Center for Visual Science

David R. Williams

Limits of Human Vision

I use advanced psychophysical, anatomical, and imaging techniques to study how the structure of the eye and brain affects visual experience. My Ph.D. thesis at the University of California, San Diego was completed under the direction of Donald I. A. MacLeod in 1979. In collaboration with Mary Hayhoe, we mapped the distribution of individual short-wavelength-sensitive photoreceptors in the human retina. These experiments demonstrated that it is possible to stimulate a single photoreceptor with light in the living human eye. They also showed that only 10 photons absorbed by a single photoreceptor are necessary to produce a visual sensation. In 1979, I joined the Technical Staff at Bell Laboratories in Murray Hill, New Jersey, working with John Krauskopf. We used a three-laser colorimeter to study the opponent mechanisms that underlie human color vision. In 1981, I joined the faculty at the University of Rochester. During my early years at Rochester, I developed a new laser interferometer to stimulate the retina with interference fringes that are not blurred by the optics of the eye. Looking into this instrument, observers could detect interference fringes at very high spatial frequencies because of aliasing by the photoreceptor mosaic. These observations revealed clearly for the first time the theoretical limitations predicted by sampling theory on human visual resolution and allowed the first measurements of the spacing of cones in the living human eye. Walt Makous, Donald Macleod, and I have also used interference fringes to measure the sizes of receptors in the living eye.

A more recent project, in collaboration with Orin Packer and David Bensinger, has provided the first color images of the primate photoreceptor mosaic that allow us to distinguish the three cone types responsible for human color vision. An example of such an image is shown below.

Image by Orin Packer and David Bensinger

We constructed a custom microscope equipped with a very sensitive camera that allows us to take pictures of the cones at low enough light levels that the photopigment in the retina are not bleached. These experiments reveal the packing arrangement and relative numbers of the three cones and clarify how the retina encodes color information for transmission to the brain.

A current research project uses adaptive optics to explore the optical quality of the eye and the organization of the human retina. We measure the aberrations of the eye with a Hartmann-Shack wavefront sensor, first developed by Junzhong Liang. These measurements are used to control the shape of a deformable mirror. This mirror can be warped into the appropriate shape to correct most of the eye's aberrations, providing the eye with the best image quality ever achieved. When a subject looks through the mirror, psychophysical experiments can be performed to study the optical and neural limits on human vision. When the experimenter looks through the mirror at the subject's retina, he has a much sharper view of the retina than has ever been possible before. With this and other advanced imaging techniques, J. Liang, Don Miller, and I have been able to resolve features at the spatial scale of single cells for the first time in the living retina. The image below shows the mosaic of cone photoreceptors in the living eye taken through adaptive optics by Austin Roorda, a former post-doc in the lab.

Mosaic of cone photoreceptors in the living eye.
Image by Austin Roorda

Additional images can be viewed here. Adaptive optics may ultimately prove valuable in the diagnosis and treatment of retinal disease. Wavefront sensing has interesting applications for improving contact lens design and laser refractive surgery.

Since 1991, I have served as Director of the Center for Visual Science, an interdisciplinary research program focused on the mechanisms of human vision. I hold appointments in the Department of Brain and Cognitive Sciences, the Institute of Optics, and the Department of Ophthalmology. In 1997, I became the first recipient of the William G. Allyn Chair of Medical Optics at the University of Rochester.

Selected Publications

Williams, D.R. (1990) Photoreceptor sampling and aliasing in human vision. In: Moore, D.T. (Ed.), Tutorials in Optics, Optical Society of America.

Williams, D., Sekiguchi, N. and Brainard, D. (1993) Color, contrast sensitivity, and the cone mosaic. P.N.A.S., 90, 9770-9777.

Williams, D.R., Brainard, D.H., McMahon, M.J., and Navarro, R. (1994) Double-pass and interferometric measures of the optical quality of the eye. J. Opt. Soc. Am. A, 11, 3123-3135.

Packer, O.S., Williams, D.R., and Bensinger, D.G. (1996) Photopigment transmittance imaging of the primate photoreceptor mosaic. J. Neurosci., 16, 2251-2260.

Miller, D., Williams, D.R., Morris, G.M., and Liang, J. (1996) Images of the cone mosaic in the living human eye. Vision Res., 36, 1067-1080.

Liang, J. and Williams, D.R. (1997) Aberrations and retinal image quality of the normal human eye. J. Opt. Soc. Am., A., 14, 2873-2883.

Liang, J., Williams, D.R., and Miller, D.T. (1997) Supernormal vision and high resolution retinal imaging through adaptive optics. J. Opt. Soc. Am., A., 14, 2884-2892.

Roorda, A. and Williams, D.R. (1999) The arrangement of the three cone classes in the living human eye. Nature. Vol 397, 11, 520-522.

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