Senses - a Baker's Dozen

 kw: book reviews, nonfiction, natural history, science, senses, physiology

I discovered something shocking about Chinese soup. At a church potluck dinner we all enjoyed a bowl of soup from a big pot one Chinese sister brought. It had an intriguing taste, similar to soups made with Star Anise, but subtly different. In my bowl I found a black pod. I was told it was the seed pod of water chestnut. I bit into it, and had quite a surprise! Suddenly the soup tasted awful. Water tasted like battery acid. Every taste was distorted, in quite unfortunate ways, for about a day. When I told this to the cook, she laughed and said, "You aren't supposed to eat Chinese spices!" I couldn't stand to eat or drink anything until the next day.

I found out that this seed pod goes by many names, including Devil's Pod. You can get them from two sources: Chinese food stores, and Etsy, where they are sold for use in crafts (not as a food item).

The words "taste" and "flavor" have different meanings. For a physiologist, "flavor" includes both taste and smell. That is but one tidbit I find in Sentient: How Animals Illuminate the Wonder of Our Human Senses by Jackie Higgins. Another is that the five kinds of taste we know may actually number seven. In addition to sweet, sour, salty, bitter, and savory ("umami", triggered by glutamine from protein), some researchers have found hints that our tongues have sensors for calcium (a different kind of salty) and fat.

We all learned that the "five senses" are sight, hearing, taste, smell, and touch. These are the senses that have visible organs. However, we also have senses of balance, hunger, and a host of others that may number more than 20. Indeed, our eyes sense two different regimes of light. When the only light in a room is a candle, but we can still see colors in all except the gloomiest corners, our sight is near the threshold of photopic vision. Light bright enough for us to distinguish colors is sensed by the color-selective cones in our retina. Moonlight, particularly when the moon is a few days past full, or a few days shy of full, is sensed by the color-blind (actually blue-green sensitive) rods in our retina, and we can see rather well by scotopic vision.

There is actually another network of light-sensing cells in our retina that connect to our body clock, so that day/night cycles reset it daily. Long-term experiments with people in caves and bunkers have shown that without this daily resetting, our body clock tends to run on a 25-hour cycle. Thus, in addition to color vision and night vision, our eyes have a third function related to our sense of time. Body temperature, blood pressure, and alertness run in daily cycles; the "afternoon sleepies" aren't just because of that big lunch you ate.

The 12 chapters in Sentient each focus on a different sense. Eleven of these are known in humans: 

• color vision
• night vision
• hearing
• touch (but this is a multi-modal group of senses with different receptors for each)
• pleasure and pain (two levels of stimulation of one set of receptors)
• taste
• smell (by itself, or as bundled into "flavor" with taste)
• desire (pheromones)
• balance
• time
• proprioception (required to touch your nose with eyes closed)

The twelfth (Chapter 11) is direction, which we'll get to shortly. There is a bonus chapter on a 13th sense, the electric sense in the bill of a platypus. Sharks and other aquatic predators also have it. Humans don't; getting an electric shock stimulates nerves directly, but there is no sensor that allows us to know the subtle electric fields around moving animals. That's probably a good thing. If we had an electric sense it would be overloaded by the pervasive 60- (or 50-) cycle hum found everywhere except unpopulated areas and Amish homes, and by the multitude of electromagnetic signals that bring our favorite programs to AM and FM radios and TV antennas.

The iconic animal in each chapter is renowned for excellence, or exceedance, in the chosen sense. For example, the Mantis Shrimp of Chapter 1 has, not just the three color sensors that we (and most primates) have, but a total of TWELVE, including one or two that see ultraviolet. Apparently, in the extremely colorful environment of a coral reef, it is thus better able to discriminate specific prey by their colors. Chapter 6 on Taste tells of a large catfish. Catfish are known to have numerous taste receptors all over their bodies; one researcher calls a catfish a "swimming tongue". The Goliath Catfish is the largest; think of a ten-foot tongue swimming around.

I was quite interested in the directional sense that some people exhibit (Chapter 11). Many animals including migrating birds are found to have pieces or chains of magnetite crystals that let them sense, and perhaps even see, the magnetic field of the earth. When migrating, they follow the direction of the force, or go at an angle to it; at the end of the trip, they may sense the steepening angle as they approach the magnetic pole, and finish their journey when the angle reaches a certain degree. Do humans have a magnetic sense? Many experiments include quite a number that seem to say "Yes", but some seem to say "No" and others are equivocal. If humans do not have a magnetic sense, that'll be odd, because so many mammals do have one, and members of nearly every other group of animals have it. We do have something, or some of us at least, for there are those who always know which way is north, or home, or another chosen direction, even after being taken somewhere blindfold.

This is so far my favorite book this year: Fascinating, packed with very interesting information, and easy to read.

I'd like to end with speculation about a fourth sense that may be located in our eyes. It is something I've noticed after I turn out the lights in my bedroom. In the dark, with my eyes closed, I seem to be able to see my hand and arm move, and also the blanket, when I rearrange the covers. This is not scotopic vision somehow seeing through my eyelids. The rods are sensitive to blue-green light peaking at 500 nm (normal green-sensitive cones peak about 540 nm). Rods cut off on the longwave side at about 600 nm, but hardly any light shorter than 600 nm gets through our red-colored eyelids, because of the blood in them.

By my bedside is a clock radio with bluish LED numerals. I have a pink filter over it so it doesn't light up the whole room (the radio is poorly designed). It is plenty bright enough for me to see the blue color. I can't see it at all through my eyelids, even when fully dark adapted. But there is a little reddish light in the room from a couple of pilot lights on equipment, and a pinkish light that comes through the windows from skyglow. When there is still enough light in the room to see faint colors it looks like this (picture edited to look like what I remember):

On moonless nights, after my eyes have become dark-adapted, it looks more like this:

The slight bluish hue is typical of scotopic vision. A scene by moonlight looks bluish, even though the moon's actual color is brown. Although rods outnumber cones 20-to-1 (120 million rods and 6 million cones), they are ganged together for greater sensitivity, which greatly reduces the sharpness of the scene.

Now, here is what I noticed about a year ago. I typically turn out the light around 11 or a little later. Even before I am fully dark-adapted, when my eyes are closed and I readjust the covers or move my hand, I can see the movement of my hand and the covers, and if—still with closed eyes—I look around the room, I can see the outlines of what is in the room, a little more faintly than if I open my eyes, but distinctly. The windows seem the brightest. Further editing of the photo from above yields this, which matches what I see:

It is perhaps twice as blurry as scotopic vision, and the light and dark areas are a little different. If I hold up my hand and wave it about it seems black. If I raise my head and look at the clock radio, with its dim blue numerals, I don't see them at all.

Is it possible that I am seeing right through my eyelids? If so, it must be by sensors that see by the pink light from the windows, and/or the faint red light of the pilot lights. It could be something else entirely.

I have considered that my brain may be conjuring a dim memory of the room, as part of a normal vision function that anticipates what is "normal", priming the visual system to detect any differences. My brain knows where my hand is; it knows how the bedcovers move when I shift them; it knows where the windows and furniture are. It could know enough to meld proprioception (knowing where my head is pointed and where my hands are) with this calculated scene so that the scene "stays put" when I turn my head.

I cannot decide at present whether I am actually seeing, or constructing, what I sense.