development and Tufts University School of Medicine DALE

M.

STACK

Concordia University and Montreal Children’s Hospital

Measuresof infantattention,particularly speed of processing, correlate with later intelligence, implying that they are tapping central processing ability. Yet, little is known about changes in speed of processing beyond the first year of life and before the child reaches school age. To assess changes in processing speed in the second to third year of life, two sequential visual events were shown to 22-, 27- and 32-month-old children. Twelve children were examined at each age using a Standard-Transformation-Return paradigm designed to address a number of limitations of attentional measures. Two coders scored attentional and affective behavioral responses while beat-by-beat heart rate was measured. Response clusters, rather than single responses, and first recognition reactions, rather than measures of habituation, were examined. Response clusters, implying mental representations (a central processing phenomenon), occurred following fewer trials of exposure for older children, indicating that speed of processing increases with age. Longer latencies to first clusters during the transformation relative to the standard phase imply proactive inhibition that also declines with age.

The authors thank Marguerite Randolph, Susan Coley, and Maryann Collins for their help with data collection and analyses; Mary Tsonis and Vicki Frank for their aid with data reduction, analysis, and graphics; and Yves Beaulieu and Cheryl-Lynn Rogers for editorial comments. The excellent reviews by Robert McCall and Lee Thompson sharpened our manuscript considerably and are gratefully acknowledged. This research was supported, in part, by grants from the Office of Special Education (No. G00760379) and the Tufts-New England Medical Center Hospital to P.R. Zelazo and R.B. Kearsley, the Carnegie Corporation of New York, the Lisa Hoffman-Alan Schouela Fund, and the Montreal Children’s Hospital-McGill University Research Institute to P.R. Zelazo. The authors acknowledge that the statements and views expressed are solely their responsibility. Correspondence and requests for reprints should be sent to Philip R. Zelazo, Psychology Department, The Montreal Children’s Hospital, 2300 Tupper Street, Montreal, Quebec, Canada H3H lP3, or Dale M. Stack, Department of Psychology, Concordia University, 7141 Sherbrooke Street West, (PY-170). Montreal, Quebec, Canada H4B 1R6. 41

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There is a long and extensive history of research on speed of information processing (Kail, 1991a, 1991 b) but it has not been established whether processing speed increases during the third year of life. If developmental changes in speed of processing are measurable during the first 3 years, an alternative psychometrically sound means to assess mental ability, independent of traditional motoric measures, is possible. The separation of information processing from the development of expressive capabilities (Zelazo, 1989) provides an opportunity to bypass a major confound characteristic of traditional tests-namely the use of motoric and expressive language measures to assess mental ability in children with motor and language delays. An information-processing measure combined with conventional tests would afford greater diagnostic precision allowing a differential diagnosis of central processing delays from delayed expressive development (Kearsley, 1981; Zelazo, 1988a, 1989; Zelazo & Kearsley, 1984). Attentional and information-processing approaches developed by a number of investigators have been used as measures of central processing ability (DiLalla et al., 1990; Fagan & Detterman, 1992; Fagan & McGrath, I98 I ; Lewis & BrooksGunn, 1981; McDonough & Cohen, 1980, 1982; Miranda & Fantz, 1974; Zelazo, 1979, 1988a, 1989; Zelazo & Kearsley. 1984). Using fixed trials and infant-control habituation procedures with both visual and auditory stimuli, these and other researchers showed that neonates through 36 months display patterns of habituation to repeated stimuli and recovery or preference to novel stimuli (e.g., Casey & Richards, 1988; Rose, Feldman, McCarton, & Wolfson, 1988; Rose, Feldman, Wallace, & McCarton, 1989; Weiss, Zelazo, & Swain, 1988; Zelazo, Weiss, & Tarquinio, 1991). The findings in each instance imply that mental representations were formed-a central processing phenomenon (Bornstein & Sigman, 1986; Zelazo, 1979, 1988a, 1988b). These two principal classes of information-processing measures, decrement of attention to redundant stimuli and recovery of attention to novelty (Bornstein & Sigman, 1986), show moderate but significant levels of continuity with measures of cognitive competence in childhood. Decrement and recovery involve encoding stimulus information and subsequent comparison of new information, and imply the creation of mental representations for events including detection, discrimination, identification, categorization, concept formation, problem solving, and retrieval (Bornstein & Sigman, 1986; McCall & Carriger, 1993; Zelazo, 1988a, 1989). Despite the potential of information-processing measures as indices of central processing ability, a number of serious questions have been raised that limit their validity and appropriateness presently (Bornstein & Sigman, 1986; Clifton & Nelson, 1976; McCall, 1982; Reznick & Kagan, 1982; Sophian, 1980). First, most studies have used only one response system, visual fixation, resulting in a confounding between the visual modality and a general central processing effect. One way to establish central processing is to demonstrate common responding across multiple response systems. However, those studies that have used multiple systems such as visual fixation and heart rate deceleration produced results that

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followed a different pattern for each response (e.g., Clifton & Nelson, 1976; McCall & Kagan, 1967). It is possible that visual fixation and heart rate follow different courses, that either one may correlate with one or more other measures such as vocalization and/or smiling but not with themselves. There is ample evidence that multiple responses occur to the processing of visual and auditory events (Hopkins, Zelazo, Jacobson, & Kagan, 1976; Reznick & Kagan, 1982; Zelazo, Hopkins, Jacobson, and Kagan, 1974); it is less clear that they co-occur in simultaneous clusters, a demanding test of correlation. A second limiting factor has been the use of two procedures-habituation and paired comparisons-to predict later intelligence. Bornstein and Sigman (1986) and McCall and Carriger ( 1993) performed syntheses and meta-analyses of infant studies predicting later intelligence and concluded that both the habituation and paired comparison paradigms are equally predictive. Moreover, response to novelty is highly predictive of later mild, moderate, and severe mental retardation (Fagan & Detterman, 1992). Zelazo, Weiss, Papageorgiou, and Laplante (1989) found that both habituation of a novel stimulus following recovery and recovery to a previously habituated stimulus following the introduction of a novel stimulus (classic dishabituation) discriminated normal from moderate and high-risk newborns. A further integration is implied. The Zelazo et al. (1989) results not only indicate that habituation may discriminate groups under similar circumstances as recovery, that is, the introduction of a novel stimulus, but, that simply a change in stimulation per se (as in dishabituation to a previously familiar stimulus), is the important determinant of recovery. These results imply that recovery to a novel stimulus and its subsequent habituation may measure different aspects of a common process, namely, the creation of mental representations. For example, it appears that smiling occurs when a match is made between an external stimulus and a mental representation of an event following some effort (Sroufe & Waters, 1976; Zelazo, 1972, 1976; Zelazo & Komer, 197 1), a view that is consistent with responsiveness to dishabituation. This matching process occurs during the active formation of a mental representation. Indeed, clusters of positive affective responses including cardiac deceleration, smiling, positive vocalization, and often motoric gestures such as clapping or pointing occur during the peak of visual attention (Reznick & Kagan, 1982; Zelazo, 1988b; Zelazo et al., 1974). The inference of a mental representation from expressive clusters is contiguous and direct. In contrast, mental representations inferred from habituation (marked decrement in responsiveness, and ultimately the absence of a response) and recovery (responsiveness to a different stimulus) procedures are removed in time and indirect logically. A third limiting factor is the heavy reliance on static visual stimuli in most prior research of infant Attention (e.g., Cohen, 1972; Fagan, 1984; Rose et al., 1988; Rose et al., 1989). Indeed, it is conventional laboratory wisdom among researchers of infant attention that children older than about 7 months do not comply as consistently as younger children to static visual stimuli, presumably

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because they get bored. Sequential stimuli have been shown to be more compelling than static stimuli, reducing the speed with which the information can be processed and permitting a gradual unfolding of the mental representation process (Zelazo, 1979, 1988a). Laplante, Zelazo, and Gauthier (1989) showed 4-month-old infants both static stimuli used by Lewis and Brooks-Gunn (1984) and sequential stimuli developed by Kagan, Kearsley, and Zelazo (1978). Infants looked proportionately longer, cried less, and were less likely to have the session terminated due to distress during the sequential events. These sequential stimuli remained interesting for children from about 3 through 36 months and produced greater affective responsiveness. Sequential stimuli have been used to evaluate rule transfer (McCarty & Haith, 1989), predictability, and its effects on infant visual expectations (Haith, Hazan, & Goodman, 1988; McCarty, 1989), and to determine whether event-related potentials can provide evidence for expectancy (Karrer et al., 1989). A fourth limiting factor concerns the restricted age range during which current information-processing procedures are applicable. Most of the basic and applied information-processing research was conducted on children between the ages of about 3 to 7 months; there is little published work using 2- to 3-year-olds (Kagan et al., 1978; Lewis & Brooks-Gunn, 1984; Lewis, Goldberg, & Campbell, 1969; McDonough & Cohen, 1982; Zelazo & Kearsley, 1984) and many questions remain. It has not been established that processing speed increases during this age, for example. From a pragmatic perspective, there is a need for valid assessment of central processing ability among 18- to 36-month-olds when delays with walking and talking and behavior problems prompt both parents and pediatricians to question mental development. The Standard-Transformation-Return (STR) paradigm was developed to elicit attention from children from the first through the third years of life (Kagan et al., 1978; Zelazo, 1979, 1988a). A standard stimulus is repeated so that a child can build an expectancy, assimilate changes to discrepant variations of the standard (transformation), and recognize the reappearance of the standard following the discrepancy (return). In order to both monitor this dynamic process and to strengthen the base on which inferences about central processing ability could be made, clusters of positive expressive behaviors rather than single response systems were developed. The STR procedure was used to test the integrity of the child’s capacity to create mental representations for events and measure the speed at which these representations were formed and announced. There is considerable evidence that children’s speed of information processing increases during the first year of life (Lasky & Spiro, 1980; Lewis, 1971; Lewis, Goldberg, & Campbell, 1969; Zelazo, 1979, 1988a). Similarly, there is a substantial history on “mental speed” among older samples that dates to the beginning of research on intelligence (Berger, 1982; Eysenck, 1987; Vernon, 1987). These studies reveal large and consistent age differences with speed of processing increasing from early childhood through adolescence reaching a peak in

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young adulthood, but virtually no research on changes in speed of processing during the second and third years (Kail, 199 la, 199lb). Our study addresses this omission using a procedure that corrects for limitations with prior research and that clearly implies central information processing. Children from three age groups during the second and third years were shown two sequential visual stimuli. It was hypothesized that first clusters of recognition responses would be displayed earlier for older children and that this faster speed of processing would be reflected in both numbers of trials and latencies to the first clusters of behavioral and heart rate reactions.

METHOD Subjects Data were collected with a cross-sectional sample of 36 normal children. There were 12 children in each of three age groups: 22 months (M = 21.6, SD = 1.2), 27 months (M = 27.4, SD = 0.7), and 32 months (M = 32.1, SD = 1.3). The sample consisted of mostly boys with four, two, and three girls at 22, 27, and 32 months, respectively. Four additional infants were excluded from the sample: one because of fretting, two because of mechanical failures producing a loss of heart rate data, and one because he did not have sufficient heart rate variability to qualify for recognition clusters, that is, he did not produce cardiac decelerations greater than six beats over three consecutive readings during the dynamic portions of the events. All children had normal intellectual development as assessed by the Bayley Scale of Mental Development (Bayley, 1969) with no mental ages less than 4 months below chronological age. All children were full-term, White, from middle-class families and experienced uncomplicated deliveries. Educational levels of the mothers ranged from the completion of high school to graduate school (M and M&I = 15 years). Children were recruited through newspaper advertisements, public service radio announcements, and professional referrals. Paradigm and Stimuli The paradigm and stimuli used to assess mental representations have been reported earlier (Kagan et al., 1978; Zelazo, 1972, 1979, 1988a, 1989). Each child was given an opportunity to develop a memory for an event, announce his or her recognition of that event following a transformed variation, and assimilate and announce the transformation itself. There were six standards during which the event was repeated, three transformation trials during which a variation of the standard was introduced and repeated, and three trials during which the standard reappeared. Two sequential visual stimuli (car-doll and light sequences) that were different in appearance but similar in dynamic properties were used. In designing the stimuli, the potential to form mental representations was considered more impor-

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tam than the control of all stimulus characteristics. In each instance, the entire sequence unfolds to produce an event constituting a trial. Moreover, an initial segment (a rolling car or a rod moving) causes a subsequent outcome (e.g., a doll falling or bulbs lighting). Both events have an expectancy period during which anticipation can occur but the event cannot be influenced. In the car-doll sequence, a toy car rolled down an incline and knocked over a Styrofoam object on impact (see Figure 1). This event was repeated for six continuous trials during the standard phase allowing the child to create an expectancy. On Trials 7 through 9 (transformation phase), the car made contact with the object but did not knock it over. During the return phase, Trials 10 through 12, the car knocked the object over on contact once again. Each standard and return trial lasted an average of 15.5 s with a mean intertrial interval of 4.0 s. Each transformation trial lasted 15.4 s. The second stimulus, a light sequence, is illustrated in Figure 2. During the standard phase, the presenter’s hand moved the rod through a 240” arc to make contact with and light three brightly colored bulbs. After the bulbs remained lit for 4 s, the presenter returned the rod to its original position. During the three transformation trials, all components were retained, but the order was scrambled. The rod moved through the arc, touched the bulbs, and immediately returned to its starting position, appearing to cause the bulbs to go on and remain lit for 4 s.

Figure 1. Sequential

car-doll

stimulus.

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light stimulus.

The presenter extended his hand over the bulbs seeming to cause them to go out. The three transformation trials were followed by three reappearances of the standard sequence. Each standard and return trial lasted an average of 1.5 s with a mean intertrial interval of 4.0 s. Each transformation trial lasted 14.5 s. Setting and Procedure Both stimuli were presented in a setting resembling a puppet theater with large black wings (cf. Zelazo, 1979). The most salient feature in the setting was a stage on which the visual events were presented. Four loo-watt bulbs illuminated the activity on the stage in the otherwise darkened room. The light sequence was always presented first followed by a lo-min break before presentation of the cardoll sequence. Each child was seated on his or her mother’s lap approximately 18 in. from the stimulus at eye level. Mothers were instructed to avoid interacting with or speaking to their child during the experiment and to maintain a stable position for the duration of the event so as not to influence their child’s responses directly or inadvertently. Coders, Measures, and Reliabilities Small Plexiglas windows embedded in the curtain permitted coders on either side of the stage to record on button boxes the occurrences and durations of specific

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targeted behaviors. One coder observed visual fixation to the stimuli, smiling, positive vocalizing or talking, and fretting. Visual fixation was coded when the child looked at any part of the stimuli. Smiling was defined as the occurrence of raised cheeks and upturned mouth. Vocalizing was defined as any positive sound, babble, or word, excluding physiologically induced sounds such as burps, grunts, and sneezes. Fretting was defined as a negative or distress sound including crying and protest sounds. A second coder recorded anticipatory looking, pointing toward the stimuli or clapping, turning to parent, waving, and twisting. Anticipatory looking was coded when the child’s eyes darted ahead of the action during the expectancy and dynamic periods of the sequences occurring primarily before the car was released (expectancy period) and while it rolled down the incline to knock the object over (dynamic period). Pointing was defined as a clear gesture toward the stimulus with a hand, often with index finger extended, clapping when two opened hands were brought together with modest force usually producing an audible sound, and waving when one or both arms were moved through an arc judged to be 60” or greater. Twisting was defined as arching the back, turning away from the stimulus, and/or bending 90” forward, and this usually preceded fretting. Turning to parent was defined as turning to make visual contact with the parent often with a positive affective quality lasting 4 s or less. Interobserver reliabilities were calculated for two pairs of independent judges who made live observations of 12 children using one set of measures (Button Box 1) and seven different children using a second set (Button Box 2). The correlation of duration of occurrence scores for each pair of independent coders for each variable were calculated for all 12 trials for all children. Seven trials were lost due to a mechanical failure resulting in 137 observations for Button Box 1. There were 84 observations for Button Box 2. Product-moment correlations yielded the following scores: fixation, Y = .95; smiling, r = .85; vocalizing/talking, r = .86; fretting, r = .90; anticipatory fixation, r = .79; pointing/clapping, r = .89; waving, r = .73; and twisting, r = .93. In addition to the behavioral measures, small electrodes were attached to each child’s sternum to permit recording of an electrocardiogram (EKG). This EKG signal was converted to a beat-by-beat recording of heart rate using a cardiotachometer. Analogs of the sequences were recorded throughout testing, either automatically or with some points indicated by a depression on a foot pedal. Each stimulus yielded a unique printed configuration that reflected precise points in the sequences throughout the 12 trials. For example, a magnet on the bottom of the toy car closed reed switches embedded at the top and bottom of the runway as the toy car passed over these locations. An additional switch was closed when the Styrofoam object was knocked over by the car. In the light sequence, automatic switches marked the rod’s contact with the lights and whether they were on or off. A foot pedal produced signal marked the beginning and end of the trial and the lifting of the rod. In both instances, a timed tape recording directed the

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actions of the presenter. The signals created a stimulus analog that was recorded on a polygraph tracing that was time-locked with the coded behavioral measures and automatic beat-by-beat recordings of heart rate. Clusters of Behaviors Each of the individual measures and beat-by-beat cardiac responses were scored to identify thefirst clear clusters of behaviors to occur to the stimulus sequences implying the formation of mental representations. The principal assumption underlying the scoring of clusters was that children form firmer mental representations for complex stimulus sequences with successive presentations (cf. Zelazo, I988a, 1988b; Zelazo & Komer, 197 1). The occurrence of a cluster of attentional and positive affective behaviors is believed to announce the excitement accompanying the formation of a mental representation well before habituation occurs. Clusters were derived from prior research with this procedure (Kagan et al., 1978) and related studies (Zelazo, 1972, 1979; Zelazo, Kagan, & Hartmann, 1975). These results indicate that smiling and vocalizing frequently accompany visual fixation and cardiac deceleration during the presentation of these sequential visual stimuli, particularly during the second and third years (Kagan et al., 1978; Zelazo et al., 1975). Moreover, smiling and vocalizing occur to moderately discrepant as opposed to novel or redundant information (McCall & McGhee, 1977; Zelazo, 1976; Zelazo et al., 1974). Independent research indicated that smiling occurs to the matching of an external event to a mental representation of that event following some effort or tension (Kagan, 1967; Sroufe & Waters, 1976; Zelazo, 1972; Zelazo & Komer, 1971), a process that is essentially identical to the formation of mental representations inferred from cognitive measures such as heart rate decelerations (McCall & Kagan, 1967). Given this research context and a large longitudinal data set (Kagan et al., 1978) collected with the STR paradigm and multiple attentional and affective responses, it was clear that clusters of behaviors occurred to the culmination of these sequences during some trials. Moreover, toddlers with expressive language not only gave similar response clusters as less verbal children, but often articulated their thoughts verbally, saying “doll fall” while smiling and pointing, for example. Using these data we required high levels of visual fixation to assure that heart rate deceleration-the principal cognitive measure-was stimulus related. Expressive and affective measures were used to confirm that cardiac decelerations were indications of mental resolution (as opposed to movement artifact, startle, or other factors). Moreover, these expressive measures including smiling, appropriate speech, vocalizing, clapping, pointing, and turning to parent to share, have independent empirical support and high face validity. One of the objectives of this study was to test prospectively the validity of these clusters for the discrimination of this relatively narrow age range during the second year of life. Identification of the first trial on which a firm mental representation was announced was the primary goal. A positive expressive cluster-a

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conservative index-was used to reflect visual attention and cardiac deceleration common to all children and one or more expressive behaviors to accommodate individual variations among children’s expressive patterns. Rules for scoring first clusters consisted of three essential criteria. First, expressive clusters required a minimum visual fixation of 80% of the trial to establish sufficient opportunity to encode and respond to the complex sequence and to assure stimulus relevance of the reactions. Second, a minimum cardiac deceleration of six or more beats during the dynamic phases of the visual events (“lights on” and “doll fall”) sustained over a minimum of three readings established the possibility of a mental resolution during attention, that is, a match between the external event and the mental representation (Kagan, 1971; Lacey, 1967). To increase the probability that the cardiac decelerations reflected mental resolution, one or more positive expressive behaviors during or immediately after the cardiac deceleration (within 3.0 s) served as the third criterion. The latter included smiling, vocalizing or appropriate speech, pointing or clapping, and turning to parent to share the event. Anticipatory fixation accompanied by a simultaneous cardiac acceleration equal to or greater than four beats during the expectancy period followed by a deceleration equal to or greater than eight beats during the dynamic period also served as an expressive cluster. An actual expressive cluster during the car-doll sequence is illustrated in Figure 3. In cases where attention and cardiac reactivity to the dynamic phase were high, but no appropriate expressive behaviors occurred, the first recognition cluster was identified using a similar approach but with less face validity. The first largest cardiac deceleration (equal to or greater than eight beats) to occur during the dynamic phase of the event that was followed by a 40% reduction in cardiac deceleration over the next two or more subsequent trials served as the point of first cluster. The trial in which the peak cardiac deceleration occurred was distinguished from orienting (Cohen, 1972; Hopkins et al., 1976) by eliminating the first presentation (Trial I) as a possible outcome. It must be emphasized that the reduction in cardiac deceleration was used to identify the peak point of responding in the formation of the mental representation and not the trial on which habituation occurred, and is clearly distinct from any traditional use of habituation. This procedure was used in only 35 of 216 (16.2%) possible instances. Detailed rules for the assignment of first response clusters were followed strictly and independently by two judges who later conferred to resolve differences to achieve consensus, an event that occurred rarely. A discussion was necessary to resolve ambiguities in only 10 of 864 trials (< I %) and less than 5% of the phases. Dependent Measures Three measures were analyzed statistically. The first trial during which a clear cluster of recognition behaviors occurred served as one dependent measure, chosen because of its similarity to measures of recovery to novelty, offering concep-

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Figure 3. Actual response cluster from polygraph car-doll sequence.

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tual consistency. A score of 1 through 6 indicated the trial during which a recognition cluster occurred for the standards with 7 reflecting no cluster; a score of 1 through 3 reflected the trial during which a cluster occurred for the transformation and return phases with a score of 4 indicating no cluster. The assignment of only one more trial for a score in cases where no clear cluster was achieved is

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conservative, as is the use of trial number; both factors constrict differences among groups. Thus, the mean number of trials to the occurrence of first clusters of responses to the light and car-doll sequences for each child served as the first unit for statistical analysis. Trial to first cluster was used as a primary dependent variable because it reflects actual numbers of exposures to these stimuli. However, this measure does have a limitation, as the standard phase is measured on a different scale from the transformation and return phases. To use two standard blocks of three trials each would not be appropriate because these six trials make up one phase; as such, once a cluster is produced, the score would have to be 4 for the next three trials even if the child clustered on the fifth or sixth trial. Because of this issue with the scale of measurement and its potential for restricting the range for data analyses, a second related dependent variable, percent latency to first cluster, was developed. Percent latency to first cluster was a measure of the length of time that each child took to announce recognition, from the initiation of the first trial to the point where the cluster began. The latency was derived from the polygraph record and subsequently divided by the total time in seconds available for the particular phase (standard, transformation, return) for each stimulus, and multiplied by 100 to produce a percent latency score. This measure offered more precision because it is a continuous rather than a discrete variable and more closely reflects actual time to elapse before achieving a first cluster. A third dependent measure, percentage of infant visual fixation, was analyzed to ensure that there were no differences in duration of attention among the three age groups. This analysis examined whether visual fixation was confounded with speed of processing.

RESULTS An Age X Event X Phase analysis of variance (ANOVA) was conducted to determine whether the two stimuli differed. There was no significant main effect or two-way interaction with event; thus, all subsequent analyses were collapsed across the car-doll and light events to produce a combined score. The standard statistical test conducted on each dependent variable was a two-way ANOVA with one between factor, age (22, 21, and 32 months), and one within factor, phase (standard, transformation, and return). The source of effects contributing to any significant interactions was isolated by simple effect analyses (Winer, 197 1). Following significant results from these analyses and any main effects that were not qualified by a significant interaction, Tukey’s tests were applied to the data to determine which of the three means were different from each other. Trial to First Cluster A main effect for age, F(2, 31) = 14.02, p -=c.OOOl, confirmed the primary prediction that first clusters would occur sooner in the sequences for older chil-

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dren. Thirty-two-month-old children produced first clusters earlier over all phases (M = 2.3 trials) than 27-month-olds (M = 2.8), who in turn responded faster than 22-month-olds (M = 3.5). Tukey’s tests showed that the 22-montholds differed from both the 27- and 32-month-olds (p < .Ol), although the 27and 32-month-olds did not differ from each other. A main effect for phase indicated that the number of trials to first recognition reaction differed among the standard, transformation, and return phases, F(2, 62) = 74.27, p < .OOOl. Recognition reactions occurred more rapidly to the reappearance of the standards (M = 1.77 trials) than to the standards themselves (M = 3.47) or to the transformations (M = 3.47). Tukey’s tests showed that the standard and return phases and the transformation and return phases differed from each other (p < .O 1), although the standard and transformation phases were similar. There was no Age X Phase interaction, F(4, 62) = 1.70, p = .16, although the means were in the hypothesized direction. The age and phase means are depicted in Figure 4 because they best illustrate the results for real number of presentations to first cluster and optimize comparison with the latency measure. The phase main effect was produced primarily by the markedly fewer trials to recognition during the return phase. Moreover, although the overall means are similar for the standard and transformation phases, the slopes are different across ages. During the standard and return phases, the number of trials to a first recognition cluster declined from 22 to 27 months (p < .Ol). However, during the transformation phase there was no corresponding decline in trials to first cluster until 32 months 0, < .Ol). There was no corresponding change for either the standard or return phases (ns) from 27 to 32 months. Thus, specific comparisons indicate a differential response to phases at different ages despite the absence of a Phase X Age interaction.

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Transformation

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PHASE Figure 4. Mean trials to first cluster for two sequential visual stimuli during three phases (STR) at 22, 27, and 32 months of age. Standard errors are represented by vertical bars.

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One reason for this result may be the restricted variance occurring with a small number of possible trials, especially during the transformation phase at 22 months; no children produced first clusters during the allowed three trials at the youngest age. Another reason could be that the scale of measurement differences between phases played a role. The latter possibility is argued against by the results from the percent latency measure; infants’ responses to the shorter transformation phase were longer than to the standard phase even at 22 months. Moreover, the measurement procedure is conservative and works against the observed finding. However, to address the first issue, at least partially, the data were reanalyzed using a nonparametric test of significance. There were no first recognition clusters to the transformation phase at 22 months, four at 27, and 10 at 32 months, x2(2, N = 36) = 17.78, p < .Ol. The results support the hypothesis that the number of children who displayed first clusters during the transformation phase increased with age. The results for the parametric and nonparametric analyses indicate that the standard and return phases distinguished 22- from 27month-old children most clearly, whereas the transformation phase distinguished 22- and 27- from 32-month-old children best. Latency to First Cluster A main effect for age, F(2, 31) = 16.68, p < .OOl, confirmed the original hypothesis that latency to first cluster would be shortest for the oldest group and confirm the findings from the trials to first cluster analysis. All age groups differed significantly from each other (all ps < .05); the 32-month-olds showed the fastest processing (M = 39.53), the 27-month-olds, an intermediate level (A4 = 52.45) and the 22-month-olds displayed the longest latencies to first cluster (M = 66.53). The standard errors for latency to first cluster by age were 4.44 at 22 months, 5.58 at 27 months, and 4. I1 at 32 months. Latencies to first cluster also differed across phases, F(2, 62) = 152.27, p < .OO1, with the longest latencies to the transformation (M = 84.15), intermediate latencies to the standard (M = 43.26) and shortest latencies to the return of the familiarized standard (M = 32.31). All phases were different from each other @ < .Ol). The finding of long latencies to first cluster during the transformation phase was examined further to determine whether the differences were real or artifactual. Because latency to the dynamic portion of the light stimulus was confounded with the changed order during the transformation phase relative to the standard phase, the dynamic portion of the car-doll stimulus alone was examined. Unlike the light stimulus where the latency to the dynamic portion (lights on) was increased by a constant of 2.5 s, the car-doll stimulus was exactly the same during the standard and the transformation trials, except that the doll fell during the standard and did not fall during the transformation phase. The results confirm that latencies to first cluster during the transformation trials were longer than during the standard trials, F(4, 60) = 2.90, p < .03, within each age. Percent latency to first cluster increased from standard to transformation phases

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at all ages: 62.1 to 100 at 22 months, 29.6 to 89.6 at 27 months, and 36.9 to 66.2 at 32 months (p < .05 for all comparisons). The data are similar for the trials to first cluster measure. These main effects of age and phase were qualified by a significant Age X Phase interaction, F(4, 62) = 4.21, p < .OOl, illustrated in Figure 5. Simple effect analyses indicated that at each level of phase there was a statistical difference for age, F(2, 31) = 5.35,~ < .Ol; F(2, 31) = 16.38,~ < .OOl; F(2, 31) = 8.34, p < .OOl at standard, transformation, and return phases, respectively. The standard errors for age at each level of phase were 3.32,0.00,4.59 at 22 months; 5.88, 4.07, 4.81 at 27 months; and 3.06, 8.18, 3.24 at 32 months for each of standard, transformation, and return phases, respectively. What is most evident from Figure 5 is that the standard and return phases differentiated the 22- from 27- and 32-month-olds (p < .05), whereas the transformation phase differentiated the 22- and 27- from 32-month-olds (p < .05). These findings are similar to those from trials to first cluster. Latency to first cluster was longer during the transformation phase than during the standard phase for 27-month-old children, indicating that they behaved much like the 32-month-olds during the standards, but more like the 22-month-olds during the transformations. Duration of Visual Fixation To ensure that differences in trial and latency to first cluster were due to differences in processing and not to differential attention to the events, the percentage duration of attention (visual fixation) to the stimuli was analyzed. No differences

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g

Figure 5. Mean percent latency to first cluster for two sequential visual stimuli during three phases (STR) at 22, 27, and 32 months of age. Standard errors are represented by vertical bars.

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in attention were found, F(2, 31) = 0.23, ns. Attention was high for all groups (M = 93.69, 93.95, 92.77 for 22-, 27-, and 32-month-olds, respectively), indicating that processing differences were not confounded with attention, and that these sequential visual events were compelling across the three age groups. Attention remained high for both standard and transformation phases, and decreased during the return phase for all groups F(2, 62) = 14.93, p < .OOl. This decrease from 95% to 89% was not surprising given that previously familiarized events were shown and infants processed this return phase fastest.

DISCUSSION Increased Speed of Processing The results of this study indicate that 32-month-old children produce clusters of behavioral and cardiac responses faster than 27-month-olds, who in turn produce clusters faster than 22-month-olds. Equal levels of visual attention at all three ages indicate that increases in speed of first clusters were independent of visual fixation, which averaged greater than 89% of the trials. To our knowledge, trial to first recognition cluster and latency to first cluster during the standard, transformation, and return phases provide the first empirical evidence that information-processing tasks can discriminate groups over this lo-month range. These results imply that speed of processing, that is, time to encode, store, retrieve, and announce the formation of a firm mental representation, decreases with age. Results from both trials and latencies to first clusters imply that 27- and 32-month-old children processed the standard and return phases at similar speeds and both were faster than 22-month-olds. Both measures revealed also that 32month-old children processed the transformation phase faster than 22- or 27month-old children, who were similar. This finding was revealed most clearly by the simple effect analyses for the latency measure. The nonparametric analysis also was consistent with the argument that processing speed increases with age; older children were more likely to produce first clusters during the three transformation trials than younger children. Thus, these data support and appear to extend downward to 22 months existing work showing increasing speed of processing from early childhood to young adulthood (Kail, 1991a, 1991b). We recognize that our paradigm is distinctly different from traditional reaction time measures of speed of processing, but we believe that these procedures, adapted for toddlers, are measuring the same process. These data are consistent with theoretical arguments by Berger (1982), Detterman (1987), Eysenck (1987), Kail(1991a, 1991b), Sternberg (1985), and Vernon (1987) that speed of processing is a fundamental and basic component of early intelligence. Toddlers presumably create mental representations for the sequential visual events used in this study, and this match is announced following fewer trials of exposure for older children. Research validating these apparent changes in processing speed using independent measures including more conventional

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reaction time measures is needed, although trials and latencies to first expressive clusters represent important findings in themselves. However, as Detterman (1987) cautioned, we do not assume that speed of processing is a simple phenomenon and other factors may account for the data in part or in total. The Role of Inhibition It is our view that speed of processing (encoding) alone cannot account for trial and latency to first cluster differences between the standard and transformation phases at 22 and 27 months. By 32 months, trials and latencies to first clusters during the standards and transformations are similar. The repeated measures comparisons show that the 22- and 27-month-old children take longer to encode and announce recognition to a moderately different transformation than to encode and announce the initially novel standard stimulus. These results imply that a longer exposure time is needed if a child has an expectation for a familiar stimulus than if he or she has no prior expectancy: Past information (standard) may slow down processing of new information (transformation). During the transformation phase the younger children behaved as though they experienced proactive inhibition-the interference with new learning by prior learning-pointing to a role for inhibition in the development of intelligence during this period as proposed by Dempster (1991) McCall (1994), and McCall and Carriger (1993). At the early ages, assimilation of the transformed stimulus may have been thwarted by perseveration of the mental representation for the familiar stimulus. It appears that as children get older, not only does speed of processing increase, but inhibition of prior information may improve, rendering a faster response cluster to the transformed version of the standard. This position is similar to McCall’s (McCall, 1994; McCall & Carriger, 1993) weaker version of the role of inhibition in which he stated that both factorsincreased speed of processing and an improved capacity to decrease the effects of proactive inhibition (perseveration)-may “compose a stable component of intelligence that mediates the observed prediction” (McCall, 1994, p. 119). However, in McCall’s (1994) stronger version of the role of inhibition, he argued that inhibition, not speed of processing or encoding, may be the important component for predicting later IQ. He stated that most of the predictability of habituation rate and total looking during familiarization using static stimuli are produced by behaviors prior to encoding. Our use of trial and latency to first cluster where we examine the build up of mental representations is consistent with this argument and, therefore, subject to McCall’s interpretation. In McCall’s (1994) stronger version of inhibition, he argued that, “a case can be made that the ability to inhibit responding to less salient stimuli, including ones made lower in salience by familiarization, is a single mechanism that can explain the prediction phenomenon from both paradigms” (pp. 113- 114), that is, habituation and recognition memory procedures. This stronger case has theoretical parsimony in its favor. At the very least, McCall’s arguments and our finding of slower responding to the

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transformed stimuli relative to the standard stimuli implicating inhibition, may focus attention on the evaluation of the mediating variables in infant information processing. We agree with McCall (1994) and Colombo & Mitchell (1990), who lamented that too little effort has been directed toward validating the key concepts operating in the formation of mental representations and their predictability to later IQ. Clusters of Behaviors The use of multiple rather than single response systems strengthens the base from which the inference of central processing ability can be drawn (Clifton & Nelson, 1976; Sophian, 1980). Multiple responses such as cardiac decelerations, smiles, vocalizations, and motoric gestures including points, claps, and momentary turns to parents following quiet vigilance, occurring in disparate systems strongly imply central coordination (Zelazo, 1988a, 1988b, 1989). Independent evidence for central processing is an essential but largely neglected condition for inferring mental representations from infant attentional behaviors (cf. Bomstein & Sigman, 1986; Zelazo, 1988b). Nonetheless, the validity of earlier research in which central processing was inferred principally from visual fixation (e.g., Bomstein & Sigman, 1986; Fagan & McGrath, 1981; Lewis & Brooks-Gunn, 1981; McCall & Carriger, 1993) is strengthened by these findings. The use of visual fixation as a criterion for stimulus-related attention rather than as the principal dependent variable led to important clarifications on two issues arising from previous work. First, this distinction yields an explanation for McCall and Kagan’s (1967) finding of asynchrony between visual fixation and heart rate deceleration. Clifton and Nelson (1976) used the McCall and Kagan finding as evidence against the inference of central processing. This study confirms that visual fixation and cardiac deceleration are asynchronous but that this disparity does not disconfirm central processing. Cardiac decelerations accompanied by expressive behaviors form first recognition clusters before visual fixation habituates below 80% of a given trial. Visual fixation assures stimulusrelated attention, whereas cardiac decelerations and positive expressive measures are used to infer the formation of mental representations. Second, sequential stimuli, unlike static stimuli, require visual tracking with the child’s eyes and head because the focus of the action moves from 18 to 3 1 in. across the stage and, thus, increase the likelihood that information processing occurs during visual fixation. These first two provisions render it virtually impossible that the main determinant of processing speed is the amount of time that the infant takes to attend to the stimulus as McCall (1976) found for “rate of habituation” using static stimuli. Similarly, it is unlikely that an infant will look at a stimulus without processing it for several trials. Data from encephalographic studies with infants show that event-related potential (ERP) waveforms change from trial to trial (Karrer & Ackles, 1987; Karrer et al., 1989). These data imply that processing of

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stimuli is synonymous with attention and extremely rapid as McCall (1994) acknowledged. Therefore, the requirement for stimulus-related attention to the sequential stimuli in this procedure lends strong face validity to the notion that differences in “speed of processing” over the three ages used in this study reflect differences in time to encode the stimuli and, as argued earlier, possibly to inhibit the familiar stimulus when a changed stimulus is introduced. There has been little hard evidence to support these notions previously. Nonetheless, a more precise measure of speed of processing and additional procedures such as simultaneous measurement of ERPs should be used in future research to validate this interpretation. Formation of Mental Representations First recognition clusters to sequential visual stimuli including positive affect imply a match between an external stimulus and a mental representation of that stimulus following some effort (Kagan, 1971; Zelazo, 1972, 1988b; Zelazo & Komer, 1971). This inference in itself has specific deductions that are testable. For example, Zelazo and Komer (1971) demonstrated that smiling, one of the expressive behaviors used in this study, followed an inverted-U function with repeated presentations of initially novel sequential auditory stimuli. The sequential visual stimuli in this study with older children showed an analogous pattern during the standard and transformation phases. Generally, first clusters occurred after three or four presentations during the standard phase, indicating a pattern of increasing responsiveness rather than immediate declines as typically reported to static visual stimuli. First clusters occurred more quickly during the return phase (with only one or two presentations) as expected for familiar stimuli following the introduction of the transformed stimuli. A second deduction from the notion of mental representations is that habituation and recovery are different manifestations of the same mental process (Bornstein & Sigman, 1986; Zelazo, 1988a, 1988b; Zelazo et al., 1974). Nonetheless, measurement of first expressive clusters is distinctly different from a traditional accounting of habituation. The equivalence of habituation and recovery indices for predicting later intelligence (Bomstein & Sigman, 1986; McCall & Carriger, 1993) implies a common determinant. The discriminative capacity of first clusters measuring the speed at which mental representations are encoded and announced and possibly the inhibition of familiar stimuli, implicate directly the initial stages of the mental representation process. Traditionally, response decrement over familiarization trials, measured by the slope of the habituated response, trials to habituation, and the magnitude of the decrement in responding served as the primary measures of analysis. In this study, habituation of cardiac deceleration was used to infer the point of maximal reactivity-the expressive cluster-rather than the decline or absence of responding. This initial phase of the mental representation process was generally ignored in earlier research.

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Mental Ability and Temperament The use of affective measures to infer central processing raises questions about the role of temperament in the measurement of mental ability. At the extreme ends-severe inhibition on the one hand and extreme noncompliance and resistance on the other-temperament could obscure true mental ability. We designed controls to reduce this possibility with the use of the STR procedure and sequential events. With the extremely inhibited child, there would be either no fixation (this did not occur in our study), too little heart rate variability (one occurrence), or more commonly, no expressive behavior associated with the heart rate and fixation criteria (occurred in 16% of first clusters). In the latter case, heart rate and fixation are sufficient, highlighting the importance of the psychophysiological and behavioral combination. With the extremely noncompliant or resistant child, there would be increased fretting and twisting, which virtually always reduces visual fixation below the 80% criterion, disrupts heart rate, and precludes the display of positive affective measures required for the scoring of recognition clusters. These children do not meet the criteria for fixation and heart rate and, thus, are not included. However, the STR procedure is less affected by noncompliance than standardized measures of intelligence and does not require compliance with the examiner and age-appropriate facility with expressive language and motor development. The results of this study indicate that the interaction of affect and temperament with mental ability deserves far more attention than accorded to date, at least for this age range. Implications of Processing Changes Over Age Changes in speed of processing over the 22- to 32-month age range are important in their own right because there is a paucity of research on processing capabilities during this period. However, establishing age-appropriate processing ability has clinical potential also, not so much in terms of predictability of later intelligence but as a contemporary measure of age-appropriate processing among children with delayed development. Processes involved in early recognition memory and attention are related to outcome even up to age 8 (Bornstein & Sigman, 1986; McCall & Carriger, 1993; Rose et al., 1989). Furthermore, Fagan (1984) suggested that infant novelty scores are related to later intelligence by means of an overall g factor that taps speed of processing. Attentional and informationprocessing approaches as potential complements to conventional tests are strengthened by the generality of the STR procedure and the multiple response systems used. Few nonverbal tests of intelligence are available during this age range. Conventional tests cannot distinguish central processing delays from delays with expressive development (Zelazo, 1988a, 1989; Zelazo & Kearsley, 1984). Evidence from this study that central processing can be assessed directly and nonverbally in young children carries potential as a contemporary measure to provide a differential diagnosis for developmentally delayed children, distin-

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guishing centrally mediated mental retardation ment.

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