The prevailing illumination affects one's ability to see spatial patterns. The long term goals of this project are to explain how optical and neural factors in the human visual system affect spatial visibility at low luminance. Prior research in this laboratory shows that the eye's optical quality is detrimental for vision in dim light. Spatial vision is also limited by the density of visual neurons and the prior research indicates that the retinal image becomes more coarsely sampled by the visual system as the light level decreases. The perception of suprathreshold patterns is altered at dim light levels and this may be caused, in part, by changes in the spatial sampling properties of vision. The proposed study is comprised on behavioral experiments on human observers. 1) The first specific aim is to study the effects of abnormal optical quality on vision at low luminance. A rapid, objective technique to measure the eye's optical quality may allow one to predict a patient's visual performance at low luminance. Efforts to improve retinal image quality may enhance the patient's night vision. 2) The second aim is to quantify the optical and neural contributions to peripheral spatial vision at low luminance. The peripheral retina is the most sensitive retinal area under dim illumination, yet the optical quality of the eye is reduced for off-axis viewing. Experiments are proposed to measure the peripheral optical quality in normal eyes and to determine whether enhancement of the peripheral optics, such as by correcting astigmatism, would improve visibility in the peripheral visual field for night vision tasks. Peripheral sensitivity also varies according to the stimulus orientation. These meridional differences in sensitivity are due, in part, to optical astigmatism but they may also have a neural origin. Neural sensitivity will be examined for different orientations by measuring spatial contrast sensitivity with interference fringes that are formed directly on the retina. 3.) The third aim is to investigate the sampling density of peripheral neurons for different stimulus orientations. The density of retinal neurons can be inferred from psychophysical studies of aliasing. Under dim illumination, aliasing reflects the sampling rate of a more proximal stage of neurons than the retinal photoreceptors. Aliasing will be examined for various stimulus orientations to determine if the sampling rate is coarser for certain orientations. Differences in the sampling rate may responsible for the meridional variation in sensitivity.