1 School of Psychology, University of East Anglia, Norwich NR4 7TJ, United Kingdom.
2 Department of Psychology, Concordia University, Montreal, Quebec H4B 1R6, Canada.
3 Department of Psychology, Durham University, Durham DH1 3LE, United Kingdom.
4 Department of Psychology, University of Cambridge, Cambridge CB2 3EB, United Kingdom.
5 Department of Radiology, University of Iowa, Iowa City, Iowa 52242.
6 Memorial Hospital of Rhode Island, Brown University, Pawtucket, Rhode Island 02860.
7 Norwich Medical School, University of East Anglia, Norwich NR4 7TJ, United Kingdom.
8 C.J. Gorter Centre for High Field MRI, Department of Radiology, Leiden University Medical Centre, 2333 ZA, Leiden, The Netherlands.
9 School of Psychology, University of East Anglia, Norwich NR4 7TJ, United Kingdom j.spencer@uea.ac.uk.

The language environment to which children are exposed has an impact on later language abilities as well as on brain development; however, it is unclear how early such impacts emerge. This study investigates the effects of children's early language environment and socioeconomic status (SES) on brain structure in infancy at 6 and 30 months of age (both sexes included). We used magnetic resonance imaging to quantify concentrations of myelin in specific fiber tracts in the brain. Our central question was whether Language Environment Analysis (LENA) measures from in-home recording devices and SES measures of maternal education predicted myelin concentrations over the course of development. Results indicate that 30-month-old children exposed to larger amounts of in-home adult input showed more myelination in the white matter tracts most associated with language. Right hemisphere regions also show an association with SES, with older children from more highly educated mothers and exposed to more adult input, showing greater myelin concentrations in language-related areas. We discuss these results in relation to the current literature and implications for future research.SIGNIFICANCE STATEMENT This is the first study to look at how brain myelination is impacted by language input and socioeconomic status early in development. We find robust relationships of both factors in language-related brain areas at 30 months of age.