DISPARITY TUNING OF BINOCULAR FACILITATION AND SUPPRESSION AFTER NORMAL VERSUS ABNORMAL VISUAL DEVELOPMENT

¹ The Smith-Kettlewell Eye Research Institute, 2318 Fillmore St, San Francisco, California, 94115, United States
² Bristol Eye Hospital, Bristol, United Kingdom
³ Smith-Kettlewell Eye Research Institute, San Francisco, California, United States

^* To whom correspondence should be addressed. E-mail: amn{at}ski.org.

	Abstract

PURPOSE: To study the pattern of facilitatory and suppressive binocular interactions in stereo-deficient patients with strabismus and in normal controls. METHODS: Visual Evoked Potentials were recorded in response to a vernier onset/offset pattern presented to one eye, either monocularly or paired dichoptically with a straight vertical square-wave grating, which when fused with the target in the other eye gave rise to a percept of a series of bands appearing in depth from an otherwise uniform plane or with a grating that contained offsets that produced a standing disparity and the appearance of a constantly segmented image, portions of which moved in depth. RESULTS: Participants with normal stereopsis showed facilitative and suppressive binocular interactions that depended on which dichoptic target was presented. Patients with long-standing, constant strabismus lacked normal facilitative binocular interactions. The response to a normally facilitative stimulus was reduced below the monocular level when it was presented to the dominant eye of patients without anisometropia, consistent with classical strabismic suppression of the non dominant eye. The dominant eye of strabismic patients without anisometropia retained a suppressive input from crossed but not uncrossed disparity stimuli presented to the non-dominant eye. CONCLUSIONS: Abnormal disparity processing can be detected with the dichoptic VEP method we describe. Our results suggest that suppression in stereoblind, non-amblyopic observers is determined by a binocular mechanism responsive to disparity. In some cases, the sign of the disparity is important and this suggests a mechanism that can explain diplopia in patients made exotropic after surgery for esotropia.

Key Words: visual development, binocular vision, stereopsis, visual evoked potential, amblyopia, strabismus

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