Analysis of Visual Function

Chris Tyler

Christopher W. Tyler, Ph.D.

Having completed my education in England, I have spent the past four decades in the United States exploring the processes of human vision. I have long been fascinated with the question of how the eyes and brain work together to produce sight and its elaboration into the learning of higher cognitive function.. In order for us to see, the light that enters the eye must be converted into a stream of nerve impulses to transmit the eye's picture to the brain. The nerve impulses contain the information about the brightness, color, shape, movement, and distance of the objects in the world. Most of my research has focussed on these basic components of visual perception, and the subsequent processing into object structures.

Since the human brain operates like an electrical system, my approach has been to apply techniques from electrical engineering to the study of vision. From color to motion to 3-D vision, I have introduced engineering concepts to describe the speed and capabilities of the visual system, with its exquisite sensitivity to subtle visual changes.

I began research on eye diseases at Moorfields Eye Hospital in London, England, with the study of flicker sensitivity in glaucoma. With careful new design, this proved to form a more sensitive test of the effects of glaucoma than any current clinical test. It can therefore assist medical professionals to diagnose glaucoma as early as possible, and thus avoid further loss of vision. The same test has generated important information about several other eye diseases such as retinitis pigmentosa (RP) and other retinal and optic nerve disorders.

Our work with electrical brain potentials has revealed a rich complexity in the responses to simple stimuli, implying that we could record many different brain circuits from outside the head. We have developed a rapid method of recording brain responses across a wide range of conditions (the "sweep VEP"), which could then be used to study infant vision. The short recording time required by this new method allowed us to measure visual development with great accuracy. We found that infants can see much better at birth than previously suspected, and have close to adult vision by about eight months of age (although they may not fully "understand" what they see at this age). We can also detect the effects of poor eye coordination more readily than could previous techniques, and help physicians to refine the treatment of infant eye problems.

My studies of retinal diseases have raised questions about how light is turned into electrical energy in the receptors of the retina (rods and cones). We are now exploring the extremely rapid reactions taking place in the first few milliseconds after light hits the photoreceptors. Much of what happens in the photoreceptors controls the rest of vision, so it is important to have a thorough understanding of these early events before the rest can be properly studied. For example, vision in the periphery is much faster than in direct view, but this can probably be explained by the structure of the retinal receptors, with profound implications for later brain processing. Our recent brain imaging studies have also shown for the first time a foveal specialization for the cortical hierarchy processing the detail information essential for reading and other high-resolution visual tasks.

My current interests include brain imaging studies and mathematical modeling of the mechanisms of human stereoscopic depth, motion and face perception and higher cognitive processing such as the learning processes involved in mathematical operations. We have developed new methods to determine the dynamics of the neural population responses underlying brain imaging signals. By designing stimuli to probe specific neural sub-populations, we can use this new methodology to explore their properties in the human brain and the changes in neural dynamics during the learning process.

Collaborators: Lauren Barghout, Heidi Baseler, Patrick Cavanagh, Chien-Chung Chen, Russell Hamer, Lani Hardage, Stan Klein, Lenny Kontsevich, Lora Likova, Lei Liu, Rich Miller, Ken Nakayama, Anthony Norcia, Mark Schira, Cliff Schor Erich Sutter.

Supported by the National Eye Institute, National Science Foundation, AFOSR, Pacific Vision Foundation and Smith-Kettlewell.

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