THE word "aniseikonia" is derived from the Greek roots ἄνισος and εἰκών meaning literally "unequal image." It may be defined as that condition of the binocular visual apparatus in which the ocular images as seen by the two eyes are unequal in either size or shape or in both. The term ocular image does not refer to the retinal image, but it is applied to the final impression which reaches consciousness in the higher brain centers.
Aniseikonia is of two principal types: (1) over-all difference, in which one image is larger than the other in all meridians, and (2) meridional difference, in which one image is larger than the other in one meridian. Combinations of the two forms may occur. If the ocular images are markedly unequal in size and shape, there may be a disturbance of binocular vision, which may or may not be apparent to the patient.
ELLIOT AJ. SIGNIFICANCE OF ANISEIKONIA IN AVIATION. Arch Ophthalmol. 1946;35(4):354–360. doi:10.1001/archopht.1946.00890200361003
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