Frontiers: Crossed Brain Connections May Be an Evolutionary Legacy of Ancient Vision

The starting point for their hypothesis was vision. Information from the right visual field is transmitted to the opposite side of the brain, while information from the left visual field is sent to the right hemisphere. This organization of visual pathways has been described in different vertebrates and is associated with an ancient visual center in the midbrain known as the optic tectum. In mammals, this structure is represented by the superior colliculi.

According to the researchers' hypothesis, the visual system's crossed organization provided the foundation around which evolution built new sensory and motor pathways. In the ancestors of mammals, many of these pathways already crossed the midline of the nervous system.

When the neocortex emerged, it was easier for it to integrate into the existing system while preserving the same pattern of connections. Completely reorganizing the ancient pathways would have required additional changes to the neural "wiring."

Crossed control may therefore have become established through the reuse of an existing neural architecture. The study offers an evolutionary explanation but does not establish the exact sequence of events in the brains of animals that lived hundreds of millions of years ago.