Biology Reference
In-Depth Information
Part III
Chromatic rod vision:
a historical account
The finding that stimulation of rods alone may give rise to qualitatively
different colour sensations came as a surprise, since it challenged the
fundamental Principle of Univariance. This principle follows from
Helmholtz's (
1896
) specific fibre-energy doctrine and implies that a
given receptor or nerve fibre does not discriminate between variation
in intensity and wavelength of a test light and hence mediates only
one sensory quality. Accordingly, when only the rod receptor system
is stimulated, variation in wavelength can be simulated by variation
in intensity - they both produce variation in brightness only.
Indeed, the Principle of Univariance had been directly
demonstrated by Graham and Hartline (
1935
). By analyzing the nerve
impulses arising in the retina of the
Limulus
(horseshoe crab), where
each photoreceptor is linked with a separate nerve fibre, they found
that the variation in the response of the single fibres with wavelength
could be simulated by suitably adjustment of the incident light energy.
Thus, when the intensity was suitably adjusted, any test wavelength
could be made to evoke the same frequency of impulses from a given
receptor cell. Hence, it appeared that single retinal receptors alone
had no power to discriminate between wavelength and intensity.
How, then, could the Principle of Univariance be reconciled
with the new discovery that test stimulation of rods may give rise
to all the principle hues of the spectrum? An answer to this question
became apparent when it was discovered that the scotopic hues
were due to rod-cone interactions. It followed that the Principle
of Univariance and the specific fibre-energy doctrine of Helmholtz
(
1896
) should only be applied to functionally independent receptor
systems.
