Binocular Connectivity with Selective Monocular Function in the Mouse Retinogeniculate Pathway

Article: Picky neurons: In the visual thalamus, neurons are in contact with both eyes but respond to only one
Source: Max Planck Institute of Neurobiology (Germany)
Published: June 18, 2021
Article: Neurons Simplify Visual Signals by Responding to Only One Retina
Source: The Scientist
Published: October 1, 2021

Ipsilateral nerve fibers (red) and contralateral nerve
fibers (green) in the mouse visual thalamus

The visual thalamus—in humans, comprised of the lateral geniculate nucleus (LGN), the thalamic reticular nucleus (TRN), and the pulvinar—relays visual stimuli from the retina to the primary visual cortex. The retinogeniculate pathway, involving the LGN, is of greatest interest to visual perception. According to a long-standing theory, signals from the two eyes are separated in the visual thalamus before being transmitted to the visual cortex. However, investigations into the visual thalamus of mice showed that a large number of neurons form synapses with retinal ganglion cells (RGCs) of both eyes. Using optogenetics, researchers confirmed this finding. However, they also discovered that very few of the neurons actually receive equal signal strength from both eyes. Rather, in most neurons, signal strength from one eye dominates over the other, with the weaker signals from the non-dominant eye not reaching threshold to trigger an action potential. The researchers hypothesize that they do not play a major role in processing visual information. In other words, although demonstrating structural binocular connectivity, these cells in the mouse visual thalamus were functionally monocular, resolving the contradictory findings in the earlier study. From a basic science standpoint, the findings showed that anatomy does not necessarily confer function; in this case, neurons in contact with one another do not necessarily communicate extensively. From the perspective of function, the study showed that even with equal access to both eyes, thalamic cells selectively establish functional connections with only one eye. Monocular connections are explained by synaptic selection rather than by anatomical overlap between RGC axons and dLGN neuron dendrites alone. Furthermore, synapses with the dominant eye strengthen while those of the nondominant eye remain immature. The scientists next plan to study how eye dominance is determined, whether there are variations in different RGC types, and whether the immature contact sites could be activated when needed, for example to treat eye conditions such as amblyopia.

My rating of this study: ⭐⭐⭐

Bauer J, Weiler S, Fernholz MHP, et al. "Limited functional convergence of eye-specific inputs in the retinogeniculate pathway of the mouse." Neuron.  18 June 2021. https://doi.org/10.1016/j.neuron.2021.05.036