Senses with Purpose

Sight and hearing are innate to us and therefore seem completely natural. But if we look inside the brain, we see that our sensory systems are extremely sophisticated and were painstakingly acquired over the course of evolution. The ancestors of humans and animals could not simply settle comfortably into their ecological niche. Rather, Adaptation was a constant necessity. When danger loomed – for example, through a crack or a rustle – it was a real survival advantage to know from which direction the sound was coming. For us, it’s easy to locate a sound source, at least approximately. But if we had to perform the necessary calculations that our brain carries out to achieve this, we’d probably break a sweat. 

A straightforward principle that the auditory system can use for this purpose: Sound takes different amounts of time to reach the left and right ears. The Ear facing the sound receives the same sound louder and sooner than the ear facing away from it. The brain uses this time difference to localize the sound. In the process, neurons in the medial superior olive (MSO) generate a Neural representation of the acoustic space. This core region is located in the ascending Auditory pathway in the brainstem of mammals. “When a sound comes from the right, the left medial superior olive is primarily activated,” says neuroscientist Benedikt Grothe of LMU Munich. “If a sound comes from the front, both the left and right medial superior olives are activated equally.” 

However, as Grothe and his colleagues have discovered, the matter is actually a bit more complicated when you look at the details. This was revealed to them by the Mongolian gerbil. In this species, a negative feedback loop via another core region ensures that the previously dominant medial superior olive reacts less strongly a few milliseconds later. “If, in our example, a new sound comes from the front, the right medial superior olive will respond most strongly, and the sound will be incorrectly localized to the left,” says Grothe. 

At first, Grothe and his colleagues were skeptical of this finding.
Ultimately, their measurements showed that mammals systematically mislocate a second sound. Could that be true? Indeed: The mislocalization could also be demonstrated in human subjects. It is reasonable to assume that the feedback loop exists not only in gerbils but in all mammals.

“Mammals perceive the acoustic space around them not absolutely, but relatively,” says Grothe. And thus differently from birds. This is because in birds, different nerve cells are activated depending on the direction of the sound source. “The neurons in birds are arranged systematically, creating a topographical map of the acoustic environment,” says the Munich-based neuroscientist. Such maps are found in the brainstem and Midbrain of birds. As a result, birds are able to precisely locate the source even when multiple sound events occur in succession. In mammals, however, there are no topographic neural maps of the auditory space.

But how can evolutionary biology explain why mammals systematically make errors in sound localization? “We are descended from tiny, nocturnal mammals,” explains Benedikt Grothe. When mammals first developed ears, they were not hunters but prey – for many, many millions of years. “To run away in the opposite direction, they didn’t need to locate the predator with 1.5-degree precision, as an owl, for example, can.” Birds, on the other hand, are descended from dinosaurs that had to pinpoint their prey’s location to be successful. In mammals, the corresponding selective pressure was absent for millions of years after the development of ears. “Moreover, mammals don’t simply make mistakes,” emphasizes Benedikt Grothe. “In fact, their spatial resolution shifts dynamically depending on the listening situation. And that, in turn, is helpful for localizing sound sources.”

Just how sophisticated hearing is in the brain is also evident elsewhere: when one considers how complex environmental stimuli are represented in the brain. Neurophysiologist Arthur Konnerth of the Technical University of Munich is interested in the cellular basis of what is known as “sparse coding.” Put simply, the question is why, for example, in the auditory cortex, only very few neurons respond to a specific stimulus with large electrical signals, says Konnerth. “Different, small groups of other neurons respond to other stimuli.” Konnerth is investigating the fundamentals of this phenomenon using high-resolution two-photon network calcium imaging, an imaging method he originally developed himself with colleagues. “This allows us to simultaneously measure the activity of numerous cells in the Cortex within the intact brain of the experimental animal.”

In a study published in 2020, a team led by the Munich-based neurophysiologist had mice listen to chords consisting of three or four notes multiple times, thereby memorizing them. After the training, the researchers observed that a few neurons responded specifically to the entire chord with intense firing. However, there was no significant response to the individual notes of the chord. Complex sensory stimuli are apparently represented specifically by individual neurons in the auditory cortex. 

Much like sound localization, sparse coding may also be innate. In ongoing studies, Arthur Konnerth and his colleagues are testing whether sparse coding is innate or learned. There are already initial indications – though not for the auditory cortex, but for the Visual cortex Early in its development, a small number of neurons respond specifically to previously unknown light stimuli with large electrical signals. Konnerth’s hypothesis: “Perhaps this is an indication that sparse coding, at least in its basic form, is established early on and forms a framework for later brain development.”

One trick definitely drawn from evolution’s experimental toolbox is the ability to keep our gaze steady while we move. It is generally assumed that the Vestibular system in the inner Ear registers head movement and then sends signals to the Eye muscles, causing the eyes to move and compensate for the head movement. For example, if you move your head to the right, your eyes move to the left. However, neurobiologist Hans Straka (1961–2022), formerly at LMU in Munich, discovered that – at least in the case of very rhythmic and uniform movements – the signals for stabilizing gaze do not come primarily from the vestibular system, but directly from the spinal cord.

To get to the bottom of evolution’s tricks, Hans Straka and his colleagues had to use a trick of their own. They developed an isolated skull specimen from clawed frog tadpoles. “The advantage is that we can remove everything we don’t need,” said Straka. “We leave the Spinal cord intact, and this specimen continues to generate locomotor signals in the spinal cord for left-to-right swimming movements of the tail.” At the same time, the researchers can record eye movements with a camera. They mounted the specimen on a motion simulator. They also built a screen around the specimen that allows them to display stripe or dot patterns. “This gives us both rhythmic locomotion and sensory signals from the inner ear in the specimen.” 

The tadpole’s head moves from side to side during swimming movements. The adult frog, after metamorphosis, moves forward in jerky bursts using its legs. As a result, the Eye movements of tadpoles and frogs differ. Straka and his colleagues found that the nervous system is so plastic that gaze stability is maintained even after metamorphosis into a frog.

The stabilization of the gaze makes evolutionary sense. When our vertebrate ancestors began to move freely and perceive other animals, they faced a problem: They did not know whether a perceived movement was caused by their own movement or by the movement of another animal. “That is why eye muscles evolved that keep the gaze and the image of the surroundings stable,” says Hans Straka. “They do this by making the eyes perform counter-movements whenever they move. A stable image of the surroundings is necessary in order to analyze it.”

Whether it’s locating sound or stabilizing vision: mammals, birds, or amphibians – they’ve all gone through the school of hard knocks that is evolution. And depending on their ecological niche, they have all acquired sophisticated sensory and neural strategies to survive. 

Further reading

Wang , M et al.: Single-neuron representation of learned complex sounds in the auditory cortex. Nat Commun, 2020 Aug 31;11(1):4361. 

Original publication on December 18, 2021
Last updated on April 25, 2026