The vision specialist often finds himself asking the question, "What are the immediate effects of my lens and/or prism therapy on a given patient?" If for example, a change in accommodation or convergence is effected by lenses, what will be the results of this change on the visual performance of the patient as far as his intersensory localizations of objects in space are concerned? Past experience by some traditional practitioners would dictate that positive lenses and prism base-in will tend to force a subject to localize farther out than his habitual localization pattern. Minus lenses·and prism base-out tend to localize closer than he normally would. This effect, they would say, is an illustration of the phenomenon known as SILO. The letters SILO stand for the phenomenon of smaller-in and larger-out. For example, if a subject views an object through minus spherical lenses or base-out prisms, he will experience the object as being smaller and closer whereas if he views the same object through plus spherical lenses or base-in prisms, he will experience the object as being larger and farther away. Our thesis deals only with half of the SILO effect, i.e., the perceived distance. Recently, some developmentalists have postulated that localization may be attributable to postural functions of accommodation. Specifically, since plus lenses move the posture out in space, the subject will localize farther out also. The opposite is true for minus lenses. On the other hand, some psychologists believe that the extraocular muscles relay information to the brain as to the position of the eyes in the orbit. Therefore, can we attribute changes in eye-hand coordination and intersensory localization to accommodation, convergence, or both? With this in mind it was our intention to investigate the above question utilizing the addition of spherical lenses and prisms over the habitually worn prescription of a subject in order to artificially change the accommodation and convergence and then measure the change in intersensory localization. Intersensory localization being the observer's ability to judgementally or behaviourally map one modality on to another.
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