Harold E. Bedell - US grants

The purpose of the proposed research is to identify and characterize the mechanisms by which persons with congenital nystagmus perceive a stable visual world, despite uncontrolled rhythmic oscillation of their eyes. Normal eye movements are accompanied by extraretinal signals of changes in eye positions which offset the retinal image motions that occur when the eyes move. These extraretinal signals are often apparently incomplete or absent in persons with nystagmus, indicating that other mechanisms must contribute to perceived visual stability. We suggest that persons with congenital nystagmus suppress visual information during all but a small fraction of each eye movement cycle, akin to the visual suppression that accompanies normal saccades. In addition, we suggest that some persons with nystagmus adapt to the repetitive oscillation of their retinal image, such that image motions produced by the nystagmus are perceptually discounted or ignored. The suppression and adaptation, which we propose contribute to perceived stability in persons with nystagmus, are envisioned to be extensions of mechanisms that also play a part in normal vision. Hence, the results of the studies proposed here to characterize the operation of these mechanisms in persons with nystagmus should improve understanding of how visual stability is achieved during normal eye movements as well as in oculomotor abnormalities such as nystagmus.

This research will investigate how persons with congenital nystagmus maintain a stable and relatively clear visual world, despite the involuntary rapid oscillations of their eyes. In normal vision, extra-retinal signals accompany voluntary eye movements and help offset the retinal image changes that occur. In persons with nystagmus, the extra-retinal signals usually underrepresent eye movement amplitude (according to patients' reports of afterimage motion in the dark) and therefore supplementary mechanisms must contribute to perceived stability of the world. We will characterize precisely the amplitude and phase of extra- retinal signals for congenital nystagmus and determine whether the perceptual and motor systems receive the same or different signals. Parallel studies with normal subjects will evaluate whether extra- retinal information is available about an involuntary eye movement, optokinetic afternystagmus. In persons with nystagmus, thresholds are elevated dramatically for unreferenced motion in the meridian of eye movements. This insensitivity to (or discounting of) particular motions could contribute to perceptual stability. Unreferenced motion thresholds might be raised (at least in part) because movement of the target is obscured by image motions resulting from nystagmus. This possibility will be tested. In addition, we will determine how thresholds for relative motion and flicker, and the perceived velocities of suprathreshold motions, are affected. Visual acuity in persons with congenital nystagmus is better than expected from the typically large and rapid motions of the retinal image. Hypothetically, this occurs because the eyes slow momentarily as the fovea passes over a target of regard. This hypothesis will be tested by measuring acuity for targets presented at different phases of the nystagmus. In addition, the contributions to reduced acuity of 3 potential mechanisms will be determined: retinal image smearing, neural modification (an amblyopia), and variability in the accuracy of slowing the fovea on target. This research will contribute to understanding how stable and clear vision is maintained in persons with nystagmus, and also in normal vision. Further, it will indicate how visual functioning might be changed if congenital nystagmus is reduced, and so may lead to improved strategies for treatment.

biomedical equipment purchase;

statistical service /center; statistics /biometry; eye disorder; vision; biomedical facility;

DESCRIPTION (Provided by applicant): In persons with congenital nystagmus (CN), rhythmic to-and-fro oscillations of the eyes produce nearly continuous motion of the retinal image. Nevertheless, individuals with CN rarely report the visual world to be moving or smeared. Recent observations indicate that the retinal image motion that accompanies voluntary smooth pursuit and saccadic eye movements in normal observers results in less perceived motion smear than does comparable motion of the retinal image when the eyes are still. The underlying premise of the studies described in this proposal is that the mechanisms responsible for the attenuation of perceived motion smear during normal eye movements are similar to those that mediate the relatively clear vision that is reported by persons with CN. The overall goal of this research project is to characterize the attenuation of perceived motion smear that occurs during normal eye movements and to evaluate possible underlying mechanisms. This goal will be addressed in the context of four specific aims: (1) Determine whether perceived motion smear is attenuated during normal involuntary eye movements, which are presumed to be neurologically similar to the involuntary ocular oscillations in persons with CN, (2) Identify whether the presence of an extra-retinal signal for eye or gaze movement, or the-perception of a stationary target in space is the necessary condition for the attenuation of perceived motion smear, (3) Determine how the attenuation of perceived motion smear during eye movements interacts with the motion deblurring of complex visual scenes that is based on metacontrast masking, and (4) Assess whether a hypothesized increase in temporal processing speed during eye movements can contribute to the reduction of perceived motion smear. The proposed program of research will clarify the mechanisms by which perceived motion smear is attenuated during normal eye movements, and will provide evidence about the likely contributions of these mechanisms to the clarity of vision reported by persons with CN.