Biology Reference
In-Depth Information
He concluded that in a rod monochromat without functional cones,
the rod receptors increased their activity as light level increased until
at bright daylight the rod monochromat became dazzled and, hence,
unable to discriminate any feature in the visual scene - seeing only a
very bright, white light.
In the normal human retina, on the other hand, where both the
rod and cone receptor systems are operative, the rod receptors were
assumed to increase their output only up to a certain light level and,
thereafter, to become increasingly inhibited by cone activity. The
cone activity was assumed to inhibit the regeneration of rhodopsin
and, thereby, reduce the rod output. Indeed, in bright daylight
rhodopsin would not regenerate at all. Under this condition, there
would eventually be no signals from rhodopsin to sensitize the 'A'
substance and, as a consequence, this substance would regenerate
to its dark-adapted value. Hence, the non-functioning of the rod
system in bright daylight was assumed to be due solely to the lack of
rhodopsin available in the rods.
Despite the inhibition from cones, however, rods were thought
to operate at light levels several log units above the absolute
dark-adapted cone threshold (e.g. in a room flooded with daylight)
giving the possibility of simultaneous rod-cone activity over a long
transitional intensity interval.
5.2.3 Rods subserving chromatic colour vision
G. E. Müller (
1923
) also made a thorough and noteworthy analysis of
the available evidence of
chromatic
functioning of the rod system.
The evidence procured was judged to indicate that decomposition of
rhodopsin by light generated a weak green-blue colour sensation in
addition to the prominent achromatic component. Yet, he could not
decide, on the basis of the available evidence, whether the rhodopsin
molecules responsible for the green-blue colour were situated in the
outer segment of rods or cones. Evidence of a rod origin was found
from colour sensations that were obtained by a rod-cone colour
mixture under mesopic conditions and could be closely simulated by
