Contralateral brain
The contralateral organization involves both executive and sensory functions (e.g., a left-sided brain lesion may cause a right-sided hemiplegia).
A decussation denotes a crossing of bundles of axonal fibres inside the central nervous system.
6.3, 6.13, 6.20 Thus, motor, somatosensory, auditory, and visual primary regions in the forebrain predominantly represent the contralateral side of the body.
Some of these exceptions are worth mentioning: According to current understanding, the contralateral organization is due to an axial twist (explained below).
[18][19][20] The visual map theory was published by the famous neuroscientist and pioneer Santiago Ramón y Cajal (1898).
According to this theory, the function of the optic chiasm is to repair the retinal field image on the visual cortex.
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.
[24] Both of them propose that the rostral part of the head, including the forebrain, is in fact effectively completely turned around.
Due to these oppositely directed compensations of the anterior head and the rest of the body, the animal becomes twisted.
[10] 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.
[10] 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.
[26] The idea of a somatic twist was inspired by the dorsoventral inversion hypothesis;[27][28] and was proposed by Marcel Kinsbourne.
In addition to providing rationale for the inverted body and the contralateral forebrain, it explains why the heart and bowels are asymmetric.
According to the axial twist hypothesis, this represents an extreme case of Yakovlevian torque,[32] and may occur when the cerebrum does not turn during early embryology.
[33] 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.
