Background: Primitive adaptations in lateral-eyed animals have programmed the oblique muscles to counterrotate the eyes during pitch and roll. In humans, these torsional movements are rudimentary. Purpose: To determine whether the human oblique muscles are vestigial. Methods: Review of primitive oblique muscle adaptations and exaptations in human binocular vision. Results: Primitive adaptations in human oblique muscle function produce rudimentary torsional eye movements that can be measured as cycloversion and cyclovergence under experimental conditions. The human torsional regulatory system suppresses these primitive adaptations and exaptively modulates cyclovergence to facilitate stereoscopic perception in the pitch plane. It also recruits the oblique muscles to generate cycloversional saccades that preset torsional eye position immediately preceding volitional head tilt, permitting instantaneous nonstereoscopic tilt perception in the roll plane. Conclusions: The evolution of frontal binocular vision has exapted the human oblique muscles for stereoscopic detection of slant in the pitch plane and nonstereoscopic detection of tilt in the roll plane. These exaptations do not erase more primitive adaptations, which can resurface when congenital strabismus and neurologic disease produce evolutionary reversion from exaptation to adaptation.
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