Panoramic Environments Influence the Functional Organization of Retinal Cells

April 7, 2023 feature

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by Ingrid Fadelli , Medical Xpress

Existing neuroscientific models propose that the visual system functions similarly to a camera, encoding the positions of different objects. However, an animal's surroundings are constantly changing and these changes could affect the processing of visual information.

Recently, researchers at the Institute of Science and Technology in Austria and LMU in Germany provided evidence supporting this hypothesis, demonstrating that the organization of neurons in the mouse retina is influenced by panoramic visual statistics, such as non-uniform light levels. The team's findings have been published in Nature Neuroscience and could significantly contribute to the current understanding of the visual system and its evolution.

Maximillian Jösch, one of the researchers involved in the study, commented, 'A key feature of every living organism is adapting to its environment to survive. Such adaptations should also occur in the computations performed by the brain, to extract relevant and dismiss less critical information. We set out to test this idea by taking advantage of the most prominent visual changes observed systematically in nature: the gradient of light intensity and contrast levels from the ground to the sky to ask if the mouse visual system evolved to consider these constraints.'

To examine the organization of sensory space that activates each neuron in the mouse retina (receptive fields) in relation to the scenes observed by mice, Jösch and his colleagues developed a new optical imaging technique. This technique enabled them to measure and track the activity of thousands of neurons in a single retina simultaneously.

Jösch explained, 'When a retina neuron is active, sending electrical pulses to the brain, ions flow inside the cell, e.g., calcium. We can visualize that activity by adding a fluorescent indicator in each neuron. When calcium flows in, the fluorescence changes. These changes in fluorescence can be recorded with a sensitive camera, and with that, we can infer how the neuron responds to different visual stimuli across the entire retina.'

The researchers conducted their experiments on extracted mice retinas, which, like most mammals, do not include the small area known as the fovea, a small slump in the retina that allows humans and other primates to see at high definition. The fovea, which makes up less than 1% of the entire human retina, is known to play a key role in the visual perceptions of which humans are more conscious. The remaining 99% of the human retina contributes to visual perceptions, from which many appear to be unconscious processes. Thus, from a human-centric perspective, this study focuses on the processing happening in the latter 99%.

The researchers found that the computations performed by neurons in the mice retina changed depending on the panoramic visual statistics of that part of the retina usually exposed to daylight. This supports their initial hypothesis that the visual system is not inherently homogenous and is in fact adapted to the external environment.

'To our surprise, we found that retinal neurons are more likely to inform the rest of the brain when a stimuli change is unexpected,' Jösch said. 'Importantly, the unexpected depends on where the neuron looks, either the sky or the ground. Thus, retina circuits systematically adapted their properties from the lower to the higher visual field to represent the world more efficiently.'

Overall, the findings gathered by this team of researchers suggest that the panoramic structure of natural scenes affects the organization of different processing strategies in different regions of the retina. This expands previous models of the visual system, highlighting its adaptive and dynamic nature.

'We usually assume that the visual system is homogenous, or in other words, that the visual world is represented like a camera, measuring each position similarly,' Jösch added. 'However, our natural surroundings are not similar; they systematically change from ground to sky. Thus, a system that evolved to live in nature should consider this. Our results indicate that living organisms' visual system has adapted to cope with natural constraints to improve the efficiency of their neuronal code.'

In the future, the recent work by Jösch and his colleagues could inspire other teams to further examine how panoramic statistics or other visual elements shape cell organization in the retina to refine our understanding of vision in general.

'We are now exploring how similar adaptations change when changing the context, for example, when adapting to different light levels occurring during the day or at night,' Jösch added.

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