Most of the human brain's estimated 86 billion nerve cells, or neurons, can ultimately engage in a two-way dialogue with any other neuron. To shed more light on how neurons in this labyrinthine network integrate information - that is, precisely how multiple neurons send and combine their messages to a target neuron - a team of researchers focused on a rare case in which information only travels in one direction: from the retina to the brain.

In this study published in the journal Cell, researchers developed a means of tracking the activity of the far-reaching ends of retinal neurons (called boutons) as they deliver visual information to the thalamus, a brain region involved in image processing.

As they relay discrete bits of visual information to the brain, different types of retinal neurons respond to distinct features of visual content such as an object's direction of motion, brightness, or size. Conventional wisdom held that these lines of information remained separated in the thalamus. Instead, the team found that the boutons from different types of retinal neurons were often organized in local clusters and that boutons in a cluster typically make contact with a common target neuron, leading to a mixing of different lines of information. However, this mixing was not random - boutons in a cluster tended to share a common sensitivity to one or more visual features.

Authors show that clusters of boutons from different axons often showed similar preferences for either one or multiple features, including axis and direction of motion, spatial frequency, and changes in luminance. Conversely, individual axons could “de-multiplex” information channels by participating in multiple, functionally distinct bouton clusters.

Ultrastructural analyses demonstrated that retinal axonal boutons in a local cluster often target the same dendritic domain. These data suggest that functionally specific convergence and divergence of retinal axons may impart diverse, robust, and often novel feature selectivity to visual thalamus.

"The selective mixing of information from this arrangement of nearby boutons may be the retina's version of Pointillism, the neo-expressionist art technique in which nearby dots of different colors are fused together to create new and diverse colors," said the senior author. "In this way, this first interface between eye and brain is surprisingly sophisticated."

https://www.bidmc.org/about-bidmc/news/2018/05/scientists-visualize-the-connections-between-eye-and-brain

https://www.cell.com/cell/fulltext/S0092-8674(18)30573-7