54development of the basic ideas of the duplicity theory

his far-reaching speculations have proved successful in promoting the development of the duplicity theory. Indeed, his theory may be seen as a test of the fruitfulness of providing comprehensive and speculative theories within vision research. In the following sections we present his theory in some detail.

5.2.1  G. E. Müller’s speculation on the phototransduction in rods

In agreement with Kühne (1877a), Parinaud (1881, 1885), König (1894) and von Kries (1894, 1929), G. E. Müller held that rhodopsin was the photopigment of twilight vision and, consequently, that its spectral decomposition determined spectral sensitivity in a dark-adapted state at low test intensities (G. E. Müller, 1897, 1923). Also, he assumed that there was a continuous approach toward a state of equilibrium between decomposition and regeneration of the photopigment when it was acted upon by light.

AtvariancewithKühne,Parinaud,KönigandvonKries,however, he presumed that rhodopsin was but an optical sensitizer (optischen Sensibilisator). Its decomposition by light sensitized and, hence, increased the turnover of a base substance A (‘Ausgangsmaterial’) to a substance W (‘W-material’). As the amount of this second substance increased, it was transformed into a third substance, substance V (‘V-material’). The latter process, then (the change from the W to the V substance in the outer segment of the rods), was thought to be responsible for the ‘white-‘ related neural activity (‘W-Erregung’) of the rods (G. E. Müller, 1923). An important and original consequence of this line of thinking was that rod sensitivity did not solely depend on the amount of rhodopsin, but also on the amount of the substance A available in the outer segment of the rod receptors.

5.2.2  Cones may inhibit regeneration of rhodopsin

In order to obtain a better understanding of how rod activity may change with light level, G. E. Müller (1923) made an extensive review of the available evidence concerning so-called rod monochromats.

the colour theories of a. tschermak and g. e. müller 55

He concluded that in a rod monochromat without functional cones, the rod receptors increased their activity as light level increased until at bright daylight the rod monochromat became dazzled and, hence, unable to discriminate any feature in the visual scene – seeing only a very bright, white light.

In the normal human retina, on the other hand, where both the rod and cone receptor systems are operative, the rod receptors were assumed to increase their output only up to a certain light level and, thereafter, to become increasingly inhibited by cone activity. The cone activity was assumed to inhibit the regeneration of rhodopsin and, thereby, reduce the rod output. Indeed, in bright daylight rhodopsin would not regenerate at all. Under this condition, there would eventually be no signals from rhodopsin to sensitize the ‘A’ substance and, as a consequence, this substance would regenerate to its dark-adapted value. Hence, the non-functioning of the rod system in bright daylight was assumed to be due solely to the lack of rhodopsin available in the rods.

Despite the inhibition from cones, however, rods were thought to operate at light levels several log units above the absolute dark-adapted cone threshold (e.g. in a room flooded with daylight) giving the possibility of simultaneous rod-cone activity over a long transitional intensity interval.

5.2.3  Rods subserving chromatic colour vision

G. E. Müller (1923) also made a thorough and noteworthy analysis of the available evidence of chromatic functioning of the rod system. The evidence procured was judged to indicate that decomposition of rhodopsin by light generated a weak green-blue colour sensation in addition to the prominent achromatic component. Yet, he could not decide, on the basis of the available evidence, whether the rhodopsin molecules responsible for the green-blue colour were situated in the outer segment of rods or cones. Evidence of a rod origin was found from colour sensations that were obtained by a rod-cone colour mixture under mesopic conditions and could be closely simulated by

56development of the basic ideas of the duplicity theory

binocular colour mixture. G. E. Müller (1923) found this similarity consistent with a rod origin, since he assumed the rod and cone pathways were separated, perhaps to beyond the optic radiation, i.e. to the visual cortex.

The chromatic rod system would, of course, differ markedly from the chromatic cone system by operating at low light levels and subserving a green-blue colour invariant of wavelength. In order to explain the origin of such an odd chromatic rod system, G. E. Müller speculated that the rod system in a previous state of phylogenetic development, in fact, could discriminate between different coloured lights, but that the development of the photopigment rhodopsin, necessary for the increase of sensitivity during the second phase of long-term dark adaptation, was antagonistic to the formation of chromatic-related red-green and yellowblue substances in the rods, so that only a primitive form of colour vision remained for the rod system.

5.2.4  Three types of cones and five pairs of opponent processes

The cone system, in contrast to the rod system, was assumed to have retained the ability to mediate the full range of colour sensations. In order to explain this marvellous ability, G. E. Müller made two basic assumptions: (1) the phototransduction in the retina, where light generates nervous activity, involved a triplex cone-receptor mechanism, and (2) colour processing was based on opponent activi­ ties. Accordingly, he considered both the Young-Helmholtz and the Hering colour theories to be over-simplifications (see foreword G. E. Müller, 1930).

A simplified diagram of his theory is shown in Fig. 5.1. For illustrative purposes, only activation of the rod and one of the three cone systems (P1) are shown. As can be seen, though, the difference between the rod and cone systems is marked. In fact, the rod system involves two white-black systems and possibly an ­undeveloped greenblue colour system, while the cone systems with ­photon-absorption processes P1 (sensitive from the extreme longwave end of the visual