Sense and Sensibility

By Brian Clegg

Here’s a question. How many senses do humans have?

There are plenty of senses we don’t have that other organisms do. For example, some birds appear to be able to detect the Earth’s magnetic field, while some aquatic creatures, like sharks, can detect the electrical fields emitted by the nervous systems of their prey.

Other organisms make use of the familiar senses in different ways. Some, for instance, can see infra-red or ultra-violet, while bats use their extraordinary auditory echo-location system to navigate.

Sensory input is usually modified by the brain to produce an unreal but helpful model. So, for instance, what we see is a construct from the input of optical nerves, not a camera-like picture. You can see this particularly strongly with optical illusions – check out my favourite one. Other animals may have different senses integrated into such a picture – dogs, for instance, may have their sense of smell more strongly integrated with sight than we do.

There isn’t a precise number that’s recognised, but give yourself a point for anything between nine and twelve, because it certainly isn’t just five. That’s all the fault of Aristotle  but we still get taught it in school. Think, for example, of the sense that tells you which way up you are. Or proprioception, the sense that gives you an awareness of the location of parts of your body, so you can touch your nose with your eyes closed.

For that matter, we have an infra-red sense – not as good as organisms that have it built into their sight, but our skin has crude infra-red detectors so that you can tell, for instance, if an iron is switched on when it is several centimetres from your hand. And then, like a smartphone, you have an acceleration sensor, in this case in your inner ear, that makes an important contribution to your sense of balance. So that’s at least nine senses.

One extra sense that many would add is the sense of pain. The burning sensation when biting into a strong chilli, or the pain you feel when a needle is pushed into your arm, seem related to taste and touch respectively – and certainly they are transmitted by the same receptors. But arguably there is a significant difference between the warning sense of pain and the other uses those sensors are put to. You can also, of course, experience internal pain – and most of us tend not to think of touch or taste being a sense that is attributed to our internal organs.

Image from Unsplash by Colin Lloyd

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Now Appearing is the blog of science writer Brian Clegg (www.brianclegg.net), author of Inflight Science, Before the Big Bang and The God Effect.