The process of optical information processing in the retina of the human eye is incredibly energy-intensive, especially when considering the relatively low weight of the retina. The retina plays a critical role in converting light into electrical signals, which are then sent to the brain for interpretation. In fact, it is one of the most energy-consuming processes in the body. However, for over half a century, the scientific community has largely adhered to what is known as the “efficient coding hypothesis,” a theory that assumes that the retina’s primary task is to process visual information in a way that conserves energy. This implies that as few nerve cells as possible should be active at once, thus maximizing the overall energy efficiency of visual processing.
According to the efficient coding hypothesis, when interpreting visual stimuli, the retina should transmit only the essential information required to understand an image, with the minimum amount of activity among nerve cells. This idea is rooted in the concept that conserving energy is paramount for the nervous system, allowing it to focus its resources on processing crucial information rather than generating excessive activity.
For over 50 years, this hypothesis largely shaped our understanding of how vision works. It suggested that, in a typical scenario, the retina’s nerve cells would stay dormant, only activating to relay necessary visual signals to the brain when necessary. Therefore, it was assumed that when stimuli such as light or shadows pass through the visual field, retinal nerve cells should selectively respond in a way that highlights critical information such as edges, shapes, and contrasts, while minimizing background details or redundancies.
However, recent research led by Professor Dr. Tim Gollisch and his team at the University Medical Center Göttingen in Germany has called into question whether the efficient coding hypothesis is universally applicable to all nerve cells in the retina. In this groundbreaking study, the team found that the coordinated behavior of retinal nerve cells does not always align with the assumptions of the efficient coding hypothesis. By observing retinal tissue preparations, they discovered that large groups of nerve cells often activate simultaneously, a pattern that goes against the idea of minimizing activity to conserve energy. The cells didn’t transmit unique or individual signals, but instead fired in unison, sending the same signals at the same time.
This synchronized activity of retinal cells might appear counterproductive from an energy efficiency standpoint, as more nerve cells should theoretically minimize the energy cost of transmitting visual information. However, the researchers found that this group activation is not random. Instead, it is associated with distinct visual features such as high-contrast images or directional movement within the visual field.
One of the more interesting discoveries from their research is the possibility that coordinated cell activity allows the brain to more easily detect particularly important visual signals, such as sharp contrasts or movement. These key features might stand out more starkly from the surrounding visual noise, offering the brain useful distinctions that could facilitate quicker and more accurate processing of critical elements of a scene.
Furthermore, this coordination could help the visual system maintain efficiency in another way, not by limiting neural activity, but by responding very quickly and briefly to salient stimuli such as a sudden contrast change or the movement of an object. Thus, the increase in neural activity may not be about generating excessive signals but about allowing rapid, brief bursts of action when needed to discriminate meaningful visual signals quickly, which would still contribute to energy efficiency by minimizing prolonged signaling.
The broader implications of this research also extend into the realm of therapeutic treatments, particularly for those affected by blindness due to retinal degeneration. In cases where light-sensitive photoreceptors in the retina have died, leading to blindness, the absence of these photoreceptors hampers the visual process, as these cells are the first to respond to environmental light and convert it into signals that travel along the optic nerve to the brain. Without these photoreceptors, the retina no longer has a way to process visual information, leading to a breakdown of vision.
However, recent advancements in technology, including the use of prosthetic devices, offer potential for restoring some vision in patients who have lost their retinal photoreceptors. Researchers like Dr. Dimokratis Karamanlis, who was a postdoctoral researcher at the University Medical Center Göttingen, have been exploring ways to utilize artificial stimulation to activate the retinal nerve cells directly in the absence of functioning photoreceptors. This effort is part of a broader trend toward developing optogenetic therapies. The crucial challenge, however, is how to generate neural activity patterns that mimic the natural functioning of a healthy retina as much as possible. The discovery of group coordination in neural activity may have significant implications for the development of these technologies. When retinal prosthetics are applied, it will be essential to ensure that the artificial activation of retinal nerve cells is done in such a way that preserves the specific, coordinated activities found in the healthy retina. This would allow the brain to accurately interpret the signals it receives, mimicking natural vision as closely as possible.
The study’s findings, which have been published in Nature, highlight that different retinal nerve cell classes exhibit varying activity patterns, with some closely following the efficient coding hypothesis and others defying it. These variations, especially the coordinated responses in groups of cells, could potentially serve as essential clues for designing therapeutic approaches in the future.
In a practical sense, these findings directly influence research at the recently established Else Kröner Fresenius Center for Optogenetic Therapies in Göttingen. This interdisciplinary center aims to introduce light-sensitive proteins into nerve cells in the retina, providing a means to restore or enhance vision in certain forms of blindness. Understanding how groups of nerve cells typically work together to process high-priority visual cues like contrast or motion will aid in tailoring new therapeutic strategies. By studying the specific patterns that must be reproduced in optogenetic treatment, researchers can develop methods that will stimulate retinal cells in the most natural, coordinated ways, enabling patients to experience vision with a greater degree of authenticity.
The discovery that certain retinal nerve cells operate together to detect particularly important visual cues is likely to be a breakthrough in not only understanding vision but also improving treatments for those with retinal damage. Patients suffering from retinal degeneration may one day benefit from retinal prosthetics designed to stimulate groups of cells in a way that mimics natural, coordinated activity. Future research will likely focus on mapping these patterns and refining optogenetic technologies so that artificial visual signals can be processed in a biologically relevant manner, improving the accuracy with which patients perceive their environment.
Reference: Dimokratis Karamanlis et al, Nonlinear receptive fields evoke redundant retinal coding of natural scenes, Nature (2024). DOI: 10.1038/s41586-024-08212-3