Contralateral brain

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 present in all vertebrates but in no invertebrate. 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 ). 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). 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.