Our Sixth Sense Is as Fine-Tuned as It Can Be

Our sixth sense is as fine tuned as it can be says Todd Squires, a physicist at Caltech. He has investigated why the natural selection process, operating over evolutionary time, settled upon specific dimensions for the vestibular semicircular canals (SCC), the set of three mutually perpendicular, fluid-filled tubes housed in the inner ear of vertebrates that give an organism its sense of balance.

Scientists sometimes recognize the perception of balance and motion as being a sixth sense, in addition to the usual five---smell, touch, sight, hearing, and taste. The balance sense organ, the SCC structures, are essentially donut-shaped, with a major radius of 3 mm and minor radius of 0.2 mm.

Furthermore, the torus is interrupted by a membrane, called a cupula, impregnated with tiny sensory hairs for sensing the sloshing of the fluid through the canals. Sensing an acceleration or rotation involves the fluid being momentarily left behind while the head (and the SCCs) rotate in a new direction. The fluid displaces the cupula, deflecting the sensory hairs and triggering a neural signal to the brain and muscles controlling the eye, and this is what gives us the sense of motion, and sometimes dizziness.

Squires addressed himself to the question of why the SCC should be roughly the same size (to within a factor of three) in mice as it is in whales. In humans, for instance, the SCC reaches its full adult size in about the 14th week of pregnancy. Why should SCCs be all of this one size, as if evolutionary pressures had “converged” on an optimal solution?

In performing studies of optimal design, Squires varied four different key physical parameters---SCC major radius, minor radius, cupula thickness and height---and discovered that the greatest canal sensitivity occurred for those parameter values manifested in actual vertebrates.

Knowing how the canals work is important for understanding various forms of dizziness (such as “top-shelf vertigo,” the light-headedness experienced by some when they tilt their heads back in looking at a top shelf) and for understanding peculiarities of some ordinary visual experiences.

For example, since the SCC output is wired into eye-control muscles, some motions can be compensated: you can read a fixed page while swiveling your head, but with your head fixed you can’t read a page swivelled by a friend. The SCC-eye feedback effect also explains why some home video, recorded while the filmer is in motion, doesn’t look so good afterwards in the editing stage, when the neuro-feedback mechanism isn’t at work. (Todd Squires, Physical Review Letters, 5 Nov 2004; tsquires@acm.caltech.edu, 626-395-4640; for further background, see Parker, Scientific American, November 1980, p118.)