Researchers from the Max Planck Institute of Neurobiology now show that although neurons in the mouse visual thalamus connect to both eyes, they establish strong functional connections only with one retina. These results—published in Neuron—settle partly contradictory results of earlier studies and demonstrate how important it can be to complement structural data with functional analyses.
The scientists developed an optogenetic method, so that they could activate ganglion cells of both eyes successively with light of different colors and measure the corresponding electrical responses in a thalamic cell. Their analysis showed that indeed a multitude of neurons in the visual thalamus receive input from both retinas. "With these results, we can explain the contradictory findings of previous research," says Tobias Rose, who led the study. "The neurons in the visual thalamus do form connections with both eyes, but are functionally monocular or, so to speak, one-eyed. That means that only the signal strength of one eye is high enough for the cells to respond."
The spatial arrangement of retinal and thalamic cells and the resulting connection possibilities alone cannot explain the functional monocularity. Instead, the scientists showed that even with equal access to both eyes, thalamic cells establish functional connections with only one eye. Accordingly, there is a clear selection of the input source. In addition, the contact sites with the dominant eye seem to strengthen, while those of the non-dominant eye remain in an immature state.
With their study, the researchers prove that the mouse visual thalamus indeed transfers visual information in separate channels—presumably similar to humans.