Nerve cells as team players
Göttingen researchers explain how the eye recognizes motion: Special nerve cells in the eye indicate a shift in the line of sight. Groups of these cells contribute to the recognition of small eye movements and help to keep the viewing direction stable so that the image of the environment becomes clear.
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/UMG/ BN, Duppé/ To take a sharp picture, the camera must be kept still, otherwise the image or the edges of objects are blurred. The same applies to our visual system. To perceive our surroundings clearly, our line of sight must be fixed, even if we are in motion. To do this, the brain constantly monitors the movement of the viewing direction in order to be able to counteract with eye movements if necessary. This system is malfunctioning in various disorders of the visual system, leading to uncontrolled eye movements, often called nystagmus. One of the results is poor visual acuity.
Nerve cells in the visual system play a central role in monitoring the line of vision. These cells are activated when the image that falls onto the eye shifts in a certain direction. Their signals can thus act as triggers for counteracting eye movements. Norma Kühn and Tim Gollisch from the Department of Ophthalmology at the University Medical Center Göttingen (UMG) discovered such nerve cells in the eye of salamanders some time ago. Now they can show how these nerve cells represent the movement of the environment through their activity.
The researchers found that the cells were unable to perform their task individually because they are not only activated by motion but also by simple changes in brightness. In order to distinguish real motion from brightness changes, the nerve cells must therefore act as a team. In the manner of real team players, they not only pass on information about the movement they perceive, but also supply a correction signal for neighboring cells. Thus, downstream nerve cells receive important information with which to process motion signals and changes in brightness separately.
“It is not the signals of individual cells that indicate the direction in which a movement has taken place. It is rather the difference between the signals of two cells which is decisive,” says Norma Kühn, first author of the publication. Groups of nerve cells transfer more information about the observed motion because they correct each other. “It’s a clear case of ‘the whole is more than the sum of its parts’, an example of ‘synergy’ between nerve cells,” says Professor Tim Gollisch, senior author of the publication. “This synergy between nerve cells is further increased by the simultaneous, that is to say synchronous, activity of the nerve cells. This leads to a more precise representation of the direction of motion,” said Kühn. “We conclude that coordinated activity in groups of nerve cells plays an important part in the visual system.”
Insights can help in the development of visual prostheses
According to Tim Gollisch, these findings are not only interesting from a basic research point of view. In the long term, he hopes that these findings will be incorporated into the development of visual prostheses. Such sensory prostheses use artificial activation, for example via small electrodes in the eye, to restore visual function after people have turned blind because the light sensors in their eyes have died. “The results indicate,” said Gollisch, “that not only must the correct cells be activated, but that the relative activation of different cell classes is also important so that the brain can correctly interpret the information coming from the eye.”
Translation of the German text from the University Medical Center Göttingen, Georg-August-Universität
>> original press release (German text)
Kühn NK, Gollisch T (2019). Activity correlations between direction-selective retinal ganglion cells synergistically enhance motion decoding from complex visual scenes. Neuron 2019 Jan 17. pii: S0896-6273(19)30004-2. doi: 10.1016/j.neuron.2019.01.003.