Biology Reference
In-Depth Information
By plotting the reciprocal of equal quantum intensity necessary
to produce a threshold response against wavelength, Granit obtained
so-called dominator and modulator curves. Two fundamentally
different dominator curves were discovered in the mixed rod-cone
retina: (1) a scotopic curve obtained under dark-adapted conditions
with a maximum sensitivity at about 500 nm closely corresponding
to the spectral curve of rhodopsin, and (2) a photopic dominator curve
obtained under light-adapted conditions with a maximum sensitivity
at about 560 nm. Thus, the change from the scotopic to the photopic
curves represented a Purkinje shift - a shift from rod to cone activity.
Accordingly, Granit found no photopic dominator curve in the rod
retinas of the rat and guinea pig where, apparently, cone mechanisms
of any significance were lacking.
8.2.1. The trichromatic colour theory challenged
Granit suggested that both the scotopic and photopic domina-
tors served pure colourless 'white' sensation, since their spectral
sensitivity curves were broad and, therefore, could not offer the central
mechanism any adequate cue for colour discrimination. Accordingly,
in sharp contrast to the Young-Helmholtz colour theory, he suggested
that information about hue could only be conveyed by narrow spectral-
sensitivity curves confined to widely separated spectral regions. In
agreement with this view, he found, in addition to the dominator
curves, numerous relatively light-resistant, narrow spectral modulator
curves across the visible spectrum. Hence, these modulators appeared
to be well suited for mediating wavelength information.
Nevertheless, although Granit stressed that the modulator
curves were too narrow to be regarded as the trichromatic units of the
Young-Helmholtz colour theory, he held that his results confirmed
the essential truth of Thomas Young's generalizations, since the
modulator curves were obtained in three different regions of the
spectrum (440-470 nm, 520-540 nm and 580-610 nm).
Granit found a more severe challenge confronting the trichro-
matic theory in the observation that brightness and chromatic
