The
contralateral organization of the
forebrain
In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hindbrain (rhombencephalon) are the three primary ...
(
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
: contra‚ against; latus‚ side; lateral‚ sided) is the property that the
hemisphere
Hemisphere refers to:
* A half of a sphere
As half of the Earth
* A hemisphere of Earth
** Northern Hemisphere
** Southern Hemisphere
** Eastern Hemisphere
** Western Hemisphere
** Land and water hemispheres
* A half of the (geocentric) celes ...
s of the
cerebrum
The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. ...
and the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direct ...
represent mainly the contralateral side of the body. Consequently, the left side of the forebrain mostly represents the right side of the body, and the right side of the brain primarily represents the left side of the body. The contralateral organization involves both executive and sensory functions (e.g., a left-sided
brain lesion
Neurotrauma, brain damage or brain injury (BI) is the destruction or degeneration of brain cells. Brain injuries occur due to a wide range of internal and external factors. In general, brain damage refers to significant, undiscriminating ...
may cause a right-sided
hemiplegia
Hemiparesis, or unilateral paresis, is weakness of one entire side of the body ('' hemi-'' means "half"). Hemiplegia is, in its most severe form, complete paralysis of half of the body. Hemiparesis and hemiplegia can be caused by different med ...
). The contralateral organization is present in all
vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
but in no
invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
.
According to the current theory, the forebrain is twisted about the long axis of the body, so that not only the left and right sides, but also dorsal and ventral sides, are interchanged. (See below.)
Anatomy
Anatomically, the contralateral organization is manifested by major
decussations (based upon the
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
notation for
ten, 'deca,' as an
uppercase
Letter case is the distinction between the letters that are in larger uppercase or capitals (or more formally ''majuscule'') and smaller lowercase (or more formally ''minuscule'') in the written representation of certain languages. The writing ...
'X') and
chiasmas (after the
Greek uppercase
Letter case is the distinction between the letters that are in larger uppercase or capitals (or more formally ''majuscule'') and smaller lowercase (or more formally ''minuscule'') in the written representation of certain languages. The writing ...
letter 'Χ,'
chi). A decussation denotes a crossing of bundles of
axonal
An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...
fibres inside the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
. As a result of such decussations: The
efferent connections of the cerebrum to the
basal ganglia
The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an ext ...
, the
cerebellum
The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebe ...
and the
spine
Spine or spinal may refer to:
Science Biology
* Vertebral column, also known as the backbone
* Dendritic spine, a small membranous protrusion from a neuron's dendrite
* Thorns, spines, and prickles, needle-like structures in plants
* Spine (zoolo ...
are crossed; and the
afferent connections from the spine, the cerebellum and the
pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of Va ...
to the thalamus are crossed.
Thus, motor,
somatosensory
In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position ( proprioception), and pain. It ...
,
auditory, and visual primary regions in the forebrain predominantly represent the contralateral side of the body.
Two of the
cranial nerves show chiasmas: (1) the
chiasm of the
optic tract (i.e., cranial nerve II), which originates from the eyes and inserts on the
optic tectum of the
midbrain
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', ...
; and (2) the
trochlear nerve
The trochlear nerve (), ( lit. ''pulley-like'' nerve) also known as the fourth cranial nerve, cranial nerve IV, or CN IV, is a cranial nerve that innervates just one muscle: the superior oblique muscle of the eye, which operates through the pu ...
(i.e., cranial nerve IV), which originates in the ventral midbrain and innervates one of the six muscles that rotate the eye (i.e., the
superior oblique muscle).
The contralateral organization is incomplete
Although the forebrain of all vertebrates shows a contralateral organization, this contralaterality is by no means complete. Some of these exceptions are worth mentioning:
*
Olfaction
The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.
In humans, ...
(i.e., smelling sense) is a noteworthy exception. Each
olfactory lobe connects to the ipsilateral centers of the frontal cerebrum.
* In
chondrichthyans (e.g.,
sharks and
skate
Skate or Skates may refer to: Fish
* Skate (fish), several genera of fish belonging to the family Rajidae
* Pygmy skates, several genera of fish belonging to the family Gurgesiellidae
* Smooth skates or leg skates, several genera of fish belongi ...
s), the thalamus does not retrieve a branch from the optic tract but only from the contralateral
optic tectum, so that the optic path
decussates twice, and the forebrain represents the ipsilateral eye.
* In large brains, some functions tend to be strongly
lateralized. For example, the
language
Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languages are the primary means by which humans communicate, and may be conveyed through a variety of ...
regions (i.e.,
Broca's and
Wernicke's area
Wernicke's area (; ), also called Wernicke's speech area, is one of the two parts of the cerebral cortex that are linked to speech, the other being Broca's area. It is involved in the comprehension of written and spoken language, in contrast to B ...
) are situated in the left hemisphere of most humans.
* Most
afferent and
efferent connections of the forebrain have bilateral components, especially outside the primary sensory and motor regions. As a result, a hemiplegia that is acquired at very young age can be completely compensated over time.
Theories
According to current understanding, the contralateral organization is due to an axial twist (explained below). A number of other explanations have been published, the most popular of which is the visual map theory (explained below). A short review of existing hypotheses is given by reference.
A
popular-science
''Popular Science'' (also known as ''PopSci'') is an American digital magazine carrying popular science content, which refers to articles for the general reader on science and technology subjects. ''Popular Science'' has won over 58 awards, inclu ...
video explains these theories in brief.
Whereas the Visual Map Theory and the Axial Twist Theory have been formulated in detail and can be regarded as
scientific theories
A scientific theory is an explanation of an aspect of the natural world and universe that has been repeatedly tested and corroborated in accordance with the scientific method, using accepted protocols of observation, measurement, and evaluatio ...
, further hypotheses tend to explain specific aspect; they propose that: a geometrical mapping argument,
a coincidence of parcellation,
or optical inversion on the retina of the eye,
might explain the contralateral organization.
Visual map theory by Cajal
The visual map theory was published by the famous neuroscientist and pioneer Santiago Ramón y Cajal
Santiago Ramón y Cajal (; 1 May 1852 – 17 October 1934) was a Spanish neuroscientist, pathologist, and histologist specializing in neuroanatomy and the central nervous system. He and Camillo Golgi received the Nobel Prize in Physiology or M ...
(1898). (See also and for English summaries.) According to this theory, the function of the optic chiasm is to repair the retinal field image on the visual cortex. The pupil in the vertebrates’ eyes inverts the image on the retina, so that the visual periphery projects to the medial side of the retina. By the chiasmatic crossing, the visual periphery is again on the outside, if one assumes that the retinal map is faithfully maintained throughout the optic tract.
The theory has a number of weaknesses. For example, the visual tracts spiral their way from the thalamic LGN
In neuroanatomy, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral projec ...
to the visual cortex. (See figure; this path is known as the optic radiation
In neuroanatomy, the optic radiation (also known as the geniculocalcarine tract, the geniculostriate pathway, and posterior thalamic radiation) are axons from the neurons in the lateral geniculate nucleus to the primary visual cortex. The op ...
.) As a result, the retinal map shows the visual periphery on the medial side. However, the central objective of the theory was to obtain a precise, faithful visual map with the medial field projecting to the medial sides of the visual cortex.
Twist theories
Two twist theories have been proposed independently: the axial twist hypothesis by de Marc Lussanet and Jan Osse
and the somatic twist hypothesis by
Marcel Kinsbourne Marcel Kinsbourne (born 1931) is an Austrian-born pediatric neurologist and cognitive neuroscientist who was an early pioneer in the study of brain lateralization. Kinsbourne obtained his M.D. degree (styled B.M., BCh., Oxon.) in 1955 and D.M. de ...
.
Axial twist hypothesis
The axial twist hypothesis was designed to explain how the pattern of contralateral organization, decussations and chiasms develops, and why this pattern is evolutionarily stable.
The evolutionary stability is truly remarkable, given that there are no known exceptions throughout the 500 million years of vertebrate evolution. According to the theory, the contralateral organization develops as follows. The early embryo is turned onto its left side, such that its left is turned to the yolk and its right is turned away from the yolk. This asymmetric orientation is compensated by asymmetric growth, to regain superficial bilateral symmetry. The anterior head region turns to the left, as shown in the schema. The forebrain is not a superficial structure, but it is so intimately associated with superficial body structures that it turns along with the anterior head. These structures will later form the eyes, nostrils and mouth.
The body behind the head compensates the asymmetric body orientation in the opposite direction, by turning to the right. (See schema.) Due to these oppositely directed compensations of the anterior head and the rest of the body, the animal becomes twisted.
The optic tract grows from the retina to the optic tectum. Because dorsal and ventral are inverted in the anterior head region, the tracts grow at first toward the ventral side, to meet in the midline to form a chiasma. Since the optic tectum lies on the dorsal midbrain, each tract then continues dorsally to the contralateral optic tectum.
The heart and bowels are internal organs with no strong integration in external body structures, so there is no evolutionary pressure to make them turn in a similar way. Rather, these organs retain their original asymmetric orientation in the body.
The axial twist hypothesis predicts that small asymmetries of the face and brain — as well as those found in the opposite direction in the trunk — remain into adulthood, and this has been confirmed experimentally.
Somatic twist hypothesis
The idea of a somatic twist was inspired by the
dorsoventral inversion hypothesis;
and was proposed by Marcel Kinsbourne.
According to the dorsoventral inversion hypothesis, an ancestral
deuterostome
Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some ...
turned on its back. As a result, vertebrates have a dorsal nervous system, whereas
protostomes
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
have a ventral one. According to the somatic twist hypothesis, not the entire animal turned on its back but just the ″somatic″ part — i.e., everything behind the eyes, mouth and nostrils, including the forebrain.
Comparing inversion, somatic twist and axial twist
The three theories are closely related.
The somatic twist hypothesis was proposed as an improvement to the inversion hypothesis, and thus has a much wider explanatory power than its predecessor. It not only explains the inversion of the body but additionally the contralateral forebrain.
The axial twist hypothesis was defined independently of the other two. In addition to providing rationale for the inverted body and the contralateral forebrain, it explains why the heart and bowels are asymmetric. Moreover, it is the only one of the three theories that is supported by evidence from embryological growth, which is the only theory that has been tested independently.
Evolution
A remarkable property of the contralateral organization is that it is present in every vertebrate. Even the most distant clades —
agnathans — possess an optic chiasm,
and even the skull impressions of early vertebrates from the
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
show the presence of an optic chiasm:
this idea was worked out by Kinsbourne.
There is molecular evidence for the inversion hypothesis in almost all groups of deuterostomes. It is not known, however, what exactly was the selective pressure that caused the inversion. Twisting and asymmetric development are well known from other
deuterostomes — such as
Hemichordata,
Echinodermata
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
,
Cephalochordata and
Tunicata. Turning toward the side or upside-down also occurs frequently in these clades.
Developmental malformations
In
holoprosencephaly
Holoprosencephaly (HPE) is a cephalic disorder in which the prosencephalon (the forebrain of the embryo) fails to develop into two hemispheres, typically occurring between the 18th and 28th day of gestation. Normally, the forebrain is formed ...
, the hemispheres of the cerebrum or part of it are not aligned on the left and right side but only on the frontal and occipital sides of the skull, and the head usually remains very small. According to the axial twist hypothesis, this represents an extreme case of
Yakovlevian torque,
and
may occur when the cerebrum does not turn during early embryology.
Cephalopagus or janiceps twins are
conjoined twin
Conjoined twins – sometimes popularly referred to as Siamese twins – are twins joined ''in utero''. A very rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence ...
s who are born with two faces, one on either side of the head. These twins have two brains and two spinal chords, but these are located on the left and the right side of the body.
According to the axial twist hypothesis, the two nervous systems could not turn due to the complex configuration of the body and therefore remained on either side.
See also
*
Brain asymmetry
*
Lateralization of brain function
References
{{evolution
Animal developmental biology
Evolutionary biology
Neuroscience
Cerebrum
Biology theories