"Gray matter and superficial white matter are physically close and may play different roles: gray matter processes information, while superficial white matter helps nearby brain regions communicate," said Yingxu Liu, PhD, a postdoctoral scholar at the Stevens INI and first author of the study. "Our findings suggest that cognitive health depends not only on how much gray matter is preserved, but also on the condition of the wiring that connects it."

How did researchers measure the wiring?

The scientists used diffusion MRI, which follows how water moves through brain tissue. This method reveals microscopic structures that conventional scans often miss.

They focused on two measurements: neurite density (the number of small projections nerve cells use to send and receive signals) and the amount of free water surrounding those projections. Lower neurite density or more free water can indicate damage, including loss of myelin, the fatty insulation around nerve fibers, inflammation, or swelling.

Participants also completed cognitive tests covering memory, executive function, language, and visuospatial ability.

Why study older adults in India?

The data came from the Harmonized Diagnostic Assessment of Dementia for the Longitudinal Aging Study in India (LASI-DAD). More than half of the broader LASI-DAD population has low literacy, and about 60% lives in rural communities.

Populations with these characteristics have rarely appeared in brain imaging studies. Including them lets researchers explore how education, geography, and social background shape cognitive aging.

In this analysis, the link between superficial white matter and language ability proved stronger among people who could not read, those with no formal schooling, and rural residents.

The researchers caution that social factors do not necessarily cause brain changes. Instead, life experiences such as education, health, and environment may combine in complex ways.

What do researchers still need to learn?

The study examined participants at one point in time. It cannot show whether superficial white matter begins to deteriorate before, during, or after gray matter loss. Follow-up work must track people over years to clarify the order of changes.

Future studies will examine how vascular health, inflammation, and proteins linked to Alzheimer's disease interact with shifts in gray and white matter. Institute director Arthur W. Toga, PhD, called for research that reflects the world's social, cultural, and geographic diversity, noting that the Indian cohort helps identify factors that may protect cognition across the lifespan.

"The findings point to superficial white matter as a possible source of resilience," said Leon Aksman, PhD, assistant professor of research neurology at the Stevens INI and senior author of the study. "Two people with a similar degree of gray matter loss may not experience the same cognitive effects if the local connections surrounding that gray matter differ in health. Following participants over time will be essential to test whether preserving these connections can help maintain cognition."

The research drew support from the National Institute on Aging, the National Institute of Mental Health, the National Institute of Neurological Disorders and Stroke, and the NIH Office of the Director.