According to current understanding, the contralateral organization is due to an axial twist ( see below), which is the only theory that has been tested independently. A number of alternative proposals have been published earlier, the most popular of which is the visual map theory. A popular-science video explains these theories in brief. alt=schematic drawing of the central nervous system, showing schematically how arrow halves are projected on the eyes' retina, crossed by the optic chiasm and mapped correctly to the visual cortex|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. 300px|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) , 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 to the visual cortex (see Figure ; this path is known as the optic radiation ). As a result, the retinal map show the visual periphery on the medial side. However, the central point of the theory was exactly to obtain a  faithful visual map with the medial field projecting to the medial sides of the visual cortex. 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. alt=caption|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 hypothesis was designed to explain how the pattern of contralateral organization, decussations and chiasms develops, and why this pattern is evolutionary 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 , as dorsal and ventral are inverted in the anterior head region, the tracts grow at first to 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 also . Rather, these organs retain their original asymmetric orientation in the body. The axial twist hypothesis predicts that small asymmetries of face, brain and ( in the opposite direction ) in the trunk  remain into adulthood, and this has been confirmed experimentally. According to the dorsoventral inversion hypothesis, an ancestral deuterostome turned on its back. As a result, vertebrates have a dorsal nervous system  whereas 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 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  the inverted body and the contralateral forebrain  it explains why the heart and bowels are asymmetric. Moreover, it is the only  of the three  that is supported by evidence from embryological growth.
According to current understanding, the contralateral organization is due to an axial twist ( explained further below), which is the only theory that has been tested independently. A number of alternative proposals were published earlier, the most popular of which is the visual map theory. A popular-science video explains these theories in brief. alt=schematic drawing of the central nervous system, showing schematically how arrow halves are projected on the eyes' retina, crossed by the optic chiasm and mapped correctly to the visual cortex|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. 300px|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) . (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 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. 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. alt=caption|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 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. According to the dorsoventral inversion hypothesis, an ancestral deuterostome turned on its back. As a result, vertebrates have a dorsal nervous system , whereas 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 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.
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