38development of the basic ideas of the duplicity theory

variation in relative sensitivity between rods and cones. In the red part of the spectrum, for example, von Kries could find no photochromatic interval, in agreement with the relatively low sensitivity of rhodopsin to red light.

Lastly, under a fifth heading, ‘Open questions’, von Kries pointed to several largely unknown but important research topics for future research such as the phototransduction process in the retina, nerve processing, photopigments of cones, mechanisms underlying rod and cone dark adaptation and interaction between rod and cone processing. Perhaps his most interesting question, though, was whether rods might contribute not only to brightness and form, but also to the so-called positive, complementary Purkinje afterimage (i.e. a complementary colour sensation that may be observed in complete darkness after about 0.2 s following a short chromatic light stimulation). Since von Kries found that this positive, complementary afterimage was not ordinarily observable at the fovea or when deep red light was employed (see Tschermak, 1902, pp. 759–768 and G. E. Müller, 1930, pp.189–198 for rejections of both these points), he suggested that the afterimage might be due to a second activation of the rods combined with a negative complementary afterimage generated by the cone mechanism. As an alternative explanation, he suggested that signals from rhodopsin situated outside the rod receptors might directly activate the cone receptor mechanism (von Kries, 1929, pp. 702, 705).

Apparently, his suggestion that rod activity might somehow be involved in chromatic colour vision processing was not accepted by the scientific community. One major objection was that several research workers were able to obtain the Purkinje afterimage at the central fovea. (For alternative interpretations of the Purkinje afterimage, see Helmholtz (1911), p. 260; Tschermak (1902), pp. 759–768; and­ G. E. Müller (1930), pp. 189–198.)

3.13  An attempt to unify the theories of Schultze and Young-Helmholtz

In his 1929 paper von Kries mainly discussed the evidence already available in favour of the duplicity theory. In a previous paper he

the schultze tradition 39

had made another important contribution by attempting to integrate evidence accumulated within the Newton and Schultze traditions into a unified theory (von Kries, 1911). In this endeavour he encountered a serious problem: no existing theory could adequately account for achromatic colour sensation. The explanation provided by the Young-Helmholtz colour theory had long been outdated. Clearly, the presumption that white sensation was generated by the three primary cone receptor types activated to about the same degree was challenged by the fact that the rod receptors also mediated achromatic sensation. Furthermore, it had been found that monochromatic spectral lights could give rise to colourless sensations under daylight conditions when the size of the test field was very small, and also when the test stimulus was exposed in the far peripheral retina (von Kries, 1911, pp. 430–432). Hence, a more comprehensive theory was called for.

In his attempt to provide a more adequate explanation of achromatic colour sensation, von Kries (1911) presumed that the whiterelated process could be provoked by two different centrally located mechanisms operating more or less independently of each other: the cone-related mechanism, with a tripartite structure, generating both chromatic and achromatic sensations; and the rod-related mechanism, with a unitary structure, reacting to different wavelengths in a qualitatively­ homogeneous manner, giving rise to achromatic vision only. Thus, day and night vision were thought to be special modes of vision based on separate and distinct parts of the visual organ as a whole (see von Kries, 1911, p. 395).

As revealed by the following quotation, however, von Kries was keenly aware of the fact that important problems remained to be solved, particularly with regard to the complex cone mechanism:

… it may be considered as extremely probable that the organisation in three components assumed in the Helmholtz theory does not apply to the organ of vision as a whole, but only to those parts that are directly exposed to the action of light and a more or less extended series of parts connected with them; and that, on the other

40 development of the basic ideas of the duplicity theory

hand, the final results, the immediate substrata of the sensations, are themselves of a different nature; and hence that somewhere along the route the three independent results of stimulus are transformed into processes of a different kind and composition. As to these processes, nothing can be said with certainty, in the writer’s opinion, except that in them the colourless sensation has some outstanding physiological significance (von Kries, 1911, pp. 431–432).

It will be seen that the duplicity theory offered by von Kries, like the Young-Helmholtz colour theory, had little to say about the actual neurophysiological colour processing beyond the receptor level. This ignorance is perhaps best illustrated by Helmholtz’s (1896) final version of the Young-Helmholtz colour theory where he simply suggested that the colour-related processes from each receptor were independently transmitted to the brain through isolated nerve fibres.