The first reference maps for the human brain have been completed

lFA DOCTOR wants to know how well a child is growing, she can turn to clinically validated charts that show exactly how that child compares to the norm for their age and gender. The doctor can look up not only how many centimeters shorter or taller the child is than the average for his age, but also exactly in which height percentile he falls. Medical diagnoses can then be made on the basis of an absolute comparison with the statistical standard.

Reference cards are an important tool in modern primary care and cover many aspects of a person’s healthy development. However, there is a big hole in their cover: the human brain. Richard Bethlehem and Simon White of the University of Cambridge and Jakob Seidlitz of the University of Pennsylvania want to solve that. Register Naturethe neuroscientists describe the most comprehensive effort to date to create a standard against which to measure a person’s brain development over a lifetime.

Their brain charts were compiled from more than 120,000 three-dimensional brain scans of more than 100,000 patients who participated in more than 100 different studies. The dataset included people of all ages, ranging from babies still developing in the womb, just over 100 days after conception, to adults over 100 years old.

With that data, the scientists cataloged how the average human brain evolved from cradle to grave, focusing on three types of brain tissue: gray matter (made up of neuron cell bodies), white matter (the filaments connecting neurons), and tissue that transports cerebrospinal fluid. (the conduit system of the brain). The scientists paid particular attention to the cerebral cortex, the outermost layer of the brain, which is responsible for higher-order brain functions. They observed gray matter in the cortex peaking in volume at 5.9 years, 2 to 3 years later than previously thought.

After characterizing the development and aging of the average human brain, the scientists modeled the distribution around it and mapped the percentile-per-percentile variation in the structure of human brain tissue. This allowed them to examine how the brains of patients with various developmental or degenerative conditions relate to more typical brains. “Our study confirmed that Alzheimer’s disease, mild cognitive impairment and schizophrenia show marked restructuring of brain tissue compared to a more typical age-sex brain,” says Dr. Seidlitz.

The catalog also yielded some surprises. It is generally assumed that autism presents differently in male and female patients, but this difference is hardly noticeable in their brain tissue. In contrast, attention deficit hyperactivity disorder (ADHD) — which is similarly represented by gender — shows the greatest mean difference in brain structure between male and female patients of any diagnosis they analyzed. Over the course of their lives, men’s brains ADHD patients appear skewed toward below-average volumes of gray matter, white matter, and cerebrospinal fluid. The brains of women ADHD patients, on the other hand, were ever so slightly skewed toward higher volumes of the same tissues.

What these differences in brain size mean is not yet clear. And the authors caution that their brain maps are not yet ready for clinical use, not least because the dataset they used has several limitations. “Unfortunately, the data we collected reflect the demographic biases of neuroscience research in general, i.e. most studies are from Europe or North America and overrepresent patients of European descent,” said Dr. Bethlehem.

To reflect the full diversity of the normative development of the human brain, a more representative data set is required. Once that is achieved, the usefulness of brain maps can be tested in a clinical setting. Hopefully one day these charts can become a useful tool in tracking a person’s brain health or detecting the earliest physical signs of brain disorders like Alzheimer’s disease.

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This article appeared in the Science & Technology section of the print edition under the headline “Frames of mind”

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