by Dean Burnett
Imagine a police sketch artist, constructing an image of a person from secondhand descriptions. Now imagine it’s not one other person who’s providing the descriptions, but hundreds. All at once. And it’s not a sketch of a person they have to create but a full-colour 3D rendering of the town in which the crime occurred, and everyone in it. And they have to update it every minute. The brain is a bit like that, only probably not quite as harassed as this sketch artist would be.
It is undeniably impressive that the brain can create such a detailed representation of our environment from limited information but errors and mistakes are going to sneak in. The manner in which the brain perceives the world around us, and which parts it deems important enough to warrant attention, is something that illustrates both the awesome power of the human brain, and also its many imperfections.
A rose by any other name …
(Why smell is more powerful than taste)
As everyone knows, the brain has access to five senses. Although, actually, neuroscientists believe there are more than that.
Several ‘extra’ senses have been mentioned already, including proprioception (sense of the physical arrangement of body and limbs), balance (the inner-ear-mediated sense that can detect gravity and our movement in space), even appetite, because detecting the nutrient levels in our blood and body is another sort of sense. Most of these are concerned with our internal state, and the five ‘proper’ ones are responsible for monitoring and perceiving the world around us, our environment. These are, of course, vision, hearing, taste, smell and touch. Or, to be extra scientific, ophthalmoception, audioception, gustaoception, olfacoception and tactioception, respectively (although most scientists don’t really use these terms, to save time). Each of these senses is based on sophisticated neurological mechanisms and the brain gets even more sophisticated when using the information they provide. All the senses essentially boil down to detecting things in our environment and translating them into the electrochemical signals used by neurons which are connected to the brain. Coordinating all this is a big job, and the brain spends a lot of time on it.
Volumes could be and have been written about the individual senses, so let’s start here with perhaps the weirdest sense, smell. Smell is often overlooked. Literally, what with the nose being right below the eyes. This is unfortunate, as the brain’s olfactory system, the bit that smells (as in ‘processes odour perception’), is odd and fascinating. Smell is believed to be the first sense to have evolved. It develops very early; it is the first sense to develop in the womb, and it has been shown that a developing baby can actually smell what the mother is smelling. Particles inhaled by the mother end up in the amniotic fluid where the foetus can detect them. It was previously believed that humans could detect up to 10,000 separate odours. Sounds like a lot, but this total was based on a study from the 1920s, which obtained the figure largely from theoretical considerations and assumptions that were never really scrutinised.
Flash forward to 2014, when Caroline Bushdid and her team actually tested this claim, getting subjects to discriminate between chemical cocktails of very similar odours, something that should be practically impossible if our olfactory system is limited to 10,000 smells. Surprisingly, the subjects could do it quite easily. In the end, it was estimated that humans can actually smell in the region of 1 trillion odours. This sort of number is usually applied to astronomical distances, not something as humdrum as a human sense. It’s like finding out the cupboard where you store the vacuum cleaner actually leads to a subterranean city with a civilisation of mole people.*
So how does smell work? We know smell is conveyed to the brain through the olfactory nerve. There are twelve facial nerves that link the functions of the head to the brain, and the olfactory nerve is number 1 (the optic nerve is number 2). The olfactory neurons that make up the olfactory nerve are unique in many ways, the most pronounced of which is they’re one of the few types of human neurons that can regenerate, meaning the olfactory nerve is the Wolverine (of X-Men fame) of the nervous system. The regenerative capabilities of these nose neurons means they are extensively studied, with the aim of exploiting their regenerating abilities to apply them to damaged neurons elsewhere – for instance, in the spine of paraplegics.
Olfactory neurons regenerate because they are one of the few types of sensory neurons that are directly exposed to the ‘outside’ environment, which tends to degrade fragile nerve cells. Olfactory neurons are in the lining of the upper parts of your nose, where the dedicated receptors embedded in them can detect particles. When they come into contact with a specific molecule, they send a signal to the olfactory bulb, the region of the brain responsible for collating and organising information about odour. There are a lot of different smell receptors; a Nobel Prize-winning study by Richard Axel and Linda Buck in 1991 discovered that 3 per cent of the human genome codes for olfactory receptor types.2 This also supports the idea that human smell is more complex than we’d previously thought.
When the olfactory neurons detect a specific substance (a molecule of cheese, a ketone from something sweet, something emanating from the mouth of someone with questionable dental hygiene) they send electrical signals to the olfactory bulb, which relays this information to areas such as the olfactory nucleus and piriform cortex, meaning you experience a smell.
Smell is very often associated with memory. The olfactory system is located right next to the hippocampus and other primary components of the memory system, so close in fact that early anatomical studies thought that’s what the memory system was for. But they’re not just two separate areas that happen to be side by side, like an enthusiastic vegan living next to a butcher. The olfactory bulb is part of the limbic system, just like the memory-processing regions, and has active links to the hippocampus and the amygdala. As a result, certain smells are particularly strongly associated with vivid and emotional memories, like how a smell of roast dinner can suddenly remind you of Sundays at your grandparents’ house.
You’ve probably experienced this yourself on many occasions, how a certain smell or odour can trigger powerful memories of childhood and/or bring about emotional moods associated with smells. If you spent a lot of happy time as a child at your grandfather’s house and he smoked a pipe, you will likely have a sort of melancholy fondness for the smell of pipe smoke. Smell being part of the limbic system means it has a more direct route to triggering emotions than other senses, which would explain why smell can often elicit a more powerful response than most other senses. Seeing a fresh loaf of bread is a fairly innocuous experience, smelling one can be very pleasurable and oddly reassuring, as it’s stimulating and coupled with the enjoyable memories of things associated with the smell of baking, which invariably ends up with something pleasant to eat. Smell can have the opposite effect too, of course; seeing rotten meat isn’t very nice, but smelling it is what’ll make you throw up.
The potency of smell and its tendency to trigger memories and emotions hasn’t gone unnoticed. Many try to exploit this for profit: estate agents, supermarkets, candle-makers and more all try to use smell to control people’s moods and make them more prone to handing over money. The effectiveness of this approach is known but probably limited by the way in which people vary considerably – someone who’s had food poisoning from vanilla ice-cream won’t find that odour reassuring or relaxing.
Another interesting misconception about smell: for a long time, it was widely believed that smell can’t be ‘fooled’. However, several studies have shown this to be not true. People experience illusions of smell all the time, such as thinking a sample smell is pleasant or unpleasant depending on how it’s labelled (for instance, ‘Christmas tree’ or ‘toilet cleaner’ – and for the record this isn’t a joke example; it’s a real one from a 2001 experiment by researchers Herz and von Clef).
The reason it was believed there were no olfactory illusions seems to be because the brain only gets ‘limited’ information from smell. Tests have shown that, with practic
e, people can ‘track’ things via their scent, but it’s generally restricted to basic detection. You smell something, you know something is nearby that’s giving off that smell, and that’s about it; it’s either ‘there’ or ‘not there’. So if the brain scrambles the smell signals, so that you end up smelling something that’s different from what’s actually producing the odour, how would you even know? Smell may be powerful, but it’s got a limited range of applications for the busy human.
Olfactory hallucinations,† smelling things that aren’t there, also exist, and can be worryingly common. People often report the phantom smell of burning – toast, rubber, hair or just a general ‘scorched’ smell. It’s common enough for there to be numerous websites dedicated to it. It’s often linked to neurological phenomena, such as epilepsy, tumours or strokes, things that could end up causing unexpected activity in the olfactory bulb or elsewhere in the smell-processing system, and be interpreted as a burning sensation. That’s another useful distinction: illusions occur when the sensory system gets something wrong, has been fooled. Hallucinations are more typically an actual malfunction, where something’s actually awry in the brain’s workings.
Smell doesn’t always operate alone. It’s often classed as a ‘chemical’ sense, because it detects and is triggered by specific chemicals. The chemical sense is taste. Taste and smell are often used in conjunction; most of what we eat has a distinct smell. There’s also a similar mechanism as receptors in the tongue and other areas of the mouth respond to specific chemicals, usually molecules soluble in water (well, saliva). These receptors are gathered in taste buds, which cover the tongue. It’s generally accepted that there are five types of taste bud: salt, sweet, bitter, sour and umami. The last responds to monosodium glutamate, essentially the ‘meat’ taste. There are actually several more ‘types’ of taste, such as astringency (for instance from cranberries), pungency (ginger) and metallic (what you get from … metal).
Smell is underrated, but taste, by contrast, is a bit rubbish. It is the weakest of our main senses; many studies show taste perception to be largely influenced by other factors. For example, you may be familiar with the practice of wine tasting, where a connoisseur will take a sip of wine and declare that it is a fifty-four-year-old Shiraz from the vineyards of southwest France, with hints of oak, nutmeg, orange and pork (just guessing here) and that the grapes were crushed by a twenty-eight-year-old named Jacques with a verruca on his left heel.
All very impressive and refined, but many studies have revealed that such a precise palate is more to do with the mind than the tongue. Professional wine tasters are typically very inconsistent with their judgements; one professional taster might declare that a certain wine is the greatest ever, while another with identical experience declares it’s basically pond water.3 Surely a good wine will be recognised by everyone? Such is the unreliability of taste that no, it won’t. Wine tasters have also been given several samples of wine to taste and been unable to determine which is a celebrated vintage and which is mass-produced cheap slop. Even worse are tests that show wine tasters, given samples of red wine to evaluate, are apparently unable to recognise that they’re drinking white wine with food dye in it. So clearly, our sense of taste is no good when it comes to accuracy or precision.
For the record, scientists don’t have some sort of bizarre grudge against wine tasters, it’s just that there aren’t many professions that rely on a well-developed sense of taste to such an extent. And it’s not that they’re lying; they are almost certainly experiencing the tastes they claim to, but these are mostly the results of expectation, experience and the brain having to get creative, not the actual taste buds. Wine tasters may still object to this constant undermining of their discipline by neuroscientists.
The fact is that tasting something is, in many cases, something of a multisensory experience. People with nasty colds or other nose-clogging maladies often complain about being unable to taste food. Such is the interaction of senses determining taste that they tend to intermingle quite a lot and confuse the brain, and taste, as weak as it is, is constantly being influenced by our other senses, the main one being, you’ve guessed it, smell. Much of what we taste is derived from the smell of what we’re eating. There have been experiments where subjects, with their nose plugged and wearing blindfolds (to rule out vision’s influence, too), were unable to discern between apples, potatoes and onions if they had to rely on taste alone.4
A 2007 paper by Malika Auvray and Charles Spence5 revealed that if something has a powerful smell while we’re eating it the brain tends to interpret that as a taste, rather than an odour, even if it’s the nose relaying the signals. The majority of the sensations are in the mouth, so the brain overgeneralises and assumes that’s where everything is coming from and interprets signals accordingly. But the brain already has to do a lot of the work in generating taste sensations, so it would be churlish to begrudge it making inaccurate assumptions.
The take-home message from all of this is that if you’re a bad cook, you can still get away with dinner parties if your guests are suffering from terrible head colds and willing to sit in the dark.
Come on, feel the noise
(How hearing and touch are actually related)
Hearing and touch are linked at a fundamental level. This is something most people don’t know, but think about it; have you ever noticed how incredibly enjoyable it can be to clean out your ear with a cotton bud? Yes? Well, that’s nothing to do with this, I’m just establishing the principle. But the truth is, the brain may perceive touch and hearing completely differently, but the mechanisms it uses to perceive them at all have a surprising amount of overlap.
In the previous section, we looked at smell and taste, and how they often overlap. Admittedly, they do often have similar roles regarding recognising foodstuffs, and can influence each other (smell predominately influencing taste), but the main connection is that smell and taste are both chemical senses. The receptors for taste and smell are triggered in the presence of specific chemical substances, like fruit juice or gummy bears.
By contrast, touch and hearing; what do they have in common? When was the last time you thought something sounded sticky? Or ‘felt’ high-pitched? Never, right?
Actually, wrong. Fans of the louder types of music often enjoy it at a very tactile level. Consider the sound systems you get in clubs, cars, concerts and so forth that amplify the bass element of music so much that it makes your fillings rattle. When it’s powerful enough or of a certain pitch, sound often seems to have a very ‘physical’ presence.
Hearing and touch are both classed as mechanical senses, meaning they are activated by pressure or physical force. This might seem weird, given that hearing is clearly based on sound, but sound is actually vibrations in the air that travel to our eardrum and cause it to vibrate in turn. These vibrations are then transmitted to the cochlea, a spiral-shaped fluid-filled structure, and thus sound travels into our heads. The cochlea is quite ingenious, because it’s basically a long, curled-up, fluid-filled tube. Sound travels along it, but the exact layout of the cochlea and the physics of soundwaves mean the frequency of the sound (measured in hertz, Hz) dictates how far along the tube the vibrations travel. Lining this tube is the organ of Corti. It’s more of a layer than a separate self-contained structure, and the organ itself is covered with hair cells, which aren’t actually hairs, but receptors, because sometimes scientists don’t think things are confusing enough on their own.
These hair cells detect the vibrations in the cochlea, and fire off signals in response. But the hair cells only in certain parts of the cochlea are activated due to the specific frequencies travelling only certain distances. This means that there is essentially a frequency ‘map’ of the cochlea, with the regions at the very start of the cochlea being stimulated by higher-frequency soundwaves (meaning high-pitched noises, like an excited toddler inhaling helium) whereas the very ‘end’ of the cochlea is activated by the lowest-frequency soundwaves (very dee
p noises, like a whale singing Barry White songs). The areas between these extremes of the cochlea respond to the rest of the spectrum of sounds audible to humans (between 20 Hz and 20,000 Hz).
The cochlea is innervated by the eighth cranial nerve, named the vestibulocochlear nerve. This relays specific information via signals from the hair cells in the cochlea to the auditory cortex in the brain, which is responsible for processing sound perception, in the upper region of the temporal lobe. And the specific part of the cochlea the signals come from tells the brain what frequency the sound is, so we end up perceiving it as such, hence the cochlea ‘map’. Quite clever really.
The trouble is, a system like this, involving a very delicate and precise sensory mechanism essentially being shaken constantly, is obviously going to be a bit fragile. The eardrum itself is made up of three tiny bones arranged in a specific configuration, and this can often be damaged or disrupted by fluid, ear wax, trauma, you name it. The ageing process also means the tissues in the ear get more rigid, restricting vibrations, and no vibrations means no auditory perception. It would be reasonable to say that the gradual age-related decline of the hearing system has as much to do with physics as biology.
Hearing also has a wide selection of errors and hiccups, such as tinnitus and similar conditions, that cause us to perceive sounds that aren’t there. These occurrences are known as endaural phenomena; sounds that have no external source, caused by disorders of the hearing system (for example, wax getting into important areas or excessive hardening of important membranes). These are distinct from auditory hallucinations, which are more the result of activity in the ‘higher’ regions of the brain where the information is processed rather than where it originates. They’re usually the sensation of ‘hearing voices’ (discussed in the later section on psychosis), but other manifestations are musical ear syndrome, where sufferers hear inexplicable music, or the condition where sufferers hear sudden loud bangs or booms, known as exploding head syndrome, which is one from the category ‘conditions that sound far worse than they actually are’.
