Biology Reference
In-Depth Information
cone threshold, the light quanta might excite a few of the 'red' cone
receptors and, thereby, increase the responsiveness of the red- and
yellow-related opponent cells. If in this state rod activity were added,
the relative activity of these opponent cells may be enhanced, and
facilitate the specific-hue threshold. It should be noted that this facili-
tation of the chromatic cone component by rod activity could only
be found under conditions where the chromatic cone component was
relatively strong and the achromatic rod component relatively weak.
Nevertheless, the conclusion had to be drawn that rod activity in
a chromatically neutral state of adaptation was capable not only of
desaturating chromatic cone activity, as previously assumed, but also
of facilitating it.
13.2 R o d - c o n e i n t e r ac t i o n s u n d e r m e s o p i c
conditions in a chromatic state of
adaptation
Ever since Schultze (
1866
) published his duplicity theory, scientists
had inquired into how the rod and cone systems interacted in mesopic
vision to produce a combined effect. As we have seen, the prevailing
hypothesis had been that rods and cones interacted in a kind of colour-
mixture process in which the cone and rod systems contributed a
chromatic and an achromatic component, respectively.
The finding that rod signals also may give rise to chromatic colour
sensations immediately raised the question of how rods and cones
interacted when they both contributed a chromatic component.
A series of experiments was performed to explore this kind of
rod-cone interaction. Three different procedures were employed: the
change in colour of a test field was measured when (1) test condition
changed from photopic to mesopic during long-term dark adaptation,
(2) the rod stimulus was simply superimposed on the cone stimulus,
and (3) light intensity of the test field was increased from scotopic to
photopic intensity levels. Under all these conditions both receptor
types contributed a chromatic component (see Stabell,
1967
b; Stabell
& Stabell,
1973
b,
1974
,
1998
).
