An acronym for small in large out. It refers to the presumed change of the perceived size of a test object that a patient experiences, while maintaining fusion when convergence or divergence is varied. When convergence is increased, with BO prisms, the object may appear to become smaller and nearer. When divergence is increased with BI prisms the object may appear larger and further away. The SILO is not universal; children tend to respond that way, but adults commonly respond in the opposite way, that is, if the test object becomes smaller they report it as moving away from them. The acronym is then given as SOLI. The response is used in visual therapy as a feedback mechanism to patients about their performance. See base setting; biofeedback; sensory fusion.