Clifton M. Schor - US grants

DESCRIPTION (provided by applicant): It is remarkable how well the oculomotor system is able to maintain precise alignment of the two eyes. If all eye movements were between distant targets and required equal movements in the two eyes the task would be relatively simple. However, naturally occurring eye movements are very often between targets that differ in viewing distance and eccentricity and so require a disconjugate component. Good binocular alignment may also be affected by development, disease, and injury. The proposed experiments examine the processes by which subsystems such as vergence, smooth pursuit, and saccades interact to produce the requisite changes in binocular alignment. For example, despite the claims of some investigators, it is still not certain whether all horizontal vergence movements are the result of control by the horizontal vergence system or if there is some capacity for disconjugate movements by the pursuit or saccade systems. Similarly, it is not clear whether the so-called disconjugate adaptation of saccades is really disconjugate adaptation of the saccadic pulse or step or is simply the addition of vergence movements onto conjugate saccades. We will address some common problems with the interpretation of prior experimentation in this area. The proposed experiments fall into three general categories: The first is a series of experiments that represents a continuation of our adaptation experiments on vertical vergence and examines the neural control of disconjugate smooth pursuit. The second will examine more closely the binocular control of saccades and the interaction of saccades with horizontal and vertical vergence movements. The third continues our exploration of the binocular control of eye movements during head tilt and the relationship between ocular counterroll and vertical skew. The results of these experiments should provide definitive answers to several longstanding questions concerning the coordination of binocular control.

An investigator requests 5 years of support to continue psychophysical and eye movement experiments on human observers. The proposed research focuses on the functional properties of the mechanisms that mediate stereopsis and initiate vergence eye movements for large binocular retinal image disparities that are normally perceived as diplopic. The proposed studies will directly test several key features of current models of sensory and motor binocular vision and provide descriptive data that are needed to further develop these models. In addition, the results may provide insight into how unilateral defocus associated with an anisometropia initiates interocular inhibitory processes that could potential disrupt binocular vision.

Spatial selectivity of stereoscopic mechanisms in human vision that process disparity for the perception of depth will be compared to that of mechanisms that control horizontal disparity vergence. The proposed studies will investigate sustained and transient components of stereopsis and vergence. Oculomotor and psychophysical experiments will examine the orientation and spatial frequency specificity and contrast polarity required for dichoptic stimuli of transient and sustained vergence and stereo perception. The results will be used to formulate a model of disparity processing for sensory and motor systems that may be applied to the analysis of vergence and stereo anomalies as well as to interactions between sustained and transient vergence responses within complex natural visual scenes. Sustained and transient disparity vergence responses will be elicited with two paradigms. In the sustained paradigm, strength of a sinusoidally varying disparity stimulus is measured by the extent to which it can elicit a vergence response from a subject attempting to fixate steadily. The gain and phase of continuous, involuntary sinusoidal vergence tracking is measured as a function of the luminance contrast, spatial frequency, interocular spatial frequency difference and orientation difference of the stereogram stimuli. This involuntary tracking paradigm provides an objective assessment of visual system responses to threshold and suprathreshold stimuli, requiring only that the subject attempt to fixate steadily. The transient paradigm is equivalent to a forced choice procedure in which there are two possible binocular matches of the step disparity stimuli, one that produces a crossed disparity and the other an uncrossed disparity. The initial velocity and latency of step vergence responses is measured to assess the relative strength of the two stimuli. These results and those of comparable measures of stereo threshold will be used to assess the spatial selectivity of binocular sensory and motor functions and to determine if a common source of disparity information is utilized b these binocular sensor and motor function.

statistics /biometry; vision; biomedical facility;

ABSTRACT NOT AVAILABLE

Traditional theories going back 154 years to Wheatstone propose that stereoscopic depth perception is stimulated early in the visual cortex by disparities between the retinal images on the left and right eyes (retinal disparity). Alternatively, stereopsis and binocular eye movements could be stimulated in sensory-motor regions of the brain by binocular disparities between perceived directions of the two ocular images (head-centric disparity). These perceived directions result from a combination of retinal image location and eye position information. Under normal viewing conditions, retinal disparities are indistinguishable from headcentric disparities, so it is not possible to determine which stereoscopic vision is based on. However, the two forms of disparity do have different magnitudes when a briefly flashed target is presented with a small interocular time delay near the onset of a rapid eye-movement (saccade). The saccade causes unequal distortions of perceived direction by the two eyes, and these differences produce stereo-depth. This is the first evidence that eye movements can influence binocular disparity stimuli for stereopsis and that binocular disparity is represented in headcentric coordinates.

With support of the National Science Foundation, Dr. Schor will investigate how rapid (saccadic) eye movements produce distortions of stereoscopic-depth perception. These depth distortions reveal how the brain represents binocular disparities that stimulate stereopsis and binocular eye movements. In addition to improving our basic understanding of stereopsis, this work has implications concerning how eye movements might influence the accuracy of depth stimulated by computer-simulated stereo displays that present alternate frames to each of the two eyes. The resulting distortions of space could influence performance in depth localization and aiming tasks.

DESCRIPTION (provided by applicant): Sensory-Motor Control of the Binocular Near Response is a basic study of the progressive reduction of accommodative amplitude that occurs with age (presbyopia) and how it influences the coordination of focusing responses and binocular eye alignment (the near response). Incipient presbyopia begins in childhood and progresses as the amplitude of accommodation declines linearly with age until the end of the fourth decade of life when virtually all eyes become unable to accommodate by changing optical power of the eyes. While research during the past century has concentrated on the biomechanical properties of the structural components of accommodation and on the age-related biomechanical changes in these structures that lead to presbyopia, advances in knowledge of how neural control of accommodation adjusts to these changes has been very limited. This proposal investigates the optimization of the near response by adapting neural control of accommodation and convergence in response to age-related biomechanical changes of the accommodation plant that underlie the natural progression of presbyopia. The proposal addresses (1) the ability to adapt accommodation to preserve youthful accommodative dynamics in response to age-related changes in visco-elastic properties of the crystalline lens, (2) the influence of adapting dynamic accommodation and convergence on their dynamic cross-coupled interactions, (3) adaptation of the static cross-coupled interactions between accommodation and convergence in response to age-related biomechanical changes of the ocular lens, (4) calibration of consensual accommodation to potential unequal aging of the two ocular lenses, and (5) decline of adaptation ability with age. Results will be interpreted with models of the dynamic properties of accommodation and their interactions with convergence, in order to understand how adaptation could extend the linear operating range of the near response with age. Results of these experiments will provide fundamental knowledge about the adaptive mechanisms that compensate for the decline of accommodation with age, and interactions between accommodation and convergence that influence the accuracy of binocular eye alignment. In addition, models developed from the experiments can facilitate the successful design and implementation of new treatments, such as accommodating intra-ocular lenses, designed to offset presbyopia.