Researchers at the University of California, Davis, are challenging the conventional view of how connections form between the optic nerves and the brain.

Early in development, cells in the retina show waves of activity long before the eyes are exposed to light. At the same time, optic nerves grow from the left and right eyes into the brain and form separate "eye-specific" layers in a brain structure called the lateral geniculate nuclei, or LGN.

Neuroscientists currently think that the pattern of spontaneous activity is essential to "instruct" the formation of separate left and right eye connections to the LGN, because if the activity in the retina is blocked, the connections to the brain do not separate into layers properly.

Researchers Andrew Huberman, Guo-Yong Wang, Lauren Liets, Odell Collins, Barbara Chapman and Leo Chalupa used an antibody coupled to a ribosomal toxin to perturb the activity of selected cells in the retina. The retinal cells still show spontaneous activity, but in a random rather than a normal pattern. Surprisingly, the axons of the optic nerve still grow into the brain normally and form normal layered LGN structures. However, when all activity was completely blocked in the developing retina, the researchers found that the normal separation of left and right eye connections did not occur.

The results show that neuronal activity in the eye permits the eye-specific connections to the LGN to develop.

"Contrary to what everyone expected, the pattern of activity in the developing retina is not a key factor in this process," Chalupa said. The findings represent a major shift in the field of developmental neurobiology of the visual system, he said.

The results are published in the May 9, 2003, issue of the journal Science.