Philip R. Zelazo

Stress During Pregnancy Affects General Intellectual and Language Functioning in Human Toddlers

Prenatal maternal stress has been shown to impair functioning in nonhuman primate offspring. Little is known about the effects of prenatal stress on intellectual and language development in humans because it is difficult to identify sufficiently large samples of pregnant women who have been exposed to an independent stressor. We took advantage of a natural disaster (January 1998 ice storm in Québec, Canada) to determine the effect of the objective severity of pregnant womens stress exposure on general intellectual and language development of their children. Bayley Mental Development Index (MDI) scores and parent-reported language abilities of 58 toddlers of mothers who were exposed to varying levels of prenatal stress were obtained at 2 y of age. The hierarchical multiple regression analyses indicated that the toddlers birth weight and age at testing accounted for 12.0% and 14.8% of the variance in the Bayley MDI scores and in productive language abilities, respectively. More importantly, the level of prenatal stress exposure accounted for an additional 11.4% and 12.1% of the variance in the toddlers Bayley MDI and productive language abilities and uniquely accounted for 17.3% of the variance of their receptive language abilities. The more severe the level of prenatal stress exposure, the poorer the toddlers abilities. The level of prenatal stress exposure accounted for a significant proportion of the variance in the three dependent variables above and beyond that already accounted for by non–ice storm–related factors. We suspect that high levels of prenatal stress exposure, particularly early in the pregnancy, may negatively affect the brain development of the fetus, reflected in the lower general intellectual and language abilities in the toddlers.

Adverse Effects of Fetal Cocaine Exposure on Neonatal Auditory Information Processing

Background. Studies with animals have shown that in utero exposure to cocaine interferes with fetal brain development by disrupting the processes of neuronal proliferation, differentiation, and migration, often leading to subsequent neurobehavioral deficits. However, studies with humans have produced inconsistent findings. Although neurobehavioral abnormalities have been observed among cocaine-exposed infants in several studies and in some cases dose-response effects have been found, the specific neurobehaviors affected vary from one study to the next. Researchers studying the effects of fetal cocaine-exposure are faced with many difficult challenges. For example, women who use cocaine typically use other substances in addition to cocaine, many of the methods available for identifying cocaine-exposed neonates are not reliable, and the available methods for assessing cocaine-exposed newborns may not be sufficiently sensitive to detect the subtle effects of cocaine on the developing central nervous system. Despite these difficulties, there is a growing body of research that suggests that fetal cocaine exposure is associated with subsequent language deficits among children exposed in utero. However, it is virtually impossible to disentangle the effects of the impoverished environments in which these children are often raised from the effect, if any, of fetal cocaine exposure. To determine the effects of fetal cocaine exposure independent of postnatal environmental effects, cocaine-exposed neonates would ideally be tested within the first few weeks of birth, and to identify early risks for subsequent language delay, well-researched auditory information processing measures could be used. Objective. The purpose of the present study was to assess the effects of fetal cocaine exposure on neonatal auditory information processing ability. To overcome limitations of some previous studies on the neuroteratogenic effects of cocaine, such as unreliable subject identification techniques, inadequate control over confounding variables, and questionable measures of central nervous system integrity, a valid measure of auditory information processing was used in a rigorous, case-control design. Method. Newborn information processing was assessed using habituation and recovery of head-turning toward an auditory stimulus across the 3 phases of the procedure: familiarization, novelty, and dishabituation. During the familiarization phase, the infant orients and habituates to a repeated word; during the novelty phase, the infant recovers head-turning to a novel word and subsequently habituates to this word; and during the dishabituation phase the infant displays renewed head-turning to the return of the original stimulus. Testing takes ∼20 minutes. This procedure has been shown previously to discriminate among infants at high-, moderate-, and low-risk for subsequent developmental delay. Twenty-five cocaine-exposed and 25 nonexposed control neonates, identified by meconium analysis, urine analysis, and/or maternal self-report, were tested on the auditory information processing procedure. The majority of infants were tested within the first few days of birth. Cocaine-exposed and control neonates were matched on birth weight, gestational age, Apgar scores, age at testing, and socioeconomic status as reflected by household income. Mothers were matched on age, weight gain, cigarette smoking, and alcohol consumption. Results. Fetal cocaine exposure was associated with impaired auditory information processing. Both cocaine-exposed and nonexposed control neonates oriented to the familiarization stimulus, but cocaine-exposed neonates displayed impaired habituation. Moreover, cocaine-exposed neonates did not recover or habituate to the novel stimulus or dishabituate to the return of the familiarization stimulus. Whereas nonexposed, control infants exhibited high levels of turning away from the familiarization stimulus during habituation (implying boredom), followed by high levels of turning toward the novel stimulus, indicating recovery of attention, the cocaine-exposed infants turned randomly. Clearly, auditory information processing of cocaine-exposed infants was impaired, despite the fact that they exhibited the same overall number of head-turns and the same high level of positive state as the nonexposed infants. Conclusions. The results imply that cocaine is a neuroteratogenic agent that impairs auditory information processing ability during the newborn period. Cocaine-exposed neonates exhibited a response pattern that is consistent with slower speed of auditory information processing. These deficits were observed within the first few days of birth, before adverse postnatal environmental influences could exert their effect. Moreover, the case-control design increased the probability that the observed information processing deficits were due primarily to the direct effects of fetal exposure to cocaine and not other prenatal factors. However, the long-term implications of these findings for the development of the infant/child are not known and must be addressed in follow-up studies.

Maternal cocaine use without evidence of fetal exposure

We report a case of lack of fetal exposure to cocaine and benzoylecgonine as evidenced by meconium and hair analysis, but exposure to nicotine and its metabolite cotinine, after extensive maternal use of cocaine and nicotine. These data suggest that the mode of maternal use of cocaine and individual differences in placental handling of the drug may protect some fetuses, and highlight the need to address interpatient variability.

Recovery of Neonatal Head Turning to Decreased Sound Pressure Level.

Human newbornss responses to decreased sound pressure level (SPL) were investigated with a localized head-turning habituation procedure. Recovery to a decrement in volume contradicts a selective receptor adaptation view because a lower intensity stimulus does not engage a separate set of receptor cells. Ninety-six infants (mean age = 41 hr) were presented with the same sound for 2 infant-controlled phases. After criterion orientation and criterion habituation to 1 of 2 stimulus intensities (72 or 78 dB), infants were randomly assigned to test trials at an intensity of 66, 72, or 78 dB; Planned comparisons of percentage of test trials with head turns toward the sound revealed that decreased SPL produced recovery of localized head turning.

The effects of delay on neonatal retention of habituated head-turning☆

Abstract Recovery of neonatal head orientation following auditory habituation was used to examine the effects of intertrial delay. One of two words served as stimuli with criterion orienting and habituation established before the delay. Fifty-six full-term neonates were assigned randomly to one of four delay groups: 10 (control), 55, 100, or 145 s. The percentage of head-turns toward the sound source (with one turn per trial) for blocks of three trials served as the principal dependent variable. Recovery of localized head-turning increased as length of delay increased, with the 100- and 145-s groups showing greater recovery than the 10- and 55-s groups. Infants presented with control trials systematically turned away from the sound source following habituation. Selective sensory adaptation, spontaneous recovery, and short-term memory interpretations of these data are considered.

Recovery and Dishabituation of Sound Localization Among Normal-, Moderate-, and High-Risk Newborns: Discriminant Validity

Abstract The discriminant validity of newborn localized head-turning to speech sounds was assessed using a familiarization-recovery-dishabituation paradigm. Neonates (72 hours corrected age) born at normal, moderate, or high risk for delayed mental development were identified using independent pre- and perinatal criteria. An infant-controlled procedure was used to assess head-turning across three phases of the experiment: (1) familiarization to a repeated stimulus; (2) presentation of a novel stimulus; (3) the return of the original familiarized stimulus. Percentage of trial blocks with head-turns toward the sound source and the difference in number of turns toward and away served as principal dependent measures. Comparable percentages of head-turning and rates of orientation and habituation occurred during initial familiarization trials. However, degree of recovery to the novel word and return of the original word and habituation to the novel word discriminated among groups, particularly between normal neonates and those at high risk for subsequent mental delays.

Generalization of neonatal habituation

Abstract The effect of delay on human newborns response to decreased sound pressure level (SPL) was investigated with a localized head-turning habituation procedure. The capacity for stimulus generalization was examined as one way to assess a memory interpretation of newborn habituation. During an initial familiarization phase, infants (M age = 40 hours) received repeated presentations of a sound until they demonstrated criterion orientation (three head-turns toward the sound in four consecutive trials) followed by criterion habituation (three consecutive trials with no head-turns or head-turns away from the sound). Then, infants received either a 10-s or a 55-s delay followed by postdelay and novelty phases of six trials each. During the postdelay phase, 12 female infants were randomly assigned to one of each of the following six conditions: presentations of the same sound at 0-, 6-, or 12-dB decrements in SPL or a novel sound following the 55-s delay; and presentations of the same sound at 0- or 6-dB decrements in SPL following the 10-s delay. The main dependent measure was percentage of trials with head-turns toward the sound during the postdelay phase. Planned comparisons revealed generalization of habituation to decreased SPL following 55 s, whereas recovery occurred following the 10-s delay. Implications for selective receptor adoptation and memory interpretations are considered. It is proposed that newborns retain the phonetic properties of the stimulus and categorize the lower volume sound as familiar.

An Information Processing Approach to Infant—Toddler Assessment and Intervention

It has been acknowledged widely that conventional tests of infant—toddler development have low predictive validity (e.g., Bayley, 1970; Brooks & Weinraub 1976; Lewis & Brooks-Gunn, 1972; McCall, 1976; Shehan, 1982; Stott & Ball, 1965; & Willerman & Fiedler, 1974). Indeed, if a child is under 18 months of age, predictive correlation to later development is virtually zero although predictability is generally better for seriously delayed children as Honzik (1976) points out. McCall (1979) summarized the stability—instability of individual differences in infant mental development and concluded that “in-fant tests do not typically reveal highly stable individual differences within the first 18 months or from infancy to later IQ” (p. 715). Predictive validity is so low for traditional tests prior to 18 months of life that considering the infant’s own performance on a mental test does not substantially increase the correlation to later childhood IQ achieved from parental education or Socioeconomic status (SES) alone (.40 to.60; McCall, 1979, p. 714).