Neuroscience Graduate Program at UCSF
Phototransduction and Vision in Primates
The light-evoked responses of photoreceptors play a key role in shaping our visual experience. We would like to understand the physical processes that determine our abilities to detect dim light, discriminate between different colors and to achieve optimal visual acuity and temporal discrimination. To study the earliest stages of phototransduction we use suction electrodes to measure photocurrent in single rod and cone outer segments of human and monkey eyes and compare this to corresponding measures of human visual perception. The brightness of colored lights and the appearance of color mixtures could be predicted from the measured wavelength dependence of the responses in single rods and cones. Our ability to detect very dim light corresponded to the signal-to-noise ratio of rod electrical activity. After exposure to bright lights, the rod signal-to-noise ratio recovered slowly, in a fashion similar to the timecourse of human dark adaptation and the fading of visual afterimages.
The photocurrent response leads to a change in membrane voltage; the voltage is further modified by voltage-activated conductances and synaptic interactions. To look at these later stages of phototransduction we use patch electrodes to record the photovoltage of primate photoreceptors. We have found that rods act as independent light detectors, unlike the rods of cold-blooded vertebrates which extensively coupled to each other through gap junctions. Primate cones, however, were found to receive input from surrounding rods. This input would be expected to degrade the spatial resolution of cone vision. We are currently exploring the pharmacological mechanisms of the coupling and investigating the effects of light adaptation. We are also investigating on the deactivation steps of phototransduction.
After the photoreceptors, the next step in visual transduction is the excitation of retinal bipolar cells. In the primate, there are at least 12 subtypes of bipolar cells with differing synaptic connections and neurochemistry. Almost nothing is known about the electrical response properties of these cells. We expect that different subtypes are tuned to extract different aspects of the visual scene. In what manner are bipolar cells specialized? How are single photon responses processed? How is information about color encoded by bipolar cell wiring? To address these questions we are making whole-cell recordings in bipolar cells from primate and mouse retina.
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Packer, O.S, Verweij, J, Li, P.H., Schnapf, J. L., & Dacey, D.M. (2010). Blue-yellow opponency found in primate S cones. Journal Neuroscience 30: 568-572.