Last revised by Mohamed Saber on 24 May 2021

Petalia is an anatomic description of cerebral asymmetry where one of the cerebral hemispheres protrudes towards the other hemisphere and thereby causes an impression on the inner surface of the skull 1,2.


The typical configuration in modern humans is the combination of a right frontal lobe petalia and a left occipital lobe petalia i.e the right hemisphere protrudes anteriorly beyond the left, and the left hemisphere extends posteriorly beyond the right. CT and MRI studies show that these petalias are more prominent in right handers. The opposite pattern is seen in left-handedness.

A related term, Yakovlevian counterclockwise torque, is a geometric torsion in hemispheres secondary to their asymmetries, where generally the wider right frontal cortex makes a displacement to left and concomitantly the left occipital pole makes a bending towards the right side, also known as occipital bending.  Some studies have shown that petalia and occipital bending have some functional significance and these are more common among patients with psychiatric diseases like major depressive disorder 3, schizophrenia 4, and bipolar affective disorder 5 in comparison to healthy controls. Occipital petalia is also a predictive imaging sign for transverse sinus dominance 3,7.

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