MOVING THE RETINA - CHOROIDAL MODULATION OF REFRACTIVE STATE

The chick eye is able to change its refractive state by as much as 7 D by pushing the retina forward or pulling it back; this is effected by changes in the thickness of the choroid, the vascular tissue behind t he retina and pigment epithelium. Chick eyes first made myopic by wear ing diffusers and then permitted unrestricted vision developed choroid s several times thicker than normal within days, thereby speeding reco very from deprivation myopia. Choroidal expansion does not occur when visual cues are reduced by dim illumination during the period of unres tricted vision. Furthermore, in chick eyes presented with myopic or hy peropic defocus by means of spectacle lenses, the choroid expands or t hins, respectively, in compensation for the specific defocus imposed. Consequently, when the lenses are removed, the eye finds its refractiv e error suddenly of opposite sign, and the choroidal thickness again c ompensates by changing in the opposite direction. If a local region of the eye is made myopic by a partial diffuser and then given unrestric ted vision, the choroid expands only in the myopic region. Although th e mechanism of choroidal expansion is unknown, it might involve either a increased routing of aqueous humor into the uveoscleral outflow or osmotically generated water movement into the choroid. The latter is c ompatible with the increased choroidal proteoglycan synthesis either w hen eyes wear positive lenses or after diffuser removal.