by Claude Needham Ph.D.
Opening presents is something usually associated with special days and holidays — Mother's day, birthdays, Christmas, Hanukah, baby showers, Lincoln's birthday, weddings, graduation, etc.
When confronted with a present, you do not have to open it to know that there is something there. You know that you don't know what it is, and you can conjecture and make a 'best-guess' based upon information that you have at your disposal. However, it is not until you remove the interceding wrapping and open it that you have full knowledge of the present.
Not being one to easily let a pun pass by, let's consider the situation of the present — as in the time sense. You know the "ever present now," "quit living in the past and enjoy the present," "don't worry so much about the future, it's the present that counts."
Where is this present that everyone is talking about? Where is it in relationship to you? Two rather large questions; both of which are rampant with pit and prat falls.
Rather than look at these questions directly, let's look at a few items that aren't so sweeping.
At the moment, you are reading this science column. If not, then someone must be reading it to you. If not, then somehow it has been recorded and is being played within your hearing. In any case the information from this page is making its way to you. Let's assume for brevity (too late, you say?) that you are reading this article.
To read this article, light must be emitted (crt) or reflected (lcd) from the screen. The light that is being emitted from the page travels toward your eyes (and the rest of the room, lest you begin to feel too egocentric) at a speed of roughly very-damn-fast. Actually it's not very-damn-fast, it's the fastest that most any reasonable particle would choose to go: the speed of light. Last time I checked, the speed of light was something like 2.99792x1010 cm/sec. This means that light can travel 29,979,200,000 centimeters in one second. At this speed, light can circle the Earth about 100 times in a second. To travel the few feet from the page to your eye will require only three nanoseconds — that's 3 x 10-9seconds.
The three nanoseconds that it takes for the light to reach your eye from this page is not a very long time. But, it is not now. Even before you have any possibility to perceive the events happening on this page, they are already in the past.
Okay, so the light is a little late getting to the eyes — what happens then? In order for this page to be perceived, it is necessary that the light traveling from the page to your eye actually be absorbed by the eye. Sounds simple enough, but it can throw a considerable monkey-wrench into the plans of any mad-scientist trying to manufacture an invisibility potion. Should you intend to make yourself (or anyone else) invisible, you will need to fix it so that you see in either the infrared or the ultraviolet. If your eyes stop enough light to see by, then they will stop enough light to be visible to an observer. Two eyeballs floating around in midair may not be what you had in mind when you started to develop an invisibility potion.
As it turns out, the eyes are wondrous at the job of focusing and absorbing light. The light impinges on the optical lens of the cornea where it is focused onto the back of the eye. During this process it is slowed ever so slightly. Light does not travel quite as fast through the lens — that is how the lens can focus the light. By slowing the center part of the image a little bit more than the outer part of the image, it is possible to cause the image to bend around the lens, focusing itself.
After being focused by the lens, the light travels through a viscous fluid filling the interior of the eyeball to fall on the retina at the back of the eye. On the retina is a whole bingy-bongy-bunch of cells called rods and cones. They get their names from their shapes. The rods are rod-like and the cones are cone-like in morphology. The rods and cones are spread one layer thick on the retina behind the nerve cells. The rods and cones are the photoreceptors — i.e. they recept the photons, or in real English, they receive the light.
The substances in the rods and cones that make the detection of light possible are the photosensitive (or visual) pigments. The major pigment in these cells is rhodopsin, which is composed of a protein molecule called opsin, and a molecule made from vitamin A called retinene. (See, Mom was right; you should eat your carrots.) When light strikes rhodopsin, the retinene portion twists and turns, splitting away from the opsin portion. This process requires only a few picoseconds — it may be one of the fastest biological processes known, but, it still takes us that much more away from now. The splitting of rhodopsin acts as a switch — triggering a nerve impulse to the brain. Each rod and cone is a light-sensitive switch with its own wire into the brain.
Visual resolution is limited to the number or rods and cones. When light strikes each cell, the chemical switch in it is turned on, sending a signal along the optic nerve to the visual cortex in the brain.
If the signal to the brain traveled at the speed of light, which is the fastest that any information is supposed to be able to travel, it would still take the signal about one nanosecond to reach the brain.
The transmission along the optic nerve is governed by a mixture of electrical transmission and chemical transmission. Consequently, it takes a bit longer than that. The velocity of propagation of the signal along the optic nerve is roughly 10,000 centimeters per second. To travel the few inches from the eyes to the brain could take one millisecond. This is a conservative estimate, since the impulse appears to jump resulting in a higher velocity than if the impulse traveled continuously along the axon. In any case, this is a lot slower than instantaneous. Pushing us still further into the past.
The optic nerve is composed of neurons — nerve cells. Neurons contain a nucleus, cytoplasm with a variety of cell stuff (cytoplasmic structures such as endoplasmic reticulum and Golgi bodies) and a lipid-protein membrane that separates the inside of the cell from the outside.
All cells, including neurons, have a steady electrical voltage difference across their cellular membranes. This is caused by differences in chemical concentrations on the two sides of the cell membrane. The three major ionic chemical components that cause this electrical potential across the cellular membrane are: potassium, sodium, and chloride. All three exist on both sides of the cell membrane, but the amounts are not evenly distributed. Little teeny-tiny pumps continually transport the ions across the cell membrane. But the little pumps aren't fair. They will transport more of one ion across the membrane than another. In addition, the membrane isn't fair about which ions it will let diffuse across the membrane.
Diffusion is the process whereby a chemical or ion will drift about, moving itself from one place to another. When a bottle of perfume is opened at one end of a room, it is the process of diffusion that lets the smell travel throughout the room. That could be good or that could be bad, it depends on the brand of perfume. The process of diffusion doesn't care what the perfume smell like; it will move the molecules of perfume about with total egalitarian disinterest. But the cell membrane is not so indiscriminate.
During the resting state, the membrane allows mainly positive potassium ions to passively diffuse into the cell. The result is lots of positive potassium ions and not very many positive sodium or negative chlorine ions inside the neuron. The outside of the neuron has a lot of positive sodium ions, a bunch of negative chlorine ions, but not very many positive potassium ions. This may not be fair, but it does allow nerve cells to work. The resulting difference in ion concentration across the membrane generates the electrical potential that produces the membrane voltage.
As mentioned earlier, a resting membrane potential based on ionic asymmetry (that means more on one side than the other) and selective permeability (that means little holes like the ones in a spaghetti strainer) is a common feature of almost all living cells. The nerve cells make use of a phenomenon called depolarization to transmit information to the brain — and visa-versa.
Depolarization of the membrane occurs when the membrane rapidly becomes permeable to sodium ion, causing an influx of sodium into the cell from from the outside the membrane where the sodium levels have been kept low. The rapid inflow of positive charge drives the membrane voltage from negative 60 mV (resting potential) to positive 40 to 50 mV (action potential).
These local changes in ion concentration activate neighboring sections of the nerve membrane and result in a nerve impulse propagating along the nerve fiber — much like dominos, one stimulating the other. In this manner, the information is transmitted from the retina of the eye to the visual cortex of the brain.
During this transmission from the rods and cones to the visual cortex, not only does the signal travel along the individual nerve cells, it must also jump the synaptic gaps between nerve cells.
The synapse is the teeny-tiny gap between neurons. The information is transmitted across this synapse by chemical transmitters migrating from one neuron to the other. These transmitters migrate across the gap much like carrier pigeons. Once the message reaches the other side the next neuron fires and the message is on its way again. Interestingly enough, it is possible to speed up or slow down the transmission rate across this gap by the presence or absence of certain chemicals. Yep, we're slipping even further into the past.
Late or not, the final destination of this message is the visual cortex. It is here that the nerve impulses coming all the way from the rods and cones of the eyes produce a chemical and electrical distribution — this we call vision.
The data that the brain works with is chemical and electrical distributions found within the brain — not the experience itself. The electrical activity traveling along the optic nerve influence the chemistry of the brain to transmit information received by the nerve cells of the retina — still not the experience itself. The nerve cells of the retina are caused to trigger by changes in the chemical structure of molecules found in the rods and cones of the retina — still not the experience. The structure of the chemicals in the rods and cones of the retina are altered by the absorption of light — nope, not the experience yet. The light is transmitted from the page to the retina through the mediums of atmosphere, cornea and eyeball fluid — but the light is obviously not the experience. The light is that which is left over after the ambient light of the room is reflected from the page. We will never experience the print on a page. At best we can see the differential absorption of light reflected from the page. Even then we don't actually see the light; we experience chemical and electrical distributions brought about by the impingement of this light on a whole set of physiological systems.
Not only are we living in the past, we are living in a filtered version of the past with incomplete data and data that has been altered to correspond to previous experience. We are very much living within our own heads.