Introduction
Brain laterality refers to the capability of human brain hemispheres to execute different functions,
which is related to their anatomical asymmetries.
The left hemisphere,
for example,
is normally dominant for language and logical processing,
whereas the right hemisphere is dominant for spatial recognition.
Some authors believe these anatomical differences,
and therefore the lateralization,
might be an evolutionary advantage.
There are many theories,
some of them based in anthropological studies,
trying to explain the mechanisms involved in these cerebral specializations.
Fig.
1 Superior view of a 3D-reconstruction derived from MRI of a normal brain.
Left and right brain hemispheres have an exceptional correspondence,
nevertheless their asymmetric anatomic areas are the key of a marked lateralization in the brain function.
The Molecular Theory
The factors that influence cerebral asymmetry involve a wide spectrum of mechanisms,
including genetical and environmental factors.
These mechanisms are still poorly understood.
Nevertheless,
an irregular distribution of molecules during the embryologic development,
probably related to the initial chirality from the centrosome or other cytoskeletal organizing center,
seems to be the main theory to explain how symmetry is broken in the CNS.
Left–right asymmetry of other internal organs has very consistent molecular regulation.
Fig.
2 Schematic ilustration (A)Neural tube development is accompanied by the formation of the notochord and induction of the floor plate.
Molecules secreted from the notochord function as morphogens to induce and maintain the ventral property and neural cell types in the spinal cord.
Similar morphogens also induce and pattern the forebrain.
Moreover,
the patterning centers are not limited to the notochord and the ventral neural tube.
(B) Morphogens secreted from the ventral (floor plate or prechordal plate) and/or dorsal (roof plate) midlines are distributed differently between the left and right. (C) The anterior cortex seems to participate on these irregular secretions,
leading to an asymmetric cortical regionalization.
Yakovlevian Torque and Petalia
The Yakovlevian anticlockwise torque is a geometric distortion of the brain hemispheres,
in which the frontal cortex is wider in the right hemisphere,
and the left occipital pole is wider and protrudes further posteriorly.
Fig.
3 Inferior view of a 3D-reconstruction MRI of brain.
The Yakovlevian anticlockwise torque demonstrating frontal cortex wider in the right hemisphere,
and the left occipital pole wider and protrudes further posteriorly
The right frontal and left occipital petalias are indentations on the inner surface of the skull related to protrusions of the surface of one hemisphere relative to the other,
and constitute two of the most prominent examples of interhemispheric asymmetry.
Fig.
4 Superior view of a 3D-reconstruction of a normal skull base.
The petalias are the impressions of asymmetric brain hemispheres on the surface of the cranial bones.
Perisylvian Region and Language
Both Wernicke’s language comprehension area and Broca’s speech area are functionally defined regions located in the vicinity of the lateral sulcus,
also known as the sylvian fissure.
The fissure is commonly shorter and runs less horizontally in the right hemisphere.
Asymmetries within this region are detectable early in development.
The planum temporale displays pronounced leftward asymmetry in most humans.
Fig.5 Left lateral 3D reconstruction MRI of a normal brain. A side view illustration of the brain,
showing the location of Sylvian fissure (lateral fissure),
Broca’s area and Wernicke’s area. The fissure is commonly shorter and runs less horizontally in the right hemisphere.
The planum temporale displays pronounced leftward asymmetry in most humans.
Handedness
Handedness is one of the most obvious lateralized behaviors in humans,
with preferential use of right hand in more than 90%,
observed in almost all cultures and ethnicities.
Preferred hand use has been observed even at embryonic and fetal stages in humans,
suggesting a genetic base.
Nevertheless studies involving identical twins with discordant handedness broaden the factors,
incorporating a strong environmental influence.
Fig.
6 Superior view of a 3D-reconstruction MRI. The central sulcus divides the primary motor cortex from the sensory cortex.
Greater cortical folding and gyrification within the motor-hand area is observed on the left for human with preferential use of right hand.
Polymicrogyria
Polymicrogyria is a cortical malformation that occurs during the development stages of brain,
characterized by many small gyri in the cortex.
When it occurs only on one side of the brain it is known as unilateral polymicrogyria.
Unilateral right polymicrogyria may manifest as an inherited trait, perhaps related to a gene that is required for the development of the right perisylvian region.
Fig.
7 Coronal T2-weighted MRI.
Unilateral right polymicrogyria may manifest as an inherited trait,
suggesting a gene requirement for the development of the right perisylvian region.
Gender
The evaluation of sexual dimorphic trait of human brain hemispheres have demonstrated that males usually show higher asymmetry in the volumes of their two hemispheres,
more robust in the parietal lobe at the posterior end of the Sylvian fissure,
and women displaying volumetric symmetry. Homosexual men showed more symmetric hemispheric volumes than heterosexual men.
The greater asymmetry of cerebral function in males is thought to underlie the greater incidence of language impairment in them following stroke,
and possibly also the increased incidence of learning disorders.
Psychiatric disorders
Disturbances in hemispheric lateralization have been reported for a number of psychiatric conditions.
Schizophrenia has demonstrated the strongest evidence.
Asymmetry of the planum temporale and the Sylvian fissure was significantly decreased in schizophrenia,
while asymmetry of the temporal horn of the lateral ventricle was not.
Studies to assess the role of hemispheric lateralization have been performed with a wide spectrum of advanced techniques, including behavioral and electrophysiological approaches,
structural and functional MRI.
Asymmetric hemispheric areas demonstrated in some psychiatric conditions are not completely linked.
Hyppocampal Malrotation
Hyppocampal malrotation is a developmental malformation and refers to an incomplete hyppocampal inversion,
associated with a constellation of findings,
including abnormally round shape,
abnormal angle of the collateral sulcus,
abnormal position and size of the fornix,
among others.
It predominantly affects the left hippocampus in male patients. Therefore it is a pathologic error in brain development rather than a normal variant, and might be related with the laterality process,
considering its left predilection.
Fig.
8 Coronal T2-weighted MRI.
Hippocampal malrotation (blue arrow) is a developmental malformation that predominantly affects the left hippocampus in male patients.