A dummies’ guide to the five basic senses

Touch

Touch is, in many ways, our most important sense. Deprive an infant of sight or hearing, and it will still develop normally. According to ‘Touch’, a book by neuroscientist David Linden, a baby deprived of physical contact in its early years will face a life of high stress levels, anxiety and fear. It doesn’t only help us understand our physical surroundings. We use it to express emotional contact, to ease loneliness and to share in joys and sorrows.

The sense that we call touch is a variety of complex systems which work in unison to transmit information from our skin, our muscles and even our internal organs to the brain. Different kinds of nerve endings detect different kinds of information in specific ways. And this complexity makes designing technologies to enhance, augment or mimic touch fiendishly complex. Nevertheless, scientists and researchers are creating ground-breaking technologies that aim to do just that.

Sight

Our eyes are so complex that “their evolution seems absurd to the highest degree”, said Charles Darwin in his 1859 book ‘On the Origin of Species’.

Yet evolved they have. Did you know that our trilobite ancestors from 544 million years ago didn’t have eyes at all? However, fossil records show that, following the Cambrian explosion, two profoundly different types of eye developed: the compound eye, which we see today in insects, spiders and crustaceans; and the camera eye, which can be found in humans and large animals.

The camera eye collects light and converts it into an electrical signal that the brain translates into images. The retina then takes the place of photographic film – its photoreceptors convert light into electrical signals that propagate through the retinal neural network before being sent through the optic nerve to the brain for further processing.

It’s one of the human body’s most complicated organs – second only to the brain – so it’s no wonder that our understanding of the nature of sight is only just beginning to advance.

Yet new developments bring huge promise: new breakthroughs in human and machine vision are happening at breakneck speed, as are innovations that bring the two together.

Smell

Possibly the most maligned sense is smell. Its role to warn us not to eat, drink or go near something overshadows its positive role, for example enhancing the taste sensation during a meal or activating memories.

All scent messages are transmitted via olfactory pathways to the olfactory bulbs which are located at the base of the brain, above each nasal cavity. Smell has a direct connection to the brain’s limbic system, the area responsible for memory and emotion. Associated learning connects a particular smell to an emotion, which means smell can alter the ‘smeller’s’ perception of the ‘smellee’s’ attractiveness, age, and even personality.

There are 400 functional olfactory genes in the human body and 465 redundant pseudogenes, showing what went before. The gene:pseudogene ratio in rats and mice is far higher than in humans, a sign of how human evolution has led us to rely on other senses for survival.

Taste

Taste (gustation) is one of the senses that determines the flavour of food. The tongue is the main organ responsible for taste in humans, being covered with thousands of taste buds, which contain taste receptors. Our sense of taste is stimulated by the chemical reaction of substances with these taste receptors and perceived by the gustatory cortex.

Humans can sense five basic tastes: sweetness, saltiness, sourness, bitterness, and savouriness (also known as umami), with starch suggested as a possible sixth. These represent distinct chemical reactions e.g. saltiness is sensed in the presence of ions of alkali metals, principally sodium.

Taste plays an important role in survival. Sweetness helps us identify energy-rich foods, bitterness helps us avoid potential toxins, and sourness helps us avoid dangerous concentrations of acid.

Sound

Hearing is the sense that allows people to absorb and process sound waves through the ear and intricately interpret them in the brain. It’s the sense most pivotal for communication. Without it, humans can’t understand speech, the primary way people converse, and one of the quickest ways to convey information. It helps people decipher the world around them, alerting them to dangers, such as the bike hurtling up behind; through someone’s tone of voice they might know if they’re a friend or a foe.

Sound can pique emotions. Music, movies and the emotion in someone’s voice can make a person feel happy or sad or evoke a specific memory. The famous composer Ludwig van Beethoven, who became deaf by the age of 44, summed up the loss of this vital sense as feeling ‘banished’ because he could no longer enjoy ‘refined conversations’ and ‘mutual exchange of ideas’, and of course, music.