The Neurobiology of Vision at the MBL - Universalizing Photochemical Theories of Vision: Hecht, Wald, and the Marine Biological Laboratory

“A basic characteristic of the scientific enterprise is its continuity. It is an organic growth, to which each worker in his time brings what he can; like Chartres or Hagia Sofia, to which over the centuries a buttress was added here, a tower there.” George Wald, Nobel lecture, 1967

In 1967 the Harvard University professor George Wald shared, with two other scientists, the Nobel Prize in Physiology or Medicine for his “discoveries concerning the primary physiological and chemical visual processes in the eye.” Wald inherited from his teacher, Selig Hecht, a universalist theory of light sensitive photochemistry. This universalist theory stipulated that the biochemical mechanisms of light sensitivity were conserved across animals. Hecht, and then Wald, argued that any organism that is sensitive to light uses the same biochemical mechanisms to do so.

In order to appreciate their contributions to the field, we must first understand the research tradition of Hecht and Wald and introduce some of the problems in the world that motivated their research, and to which they offered successful answers. First, the ‘duplicity theory’ of human vision, which is covered in Section 1, is motivated by a phenomenon called the Purkinje shift. In low light, color sensitivity decreases, shifting the perceivable colors towards the red end of the light spectrum during dark adaptation and making blue-end visual spectra disappear rapidly in low light until fully achromatic night vision takes over. The question then is, are there two systems of vision, one achromatic and for night/dark vision, the other for chromatic and for day/light vision? Or is there one system that handles both sensory settings? Duplicity theorists insist that the Purkinje shift (that is, the fact that night vision is achromatic and slowly takes over for day vision when light levels drop) is evidence that there are two systems, served by different photopigments in the retina.

In addition to the problem of light and dark adaptation, photochemical theories of vision need to solve the problem of how one photopigment could possible be infinitely exposable. The idea that the eye works like a kind of biochemical camera motivated scientists to wonder about just how the eye never never runs out of exposable film, so to speak. The eyes 'film' is composed of photopigments. Kühne and Boll, two figures we will meet shortly,  demonstrated that photopigments regenerate after multiple episodes of bleaching from light exposure. But how exactly that process worked was a mystery until first Hecht and then Wald filled in the theoretical and molecular details.