Making Sense of the Senses, Part 1


By Wil Forbis

Jan 1, 2011

The Our Wacky Brain collection:

In my first "wacky brain" article, I argued (or more correctly, regurgitated other people's arguments) that emotions were nothing more than sensory experiences. I blithely stated that the emotional experience could be neatly summed up as the activation of various nerves (particularly nerves related to the sensations in our viscera) and that there was no additional component such as an ethereal spirit or mind (as is often posited by those with a more romantic viewpoint.)

This presumption brings up an interesting question: how do we sense the world? We all have a general understanding of how eyes, ears, noses, tongues etc. work, but now is a good time to dig a little deeper into the workings of our senses (or sensory "modalities," to use a term often thrown around by scientific eggheads.)

Of course, discussing the operation of our sensory organs with any level of detail is beyond the scope of a single article --- entire books have been written on the eyes alone. I'm going to provide a general overview of each sense and then include links for further investigation.

Before I dive into a discussion of the individual senses, I want to throw out one warning. Neuroscience has a pretty good understanding of what's happening on a functional and physiological level when we use our senses. The experience of sight, for example, can be explained in terms of nerves in the retina sending signals that activate networks of neurons in the parts of the brain responsible for vision. But that doesn't explain sensations from an experiential viewpoint. Why do we experience red as red? Why do we hear C sharp on the piano the way we do? Why does cold feel the way it does? These are still unanswered questions (though there are theories) and worth keeping in mind.

Sight
If there's a general architecture that can describe all the senses, it's this: sensory organs perceive information about the world and then pass it to the brain through nerves. The eyes are perfect example. Light waves hit a sheet of light-sensitive cells in the back of the eye called the retina. Different retina cells respond to either color or brightness, and send this information to the brain via two optic nerves (one for each eye.)

The retina can be thought of as a miniaturized representation of the same view the eye "sees." (It's an inverted view --- a red dot in the upper left-hand corner of your field of vision will stimulate a retinal neuron in the lower right-hand part of the retinal sheet --- it's up to the brain to flip things around later. The prominent graphic at this link provides a nice view of things.) If nature wanted to keep things simple, it could have decided to pass that representation through the optic nerve to another network of neurons in the brain and somehow form a camera ready image in your consciousness. But nature seldom like simplicity; instead there are many neural networks in the brain that process vision and each of them handles specific elements of the image you are looking at. Some handle brightness, some straight lines, some color, some curves, some motion etc.

Why is this interesting? Our sensation of vision is that of one unified image. But our brain is really processing many different components of the visual "data" coming through the eyes and gluing them together. And people with brain damage can lose some of the processes of vision but not all. Some people are colorblind, some people are motion blind, some people are text blind and some people lack the ability to recognize faces.

Further reading:
Wikipedia: retina
Wikipedia: visual cortex

Sound
Like eyes, ears perceive waveforms, particularly waveforms caused by vibrations of air molecules (or whatever makes up your surrounding environment; you can hear underwater for example.)

How does the ear perceive these vibrations? Inside the ear is the famed spiral shaped cochlea. The cochlea contains tiny hair cells, each of which respond to specific frequencies of vibrations. High frequency sounds (like a screaming violin) impact the hair cells at the front (e.g. closest to your ear hole) of the cochlea, low-frequency sounds (like a tuba) register further down. In this sense, the cochlea is a bit like an inverted piano, though instead of producing sound, the cochlea is perceiving it.

Each ear's cochlea is connected to an auditory nerve which passes the information to the brain, primarily to an area called the audio cortex. In the same way that there are multiple areas of the brain to process different elements of sight, the audio cortex has multiple neural networks to process different elements of sound. In these networks, the piano rule holds --- low frequencies cause activations of neurons that one end, high frequencies activate neurons at the other.

Further reading:
Wikipedia: auditory cortex
Wikipedia: cochlea
This Is Your Brain on Music (book)

Smell
Seeing and hearing are about detecting different kinds of waves. Smell is about detecting chemicals.

How does the sense of smell work? You know the drill by now. The smell organ --- the nose --- has receptors (hundreds of them) which get excited by different features of molecules (thus one molecule can excite different receptors in different ways.) This excitation is passed along a special nerve --- and the olfactory nerve --- to the brain.

There's a couple of unique aspects of the sense of smell. Other senses first get routed through a part of the brain called the thalamus which then routes the sensory information to different parts of the brain. One part of the brain that all senses get directed to is the limbic system, which plays an important role in emotional processing. Unlike most senses, smell bypasses the thalamus and goes directly to the limbic system. As a result, smell can trigger emotional reactions much more easily than the other senses. The onslaught of a long forgotten memory brought about by a particular smell is sometimes referred to as a "Proustian memory" because author Marcel Proust so richly described the experience.

Why is it useful to have smell directly tied to emotion? Our sense of smell helps us determine whether something is edible. An extreme emotional response, such as disgust to inedible or poisonous foods, is particularly helpful in keeping a creature alive.

What else makes the sense of smell so freaky? One word: pheromones. A pheromone is, to quote Wikipedia, "a secreted or excreted chemical factor that triggers a social response in members of the same species." Humans can detect the pheromones of other humans through their sense of smell, with all sorts of strange results, perhaps the most famous being the "Wellesley effect." Stick female mammals in an environment where they can smell each other's pheromones (like a college dorm), and their cycle of menstruation will sync up. Furthermore, the more submissive girls sync up to the dominant e.g. bossiest female. (There's been some controversy in regards to the existence of Wellesley effect; this link has a good general overview.)

More pheromone madness: in 1985, Swiss biologist Claus Wedekind observed that women were attracted to the smells of men who had an immune system dissimilar from their own. (The evolutionary logic for this being that we want to have sex with people who will enable our offspring (e.g. shared genes) to survive and thrive. The children of parents who each have a specific set of immunities will be especially strong.) However, as noted in the book "Sex at Dawn," birth control pills seem to inhibit women's ability to "smell out" partners with complimentary immunity profiles. The book posits a scenario where a woman on the pill meets a fellow, marries him and decides to have children. She goes off the pill and for some indecipherable reason starts to find him unattractive. (Hint: they share similar immunities!) I'd always presumed that my girlfriends eventually tired of me because of my boorishness, excessive cursing and philandering. It turns out there may be a much more sinister reason.

Further reading:
Wikipedia: olfactory receptor
Wikipedia: pheromones
Proust Was a Neuroscientist (book)

Taste
Taste is the 2nd of the chemical sensory modalities. As most people know, the taste organ, the tongue, has receptors known as taste buds. Classic taste theory posits that there are four types of taste: sweet, salty, sour and bitter. But the existence of a 5th receptor for taste,referred to as umami, was confirmed in 2000. Umami is the rich, very satiating taste we tend to associate with beef broth, soy sauce and cheese.

Just as you'd expect, the taste buds connect with nerves which go to the brain. And in the same manner that different areas of the cochlea process different kinds of sound, different parts of the tongue are wired for different tastes. Generally speaking, taste buds at the tip of the tongue are wired for sweetness, buds at the back perceive bitter while buds in the center and on the side map to salty and sour tastes.

Further reading:
Wikipedia: taste

Touch
In some ways, touch is the most complex and controversial of the senses. Like our other senses, nerves in the sensory organ (skin, for the most part) transmit information about... what? We might consider the perception of pressure or vibrations as integral to touch, but what about perceptions of pain, temperature, itching or kinesthesia (our sense of where parts of the body are)? How about internal sensations like a stomach ache (which is key to how we perceive our emotional state)? These various sensations are sensed using different types of nerve receptors and some academics argue they should be defined as separate senses. However, in the general interest of keeping the total number of senses low, we tend to group together senses that require us to be in physical contact with the object we are sensing.

Where do we have the greatest amount of touch sensitive nerve endings? The fingertips, and --- you guessed it --- the sexual organs. (That's why it hurt so much the last time your penis got stuck in a waffle iron.)

Further reading:
WiseGeek on touch

Other senses
The traditional view is that humans have five senses (blame Aristotle for this one.) However there are several other sensory modalities humans use, including our sense of balance, time, the various sub-senses related to touch described above, and internal senses like the feeling that you need to take a pee.

Of course, we here at acid logic feel the most important sense is the most ignored: the sense of humor. (Thank you, I'll be here all week!)

Nonhuman senses
To make things even more complex, nonhuman species sometimes have additional senses like electroreception (the ability to detect electric fields) and magnetoreception (the compass like ability to detect magnetic fields.) However, humans may be catching up to animals on the last one --- this article describes a woman who has implanted magnets in her fingertips.

In some ways, the architecture of the senses is remarkably simple. Sensory information comes in through an input (your eyes, your ears etc.) is then routed to the brain's gatekeeper, the thalamus*, and from there is off to different sections of the brain that are responsible for processing specific components of sensory input.

* As noted above, the sense of smell bypasses the thalamus.

But consider this: we don't experience the world as separate streams of of sights, smells, sounds, tastes and tactile sensations that are completely separate from each other. We have a unified experience of our environment. In part two (next month), I'll look into the question of how we take information gleaned via our different senses and apply them to the various "objects" we encounter. For example, how do we glue together the image of our dog and the smell of our dog and the feeling of our dog's hair and the sound of his bark into the singular concept of "my dog Rover"?



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