A comparison of executive function in very preterm and term infants at 8 months corrected age
Introduction
Advances in medical technologies have resulted in the increased survival of infants of progressively lower birthweight and shorter gestation. Compared with full-term infants, those born early are at increased risk of subsequent difficulties, in behaviour and learning [1]. The risk of these adverse outcomes increases as gestation age and birthweight decrease [2].
Executive function (EF) deficits have been linked to a range of problems, including attention deficit disorder (ADD), learning difficulties, and autism [3], all of which are more prevalent in children born preterm [4]. EF encompasses goal directed behaviour that is deliberate, conscious, and purposeful [5]. While dependent upon basic cognitive processes, such as perception and memory, the behaviours and processes referred to as EF are associated with higher-order cognitive abilities, that require holding plans or programs in mind until executed (working memory), inhibiting irrelevant action (inhibition), and planning a sequence of actions (planning) [3], [6]. These components of EF have been identified in factor analytic studies e.g., [7], [8] as well as studies of patients with frontal lobe lesions (e.g., [9]).
Preschool and school aged children who were born preterm have been found to have poorer performance on various tasks measuring EF than their full-term peers [10], [11] with those preterm children who had medical complications in the perinatal period being more impaired on EF measures than other preterm children [12].
It is generally agreed that EF is primarily governed by the prefrontal cortex of the brain [13] and the emergence of EF abilities coincides with the development of this part of the brain [14]. However as the prefrontal cortex is late to mature and develops rapidly in infancy, it may be particularly vulnerable to disturbance in preterm infants. For example, disruption of pathways in the prefrontal lobe may occur because of damage to other neural circuits that connect to the frontal lobes and are therefore implicated in EF development [15].
During the first year of life, significant development of EF takes place. A study of working memory in infants from 8 to 12 months of age, showed that on the AB task (see description of the AB task below in Section 2.3) the time delay between hiding the object and allowing the infant to retrieve it could be significantly increased as the child got older without incurring increased errors [16]. At a similar age infants have also been shown to begin to inhibit irrelevant responses [17], to become planful, and to be able to carry out relatively complex sequences of goal directed behaviour [18]. It has been suggested that any deviation in the development of EF during this critical period may result in enduring learning, attention or behavioural problems [12], [19].
Most previous studies of cognitive development in preterm infants have relied upon either the use of standard developmental assessments [20] which are poor predictors of later learning and behaviour [4], or the use of sophisticated technology to assess perception, memory or attention [21], [22]. The present research used practical and simple measures of EF that could be applied easily in clinics or field settings. It has been argued that the AB task measures working memory and inhibition [23] and that tasks which require the execution of a series of steps to achieve a goal measure planning [24]. Therefore these were the assessment tools of choice for comparing the performance of very preterm infants (who are known to have a high risk for learning and attentional problems) and full-term infants (who have a much lower risk for these difficulties). It was hypothesised that differences on measures of EF would exist between the two groups, even when the influence of confounding variables was taken into account. If this is the case these measures of specific cognitive abilities, which are associated with behaviour and learning, may provide a means of identifying children at risk of school difficulties at an early age.
Section snippets
Subjects
Both very preterm and full-term infants enrolled in the study were healthy, had no diagnosed disabilities, lived within a 50-kilometre radius of the Mater Mothers' Hospital, and came from English speaking families. All the very preterm infants were born at the Mater Mothers' Hospital, Brisbane, between May 1998 and July 1999, with gestational ages of ≤ 32 weeks and birthweights of < 1250 g. They were recruited into the study by the clinic co-ordinator when they attended for their routine
Results
Table 2 shows the results of ANOVAs and Chi-squares to examine differences between preterm and full term infants on potential confounding variables.
Very preterm infants had significantly lower scores than full-term infants on the psychomotor development scale (PDI) of the BSID II but no significant differences were found between the groups on the mental development scale (MDI) or any of the other variables. Analysis of variance (ANOVA) (unadjusted model) was firstly used to compare very preterm
Discussion
The present study found that at 8 months corrected age the performance of very preterm infants was inferior to that of full-term infants on EF tasks of working memory, inhibition to distraction, and planning. These findings reinforce the call from a number of researchers for the measurement of more specific cognitive abilities when assessing the development of very preterm infants [36]. Furthermore as very preterm infants performed more poorly than full term infants on measures of EF and as
Acknowledgement
The first author received financial support from an Australian International Postgraduate Research Scholarship and from the School of Early Childhood at the Queensland University of Technology. The authors also wish to acknowledge the support of the Growth and Development team at the Mater Children's Hospital, Brisbane.
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