Fig. 2. The heterotropic deviation may be sufficiently small to cause the object of regard to be diplopic, but the peripheral objects displaced equidistant to the object of regard from the horopter may still be within Panum's area. Thus it is possible for a patient with a small heterotropic deviation to have extramacular fusion in the absence of macular fusion. (From Symposium on Strabismus, Transactions of the New Orleans Academy of Ophthalmology. St Louis: CV Mosby, 1971.)