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With my head resting against the window inside a high-speed train, I enjoy watching the vegetation along the tracks forming lines moving far faster than the mountains on the horizon. What in our vision system manages to estimate speed of movement like this? Our brain probably has complex neuronal networks responsible for this task. Recent studies in fact have shown that pre-processing movement-related visual information occurs well before the brain - in the retina!
Contrary to what we were told in school, the retina is not just a light sensor. The layer of neurons that covers the back of our eyes and translates light into electrical impulses does not send raw data to the brain. It conducts a prior in-depth analysis of our field of vision and carries out complex processing of the stimuli before sending signals about our movements to the brain. How? The retina has around thirty independent networks that each process one aspect of light stimulations and sends this information to various regions in the brain. The principal networks for processing colour and light intensity are well known. But we are only beginning to understand the one that deals with movement. David Berson’s team at Rhode Island’s Brown University has just shown that the neurons in this network are only sensitive to specific directions in the field of vision (1). More specifically, the neurons discharge, i.e. release an electrical pulse, for only four preferred orientations; up, down, forwards and backwards. At each point in the retina, these four orientations form two orthogonal directions perpendicular to each other; the up-down axis and the forward-back axis. As the retina forms a hemisphere, these directions rotate slightly from one point to another. So much so that over the whole of the retina, they form a network of curved lines which in fact correspond to the lines of optical flow. Just like the ones observed at high speed on the train! Or like those formed by the stars seen through the cockpit of the Falcon Millennium in the Star Wars saga, when it reaches warp-drive speeds. Our retina is therefore perfectly suited to the way we move within our environment. Proof of this can be seen in mice, whose lower retina is covered mainly with receptors sensitive to blue light. Exactly where light from the sky is projected (on the retina, the visual field is inverted). The upper part is dominated by green colour receptors. Green... like the colour of the grass the mice run through (2)! It is fascinating to note how the organisation of retinal neurons reflects that of the outside world.
Far from just being a biological camera, the retina continues to evoke wonder in the neuroscientific community. A colleague at the Institute of Vision in Paris recently told me how amazed he was at the complexity of the neuron responses he observes. Light up just one part of the retina, and the neurons remaining in the dark also start to discharge. Why? Because this organ doesn’t just process light information as a set of separate points. The neurons are interconnected and communicate through various points on the retina. This complex signal processing network, refined by millions of years of evolution and adapted to the needs of the animal, is highly efficient.
Today, numerous promising projects are attempting to create artificial retinas for patients who have lost the use of their eyes. It will probably be a long time however before prostheses that match the extraordinary performances of nature become available.
(1) S. Sabbah et al., Nature, 546, 492, 2017.
(2) A. Szél et al., J. Comp. Neurol., 325, 327, 1992.
Adrien heads at McGill University in Canada a research laboratory devoted to studying the neuronal processes involved in spatial navigation and memory.