Axial twist theory

The axial twist theory ( axial twist hypothesis) is a proposed scientific theory to explain the optic chiasm and contralateral brain of vertebrates. It suggests that the rostral part of the head is "turned around" with respect to the rest of the body. This end-part would consist of the face (eyes, nose, and mouth) as well as part of the brain (cerebrum and thalamus). The central nervous system would end up with a 180 degree twist right at the optic chiasm, and other body parts would be slightly moved as well. According to the hypothesis, the vertebrate body has a left-handed chirality.

The axial twist theory competes with a number of other proposals. None of the proposed theories explaining this phenomenon, including axial twist theory, have gained general recognition. No evidence supporting the proposed developmental twist has been found.

According to the axial twist theory, the embryo turns on its left side during development, followed by two 90 degree twists to regain external bilateral symmetry. This leads to a 180 degree turn between the forebrain and midbrain where the optic chiasm is formed.

In order for the optic tracts to connect contralaterally in the optic chiasm, the theory assumes that the retinal axons preferentially target neurons that were closest to them before the twist. Evidence for this strong assumption has not been found, and research suggests that axons function equally well connecting to the mirrored, ipsilateral neurons. The authors of the axial twist theory suggest some "molecular goalposts" to guide axons across the CNS midline, though this assumption is unsubstantiated. Furthermore, the "goalposts" would now be the factors driving the decussation, eliminating the need for a twist to happen at all.