3  The Schultze tradition

3.1  The duplicity theory of Max Schultze

The first section of Helmholtz’s classical ‘Handbuch der Physiologischen Optik’ was published in 1856, the second (containing the Young-Helmholtz trichromatic colour theory) in 1860, the third partly at the beginning and partly towards the close of 1866, while the complete work was published in 1867. The Young-Helmholtz colour theory was, therefore, known to Max Schultze (professor in anatomy and the director of the Anatomic Institute in Bonn, Germany) when he published his important work in 1866. Indeed, he accepted its major assumption that three different cone fibres conveyed independent, qualitatively different colour-related processes.

Yet, in addition to the cone receptors and their functions, he introduced the much more numerous rod receptors presuming that they were responsible for night vision, and that they mediated achromatic sensation only. This theory of Schultze may be seen as the third major paradigm shift in vision research. It entailed a profound new insight into visual processing and generated basic questions about differences and similarities as well as about possible interactions between the information processing of the two different receptor systems.

3.2  Evidence in favour of the theory

Schultze’s hypothesis that the retina contained two basically different kinds of receptor was based on extensive, comparative histological studies. Although he was aware of the fact that the structure of a receptor type in different species, and even in the same retina, may differ markedly, he found rods and cones in general to differ both with respect

the schultze tradition 23

to structure and occurrence. Thus, he pointed to several characteristics that may be summarized as follows (Schultze, 1866, pp. 247–251):

1.  Rods are generally longer than cones, and the rod and cone outer segments have rodand cone-like shapes, respectively, as the names indicate.

2.  The terminal of the inner segment of the cones is much thicker than that of the rods and divides into several thin fibres.

3.  Cones of many diurnal bird species possess yellow, red or colourless strongly light-refracting droplets (globules) between their inner and outer segments, positioned so that light has to pass through them before reaching the outer segment.

4.  Several diurnal (lizard, snake, etc.) and nocturnal (bat, mole, shark, hedgehog, mouse, skate, etc.) species have only cone and rod retinas, respectively.

5.  The central fovea of humans, apes and falcons contains only cones.

Based on points 4 and 5, Schultze (1866) concluded that cones and rods mediated day and night vision, respectively, and that both receptor systems functioned as light and space receptors. In order to decide whether both receptor systems might also mediate chromatic sensation, Schultze (1866) compared day and night vision in humans. He referred to the extensive study of Aubert (1865) who had found that colour vision changed qualitatively during dark adaptation with a gradual transition from chromatic to achromatic vision where, eventually, only brightness differences were observed. Consequently, Schultze (1866) concluded that only the cone receptor system could mediate chromatic vision. In accordance with this conclusion, he pointed to the fact that colour vision deteriorated when a test field was moved outward from the rod-free fovea, while the number of rod receptors activated increased.

3.3  One or several types of cone?

Having presented strong arguments in favour of the assumption that only cones may mediate chromatic colour vision, Schultze (1866) sought to develop his theory a step further by attempting to answer the fundamental question of whether there were one or several types of cone in the retina.

In his investigation of the retinas of mammals and fish he could find no anatomical evidence for different types of cone

24development of the basic ideas of the duplicity theory

representing the three different primaries of the Young-Helmholtz colour theory – the cones appeared essentially similar. Hence, he had to conclude that these retinas contained only one type of cone. Yet, he could point to another characteristic of the cones that might account for the triplicity of colour vision: each cone had several outgoing fibres connected to the inner segment, while the rod receptors had only one fibre connected. Reasoning from the plausible assumption that each cone needed several fibres in order to handle both achromatic and chromatic information processing, while a rod receptor, mediating a simple achromatic sensation, needed only one fibre, he came to the conclusion that the different fibres connected to each cone receptor were capable of reacting in a qualitatively different manner when excited by light of different colours (Schultze, 1866, p. 258). Thus, Schultze, in effect, suggested that the different fibres connected to the inner segment of each cone in mammalian and fish retinas constituted the basis for colour vision, serving independent, qualitatively different nerve processes.

In the retinas of diurnal birds, on the other hand, he did find evidence for three different kinds of cone receptor. Thus, he found cones with red, yellow and colourless droplets (globules) situated at the junction of the inner and outer segments, so that light had to penetrate them before entering the outer segments. The droplets, therefore, appeared to function as colour filters, and Schultze came to the conclusion that these different cone receptor types represented, respectively, the red, green and violet receptor mechanisms of the Young-Helmholtz colour theory. As an alternative, he suggested that cones with colourless droplets might function in a similar way to the cones of humans, being capable of generating the whole colour gamut.

The interesting finding that the outgoing fibre of the cones with coloured droplets was relatively thin and slender, like that of the rods, was interpreted as consistent with his prediction that each cone needed only one fibre when the retina contained several types of cone receptor.

the schultze tradition 25

Lastly, Schultze (1866) suggested that cones provided better spatial resolution than rods. However, presupposing that acuity was based on the number of receptors per unit retinal area, he encountered a serious problem when attempting to explain the reduction in acuity with retinal eccentricity. If each receptor sent an independent signal to the brain, acuity should increase, not deteriorate with eccentricity, due to the increasing number of small rods that far outnumbered the cones. In want of a better explanation of this apparent inconsistency, Schultze (1866) speculated that the better acuity performance of cones, somehow, was related to their ability to mediate colour vision. Thus, cones were assumed to perform better than rods in acuity tasks, because several fibres were connected to the inner segment of each cone and thereby provided greater information processing capability.

3.4  Phototransduction is photochemical in nature: Boll and Kühne

At the time Schultze (1866) published his duplicity theory, there was no consensus on phototransduction. Although it was known that light energy was transformed into nervous energy in the receptors, the transformation mechanism was completely unknown. Indeed, Newton’s idea, presented nearly two centuries earlier, that light produced physical vibrations in the nerve fibres was still considered a viable alternative (Helmholtz, 1867, pp. 214–215).

The works of Boll and Kühne, published a few years later, however, were a breakthrough in our understanding of the phototransduction process. Boll (1877) discovered that the photopigment rhodopsin, situated in the outer segment of the rods, was bleached by light and regenerated in the dark and suggested that these photochemical processes might be a foundation of visual colour processing. Kühne (1879) disagreed with this suggestion and instead, on the basis of very extensive research, put forward his general ‘optochemische Hypothese’ (optochemical hypothesis) where he assumed the existence of photochemical see-substances (Sehstoff) both in rods and cones (Kühne, 1879, pp. 326–332).