irrelevance of constancy

A steady sound, with constant frequency, intensity, and spectrum is annoying. Moreover, after a while, our conscious present would not register it any more. Only when that sound is turned off, may we suddenly realize that it had been there.

Roederer - The Physics and Psychophysics of Music (Page 6)

Auditory constancy is filtered out by the brain. Unchanging aspects of an ambience are not deemed important because they contain no information with regards to the dynamics unfolding in the ambience. Here is a simple mathematical formulation of this idea. Interpret X as a constant feature of the setting, and Y as an ongoing arbitrary event that can assume numerous different trajectories with non-trivial probabilities:

(Here X may have values other than "a" as well, but this region of inconstancy is assumed to be probabilistically insignificant. The lemma is copied from this book.)

The same idea also governs the way the brain treats visual inputs. Retina receptors can detect only fluctuations in illumination. However, there is an ingenious mechanism that allows us to detect visually steady objects as well. We have jittery eyeballs that artificially introduce the necessary fluctuations. Even if the book in front of you is staying still, since your eyeballs are active the image of the book on the retina is moving around.

We live in a constantly changing world, so nervous systems have evolved to detect changes in our environment. Motion in our visual field might indicate that a predator is approaching or that prey is escaping. Stationary objects generally pose less of a threat and so tend to be studiously ignored owing to neuronal adaptation mechanisms. Some nervous systems are specialized to detect only motion signals; for example, a frog might not see a resting fly, but will react rapidly as soon as the fly takes off. In a sense, this ability to see only moving objects might be true of all visual systems. Even our own visual system can detect stationary objects only because the images projected onto our retinas are never stationary for long. Involuntary fixational eye movements prevent the adaptation of our neurons to an unchanging environment. The movement of the retinal images is what keeps stationary objects of interest from fading perceptually. (Source)

(The last sentence here may be misleading. Not just the stationary objects of interest, but all stationary objects are kept from fading. In other words we can not consciously bring stationary objects in and out of our perception.)

Evolution has come up with a patch to compensate for a previous neural adaptation that ignored visual constancy. Now the brain is able to register steady images even while we are not moving our heads.

A similar counter-adaptation has not occurred in the auditory realm. Nevertheless we are able to overrun the brain's tendency to stop registering steady sounds. For instance, because of its underlying meaning, we will continue to register a civil defence siren despite its steady character.

P.S. Car alarms are extremely annoying because they are designed to be so. The manufacturers make sure that there is enough non-steadiness built into these sounds so that our brains can not filter them out. At this very moment I am being subject to one of these sounds, and I feel like I am going nuts. I wish I had the capacity to ignore any sound at will... All I can do now is to massage my forehead in a rhythmic fashion and fully subjugate myself to the pulse of the alarm.