Idiot Brain (20 page)

The potency of smell and its tendency to trigger memories and emotions hasn't gone unnoticed. Many try to exploit this for profit: real 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 odor 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 labeled (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 practice, 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 odor, how would you even know? Smell may be powerful, but it has 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, tumors 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 recognized 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 recognize 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 noses 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 Spence
⁵
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 odor, even if it's the nose relaying the signals. The majority
of the sensations are in the mouth, so the brain overgeneralizes 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 swab? 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 recognizing 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 traveling 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 deep 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.”

Regardless of this, the human brain still does an impressive job of translating vibrations in the air to the rich and complex auditory sensations we experience every day.

So hearing is a mechanical sense that responds to vibration and physical pressure exerted by sound. Touch is the other mechanical sense. If pressure is applied to the skin, we can feel it. We can do this via dedicated mechanoreceptors that are located everywhere in our skin. The signals from the receptors are then conveyed via dedicated nerves to the spinal cord (unless the stimulation is applied to the head, which is dealt with by the cranial nerves), where they're then relayed to the brain, arriving at the somatosensory cortex in the parietal lobe which makes sense of where the signals come from and allows us to perceive them accordingly. It seems fairly straightforward, so obviously it isn't.

Firstly, what we call touch has several elements that contribute to the overall sensation. As well as physical pressure, there are vibration and temperature, skin stretch and even pain in some circumstances, all of which have their own dedicated receptors in the skin, muscle, organ or bone. All of this is
known as the somatosensory system (hence somatosensory cortex) and our whole body is innervated by the nerves that serve it. Pain, aka nociception, has its own dedicated receptors and nerve fibers throughout the body.

Pretty much the only organ that doesn't have pain receptors is the brain itself, and that's because it's responsible for receiving and processing the signals. You could argue that the brain feeling pain would be confusing, like trying to call your own number from your own phone and expecting someone to pick up.

What is interesting is that touch sensitivity isn't uniform; different parts of the body respond differently to the same contact. Like the motor cortex discussed in a previous chapter, the somatosensory cortex is laid out like a map of the body corresponding to the areas it's receiving information from, with the foot region processing stimuli from feet, the arm region for the arm, and so on.

However, it doesn't use the same dimensions as the actual body. This means that the sensory information received doesn't necessarily correspond with the size of the region the sensations are coming from. The chest and back areas take up quite a small amount of space in the somatosensory cortex, whereas the hands and lips take up a very large area. Some parts of the body are far more sensitive to touch than others; the soles of the feet aren't especially sensitive, which makes sense as it wouldn't be practical to feel exquisite pain whenever you step on a pebble or a twig. But the hands and lips occupy disproportionately large areas of the somatosensory cortex because we use them for very fine manipulation and sensations. Consequently, they are very sensitive. As are the genitals, but let's not go into that.