58development of the basic ideas of the duplicity theory

(Y-B) and white-black (W-S) opponent processes take place. To explain the variety of colour experiences, G. E. Müller needed yet another level of complexity. Thus, in addition to the opponent processes in the see-substances, he introduced red-green (r-g) and yellow-blue (y-b) opponent processes in a red-green (r-g) and yellow-blue (y-b) substance located in the outer segments of the cones.

5.2.6  The P1 system

As illustrated in Fig. 5.1, the P1 process activates both the red-green and yellow-blue substances in the outer segment of the cones. Thus, P1 activates the r-process in the red-green substance and also, weakly, the y-process in the yellow-blue substance. The r-process, then, activates red (R)-, yellow (Y)- and white (W)-related processes in the see-substances, while the y-process activates yellow (Y)-, green

(G)- and white (W)-related processes.

5.2.7  The P2 system

The P2 process first activates the g-process in the red-green substance in the outer segment of the cones and the y-process in the yellow-blue substance. Then, the g-process activates the green (G)-, blue (B)- and black (S)-related processes in the see-substances, while the y-process activates the yellow (Y)-, green (G)- and white (W)-related processes like the y-process of P1.

5.2.8  The P3 system

Finally, the P3 process first activates the b-process in the yellow-blue substance in the outer segment of the cones and then the blue (B)-, red

(R)- and black (S)-related processes in the see-substances. In addition to this complex processing, P1, P2 and P3 all directly activate the white (W)-related process.

Both the processes in the red-green and yellow-blue substances in the outer segment of the cone receptors and the processes in the see-substances were assumed to be of an antagonistic nature and to closely follow the same reaction pattern. They may, therefore, all be

the colour theories of a. tschermak and g. e. müller 59

illustrated by G. E. Müller’s model for the r-process in the red-green substance presented in the following simplified diagram:

Red ligh  → photon-absorption process P1 → A → r → g

As can be seen, red light activates P1 that increases the turnover of the base substance A to a substance r. As the amount of this second substance increases, it is transformed into a third substance g. The change from r to g generates the r-process, while the g-process is generated when the process goes in the opposite direction, from substance g to substance r. The r substance may then be transformed back to A.

This complex and highly speculative colour theory of the rod and cone systems may be seen as an attempt to cover all the major facts of colour vision available. The theory deviates markedly from both the Young-Helmholtz and the Hering colour theories. At variance with the Young-Helmholtz theory, it involves both opponent colour processing and rod activity, while in opposition to Hering’s theory, it involves rod activity and operates with a triplex photochemical cone mechanism. Moreover, in contrast to Hering, who assumed the so-called ‘Urfarben’ (pure red, pure yellow, pure green and pure blue) to be correlated with homogenous, uncompounded material processes, G. E. Müller (1930) assumed that these colours were determined by complex processes in the outer segments of the cones. A pure yellow colour of the Hering type, for example, would be determined by both P1 and P2 and, hence, by r-, y- and g-processes in the outer segment of the cone receptors.

5.3  Evaluation of G. E. Müller’s colour theory

With hindsight, it seems clear that G. E. Müller’s broad speculation on the actual colour-related processes in the visual pathways, based on insufficient evidence, did not entail a new, deeper understanding of rod and cone functions. Indeed, alternative theories that explained the same colour phenomena equally well were developed. An interesting example is the colour theory presented by Schjelderup

60development of the basic ideas of the duplicity theory

(1920). In addition to the processing put forward by G. E. Müller, Schjelderup postulated a stage of colour processing in the cortex where six independent physiological processes (red-, yellow-, green-, blue-, whiteand black-related processes) operated. By further assuming that each of these independent chromatic processes of the cortex could be lacking or non-functioning, he held that he could explain the different types of colour-blindness better than G. E. Müller (see Schjelderup, 1920).

Even though the speculations of G. E. Müller did not provide important new information about colour processing, they revealed the lack of factual knowledge available and, thereby, stressed the need for the development of new instruments and techniques designed to map the anatomical details of neural elements, and to monitor actual biochemical and physiological processes. The next developmental period of the duplicity theory from 1930–1966 met this need.

Part II  The development of the duplicity theory from 1930–1966

In this developmental period, profound new knowledge about the anatomical and neurophysiological properties of the retina emerged as a result of advances in sophisticated instrumentation and research techniques. This knowledge greatly influenced the development of the duplicity theory: it provided an insight into the rod and cone processes, and also created and paved the way for new ideas of rod and cone functioning. Outstanding contributions to the development were provided by Polyak, Hartline, Kuffler and Granit. Using the Golgi impregnation method, Polyak’s investigation of the primate retina helped to elucidate the extremely complex structure of the many types of retinal cells and the character of their connections. Hartline, Kuffler and Granit, using microelectrodes for ­registration of the action potential from individual nerve fibres in response to illumination, increased our knowledge about the relationship between light stimuli and nerve impulses in the retina.