The ability of our eyes and brain to work together to encode the direction in which images are moving is fundamental to our sense of vision. Researchers have now identified how our eyes process information about the direction of moving stimuli.
Imagine you are walking down the street. Even though your eyes are looking straight ahead, the images your eyes are perceiving are changing step by step. As you move forward, your field of view flows backwards. When you look left, your field of view flows to the right, and when you look up, your field of view flows downwards.
To make sense of this, the brain requires signals to calculate how the field of view is moving.
Researchers only had a vague idea of where these signals originate in the eyes, but new research now reveals that the process is very different from what was previously thought.
“Spatial navigation is important to enable us to orient ourselves in our world – and find our way home again. Five years ago, my research group showed that errors in retinal programming cause various diseases, and in this new study we mapped the mechanism behind retinal programming,” explains a researcher behind the new study, Keisuke Yonehara, Associate Professor and Group Leader, Danish Research Institute of Translational Neuroscience (DANDRITE) and Department of Biomedicine, Aarhus University.
The research has been published in Neuron.
Studied retinas from mice using advanced electron microscopy
Keisuke Yonehara and colleagues wanted to better understand how the eyes and the brain coordinate the perception of the visual field during movement.
The researchers closely examined mice retinas while stimulating them by using a video projector to simulate movement and used fluorescent proteins, electron microscopy and electrodes to record the retinal response to movement.
“We scanned axons that formed synapses with a laser to form an image of their activity. We then analysed the images using image-processing software and found that some neurons and synapses only responded to movement in specific directions,” says Keisuke Yonehara.
Visual information is processed in a surprising location in the neurons
The research primarily shows where visual information is initially processed.
Previously, researchers thought that this took place in the last link of the neurons to the retina – before the signal is sent on to the brain.
However, the new study shows that the information is processed much earlier in the visual pathway –in the stretched nerve fibres of neurons called axons.
Neurons have extensions called dendrites, which receive electrical stimulation from other cells. The signals from other cells are transmitted through the axons. The researchers thought that dendrites receive and process signals and that the axons transmit these signals.
“Until recently, we thought that the axons only transmit signals, but we found that they also receive signals and are involved in calculating the direction in which images are moving and how we perceive our own motion through the visual field,” explains Keisuke Yonehara.
Potential to cure eye diseases and create artificial brains
According to Keisuke Yonehara, these results are important because they provide greater insight into how we visually perceive the world around us, including having positive interactions with inanimate and animate objects (and the ground too!) and being able to recognize places we have visited previously.
Three types of retinal neurons are involved in orienting us in our environment.
Photoreceptors capture light from the surroundings and form a signal and send it to a type of neuron called interneurons. They then transfer the visual information from the photoreceptors to the inner retina, where ganglion cells form the output to be sent to the brain.
The new research shows that direction selectivity from the environment is already being processed in the axons from the interneurons.
“Researchers thought that signal processing only takes place in the dendritic part of neurons, and our discovery therefore represents a completely new mechanism by which the neurons process signals. This mechanism may be more common in the interactions between neurons than we previously thought, and it should therefore also be investigated further in the future,” says Keisuke Yonehara.
Keisuke Yonehara adds that the results are not only interesting as basic science but can also be applied to understanding and developing treatments for many eye diseases and to improve understanding of how artificial brains could be created using electrical connections.