10  Status of the duplicity theory in the mid 1960s and its further development

At the end of the mid-twentieth century period (1930–1966), spectrophotometric measurements of spectral absorption in single ­photoreceptors had conclusively demonstrated that the retina contained three different types of cone, each cone with only one photopigment, together with the rods that contained only rhodopsin. Hence, it had been proved that the speculative idea of König (1894) and Willmer (1946, 1961) that rods may function as the primary ‘blue’ receptor, Willmer’s and Granit’s ideas of ‘day rods’, Polyak’s idea that the trichromacy of colour vision was based on three types of bipolar cell, and the dominator-modulator theory of Granit, were all wrong. Moreover, Lie (1963), in his important and comprehensive study, had found that rods under scotopic test conditions mediated achromatic colour sensations only, and that under mesopic test conditions they contributed an achromatic component, desaturating the chromatic cone component.

Apparently, with the exception of the idea of rod-cone interaction, the duplicity theory had become strikingly similar to the old, orthodox conceptions formulated by von Kries (1929).

10.1  Elaboration and revision of the two most basic assumptions of Schultze’s duplicity theory

As can be seen, the developmental history of the duplicity theory in the 100 years between 1866 and 1966 may be characterized by complex and comprehensive theory constructions. Outstanding examples are the theories provided by Schultze (1866), Parinaud (1881, 1885), König (1894), von Kries (1929), G. E. Müller (1930), Polyak (1941), Granit (1947, 1955) and Willmer (1946, 1961).

106 development of the duplicity theory from 1930–1966

Yet, important contributions to the development of the theory were also made by workers who focused on more limited aspects of the theory. The most important contributions were provided by research workers who made penetrating and thorough studies to uncover the underlying mechanisms of the most basic assumption of the duplicity theory, namely that rods alone operated in night vision and cones in day vision.

Schultze (1866) could not provide any explanation of this difference in rod and cone sensitivity, either in photochemical, anatomical­

or physiological terms. Following Boll’s (1877) discovery of the bleaching and regeneration of rhodopsin, however, important insights into the sensitivity regulation mechanisms were provided by Kühne in the nineteenth century and by Hecht, Wald, Rushton, Barlow and Lamb in the twentieth century. In fact, their ­contributions may be seen as a mainthread in the developmental history of the duplicity theory.

Yet, contributions were also provided by research workers who presented evidence opposing Schultze’s (1866) second major assumption that rods mediated achromatic vision only. Indeed, the evidence offered strongly suggested that the rod system was an important contributor to chromatic colour vision and that rod signals served achromatic vision only under scotopic conditions when the eye was in a dark-adapted and chromatically neutral state of adaptation. Thus, it was found that (1) rod signals of the primate retina were transferred to ganglion cells via cone pathways (Fig. 10.1) and fed both spectrally opponent and non-opponent colour cells (De Valois, 1965; Wiesel & Hubel, 1966; Daw et al., 1990; Lennie & Fairchild, 1994; Wässle et al., 1995); and (2) rod signals could give rise to all sorts of hue sensations when interacting with cone activity (Stabell, 1967a; Stabell & Stabell, 1973a, 1994; McCann & Benton, 1969; Stromeyer, 1974a, b; Buck, 1997). Obviously, a major reformulation of the duplicity theory was called for.

In the following part we will describe how our knowledge of the sensitivity regulation mechanisms of rods and cones and of the

status of the duplicity theory in the mid 1960s 107

Rod

Cone

DB

Fig. 10.1  Simplified illustration of how rod signals may affect cone activity in the primate retina. Signals from rods enter cone pathways via gap junctions between rod and cone receptors, and between AII (rod amacrine cells) and depolarizing cone-bipolar cells (illustrated by the DB cell in contact with the cone). A rod depolarizing bipolar cell (the DB cell in contact with the rod) is also shown. Moreover, signals from rods may affect hyperpolarizing cone-bipolar cells (illustrated by the HB cell) through conventional chemical synapes. ON G and OFF G represent onand off-ganglion cells, respectively.

mechanism of chromatic rod vision developed, tracing the development back to its origin in the nineteenth century. We start with the development of the ideas of chromatic rod vision.