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3/15/2006 12:01:00 AM

Even small variations in birth weight are related to early cognitive development in boys

Studies have long shown that low birth weights can have an impact on a child's later development. New research by a Rice University psychologist suggests that even small variations among infants born within a normal weight range are related to their early cognitive development, which further emphasizes the importance of prenatal care.

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Researchers have shown a number of connections between low infant birth weight and cognitive and motor development later in life. A new Rice University study also has found that normal birth weight -- at least, among male infants -- is related to how readily they focus on a visual stimulus, an ability that later may play a role in some attention deficit hyperactivity disorders.

"Infant boys who are heavier at birth, but within the normal weight range, are more likely to focus toward a visual stimulus," said James Dannemiller, a Rice University psychologist whose study was one of the first to show that birth weight in the normal range could have a measurable effect on this aspect of infant development.

"On the other hand, while girls visually orient on average as well as boys, their birth weights appear to have no connection to this type of attention."

According to Dannemiller, visual orienting may be involved in some types of attention deficit hyperactivity disorder (ADHD). "While ADHD is often thought of as an inability to sustain attention, it also may be the result of overactive orienting with anything in the environment grabbing the person's attention," he said.

The discrepancy between genders in Dannemiller's findings is similar to those reported in a study in England that saw a correlation between normal birth weight and intelligence measures in adult life. In that study, researchers also reported a stronger correlation for males than females.

In his study published in Infant Behavior and Development , Dannemiller chose to focus on visual orienting as one of the earliest dimensions of infants' typical development.

"Visual orienting is the earliest and most-developed of our abilities to focus and plays a role in our cognitive development," explained Dannemiller.

"The fact that it is associated with something physiological, namely birth weight, suggests the further importance of prenatal development."

Dannemiller also discovered that abnormally large male and female babies within his sample -- those with birth weights of more than 10 pounds -- showed less inclination to orient their eyes toward a visual stimulus. One of the causes of this condition, called "macrosomia," is uncontrolled diabetes in mothers during pregnancy. None of the mothers of the 30 macrosomic infants in Dannemiller's sample had this disorder, however.

"Currently, I am gathering more details on the pregnancies of these mothers to determine the reasons behind their babies' high weights and why, in their cases, the ability to orient decreased for both male and female infants," Dannemiller said.

To examine the relation between birth weight and visual orienting, he studied a sample of 944 infants between 2 and 5 months old. Information about the babies' birth weight and gestational age was collected from the parents.

The babies were repeatedly shown a display with small, vertical bars randomly distributed across a monitor. One of these bars either on the right or the left of the screen oscillated horizontally. In half of the trials the movement was located on the left side of the screen, and in the other half on the right. To gauge where the babies' eyes focused, Danemiller positioned observers who were unable to see the monitors but were able to see whether the infants' eyes moved to the left or right.

Danemiller cautioned that the variation in birth weight associated with visual orienting is too small to be meaningful at the level of the individual child. As he explained it, if the orienting measure were analogous to an IQ test with a standard deviation of 15 points, then the effect size would equal three IQ points per kilogram of additional birth weight.

The study's results, however, do offer clues as to how various prenatal factors within a population are related to behavioral development in infants and children.

"Disorders of attention can impair the child's ability to learn, so it is important to understand how these processes develop during the early period when infants are just beginning to explore their worlds visually," Dannemiller said.

Dannemiller is the Lynette S. Autrey Professor of Psychology at Rice. His studies on the early development of visual attention and basic perceptual processes have been published in Developmental Science, Spatial Vision, Infancy, Infant Behavior and Development and Journal of Vision. His research has received support from the National Institute of Child Health and Human Development.

For more information on this research, contact Dannemiller at dannemil@rice.edu or B.J. Almond in the Office of News and Media Relations at balmond@rice.edu.