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How We Keep Our Eyes on Target

They say it’s harder to hit a moving target. The difficulty increases even more if the weapons platform is moving, too. Consider the tank chase scene in Indiana Jones and the Last Crusade. Indy manages to avoid the guns on a lumbering tank by never staying put.

Unless on very flat and horizontal terrain, World War I and II tanks had to stop in order to aim and fire at targets—until American research engineer Clinton R. Hanna invented a gyrostabilizer for tank guns in the early 1940s.1 This device allowed American Sherman tanks to drive and shoot at the same time by keeping the gun barrel fixed on the target with automatic up-and-down movements.

Hanna’s stabilizer design has four functional parts: (1) an electrically motor-driven gyroscope, rotating at 12.5–16k RPM; (2) a motion detector that produces variable electronic current; (3) a hydraulic pump system that responds to the electronic signals; and (4) a cylinder-and-piston assembly that adjusts the altitude of the gun barrel. The device actually requires one more “part”—something that identifies a target. In the tank, that would be the tank commander or the gunner. Once the stabilized tank gun was trained on a target and the stabilizer engaged, the gun would be automatically adjusted to remain fixed on the selected target until disengaged, but only in the single axis (altitude).

For more complicated movements, like those of an airplane in flight, five more sensors are needed to address all possible motions. There are three planes of motion in space and two types of motion for each: linear acceleration (translation) and rotation. The three planes of motion for rotation are: (1) roll (as in tipping your head to the left and right, with the axis of rotation being longitudinal, like a rod passing through the base of your neck from front to back); (2) pitch (as in nodding your head up and down to indicate “yes,” with the axis of rotation being lateral, like a rod passing between your ears); and (3) yaw (as in shaking your head to indicate “no,” with the axis of rotation being vertical, like a rod passing through your spine and top of your head). These axes are called the xy, and z axes, respectively. (The three linear accelerations I hope to address in a later article.)

As you were reading the above, you might have simultaneously exercised the head motions described. How did your eyes remain fixed on the “target” of the text that you were reading as you moved your head? It turns out we, too, have a stabilizing system for our eyes!

Stabilizing Vision

Every function of a gun stabilizer is also found inside the human head. Focusing here only on the movement of rotation, we have a system of semicircular canals (or ducts) in our inner ears that act like liquid “gyro”-detectors in each of three axes (see here). These anterior, posterior, and horizontal canals are partially circular hollow tubes in right-angle relationship to each other and filled with a liquid called endolymph. Each tube ends in a widened section containing the Crista ampullaris. This feature has a gelatinous cupula that serves as a sail to capture the relative movements of the endolymph against it. The cupula is attached to a base of sensory hair cells that extend hair-like filaments into the cupula.

Since the endolymph is denser than the cupula, when we rotate our heads within one of the axes, the appropriate canal (or combination) is rotated but the liquid inside resists movement as it lags behind and thereby deflects the cupula and the tiny hairs extending into it. These tiny hairs produce variable electrical nervous impulses when deflected by the movement of the contained liquid. The semicircular canals are our “gyros” and detectors!

Like a tank commander, we engage our visual attention when we identify a visual “target” and disengage it at other times. The signals of the semicircular canals are conducted along cranial nerves to the brainstem. To put it simply, the brainstem serves like a tank’s hydraulic pump, commanding an adjustment in response to the detected movements. The commands from the brainstem run along other cranial nerves to the extraocular muscles of the eyes (serving as our cylinder-and-piston assembly), executing movements in exactly the opposite direction of the head’s turn and to the same angular deviation such that the eyes remain on the identified target.

This vision stabilization function is known as the vestibulo-ocular reflex (VOR). Knowledge of this anatomy and physiology allows a physician to assess the integrity of a comatose patient’s brainstem using either a test known as the “doll’s eyes test” or a convection-inducing caloric test.

The actual anatomy and physiology of the VOR is much more complicated, but the basic functional attributes are as I described here. Inside our head we have two such mirrored systems for a degree of redundancy—a feature that would make any aerospace engineer proud! Each system is located deep in each ear in the bony labyrinth of the temporal skull bones. The designed nature of this system should be evident to any unbiased evaluator, but this is only part of the story.

Despite the interoperable physical traits of a device to enable stable tracking, you still have to have an entity that identifies a target. This is not explainable by biochemical laws, but is rather a metaphysical—or more properly, ontological—function. Our world is filled with objects to look at, but only minds can judge context and assign relative value to those objects. To interpret value there must exist an intelligence that is outside and above (i.e., transcendental to) the equipment; such is the uniform observational experience of humanity. As is argued by atheist philosopher Thomas Nagel, the current neo-Darwinian view of life’s origins cannot account for a transcendental capacity.2

Dr. Eddy M. del Rio

Dr. Eddy M. del Rio received his MD from Saint Louis University in 2004, and currently serves as a practicing physician for the Veterans Health Administration in the greater Springfield, MO region.

  1. Gold V. Sanders, “Why Our Tanks Can Score Hits on the Run,” Popular Science 145 (September 1944): 82–85.
  2. Thomas Nagel, Mind and Cosmos: Why the Materialist Neo-Darwinian Conception of Nature Is Almost Certainly False (Oxford: Oxford University Press, 2012).