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An unusual fold appears in only 30-60 percent of the population - and is related to emotional and mental qualities, as well as to the timing of appearance of dementia

We all know the typical appearance of the brain, which is covered with folds. The vast majority of these folds are identical in shape in all humans - but there are exceptions. A certain fold only appears in thirty to sixty percent of the population, and might appear in the right or left side of the brain, or in both. Previous studies have indicated a link between this fold and certain emotional and cognitive (cerebral) qualities. A new study currently presents a link between this fold and the timing of appearance of a certain type of dementia. The underlying mechanism that links this fold and mental qualities remains unclear.  


Jane-R-Garrison_Charles-Fernyhough_Simon-McCarthy-Jones_Mark-Haggard_The-Australian: A certain fold only appears in thirty to sixty percent of the population, and may appear in the left or right parts of the brain, or in both. The fold, called Paracingulate sulcus, indicated in red in the image | From Garrison et al., Nature communications 2015

The Folded Landscape Of The Brain

The human brain contains about eighty six trillion nerve cells, and a similar amount of auxiliary cells. The neural tube is already formed during early stages of fetal development, and preliminary structures are formed during the first few weeks of the pregnancy, which will later develop into the different parts of the mature brain and the spinal cord. During later developmental stages some of the stem cells will differentiate into nerve cells, grow, send out extensions and even migrate. During this stage the brain receives its typical heavily folded appearance. The purpose of the folds - as in other systems in the body, like the colon - is to improve the ratio of surface to volume, or simply, to compress many cells in a small volume and still allow them access to the environment, so that they will be able to receive oxygen and nutrients from the bloodstream. 

Not all mammals have a brain decorated with folds. Mice, for example, which are an accepted model animal for research, have a completely smooth brain. They have no problem with that, but when the folds do not form in species that have brains with many folds, their absence causes severe functional impairments. Smooth Brain Syndrome can sometimes occur in humans, resulting in the brain developing without folds. This syndrome is caused by mutations in genes that are in charge, amongst others, of nerve cell migration. Infants that suffer from smooth brain syndrome experience intellectual disabilities, epilepsy, significant developmental difficulties and a shortened life span. Brain folds are formed during fetal development - initial folds are formed at the beginning of the formation of fetal tissues, and most folds are formed during the third trimester. Usually, the folds in the right and left halves (hemispheres) of the brain are quite symmetrical, but the symmetry is not absolute.  

The mature brain of most adults has a similar appearance, with the same structure and folds. Researchers have drawn a map of the brain folds and given them names, and so brain scientists can describe a certain area in an individual and researchers from all over the world will know exactly which area he is referring to and examine it in the brains of other people. To differentiate between the folds and protruding areas between folds, there are different names for the protruding ridges (Gyrus) and the depression or crack between two folds (Sulcus).  


The mature brain of most people have a similar appearance, the same structure with the same folds - but there are exceptions. The fold decorated structure of the brain | Shutterstock, A Step BioMed

Not In Everyone

And yet, not all brains share the exact same structure. One of the biggest difference factors is a certain Sulcus, which only develops in thirty to sixty percent of the population. The fold, called Paracingulate sulcus, is part of an area called anterior cingulate cortex. This special fold is formed in later stages of development and can also form in a non-symmetrical manner, meaning in only one hemisphere of the brain. It is mostly found in the left hemisphere.   

Though researchers believed that this fold exists only in humans, a study that examined its prevalence in additional animals demonstrated that the paracingulate sulcus also exists in members of the great apes family, including chimpanzees, gorillas and orangutans. In these species as well, the fold is only found in part of the population. However, it appears that the fold does not occur in monkeys and other animals.  


Paracingulate sulcus is also found in other great apes, including chimpanzees, gorillas and orangutans. A chimpanzee’s brain | Images / Science Photo Library

Managerial Fold

In humans, presence of the fold is probably linked to cognitive abilities: it was found that people who have such a fold in the left hemisphere of their brains tested higher in tasks that test for managerial functions, like flow of speech, spatial perception and working memory. The researchers suggest that the fold enables more effective connectivity between the anterior cingulate cortex and the prefrontal cortex - an area where complex cognitive activities take place, like planning, decision making and abstract thinking. And yet, the study was conducted on a small number of subjects and further research is required to confirm this suggestion. 

Other studies have found a link between this fold and cognitive and emotional characteristics. For example, a study conducted on teenagers found a link between a symmetrical fold and the sensation of closeness with other people. 

A recently published study examined the effect of the fold in people with frontotemporal dementia, a disease that causes nerve cells in the temporal and frontal lobes to degenerate. The disease represents about twenty percent of dementia cases. Symptoms usually emerge in people during their fifties and include dramatic changes in personality and behavior, as well as difficulties in speaking and using language. Life expectancy once the disease breaks out is estimated at eight to fourteen years. The disease gained publicity when actor Bruce Willis was diagnosed with it earlier this year. Frontotemporal dementia has a genetic component, which means that people who are carriers of mutations in certain genes are at higher risk than others to have the disease, while there are cases when the disease occurs with no known genetic background. 


The disease gained publicity when actor Bruce Willis was diagnosed with it | Wikipedia, Gage Skidmore

The Link Between The Fold And Dementia

In a new study, researchers examined 186 patients who developed frontotemporal dementia and had no known genetic background for the disease, and found that the age at which patients who had a paracingulate sulcus in their right hemisphere were diagnosed with the disease was on average two years older, compared to other patients, meaning that their symptoms appeared later in life. 

The researchers compared these findings to additional factors, such as the overall volume of brain tissue of the patients and the number of years of education they have had, but found no other factors that were linked to the late diagnosis. 

So is such a fold in the right hemisphere a good thing? not sure. The researchers found that people who have the fold did develop symptoms later in life, but the average age of death of people with the fold and without it was nearly identical. This means that life expectancy following diagnosis was shorter in people with this fold in their right hemisphere. 


The disease causes cells in the temporal and frontal lobes to degenerate. MRI scan of a frontotemporal dementia patient |  Zephyr / Science Photo Library

The Fold, Or The Adjacent Tissues?

The studies we have described suggest a link between this unusual fold and some qualities of the human nervous system: from cognitive abilities, though emotional qualities and to the timing of the appearance of a certain type of dementia. However, all of these studies still do not provide an answer to an important question - what does this link stem from? Which goes to say, why does the presence or absence of the fold affect all of these qualities? It is possible that it is not the fold that is responsible for all of this, but the alterations that its presence or absence cause to the adjacent tissues. Near the paracingulate sulcus there are areas that are related to emotional processing, decision making and communication with other areas in the brain. All of these functions could be slightly affected by structural alterations in their vicinity.  

Further research is required to understand how the presence of the fold in the right or left hemisphere of the brain affects all the other qualities we have described. Better understanding of this fold is not only important in itself, but also as it may lead to better understanding of frontotemporal dementia and finding better ways to delay the disease.