Fading sensations

I read a fascinating article (pdf, 863k) in the August 2007 Scientific American that has implications that seem broader and broader the more I reflect on them.  According to the two neurophysiologists who authored the piece, when it comes to our eyes, constant input fades away – as they put it, "an unchanging stimulus leads to neural adaptation in which visual neurons adjust their output such that they gradually stop responding." In other words, because of the wet chemistry of our retinal light sensing cells, unchanging stimulation ceases to produce a reaction. Since unchanging objects do not generally pose a threat, the article explains, "animal nervous systems have evolved to detect changes in the environment." Frogs, it turns out, are an "extreme case" of this because things not moving are actually invisible to them (the dinosaurs in the original Jurassic Park were supposed to be the same way; I was always curious about the basis for that.)

The reason our eyes are constantly in motion, jerking around with tiny little hops (called saccades and microsaccades) even when we think we're staring at a fixed point, is that human eyes create their own motion so that our visual field isn't constantly fading.

To prove to yourself that this neural fading (called the Troxler Effect) is real, check out this optical illusion (there's also an interactive version, and another illusion based on the same principle and just a whole bunch of cool optical illusions in general).

In the lab they can completely halt these microscopic eye motions, and they have found that your entire field of vision then fades away. In fact, if it were not for this effect, you would constantly see blood vessels in your eyes (which move with your eyes & therefore are faded).

(I wonder if this is in some way analogous to what takes place with video compression.  As I understand it, most compression algorithms work by recording only those portions of each frame that are different from the preceding. So those parts of the image that are unchanging are effectively ignored – just like with our vision. I wonder if our cells evolved this way not just because of the adaptive advantages of identifying motion as the authors mention, but also because there are processing efficiencies involved for the computation performed by the brain (note that according to the authors, the fading effect takes place at the level of the neuron, not at the processing level of the brain, where other "fadeouts" – for example, someone nagging you – must occur.)

But for me the most exciting point of the article, an aside which made it leap out as something of broader interest and relevance than just an ocular biological quirk, is that:

Neural adaptation takes place in all the senses, including touch.  For instance, you may feel your shoes when you first put them on in the morning, but the feeling goes away after a while.  You probably do not want to be aware of your shoes 16 hours a day, after all.  If you wiggle your toes, however, you can feel your shoes again. Similarly, fixational eye movements constantly "wiggle" the images on the retina so that your vision never goes away.

That just makes sense in a ton of ways.  It explains why smells fade, why you can't tell what your living room smells like, why you taste a glass of wine most vividly with the first sip, and by the last, it's pretty much just wine (or is it just me?).  It explains why you can enjoy food more if you alternate bites of two dishes, or eat food (even a sandwich) that is made up of mixes of different foods so that all the bites are constantly changing mixes of ingredients.  And many other things. It was just a major "aha" for me.

Also, as a barefooter I cannot resist pointing out the personal aptness of the example the authors chose to use above: the loss of sensation that comes from wearing shoes.  When we go shoeless every step brings fresh sensations.  Combined with the fact that, as a doctor friend tells me, our feet have more nerve endings than any other part of our body except for our hands and our sex organs, this helps explain why barefooting opens up an entire new world of sensation that is otherwise deadened and closed off to us. (Update: see this post on world of barefoot sensation)

This neural adaptation also represents a danger for athletes, however.  Every runner knows that when you are feeling pain, it will usually fade if you just keep running.  Often, "running through" pain, as it's called, is an excellent thing, keeping various harmless aches and pains that we all experience from immobilizing us.  But in some cases, it can be a dangerous clouding of urgent bodily signals that we are doing mounting damage to our bodies, as with tendonitis or a bone fracture.

Perhaps not just bodily pain but also psychic pain also fades with time – the pain of grief, of regret, of guilt, of embarrassment or humiliation.  Maybe if such pain doesn't fade away it is because there there are psychological "microsaccades" that keep the pain fresh.

Also, pleasure.  Even the finest delicacy, if eaten too frequently, begins to fade in the pleasure it gives.  How does the rich person, who can eat at the finest restaurants every night, ever derive the pleasure from the experience that a poorer person, who can only do so rarely, will?  How does one "wiggle" one's happiness?  Perhaps we can think of pleasurable activities that involve wiggling of one kind or another, but this line of thinking confirms what I've always believed about happiness, which is that it is immensely relative.  It's always seemed to me that people are happy when things are getting better for them, and unhappy when things are getting worse, and when things stay the same, people's satisfaction level floats back to an even keel or personal default psychological state.  (Complicating this is the issue of expectations, which is what defines improvement or decline for people.) 

This implies that the happiest person is he or she who starts off life in very dire circumstances and whose life steadily improves over its course.  The unhappiest person would be the person whose life follows the reverse course.  People whose circumstances remain the same – whether they are born in a shack or in a mansion, would be predicted to experience roughly equal levels of happiness.

Just as with neural adaptation, it is movement that produces sensation, whether good or bad.

Of course, that's assuming a certain baseline level of physical comfort, I imagine.  While the person who sleeps on a rough floor will probably get used to it (sensory adaptation again) and be none the less happy for it, hunger and disease are not likely to be subject to the effect.

I previously posted on the ancient Greek thinker Democritus, who said:

you should consider the lives of those in distress, reflecting on their intense sufferings, in order that your own possessions and condition may seem great and enviable, and you may, by ceasing to desire more, cease to suffer in your soul. . . . If you keep to this way of thinking, you will live more serenely, and will expel those not-negligible curses in life — envy, jealousy and spite.

In light of the Scientific American article, Democritus's advice to reflect upon the worse circumstances of others now sounds to me like a form of psychological saccade or "wiggling" that will improve your own happiness by making your soft plush bed and other luxuries and attainments seem fresh once again.  The same effect could be achieved by reflecting on how much better your life is than it once was.  And envy and jealousy – as well as regret – mean imagining your life as better than it is, which is psychologically the same as experiencing a downward movement in your circumstances, which results in unhappiness.