How many human senses are there?

The human senses, said Aristotle, are a gateway to reality. Back in 350 BC, he defined five ("and only five") senses: sight, hearing, touch, smell and taste. Until recently, many experts believed that still to be true - perhaps with the addition of a "sixth sense" of intuition. However, scientists in the UK, the US and other countries have now learned that humans have more senses than we thought. Some have defined as many as 53!

The senses work together

Most neurologists agree that a sense comprises a group of sensory cells that respond to our surroundings by sending and receiving signals to and from a specific part of the brain. Our senses are rooted in biology and evolution: from the first bacteria swimming in oceans "sensing" food in the water, to early humans scanning their environment for danger and opportunity. The senses help us to regulate and protect our bodies.

Our senses also work together: what one sense perceives can affect another. In 2011, scientists at the University of Southern California (USC) discovered that, when we look at an object, our brain not only processes what we see, but remembers what things feel like as well. So, for example, when volunteers were asked to watch a video of hands touching various objects, it triggered the part of their brain normally associated with touch.

Scientists say that additional human senses include proprioception (awareness of the position and movement of the limbs and body); equilibrioception (sense of balance); thermoception (sense of temperature); and nociception (sense of pain). Some scientists include interoception as an additional sense: the body's ability to recognize internal factors such as hunger, thirst, exhaustion and pain.

Adapting to today's world

Not all "senses" are genuine, however. "Some things ... such as the sense of direction," says Claire Francis of the UK-based Sensory Trust, "are defined by neurologists as ‘post-sensory cognitive activities'." And not everyone experiences senses in the same way, either: people with synaesthesia can see sounds or words as colours, and associate sights with smells.

In our changing world, some of the senses we once used for survival have become weakened or enlarged, which scientists believe can cause anxiety and stress. Human senses adapt to the world we live in, which includes technology. Do you ever feel your phone vibrating or hear it ringing when it actually isn't? If so, you may be experiencing "phantom vibration syndrome". Experts say this phenomenon is quite common among mobile-phone users, but didn't exist before mobiles were invented. It's not always easy to make sense of our senses.