COLOR AND CONNECTIVITY IN CONE PHOTORECEPTORS

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The transformation of an image on the retina into a perception of physical objects begins with the transduction of photon energy into electric signals in rod and cone photoreceptors. Prior to leaving the eye the signals of the -100 million photoreceptors are compared, compressed, averaged, analyzed and reduced into 1 million channels of information flowing in the optic nerve. These ganglion cell nerve fibers carry the information of form, brightness, color and motion encoded as action potentials. Photoreceptors were once thought to act as independent light sensors, like the individual grains of a photographic film, but this appears not to be true. In fact the photoreceptors, the first layer of nerve cells containing visual information, are in contact with one another and therefore may also be responsible for the first level of visual information analysis by the nervous system. There are two hypotheses that will be tested with this grant: Hypothesis I: The subtypes of human red and green cone pigments are expressed randomly one gene per cone cell. A direct examination of human cone spectral sensitives will be compared to the analyzed sequences of the visual pigments proteins. Electrical recording will give the precise descriptions of the spectral sensitivity functions and in parallel the number and variety of visual pigment proteins expressed will be determined. Specific questions are: What ate the spectral sensitivities of human cones? Are multiple copies and varieties of the visual pigment genes functionally expressed of visual pigment genes limited to one per cell? Hypothesis 2: Spectral classes of cones are distinct cell types, that form coupled networks which exclude other spectral classes of cones. Physiologic and anatomic techniques will be used to evaluate of the size and spectral characteristics of a cone's receptive field, this entails a quantitative determination of the strength of synaptic contact between neighboring cones. The presence of three spectral classes of cones in humans raises the issue of how the eye maintains the integrity of its three color "channels" when pooling of information is likely to occur even within the photoreceptor layer itself. The following questions will be approached: (1) Ate mammalian cones eclectically coupled to their neighboring photoreceptors? (2) Are synaptic interactions with neighboring photoreceptor limited to those of the same spectral type? (3) What is the lateral spread of excitation in human and non-human mammalian cones, and (4) are the physical dimensions of the lateral processes between neighboring photoreceptors quantitatively related to the physiologically measured space constant of the cell?