Researchers at UC 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.