Age-related and individual differences in the performance of a delayed response task (the A-not-B task) in infant twins aged 7-12 months.

The aim of the present work was to analyze the age-related dynamics and nature of interindividual differences in performing the A-not-B task, which addresses working memory in children aged 7-12 months. The cohort consisted of 150 children aged 7-12 months from mono- and dizygotic twin pairs. Working memory was assessed in terms of the maximum delay which the children could tolerate in a delayed response test (the A-not-B task). Performance of the task improved with age, such that a sharp change in the ability to tolerate the delay occurred between nine and ten months of age. Mental development as assessed using the Bayley scale showed a significant correlation with the duration of the delay in the task only after a threshold period (9-10 months of age) in the development of working memory. Analysis of intrapair correlations in mono- and dizygotic twins showed that interindividual differences in the A-not-B task delay at age 7-9 months were completely determined by individual environmental factors (including measurement errors), while at age 10-12 months the leading role in the interindividual variability in this measure was taken by the systematic environment. These results show that the abilities to tolerate delays at the beginning and end of the second six months of life are based on different mechanisms.

[Developmental and individual differences in delayed response task performance (A-not-B task) in 7- to 12-month-old infant twins]. / Vozrastnye i individual'nye razlichiia v vypolnenii zadachi na otstrochennyi otvet (proba A-not-B) u detei bliznetsov 7-12 mesiatsev zhizni.

The purpose of the study was to explore developmental and individual differences in the working memory that was assessed by performance of the A-not-B task in 7- to 12-month-old infant twins. The sample included 150 infant twins participating in genetic project, 18 of them were tested at 8 and 11 months of age. Information concerning birth weight and gestational age was collected from birth records. All infants were tested using Bayley Scales of Mental and Psychomotor Development. The infant's working memory (WM) was tested during A-not-B task performance, which was shown to depend on the maturity of the prefrontal cortex. The behavioral data were analyzed off-line using videotape records. The one-way ANOVA revealed that the AB performance improves with age. A highly significant improvement of the ability to tolerate the delay in A-not-B task was observed between 9 and 10 months of life. There was no longitudinal stability in individual infant performance of A-not-B across the period of 7-12 months of age. The AB delay was significantly correlated with psychomotor and mental development only after the crucial period of the rapid increase in the AB delay (9-10 months). The analysis of intraclass mono- and dizygotic correlations showed that individual differences in the WM (tolerance to AB delay) at 7-9 months and at 10-12 months are of different nature. There was a significant effect of shared environment on AB delay variance in 10-12-month-old group, whereas down to this age no systematic influence on individual variability was observed. These findings suggest that the ability to tolerate the AB delay can be provided by different brain mechanisms in 7-9 and 10-12-month-old infants.