A new look at neurobehavioral development in rhesus monkey neonates (Macaca mulatta)

Abstract

The Brazelton Neonatal Behavioral Assessment Scale (NBAS) evaluates a newborn infant's autonomic, motor, state, temperament, and social-attentional systems, which can help to identify infants at risk of developmental problems. Given the prevalence of rhesus monkeys being used as an animal model for human development, here we aimed to validate a standardized test battery modeled after the NBAS for use with nonhuman primates called the Infant Behavioral Assessment Scale (IBAS), employing exploratory structural equation modeling using a large sample of rhesus macaque neonates (n = 1,056). Furthermore, we examined the repeated assessments of the common factors within the same infants to describe any changes in performance over time, taking into account two independent variables (infant sex and rearing condition) that can potentially affect developmental outcomes. Results revealed three factors (Orientation, State Control, and Motor Activity) that all increased over the 1st month of life. While infant sex did not have an effect on any factor, nursery-rearing led to higher scores on Orientation but lower scores on State Control and Motor Activity. These results validate the IBAS as a reliable and valuable research tool for use with rhesus macaque infants and suggest that differences in rearing conditions can affect developmental trajectories and potentially pre-expose infants to heightened levels of cognitive and emotional deficiencies.

Keywords: IBAS scale; Motor Activity; Orientation; State Control; exploratory structural equation modeling; second-order latent growth model.

Figure 1.

Expected mean trajectories for mother- and nursery-reared animals and corresponding 95% confidence intervals of the expected trajectories of individual animals within these groups for Orientation (1a), State Control (1b), and Motor Activity (1c). The mean trajectories for each group are displayed using bold lines and 95% intervals of the within-group, between-animal differences in change are displayed by the shaded areas. Estimates are based on the validation sample. The variances of the random intercept and slope correspond to the between-animal variability in the factor scores at 7 days of age and in the linear rates of change, respectively. Assuming that the random effects are normally distributed, then approximately 95% of the individual intercepts and slopes are expected to range about their respective mean values by ± 1.96*SD of the corresponding random effect. For instance, the mean intercept of Orientation (1a) for nursery-reared animals was equal to 0.35 and the SD of the random intercept was 0.41. It follows that approximately 95% of intercepts for nursery-reared animals are expected to range from 0.35 ± 1.96*0.41 or −0.45 to 1.15. These values are shown for each of the three factors by the shaded areas. The lightest shading represents expected animal-level trajectories for the mother-reared animals and the darkest shading represents expected trajectories for the nursery-reared animals. The overlap between groups is represented by the medium shade of gray. As shown, there is overlap between groups in the expected range of the individual-level trajectories for each other the three factors. Thus, even though there were statistically significant differences in the mean factor scores between groups, there was considerable overlap in the expected trajectories of the individual animals.