Semicircular Canal Size and Shape Influence on Disorientation.

INTRODUCTION: Orientation in a 3-dimensional environment increases demands on the vestibular system. The anterior, lateral, and posterior semicircular canals sense pitch, roll, and yaw acceleration, respectively. The macular end organs sense linear acceleration and provide gravity reference. Creatures that evolved to fly (birds) share the same sensory systems and probably the same limitations as creatures that did not evolve to fly. However, bird semicircular canals have been noted for their large size relative to body mass since 1907.

METHODS: A brief literature review was conducted regarding the morphological structure of the vestibular systems in birds, and their evolutionary and functional significance as compared to that in humans.

RESULTS: For any given body mass, qualitatively more "agile" species have semicircular canals with larger radius of curvature. Specifically, the anterior and lateral canals were found to be preferentially larger than those of nonavian theropods and humans. Avian canals rarely approach circularity and all three canals typically undergo torsional excursions out of their respective planes.

DISCUSSION: Increase in the length of the semicircular canal is associated with greater sensitivity to rotational stimuli. Highly maneuverable birds of prey have the largest overall canal dimensions relative to body mass, most plausibly to match the rapid body rotations that characterize variable speed maneuvering during pursuit. By comparison, the semicircular canals in humans are relatively smaller and lack the adaptive morphological asymmetry in agile flying species. This teleological symmetry may present another inherent but less understood vestibular limitation during aerobatic maneuvers, which could lead to spatial disorientation.Cheung B, Ercoline W. Semicircular canal size and shape influence on disorientation. Aerosp Med Hum Perform. 2018; 89(8):744-748.