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In-Depth Information
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
