Do You Really Need Your Oblique Muscles?
Adaptations and Exaptations
Michael C. Brodsky, MD
Arch Ophthalmol. 2002;120:820-828.
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.
From the Departments of Ophthalmology and Pediatrics, University of
Arkansas for Medical Sciences, Little Rock.
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