Biology Reference
In-Depth Information
Similarly, a review on change in brightness discrimination with
changes in background intensity showed no clear breaks in the extrafoveal
curves. The results, therefore, were judged to indicate overlapping and
interaction of the brightness functions of rods and cones.
Furthermore, evidence presented by Lythgoe (
1940
) strongly
indicated that rod and cone impulses may also interact to determine
the fusion frequency of a flickering light. Measuring the fusion
frequency in the extrafoveal retina during dark adaptation whilst the
illumination of the flickering light was kept constant, he found, in
accord with the duplicity theory, that the fusion frequency fell or rose
depending on whether the test illumination activated the photopic
or scotopic mechanism, respectively. However, the results displayed
no clear-cut change-over from fall to rise during the dark-adaptation
period. Instead, they strongly indicated that cones determined the
critical frequency measurements during the first part of the dark-
adaptation curve, both receptor mechanisms during the middle part
and only rods during the last part.
Saugstad and Saugstad (
1959
) also presented evidence that
challenged the basic assumption of Schultze (
1866
) that rods mediated
achromatic vision only. Thus, evidence in support of the assumption
that rods may mediate colours of short wavelengths was found in the
results obtained by Hecht, Haig and Chase (
1936
/1937). Measuring
the dark-adaptation curve following moderate light adaptation, they
showed that the change from coloured to colourless perception for a
violet test light in the extrafoveal retina could be obtained below the
cone plateau level.
Furthermore, Kohlrausch (
1931
) obtained dark-adaptation
curves with only one branch in the extrafoveal retina despite the fact
that colours could be observed at the threshold level during the first
part of the curve. Saugstad and Saugstad (
1959
) interpreted this result
to mean that either the form of the curves was due to interaction
between rod and cone activities, or, presuming that the curves were
determined by rods alone, that rods mediated the colour sensations
observed.
