Vision: from photon to perception.

A National Academy of Sciences colloquium entitled "Vision: From Photon to Perception" was held at the Beckman Center of the Academy in Irvine, California, on May 20-22,1995. The meeting was organized by John Dowling, Lubert Stryer (chair), and Torsten Wiesel. The aim of the colloquium was to bring together leading scientists and students from different disciplines of vision research ranging from physics to psychology to define and explore the most challenging questions in the field. One hundred forty scientists participated in the colloquium. We are indebted to Silicon Graphics, Inc., and the Ruth and Milton Steinbach Fund, Inc., for generous grants that helped bring graduate students to the meeting. The major topics discussed were as follows. (i) How is light converted into a nerve signal? (ii) How are the outputs of rod and cone cells processed by the retina? (iii) How does the visual system develop and how did it evolve? (iv) How do we perceive color, depth, and motion? The colloquium began with a spirited opening lecture by David Hubel on the process of discovery in vision research. The first session, "From Photon to Nerve Signal" (chaired by Lubert Stryer), focused on transduction processes in vertebrate and invertebrate photoreceptor cells. The second session, "Development and Circuitry" (chaired by John Dowling), dealt with the development of the retina and lateral geniculate nucleus and with signal processing. Higher-order processes occurring in the visual cortex were considered in the third session, "Representation and Perception" (chaired by Francis Crick). Torsten Wiesel gave a reflective closing lecture on the pursuit of knowledge and the future of research in neurobiology. Denis Baylor (1) began the first session by providing an account of how the absorption of a photon by a retinal rod or cone cell leads to the generation of an amplified neural signal. Photoexcited rhodopsin triggers the activation of transducin, a G protein, which in turn stimulates a cyclic GMP phosphodiesterase. The consequent hydrolysis of cyclic GMP directly closes cation-specific channels in the plasma membrane. The resulting hyperpolarization is sensed at the synapse, where it decreases the rate of transmitter release. This light-triggered cyclic GMP cascade is one of the best understood signal transduction processes in nature. The challenge now is to elucidate the molecular events mediating recovery of the dark state and adaptation to background light. The remarkable reproducibility of the single-photon response also needs to be understood in molecular terms. Investigators are now focusing on the negative feedback actions of the light-induced fall in the cytosolic calcium level. Baylor presented several incisive recent experiments comparing phototransduction in normal and transgenic mouse rods harboring mutant genes in the deacti-