Biology Reference
In-Depth Information
light did not raise the quantum demand at the receptor level (i.e.
the threshold for rod-receptor signals did not increase) and that all
adaptation occurred at the pool.
This conclusion was also supported by experiments on dark
adaptation where the bleaching light was sent by means of a brief
electronic flash either through a grating of 0.5º period to give a
striped bleach or through a neutral filter to pass an equal amount of
light spread evenly on the retina. The test flash was under the latter
condition a grating of 0.5º period, while it was an equalizing neutral
test light in the former. Again, no significant differences between the
two conditions were obtained.
Rushton (
1965a
) argued that if sensitivity regulation occurred
at the receptor level, the dark bars of the grating should have shielded
the retina from the flash of the bleaching light. The retina should thus
have been fully dark adapted at the shielded regions, while every rod
in the bleached area should have been substantially bleached when
the same total bleaching light was evenly distributed on the retina.
Hence, from his results he concluded that bleaching with the grating
or with the same light spread evenly had about the same effect on
the threshold level, and also that the threshold level obtained by the
test flash was the same whether the test energy was concentrated
by the grating or spread evenly - all in harmony with his AGC-pool
hypothesis.
20.2 Anatomical and electrophysiological
evidence
Anatomical and electrophysiological evidence supporting Rushton's
post-receptor adaptation theory was presented by Dowling (
1967
).
Together with Boycott he had examined the synaptic organization
of the retina in several types of vertebrate retina, including the
primate, using an electron microscope. Striking support for Rushton's
AGC-pool hypothesis was obtained. Thus, they found widespread
reciprocal synapses between bipolar cell terminals and amacrine-cell
processes. The anatomical evidence indicated that bipolar cells made
