Retinal thinning on OCT Central thinning of the outer nuclear layer of the retina 2024-02-24T10:53:49Z Central thinning of the ONL Reduced thickness of the outer nuclear layer in the middle region of the retina. The outer nuclear layer (ONL) of the retina contains the nuclei of the cone and rod photoreceptors. Loss of the cellular machinery found in these nuclei causes irreparable loss of the photoreceptors and the capacity for visual function. This feature can be appreciated by directional optical coherence tomography.