Yakovlevian torque (also known as occipital bending (OB) or counterclockwise brain torque) is the tendency of the right side of the

human brain The human brain is the central organ (anatomy), organ of the human nervous system, and with the spinal cord makes up the central nervous system. The brain consists of the cerebrum, the brainstem and the cerebellum. It controls most of the act ...

to be warped slightly forward relative to the left and the left side of the human brain to be warped slightly backward relative to the right. This is responsible for certain

asymmetries Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pr ...

, such as how the

lateral sulcus In neuroanatomy, the lateral sulcus (also called Sylvian fissure, after Franciscus Sylvius, or lateral fissure) is one of the most prominent features of the human brain. The lateral sulcus is a deep fissure in each hemisphere that separates th ...

of the human

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

is often longer and less curved on the left side of the brain relative to the right. Stated in another way, Yakovlevian Torque can be defined by the existence of right-frontal and left-occipital petalias, which are protrusions of the surface of one hemisphere relative to the other. It is named for

Paul Ivan Yakovlev Paul Ivan Yakovlev (December 28, 1894 – June 16, 1983) was a Russian-American neuroanatomist who worked at Harvard Medical School. He is the namesake of the Yakovlevian torque, an asymmetry of human brains. He made contributions in the "origins ...

(1894–1983), a Russian-American

neuroanatomist Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defi ...

from

Harvard Medical School Harvard Medical School (HMS) is the graduate medical school of Harvard University and is located in the Longwood Medical and Academic Area, Longwood Medical Area of Boston, Massachusetts. Founded in 1782, HMS is one of the oldest medical schools ...

Effects

Handedness

A 2012 literature review showed that morphometry studies had consistently found that handedness-related effects corresponded to the extent of the Yakovlevian Torque; increased torque, as measured by increased size of the right-frontal petalia and the left-occipital petalia, tends to be more common in right-handed individuals. Individuals with mixed-handedness or left-handedness show reduced levels of Yakovlevian torque.

Developmental stuttering

Reduced right-frontal and left-occipital petalias and reversed petalia asymmetries (that is, left-frontal and right occipital petalias) have been associated with developmental

stuttering Stuttering, also known as stammering, is a speech disorder in which the flow of speech is disrupted by involuntary repetitions and prolongations of sounds, syllables, words, or phrases as well as involuntary silent pauses or blocks in which the ...

in both adults and pre-adolescent boys. This may be tied to the

housing

Broca's area Broca's area, or the Broca area (, also , ), is a region in the frontal lobe of the dominant hemisphere, usually the left, of the brain with functions linked to speech production. Language processing has been linked to Broca's area since Pier ...

, which plays a significant role in production of language.

Bipolar disorder

Increased size of the left-occipital petalia, resulting from an abnormally high degree of Yakovlevian torque has been associated with

bipolar disorder Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of depression and periods of abnormally elevated mood that last from days to weeks each. If the elevated mood is severe or associated with ...

. Maller et al. 2015 found that increased asymmetry of the occipital lobe, or occipital bending, was four times more prevalent in subjects with bipolar disorder than in healthy controls. This applied both to patients with bipolar disorder type I and type II.

Presence in primates

Yakovlevian torque is found in modern humans and fossil hominids, appearing reliably as early as ''

Homo erectus ''Homo erectus'' (; meaning " upright man") is an extinct species of archaic human from the Pleistocene, with its earliest occurrence about 2 million years ago. Several human species, such as ''H. heidelbergensis'' and ''H. antecessor' ...

''. The patterning of petalias in extinct human ancestors is examined via

endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...

s, wherein a cast is made of the

cranial vault The cranial vault is the space in the skull within the neurocranium, occupied by the brain. Development In humans, the cranial vault is imperfectly composed in newborns, to allow the large human head to pass through the birth canal. During bi ...

: the asymmetries of human ancestors can be measured from these casts because petalias leave impressions inside the cranial vault. Some authors have reported that similar petalia patterns are found in a number of

primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter includin ...

including

Old World monkey Old World monkey is the common English name for a family of primates known taxonomically as the Cercopithecidae (). Twenty-four genera and 138 species are recognized, making it the largest primate family. Old World monkey genera include baboons ...

New World monkey New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboid ...

s and

Great apes The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); '' Gorilla'' (the ...

, but others report different protrusions;{{cite journal , last1=LeMay , first1=M , title=Morphological cerebral asymmetries of modern man, fossil man, and nonhuman primate , journal=Annals of the New York Academy of Sciences , date=1976 , volume=280 , issue=1 , pages=349–366 , doi=10.1111/j.1749-6632.1976.tb25499.x , pmid=827951 , bibcode=1976NYASA.280..349L , s2cid=46070451 these differences seem to be tied to which techniques are used to measure the petalia, so it is not well-understood if all primates demonstrate Yakovlevian torque.

References

Yakovlevian torque Biophysics Brain Brain asymmetry