Researchers studied infants born before 30 weeks gestation that were enrolled in the Victorian Infant Brain Studies cohort from 2001-2003. They determined the number of days that infants received breast milk as more than 50 percent of of their nutritional intake from birth to 28 days of life. Additionally, researchers examined data related to regional brain volumes measured by magnetic resonance imaging (MRI) at each baby's term equivalent age and at seven years old, and also looked at cognitive (IQ, reading, mathematics, attention, working memory, language, visual perception) and motor testing at age seven.

The findings show that, accross all babies, infants who received predominantly breast milk on more days during their NICU hospitalization had larger deep nuclear gray matter volume, an area important for processing and transmitting neural signals to other parts of the brain, at term equivalent age, and by age seven, performed better in IQ, mathematics, working memory, and motor function tests. Overall, ingesting more human milk correlated with better outcomes, including larger regional brain volumes at term equivalent and improved cognitive outcomes at age 7.

The study was moreso focused on brain development in premature babies. The IQ tests were sort of an afterthought to prove correlation, but the MRI scans at term equivalent age and at seven is what's really telling, especially since, in NICU, each baby's environment is essentially identical.