Ocular torsion: sensorimotor principles.

Eye movements are basically limited to two degrees of freedom, as described by Donders' and Listing's laws. Complex patterns of innervation maintain the geometric constancy of the retinal meridians, presumably via visual feedback acting through orientation-specific neurons in the cortex. Interactions between torsional and vertical movements of the eyes can be demonstrated by stimulating ocular torsion and detecting secondary vertical deviations. Geometric considerations show that significant peripheral disparities occur with convergence and eye movements, dictating large Panum's fusional areas in the periphery. Good visual acuity in man must thus be limited to the fovea, but the large peripheral Panum's fusional areas allow sensory cyclofusion of up to 8 degrees. In addition, motor cyclofusion occurs in the presence of large-field stimuli, further helping to minimize problems from torsional diplopia both in normal individuals and in patients with ocular motor pareses.