Suzanne P. McKee - US grants

Binocular correspondence is the study of how the two monocular images are correctly fused in normal binocular vision, creating a sense of depth and permitting the detection of small disparity differences as in fine stereoacuity. Psychophysical experiments on a repetitive stimulus, the ambiguous stereogram of the "wallpaper effect", will be used to probe the operations of normal fusion. The basic stimulus for the proposed research is a small grid of bright points displayed haploscopically. Manipulation of the disparity of the edges of this display can force the fusion of this target in several different depth planes, without the intervention of changes in convergence. Two theoretical models of fusion will serve as a framework for the proposed experiments. Specifically, the proposed research will examine three questions: 1) If the edge effects are propagated via cooperative interactions between local disparity detecting neural elements, what are the spatial and temporal arrangements which produce the change in the apparent depth of this target? 2) If the edge effects represent disparity responses of low spatial frequency channels, how do the low spatial frequency channels surpress the disparity signals of higher spatial frequency channels? 3) If fusional processes depend on interactions between competing disparities, how is the stereoacuity for a target lying one plane affected by adjacent targets lying in other depth planes? This research will supply fundamental information about normal binocular fusion. A preliminary study on patients with abnormal fusion, also part of the proposed research plan, will examine how abnormal binocular mechanisms respond to these grid stimuli.

The research program at the Smith-Kettlewell Eye Research Institute includes projects on oculomotor processing, binocular vision, image motion processing, the development of spatial and temporal vision in human infants, surgical techniques in strabismus, psychophysical measurements in amblyopic observers, neural plasticity, retinal aspects of temporal processing, diagnostic use of flicker measurements in glaucoma, and special techniques for measuring local ERG's. All of these projects have benefitted greatly from past support of CORE modules. Continued support for the Electronics Services Module (hardware design and manufacture), the Computer Software Services Module (software development and maintenance) and the Administrative and Secretarial Module (manuscript and grant typing) is requested. These services are an integral part of the resources of this institution, providing assistance to both on-going and pilot projects and encouraging the collaborative arrangements that have long been typical of Smith-Kettlewell research.

DESCRIPTION (provided by applicant): We will design an objective test, based on the Visual Evoked Potential (VEP), to measure amblyopic suppression. Our basic tool is the VEP monocular response to periodic vernier onset/offset transitions in bar stimuli, and its suppression during dichoptic stimulation with matching contours. What is unique about this approach is that it identifies which eye is being suppressed as well as the visual acuity of each eye. Although we will develop and validate this test in adult subjects, our dichoptic visual display system has been specifically designed for ultimate use with infants and pre-verbal children. We also will use a new multi-channel recording system (Geodesic Sensor Net) to explore the neural basis for the differences between normal and abnormal binocular processing. About 4% of all children suffer from strabismus and/or anisometropia during early development. These abnormalities place an individual at risk for a variety of visual deficiencies. The most serious outcome is amblyopia in one eye. Amblyopia refers to a loss of visual acuity, without obvious organic cause, that cannot be corrected by wearing spectacles or contact lenses. Perhaps one third of strabismics and anisometropes suffer some loss in visual acuity. The proposed test could assist with the diagnosis of amblyopia in infancy, as well as in monitoring progress during treatment, thereby improving the chance of a favorable outcome.

DESCRIPTION (provided by applicant): Disparity tuning is a pervasive property of neurons in almost all visual areas in primate cortex. We will use a novel neuroimaging technique to examine the flow of disparity information from V1 to extra- striate areas in human cortex. This technique relies on source localization using high density EEG coupled to structural and functional MRI anatomical measurements. We will measure the neural response to repetitive changes in disparity in four widely separated cortical areas, V1, V3A, V4 and hMT+ - all known to be important in primate disparity processing. Our first aim examines the sensitivity of the cortical response in these four areas to dynamic random dots, modulated by horizontal or by vertical disparity. We will also explore the response to anti-correlated random dots which are known to drive single units in many cortical areas. Our second aim evaluates the contribution of disparity-modulation to surface segmentation. We will compare the response patterns produced by monocularly modulated changes in figure-ground segmentation to those generated by disparity modulated changes in figure-ground segmentation. Our third aim explores how the loss of stereopsis affects the cortical responses of the strabismic observer. We will specifically look for the cortical locus of strabismic suppression, as well as for the locus of the motion asymmetry that is a defining characteristic of infantile esotropia. PUBLIC HEALTH RELEVANCE Strabismus is a developmental abnormality that affects 3 - 5% of the population, resulting in the loss of stereopsis. The proposed research will measure the cortical response associated with two anomalies of strabismus: strabismic suppression and the motion asymmetry previously observed in VEP measurements. The motion asymmetry is a useful marker for judging the efficacy of treatment, particularly surgery, so understanding its cortical origin will enhance its utility.