, and visual primary regions in the forebrain predominantly represent the contralateral side of the body.
Two of the
optic tectum of the midbrain; and (2) the trochlear nerve (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 (i.e., smelling sense) is a noteworthy exception. Each olfactory lobe
connects to the ipsilateral (same-side) centers of the frontal cerebrum.
In
decussates twice, and the forebrain represents the ipsilateral (same-side) eye.[3][4]
) 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 sometimes be completely compensated over time.
Theories
Main article:
Axial Twist theory
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.
popular-science video explains these theories in brief.[6]
The Visual Map Theory and the Axial Twist Theory have been formulated in detail and can be regarded as
scientific theories
, and are explained in detail below.
Other hypotheses tend to explain specific aspects of the phenomenon. One proposes that crossing generally provides better geometrical mapping.[7] According to another view, the crossing is a coincidence that has been conserved by parcellation.[8] A third hypothesis proposes that the crossing results directly from optical inversion on the retina of the eye.[9]
An old notion, first worked out by Jacques Loeb, is that the contralateral organisation might have an advantage for motor control,[10][11]
but simulations by Valentino Braitenberg have shown that both ipsi- and contralateral connections are of major importance for control.[12]
Further studies have asked if there is a topological[13]
or functional advantage of the decussations.[14][15][16]
Visual map theory by Cajal
Cajal's schema of the visual map theory. O=Optic chiasm; C=Visual (and motor) cortex; M, S=Decussating pathways; R, G: Sensory nerves, motor ganglia.Transformations of the visual field toward the visual map on the primary visual cortex. U=up; D=down; L=left; R=right; F=fovea
The visual map theory was published by the famous neuroscientist and pioneer Santiago Ramón y Cajal (1898).[17] (See also [18] and [19] 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.
LGN to the visual cortex. (See figure; this path is known as the optic radiation
.) 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.
Axial twist
Main article:
Axial Twist theory
Two twist hypotheses have been proposed independently: the axial twist by de Marc Lussanet and Jan Osse
Whereas the somatic twist hypothesis focuses purely on the
predictions that have been tested independently.[21]
Axial twist theory
Schema of the developmental twist, according to the axial twist hypothesis. A, B: The early embryo turns onto its left side; B, C: Symmetry is retained by a further left turn in the anterior head region and a compensating right turn in the rest of the body. D, E: Growth of the optic tract leading to the optic chiasm. Colors refer to early embryo: Red=right side, blue=left side, black=dorsal, white=ventral.
The axial twist theory was designed to explain how the pattern of contralateral organization,[5] decussations and chiasms develops, and why this pattern is so evolutionarily stable,[22] having 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 scientifically.[21]
Comparing inversion, somatic twist and axial twist
According to the dorsoventral inversion hypothesis, an ancestral
protostomes
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.
The somatic twist hypothesis was proposed as an improvement to the inversion hypothesis, and thus has a much wider explanatory power than its predecessor, but is also more complicated. It not only explains the inversion of the body but additionally the contralateral forebrain. It does not explain, however, how the twist might develop in the vertebrate embryo, nor does it address the possible evolution.
The axial twist theory 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, and it is the only theory that has been tested independently.[21]
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,[2] and even the skull impressions of early vertebrates from the Ordovician show the presence of an optic chiasm:[25] this idea was worked out by Kinsbourne.[20]
There is molecular evidence for the inversion hypothesis in almost all groups of deuterostomes.
sea stars which turn their mouth downwards after the larva has briefly settled with the mouth turned up, or the adult lancelet
which buries obliquely with its mouth turned up, or many fish which tend to turn around when feeding from the water surface).
Developmental malformations
In holoprosencephaly, 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,[28] and
may occur when the cerebrum does not turn during early embryology.
Cephalopagus or janiceps twins are
conjoined twins who are born with two faces, one on either side of the head. These twins have two brains and two spinal cords, but these are located on the left and the right side of the body.[29] 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.[5]
^Chris Smith (11 Aug 2023). "Question of the Week". www.thenakedscientists.com (Podcast). Cambridge University. Event occurs at 23:30. Retrieved 14 Aug 2023.
^Ramón y Cajal, Santiago (1898). "Estructura del quiasma óptico y teoría general de los entrecruzamientos de las vías nerviosas. (Structure of the Chiasma opticum and general theory of the crossing of nerve tracks)" [Die Structur des Chiasma opticum nebst einer allgemeine Theorie der Kreuzung der Nervenbahnen (German, 1899, Verlag Joh. A. Barth)]. Rev. Trim. Micrográfica (in Spanish). 3: 15–65.
