How does the eye talk to the brain?

Two eyes are better than one! Our brains integrate two slightly different images coming from our two eyes to give us depth perception. How does this happen? To answer this, we will look at where the neural pathways from the two eyes converge. Retinal ganglion cells (RGCs) are neurons in the eye that send visual information to the brain. RGCs are divided into many subtypes based on the shapes of the cells. The lateral geniculate nucleus (LGN) is the primary brain region where RGCs first deliver visual information to brain cells. Previous studies have suggested that some LGN cells receive input from only one eye (Chen and Regehr, 2000; Sincich et al., 2007), while others receive input from both eyes (Hammer et al., 2015; Morgan et al., 2016). To fully understand how the brain integrates the images coming from two eyes, we need to understand the pattern of connectivity between RGCs and LGN cells at the level of individual neuronal connections.

To reveal this pattern in mice, a recent study by Rompani et al. (2017) identified exactly one LGN cell from each mouse and looked at all the RGCs that synapsed with that one LGN cell. They wanted to look at (1) how many of those RGCs came from only one eye (monocular) vs. both (binocular) and (2) whether those RGCs belonged to few or many subtypes. To see all the RGCs that synapsed to a single LGN cell, the authors used a viral tracing method to label LGN cells by making them glow. Using the label as a guide, the authors introduced a modified rabies virus into a single LGN cell. The modified rabies virus infects any cell that synapses onto the LGN cell and therefore the modified rabies virus infected all the RGCs that have input to that LGN cell. Finally, the rabies virus was modified such that all infected cells get labeled. Thus, all the RGCs that synapse onto that single LGN cell were labeled.

When the authors looked at the labeled RGCs, they found that the RGCs formed circular clusters in the retina with no more than one cluster per retina (Figure 1D). This is consistent with the long-standing idea that RGCs map onto the LGN depending on where they are spatially in the retina. They found three types of input patterns: relay mode, combination mode, and binocular combination mode. In relay mode (28% of LGN cells), LGN cells received input from only the contralateral eye and from the same or mostly the same RGC subtype (Figure 3D). In combination mode (32% of LGN cells), LGN cells received input from only the contralateral eye and RGCs of many subtypes (Figure 3H). In binocular combination mode (40% of LGN cells), LGN cells received input from both eyes and RGCs of many subtypes (Figure 4A). For the first time, we can see a pattern by which RGCs are connected to individual LGN cells!

This study identified important characteristics about how visual information from two different eyes converge in the brain. By labeling all the RGCs that synapse to a single LGN cell, the authors were able to describe RGC subtype input patterns and monocular vs. binocular input. These findings have important implications for functional relevance, as relay mode in other sensory systems suggest innate connections while combination mode is associated with learning and plasticity. High resolution data like this is needed to understand how the brain integrates visual information from two different eyes to generate image formation and depth perception.

References

Chen, Chinfei, and Wade G. Regehr. "Developmental remodeling of the retinogeniculate synapse." Neuron 28, no. 3 (2000): 955-966.

Hammer, Sarah, Aboozar Monavarfeshani, Tyler Lemon, Jianmin Su, and Michael Andrew Fox. "Multiple retinal axons converge onto relay cells in the adult mouse thalamus." Cell reports 12, no. 10 (2015): 1575-1583.

Morgan, Josh Lyskowski, Daniel Raimund Berger, Arthur Willis Wetzel, and Jeff William Lichtman. "The fuzzy logic of network connectivity in mouse visual thalamus." Cell 165, no. 1 (2016): 192-206.

Rompani, Santiago B., Fiona E. Müllner, Adrian Wanner, Chi Zhang, Chiara N. Roth, Keisuke Yonehara, and Botond Roska. "Different modes of visual integration in the lateral geniculate nucleus revealed by single-cell-initiated transsynaptic tracing." Neuron 93, no. 4 (2017): 767-776.

Sincich, Lawrence C., Daniel L. Adams, John R. Economides, and Jonathan C. Horton. "Transmission of spike trains at the retinogeniculate synapse." Journal of Neuroscience 27, no. 10 (2007): 2683-2692.