Visual processing starts with an optical image formed on the retina by the optics of the eye. The retina senses the image with its mosaic of cone and rod receptors that are sensitive to different wave length light and transmits the information to the brain. The retina is a highly complex organ enormously adaptable to ambient light conditions over a range of 10 to the power of 10. At night, resolution (acuity) and discrimination of colors are sacrificed for sensitivity in detection through switching from the cone to the rod receptors as well as modifications of the processing in several layers of specialized cells.

The retina is very inhomogeneous in its architecture as well as in its function. The very center that you are using as you read this text is specialized for high acuity under daylight condi tion while the more peripheral areas excel under moonlight illumination. This is accomplished through different distributions of rod and cone receptors as well as by other cells and connec tivities involved in retinal processing. The noninvasive investigation of these local functional differences in the human retina are the main focus of our studies.

Direct access to local retinal function is possible by means of the electroretinogram, a bio electrical response to visual stimulation that originates in the retina and is derived from the eye by means of a special contact lens. A stimulation and analysis technique developed in this labora tory permits us to extract the responses simulta neously at hundreds of retinal locations from this signal and derive the response topography. Through different modes of visual stimulation specific retinal mechanisms can be preferentially stimulated and their distribution studied. The distribution of response components can vary enormously. For many of these components the exact source within the retinal layers is not yet known. In some instances we have already been able to identify specific sources by matching up response topographies with the distribution of known anatomical properties.

Clinical Applications

A thorough understanding of the highly complex electroretinogram will clear the way to many future clinical applications of response topography mapping. The first indication of disease affecting the retina is often a local reduction in retinal performance. The ability to test retinal function locally and layer by layer is, therefore, of great importance. Perimetry, the only local functional test of the retina currently avail able to the clinician, requires the patient's con trolled response, is very time consuming, and is somewhat subjective. We expect that in many applications electroretinographic topography mapping will replace perimetry because of its speed and lack of active involvement of the patient. In addition, it may lead to new clinical tests, as it permits access to different, highly specific local information concerning retinal performance.

Co-authors: Duong Tran; Shuang Wu, Ph.D.; Adam Reeves, Ph.D.

Supported by the National Eye Institute.