Why do we see the world stable even when we move our eyes?

While reading this text, your eyes jump over the line, pause briefly, and then move on to the next jump. Such eye movements — so-called saccades — do not only occur when reading. We humans and other primates perform them two to three times per second, usually without being aware of it. Most remarkable, however, is that we do not perceive the resulting movement of the image on our retina. Rather, the environment appears stable. How can this be?

First of all, we should clarify the question of why we move our eyes so often at all — much more often than the heartbeats. This is because the retina is organized differently than, for example, the photosensitive chip of a cell phone camera. At the back of our eyes, photoreceptors convert light from the environment into neuronal signals that are then transmitted to the brain. However, the receptors are distributed very unevenly. Only in a small area, the fovea, are they so close together that they produce a high-resolution image. Therefore, we have to constantly change our line of vision to see all the details in the room, and we have to do this extraordinarily fast. If we were to swivel the mobile phone camera back and forth at a similar speed, all we would see in the recorded video would be a colorful veil.

But why does everything appear unshaken and clear to us while the image of the surroundings races across our retina at such high speed? Neuroscientists have been asking this question for more than 150 years. The mystery has still not been finally solved.

According to current research, several processes are involved in this perceptual phenomenon, known as spatial stability. Firstly, vision is actively suppressed at the moment of rapid eye movement. Although we are not blind at this time, processes in the brain that have not yet been understood lead to some nerve cells being less active than when we look at an object calmly.

As other studies have also shown, neurons in certain brain regions anticipate the effect of saccades on the visual impression. Individual nerve cells only respond to a small area of the visual field, which is also called their receptive field. Only the sum of all neurons thus covers the entire visual field. Recently, animal experiments have shown that some cells in the parietal lobe of the cerebral cortex shift their receptive field to their new location in space even before the beginning of a saccade. They can virtually see into the future: The neurons respond to stimuli at the location to which they should only be sensitive shortly before the saccade begins. This process probably makes a decisive contribution to the phenomenon of spatial stability. However, how these cells do this is still not understood and is the subject of intensive research.

Some neurons can see into the future

According to a new theory, there could be something else behind it: Possibly the brain is permanently calculating the position of the image on the retina with the position of the eyes in the head. Such a coding would be stable relative to the position of the head or the rest of the body. However, further investigations are needed to prove this with certainty.

The puzzle of spatial stability is one of the greatest of all neurosciences. It will therefore probably take a while to solve it finally. Until then, the fascination with this seemingly simple and yet so complex achievement of our brain remains.