Biology Reference
In-Depth Information
9.2.3 Evidence against Schultze's duplicity theory
Despite the strong supporting evidence that the retina contained
two different types of photoreceptor subserving different functions,
Saugstad and Saugstad (
1959
) also presented a long list of carefully
conducted studies that clearly showed that the duplicity theory
needed revision and elaboration. Their main concern was the
question of rod-cone interaction. On the basis of their interpretation
of the theory, they predicted that there should be an abrupt change-
over from rod to cone function as intensity increased from scotopic
to photopic levels, or vice versa, irrespective of function measured.
Opposed to this prediction, however, the available evidence suggested
that, for several visual functions, rods and cones were activated
simultaneously and produced a combined effect over a considerable
range of intensities. A number of classical studies supported this
conclusion.
Firstly, Walters and Wright (
1943
) had found that when the
test intensity increased from scotopic to photopic levels, the change
of the spectral sensitivity curve, measured psychophysically in the
extrafovea, was gradual and continuous over a very wide intensity
range and not abrupt as would be expected from the duplicity theory.
Furthermore, they found that the spectral sensitivity curve obtained
at the highest intensity level employed (more than 3 log units above
absolute cone threshold) was not identical to that obtained at the
fovea, but displaced toward the rod curve. These results, then, strongly
suggested that rod and cone signals may function simultaneously and
interact with regard to brightness.
The same interpretation could be applied to the results
obtained by Blanchard (
1918
) on light adaptation. He found that the
incremental-threshold curve of an extrafoveal, white test light (i.e.
the logarithm of the absolute threshold level of the test light plotted
against the logarithm of the intensity of an adapting background
light) yielded a continuous straight line instead of an abrupt change
in the gradient where the transition from the rod to the cone system
was expected to occur.
