Senses in Motion

It seems so simple. In, out. A stimulus comes in, a response goes out. For example, if a ball flies straight toward us, nerve cells in the Retina of the Eye are activated. These cells transmit the information in the form of electrical signals to the brain, which then processes the information: the ball, its direction of flight, and its speed. It then sends commands to the muscles. In response, our arms and hands move, and we catch the ball.

This chain of processing separates the sensory system from the motor system and seems straightforward at first glance. But as is so often the case, the reality is more complicated. This is because both systems interact early on in neural processing. It’s not just the sensory system that influences the motor system; the motor system also influences the sensory system. 

One thing becomes clear here: The brain doesn’t just react – it acts. It tries to look into the future. “Our brain is constantly making predictions about what input from the sensory organs – such as the eyes – will be like,” says neurobiologist Mark Hübener of the Max Planck Institute for Biological Intelligence in Martinsried. “Many neurons are busy comparing the expected sensory input with the actual input.” Essentially, this touches on a very old question that the eminent German physiologist and physicist Hermann von Helmholtz already posed in the 19th century: How is it that our image of the world remains stable and does not wobble, even though we are constantly moving our eyes and head, causing the image on the retina to shift constantly?

If you pan a camera back and forth, it results in a blurry image. And in principle, it should be the same with vision. But it isn’t. “The idea is this: The Visual system receives a copy of the movement commands that control the movement of the eyes and head,” says Hübener. Researchers refer to this as an efferent copy. “In this process, the sensory input expected based on the strength and direction of our eye and head movements is compared with the actual input,” explains the neurobiologist. If there is no difference, our image of the world remains stable. This effectively filters out our own movements from the visual input. Incidentally, it is possible to trick this system. If you gently press your eyelid with a finger to carefully move the eyeball back and forth, the world – which is actually static – suddenly appears to move. 

To counteract eye or head movements, the brain also relies on compensatory movements. This is being researched by biologist Boris Chagnaud from the University of Graz. “The efferent copies inform the visual system when and how our head is moving, and the oculomotor system then compensates for these movements,” says Chagnaud. The oculomotor system specifically controls the movements of the eyes. “It moves the eyes in the opposite direction of the head, thereby keeping the gaze stable,” says the biologist. And the Spinal cord also plays a role. When walking, we always move slightly up and down, while simultaneously swaying a little from side to side. As a result, the position of the eyes also changes. “The spinal cord helps to compensate for these passive eye movements,” says Boris Chagnaud. It sends copies of the motor signals to the oculomotor system, among other systems. The idea here is always to keep the impact of movements on the sensory systems as low as possible. 

In the process, our own nerve cells sometimes really trick us. This becomes evident when reading. We make purposeful Eye movements to focus on sections of a line. However, rapid eye movements – called saccades – don’t align well with sharp vision. Therefore, efferent copies inform the Visual system about the upcoming eye movements. “Even though we don’t notice it: During saccades, a part of the brain – the so-called frontal eye field – blocks out the moving visual information,” says Boris Chagnaud. We don’t notice this, so we don’t see individual frames, but rather, in a sense, a continuous film. “In this way, the brain constructs an impression of the environment that is not perceived by our eyes in this manner,” says Boris Chagnaud. 

The extent to which the motor system influences the Visual system becomes apparent when the lights go out. Mark Hübener, together with colleagues, demonstrated that motor signals appear in the Visual cortex of a mouse’s brain during movement – even when the scientists had turned off the lights or when both retinas of the test animals were defective.

Neuroscientist Georg Keller from the Friedrich Miescher Institute in Basel, together with colleagues, discovered that motor signals travel from the Anterior cingulate cortex and the secondary motor Cortex to the visual cortex. Both areas are involved in controlling movement. “These signals could be a neural correlate of the efferent copy,” speculates Mark Hübener. In this way, the visual cortex is supplied with information about how much the visual world is likely to change as a result of the mouse’s movements. 

“Another possibility is that expectations are derived from the experiences we have throughout our lives.” In that case, the visual system would not need a direct copy of the motor commands to generate expectations. Hübener gives an example: “We’ve already experienced millions of times how much the visual world shifts with a certain Eye movement. So we don’t need to generate an efferent copy from scratch every single time.” But of course, this learned experience must also be transmitted to the visual cortex via action potentials. 

Yet the motor system apparently already has a say at a processing stage earlier than the Visual cortex At least that is what the research by Laura Busse, a neuroscientist at LMU in Munich, suggests. She measured the electrical activity of neurons in the Lateral geniculate nucleus (LGN) of the thalamus – a part of the Visual pathway – in mice. “On average, the cells in the LGN responded more strongly when the mice were moving,” says Busse. “In addition, the cells’ ‘volume control’ – much like on a radio – was turned up higher, so that the neurons could react with greater sensitivity.”

One possible explanation: motor activity influences sensory activity even at the level of the thalamus. Laura Busse, however, remains cautious. Another explanation is also conceivable. When a person moves, they are generally more activated, leading to increased release of neurotransmitters such as acetylcholine, which influence the activity of nerve cells. “So it could be that it is not the motor influence that is responsible for the stronger response of the neurons in the LGN, but rather the generally higher state of arousal – or a combination of both.” 

Mark Hübener, on the other hand, goes even a step further. There are even tentative indications that, as early as the level of the retina, the activity of neurons is influenced by movement. For Hübener, all these findings point to one conclusion: “The comparison of expected and actual sensory input does not take place at just one location in the brain, but at all possible levels of processing.” This comparison is important for the brain. After all, if the world changes in a predictable way due to one’s own movements, that is largely irrelevant. “If, on the other hand, there are more significant changes in the world – for example, because another animal is moving toward me and attacking me – then the brain and the organism must react.” 

Further reading

• Leinweber, M. et al.:  A Sensorimotor Circuit in Mouse Cortex for Visual Flow Predictions. Neuron 2017 Dec 6;96(5):1204.
• Straka, H. et al.: A New Perspective on Predictive Motor Signaling. Curr Biol, 2018 Mar 5;28(5):R232-R243