The contralateral organization of the forebrain (Latin: contra‚ against; latus‚ side; lateral‚ sided) is the property that the hemispheres of the cerebrum and the thalamus 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 may cause a right-sided hemiplegia). The contralateral organization is only present in vertebrates.
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 also ).
Anatomy
Anatomically, the contralateral organization is manifested by major decussations (based upon the Latin notation for ten, 'deca,' as an uppercase 'X') and chiasmas (after the Greek uppercase letter 'Χ,' chi). A decussation denotes a crossing of bundles of axonal fibres inside the central nervous system. As a result of such decussations: The efferent connections of the cerebrum to the basal ganglia, the cerebellum and the spine are crossed; and the afferent connections from the spine, the cerebellum and the pons to the thalamus are crossed. Thus, motor, somatosensory, 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; 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 centers of the frontal cerebrum. In chondrichthyans (e.g., sharks and skates), 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 (e.g., humans, elephants and whales), some functions tend to be strongly lateralized. For example, the language regions (i.e., Broca's and Wernicke's area) 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
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 video explains these theories in brief.
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 aspect of the phenomenon. One proposes that crossing generally provides better geometrical mapping. According to another view, the crossing is a coincidence that has been conserved by parcellation. A third hypothesis proposes that the crossing results directly from optical inversion on the retina of the eye.
An old notion, first worked out by Jacques Loeb is that the contralateral organisation might have an advantage for motor control, but simulations by Valentino Braitenberg have shown that both, ipsi- and contralateral connections are of major importance for control.
Further studies have asked if there is a topological
or functional advantage of the decussations.