Background: The rate of ocular growth can be modified by the imposition of either plus or minus lenses before the eyes, which creates changes in retinal defocus. Several hypotheses with relatively complicated mechanisms have been proposed to explain these changes, but the underlying mechanism has remained elusive.
Material/Methods: Our new analysis using schematic models, however, provides a relatively simple and logically-consistent explanation of how retinal defocus magnitude alone during ocular growth gives the requisite sign for an appropriate change in ocular growth rate.
Results: During a normal genetically-determined incremental increase in axial length, the presence of either an imposed plus or minus lens results in an increase or decrease, respectively, of the blur circle magnitude. Neuromodulators in the retina are proposed to regulate the sensitivity to retinal-image contrast by means of a local feedback mechanism, and the alteration in retinal-image contrast associated with the change in blur circle causes an increase or decrease, respectively, in the rate of release of neuromodulators as well as the rate of proteoglycan synthesis, the latter being associated with the structural integrity of the sclera.
Conclusions: This provides the critical sign, as well as amplitude, information needed to modulate appropriately the rate of eye growth, to result in a decrease or increase, respectively, in the rate of ocular growth.

Keywords: contrast, Myopia, axial length growth rate, retinal-defocus, refractive error development