The processing of optical information in the retina of the eye, where the light-sensitive photoreceptors and the first nerve cells are located, is one of the most energy-intensive processes in the organism, especially in view of the low relative weight of the retina.

For more than 50 years, the so-called “efficient coding hypothesis” has determined the scientific understanding of visual processes in the eye. It states that it is the retina’s task to process visual information as efficiently as possible in order to conserve energy resources. This means that as few nerve cells as possible should be active at the same time when it comes to producing electrical signals to transmit visual information to the brain.

A team of scientists led by Prof. Dr. Tim Gollisch, research group leader at the Department of Ophthalmology at the University Medical Center Göttingen (UMG), Germany, has now discovered that the efficient coding hypothesis does not apply to all nerve cells in the eye. For a number of cells, the researchers were able to observe in retinal tissue preparations that entire groups of cells are often active at the same time.

This coordinated interaction of nerve cells appears to contradict efficient and energy-saving information transmission, as the individual cells transmit the same signals. The researchers were able to show that the joint activity of the cells does not occur at random, but that certain groups of cells become active at the same time when either high-contrast images come into the field of vision or movements in certain directions are observed.

“This coordinated cooperation of the nerve cells could enable the brain to distinguish particularly relevant optical signals, such as the recognition of contrast or movement, from other less important influences, such as changes in brightness, for example, when a cloud moves in front of the sun, making everything slightly darker.

“On the other hand, the cell groups appear to ensure energy efficiency by reacting particularly briefly to corresponding sensory stimuli,” says Prof. Gollisch, last author of the study.

“The findings offer potential for the treatment of blindness. This applies in particular to blindness caused by degeneration processes, for example, when photoreceptors in the retina die. These take up light from the environment and convert it into electrical signals, which are transmitted by nerve cells to the brain to process the visual information.

“When the photoreceptors die, there is no more signal transmission via the corresponding nerve cells. If these nerve cells are activated artificially, i.e., by a visual prosthesis, it is important to induce appropriately coordinated nerve cell activity so that the brain receives signals that are as true to life as possible in order to interpret them correctly,” says Dr. Dimokratis Karamanlis, former postdoctoral researcher at the Department of Ophthalmology at UMG and first author of the study.

These results have been published in Nature.

Background

One basis of the efficient coding hypothesis is the observation that, for example, when looking at a large white surface, the nerve cells that are mainly active are those that perceive the boundaries of the surface. Cells that recognize the “inside” of the surface are suppressed in their activity in order to save energy. The fact that the inside of the surface, i.e., between the boundaries, is also white, is something that the brain can also work out without these signals.

However, large white areas that are in the field of view for long periods of time are hard to find in actual nature. The researchers therefore tested how retinal tissue samples react to natural photographs. To do this, the pictures were projected onto the samples and moved in a way that corresponds to natural eye movements.

By simultaneously measuring the electrical activity of a large number of nerve cells, the researchers were able to show that certain classes of cells adhere well to the efficient coding hypothesis and react separately from one another. Other prominent cell classes, on the other hand, do not follow the hypothesis and tend to become active together.

Outlook

The findings of the study will be directly incorporated into the development of new therapeutic approaches at the recently founded Else Kröner Fresenius Center for Optogenetic Therapies in Göttingen. In certain forms of blindness, light-sensitive proteins are to be introduced into the nerve cells of the eyes in order to activate these cells with light.

“The results will help us to understand which activity patterns of the cells are necessary for the natural recognition of certain visual impressions. The aim of therapy development will then be to generate these patterns artificially,” says Prof. Gollisch, who is participating in the new center. Corresponding studies with patients in Göttingen are due to begin in a few years’ time.

Provided by
Universitätsmedizin Göttingen – Georg-August-Universität