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Ophthalmic Research Symposium with Olivier Marre - The Summary

On May 16th, the Institute for Ophthalmic Research Tübingen invited Prof. Olivier Marre from the Institut de la Vision in Paris to give a talk in the Ophthalmic Research Symposium. His talk was hosted by Prof. Thomas Euler and titled „Models and circuits of surround modulation in the retina”.

Prof. Olivier Marre, Institute de la Vision, Paris, France

FIA Ophthalmic Research Symposium, Edition May 2019 - Prof. Olivier Marre, Institute de la Vision, Paris, France

Prof. Marre first gave a very clear general introduction into how context affects visual perception. As a demonstration, he showed a visual illusion (as below), in which the circles actually are of the same color; however, the perception of color changes depending on the surrounding context for each circle (i.e. the colored stripes). Although such visual illusions often arise in higher visual areas, context-dependent visual processing already begins at the level of the retina. Specifically, Prof. Marre talked about his investigations into the questions if and how the responses of individual retinal ganglion cells (RGCs) are modulated by context and if a single type of RGC can encode different visual features depending on context. Further, he touched upon the overarching question of how the brain reads out and interprets the output of the retina.

At the retina level, a classical view is that one type of RGC extracts a single feature of the visual stimulus, such as contrast, whether an edge is present or if an object moves. In his group, they performed large-scale recordings of isolated mouse retina with microelectrode arrays (MEA). They presented stimuli – e.g. a jittering bar – directly above the recorded RGCs (receptive field center) with and without a second bar placed further away from the cells (in the far receptive field surround). They focused on transient OFF RGCs and found those cells responding differently to dark bars presenting in the center vs the surround. In addition, the surround responses were synchronized among neighboring cells of that type. Using computational modeling, they found that linear-nonlinear model could explain the center responses quite well but not the surround responses. By adding a layer of subunits (two-layer subunit model), roughly representing the retinal bipolar cells, the model performed as well in explaining the surround responses.

Next, they explored what would happen with two simultaneously presented dark bars – one in the center and the other one in the surround. Here, they found that distant (surround) responses were “silenced” by central stimulation, and that this could be predicted using by a “full” model containing a global gain control unit after the summation of center and surround components. Taken together, these data demonstrate that a single type of RGCs can indeed encode several features, suggesting that the same retinal circuit is able to “multiplex”, that is performing different computations depending on the context.

Finally, Prof. Marre discussed the possible cellular mechanisms underlying the surround modulation. To this end, he and his collaborators devised an exciting new method that combines two-photon computer-generated holography and optogenetic tools to target and stimulate individual rod bipolar cells (RBCs) during MEA recording. They observed that individual RBCs in the surround of a transient OFF alpha RGC can indeed activate this RGC, suggesting that RBCs may contribute to OFF alpha RGCs’ surround responses via glycinergic amacrine cells, such as the so-called AII amacrine cell.

In summary, Prof. Marre demonstrated that a single type of RGCs can multiplex distinct computations, e.g. depending on if a stimulus appears in its receptive field center or far surround. These findings indicate that retinal coding schemes, in particular for more complex stimuli, are much richer than previously thought.

Summary by Zhijian Zhao and Yongrong Qiu of the Euler Lab - the Ophthalmic Research Symposium May Edition was hosted by Prof Thomas Euler