The morphological basis to the mammalian vestibulo-ocular system: the influence of skull architecture locomotion and phylogeny

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Fix your eyes on a distant object and walk towards it. As you walk, your eyeballs constantly move around to compensate for the movements of your head. The structures responsible for monitoring head movement and feeding this information to the muscles of the eyeball are the semicircular canals of the inner ear. There are three of these half-moon shaped canals, one for each rotation of the head (e.g. tip up-down, rotate left-right and tilt left-right). These supply information via the brain to three pairs of muscles that can move the eyeball in the opposite direction to that of the head (e.g. tip down-up, rotate right-left and tilt right-left). Both the canals and muscles pairs are critical to the lives of all vertebrates. Without rapid communication between the canals and the muscle pairs, the compensatory movements of the eyes would lag behind movements of the head during motion, the image on the eye's retina would blur, and you would soon feel dizzy and eventually fall over. These are the severely debilitating symptoms experienced by patients with conditions like Meniere's disease. Clearly the eye muscles must move quickly to counteract movements of the head sensed by the canals. You might therefore imagine that the plane through each pair of muscles is aligned with the plane of the semicircular canal providing the primary input (e.g. up-down canal lies parallel to the muscle pair that moves the eye down-up). That way the signal output would closely match the direction of action required (e.g. a computer cursor is most easily moved up and down by moving the mouse up and down rather than side to side or diagonally). Such close spatial matching of signal and action would minimize the need for time-consuming readjustments of the signal by the brain. Because an aligned arrangement would seem to be the simplest and most logical, people have assumed that the planes are indeed parallel, or near to parallel. Over decades this assumption has gradually gained in credence, even appearing in some textbooks and as the basis for mathematical models of inner ear and eye muscle function. However, it turns out that there is evidence, albeit very limited, to suggest that the planes are not parallel and that the brain must therefore do some processing of the signal arising from the canals. The problem is that because the eye muscles and canals are not generally accessible to physical inspection, the available data is too little to ascertain the extent of the difference between the planes and whether such differences are the same in all animals, particularly those used in research. This means several important questions remain unanswered: First and foremost, are the mathematical models used in research to understand, for example, motion sickness employing the appropriate numbers? Second, is the difference in orientation connected to whether you normally move around rapidly or slowly, or on two or four legs? Or, is it primarily due to differences in the position of the eyes within the skull: for example is the arrangement the same in cats where the eyes face forward as it is in rabbits where the eyes face sideways. To answer these and other questions I propose imaging the heads of 100 dead mammals and measuring the difference in the plane of each pair of eye muscles and the corresponding semicircular canal. Twenty to forty rhesus monkeys and rabbits will be examined to determine variations within species. The remaining 40-60 specimens will be sampled to represent a wide range of sizes, types of body movement and skull shapes. I propose imaging the mammals with a magnetic resonance imager (MRI). This will enable me to look into the head without physically damaging it. Associations of the divergence of the planes with body size, mode of body movement and skull architecture will be evaluated with a set of complex mathematical tools called geometric morphometrics.