Ready to receive on multiple channels: sensory illusions

From the stone age, human beings have been equipped with the sensory organs that they need for survival. Today, these fantastic organs are still relevant for us in navigating the challenges of urban environments.

By Roland Knauer

In the second part of ‘Ready to receive on multiple channels’, our author explained how taste, touch and sight work. For example, human eyes often register an eagle in the sky as just a black dot as these birds of prey with a wing span of over two metres fly so high. However, from up there the eagle with its razor-sharp eyesight has no problem spotting a squirrel, which weighs just a few hundred grams and is much smaller than the eagle.

Human eyes are nowhere near as sharp. After all, having eyes like a hawk would be of little use to us as we rarely need to spot things from such heights and would not be able to catch the squirrel anyway as we lack the eagle’s ability to swoop down through the air.

The sense of sensory illusions

Instead, humans are able to recognise shapes incredibly well and can see in 3D, allowing us to easily determine whether the animal stalking through the grass a few metres away is tasty prey or a dangerous predator. This ability to recognise shapes works so well that when we look at a picture of three circles with a segment cut out of them and three bent lines that are arranged cleverly, we see a triangle, even though there is no actual triangle to be found.

This effect, known as Kanizsa’s triangle, is just an illusion in which the brain connects the shapes in such a way that it sees a familiar structure. However, sensory illusions can be useful in certain situations. For example, if someone sees a hand sticking out of murky water they will grab it, as the brain completes the picture of a human, who they know is in danger of drowning. The areas of the human brain that process visual information are therefore remarkably large. The areas responsible for hearing, by contrast, are a little smaller, although this sense is also very important.

This ratio is exactly the opposite in dolphins. These marine mammals are unable to see very far even in clear water so – like bats which are unable to rely on their eyesight when hunting at night – they have developed a way of seeing with their ears. They make high-pitched noises which are able to travel long distances and use the echoes in a similar way to visual stimuli to complete the picture. As dolphins also use these noises to communicate, the area of the brain responsible for hearing is much larger in these animals than in humans.

Language and vibration

Dogs, on the other hand, have ultrasonic hearing and are able to hear sounds with frequencies of 50,000 Hertz while the hearing range of humans only goes up to 20,000 Hertz. This is why a dog can hear an ultrasonic whistle used by its owner, even though the owner themselves hears nothing. However, humans have fantastic hearing in the range between 2000 and 4000 Hertz. ‘This is the range that our language lies in,’ explains Hermann Handwerker.

Our senses perform best in the areas where they are needed. For this reason, we are also able to ‘hear’ with our skin. The skin contains sensory cells that measure vibrations with frequencies of several hundred Hertz. ‘This allows us to feel the stamping of a herd of animals in the far distance,’ Hermann Handwerker summarises. This sense might not be particularly useful during a shopping trip in the city but for hunters in the Stone Age it provided important information about which general direction it would be best to go in on their search for prey.

An adaptive filtering system

Some of our senses work flat-out without us even noticing. For example, we have sensory cells that measure how much the carotid artery stretches with each heartbeat. This information helps to keep blood pressure relatively constant without us consciously doing anything. Other sensory cells in the stomach register when it becomes too full and, if worst comes to worse, reverse the process of food entering the stomach. This sense also reacts to certain molecules that occur in the case of poisoning, preventing the worst by causing vomiting.

However, this effect can also occur when the vestibular system receives strong stimuli. This is why some people get seasick in rough conditions or carsick on winding mountain roads. Our brains are constantly flooded with information from our sensory organs, which would probably result in chaos if important signals weren’t separated from unimportant ones automatically. This filtering system adapts to the current situation, ensuring that a breastfeeding mother is woken by the tiniest noise from her baby although she would have slept through louder noises before the birth.

The opposite also happens, as people can get used to their partner’s regular snoring, allowing a couple who have been married for years to sleep peacefully. Unless the snoring stops – then the other person wakes up immediately and can save a life if the snorer has stopped breathing. Once again, the human senses are perfectly adapted for the situations we face in life.