The depth of small grooves on the brain’s surface is connected to stronger network connectivity and better reasoning ability, according to a new study.

While many grooves and dimples on the brain are unique to humans, they are often disregarded as an uninteresting result of fitting a large brain into a small skull.

However,neuroscientists are discovering that these folds are more than just artifacts. The depth of the smallest of these grooves appears to be related to increased interconnectedness in the brain and improved reasoning ability.

Researchers at the University of California, Berkeley, discovered that the depths of some small grooves in children and adolescents correlated with increased connectivity between brain regions-the lateral prefrontal cortex and lateral parietal cortex-involved in reasoning and other high-level cognitive functions, according to a study in The journal of Neuroscience.

The grooves may bring these areas closer together, shortening the connections between them and speeding up communication.

According to the researchers, variability in these small grooves, known as tertiary sulci, may help explain individual differences in cognitive performance and could serve as diagnostic indicators or biomarkers of reasoning ability or neurodevelopmental disorders.

“the impetus for this study was having seen that sulcal depth correlated with reasoning across children and adolescents,” says Silvia Bunge, professor of psychology and a member of UC Berkeley’s Helen Wills Neuroscience Institute (HWNI).

“Given our previous findings, our former postdoctoral fellow Suvi Häkkinen aimed to test if sulcal depth was correlated with reasoning performance and to test if patterns of coordinated activity within a lateral prefrontal-parietal network could explain this relation between sulcal depth and reasoning.”

“We had explicit predictions about which tertiary sulci in the lateral prefrontal cortex would be functionally connected to tertiary sulci in the lateral parietal cortex, and that panned out,” added Kevin Weiner, UC Berkeley associate professor of psychology and of neuroscience and a member of HWNI.

“Prefrontal and parietal cortices aside, the hypothesis is that the formation of sulci leads to shortened distances between connected brain regions, which could lead to increased neural efficiency, and then, in turn, individual differences in improved cognition with translational applications.”

“The cortex is sort of haphazardly crunched up into the brain-that’s what I was always taught,” Bunge says.”Kevin came along and changed my mind about sulci.”

The Significance of Brain Folds

Most animal brains, including those of mammals, have smooth surfaces. Primates have hills and valleys covering their cerebral cortex. While marmosets, New World monkeys, have shallow, barely perceptible sulci, human sulci are deeply incised, with 60% to 70% of the cortex buried in these folds.

Cortical folding patterns in humans change with age, establishing their final structure late in prenatal progress while becoming less prominent in old age.

“While sulci can change over development, getting deeper or shallower and developing thinner or thicker gray matter-probably in ways that depend on experience-our particular configuration of sulci is a stable individual difference: their size,shape,location,and even,for a few sulci,whether they’re present or absent,” says Bunge,who studies abstract reasoning in young people,from 6 years of age through young adulthood.

“The cortex is sort of haphazardly crunched up into the brain-that’s what I was always taught,”

The smallest grooves, many of which are uniquely human, are called tertiary sulci because they appear last in prenatal development and are never as deep as the major or primary sulci that are most evident on the cerebral surface.

Scientists have speculated that the tertiary sulci emerge in parts of the human brain that have expanded the most throughout evolution and have a protracted development, and that they are likely associated with aspects of cognition-reasoning, decision-making, planning, and self-control-that develop over a protracted adolescence.

Though, prior to this study, there was no evidence of a link between tertiary sulci and brain connectivity. The UC Berkeley study is one of the few in recent years to provide such evidence.

Uncovering the Role of Sulci in Cognition

Weiner and Bunge stated that they were never taught how to define tertiary sulci as undergraduates; they frequently examined scans of average brains that did not match any specific individual.

Weiner noticed this discrepancy as an undergraduate.

“At the time, all I knew was that I had some cortical squiggles that weren’t in the average brain atlases that we had in the lab. So the question I asked my mentors, Sabine Kastner and Charlie Gross, was: Do I have different structures that aren’t in our atlases or are structures missing from these atlases?” he says. “That sent me down a 15-year rabbit hole studying one particular tertiary sulcus in the visual cortex.”

That work revealed that a specific sulcus, the mid-fusiform sulcus, varied in length from 3 millimeters to 7 centimeters