Biology Reference
In-Depth Information
Furthermore, by his so-called indirect adaptation technique,
where the adaptation field was located at one side of the test field,
Wright obtained extremely rapid regeneration rates, indicating that
an electrical adaptation mechanism, acting 50-100 times faster than
the chemical adaptation process, was also in operation.
Lastly, a quite unexpected observation was obtained when
the 'blue' recovery curve of a blue (460 nm) test patch was recorded
following yellow adaptation. Since yellow light would not be expected
to activate the 'blue' receptors to any marked degree, one would
predict that sensitivity to blue light under these conditions would
remain relatively stable. Contrary to this expectation, however, he
found an extreme reduction of the sensitivity of the blue test light
immediately after the yellow adaptation (later termed transient
tritanopia). Equally surprising, he found that, following adaptation to
almost any radiation of the spectrum, a test colour chosen anywhere
from the red-yellow-green range of the spectrum appeared to be
admixed with a positive amount of blue. Even adaptation to a blue
colour tended to produce blue when, for instance, a yellow test was
employed. No blue afterimage that could explain the positive blue
was seen in the dark. Hence, since the positive blue observed could
hardly arise through any direct action which the test patch exerted
on the 'blue' receptors, Wright (
1946
) suggested that the positive blue
arose from a secondary reaction occurring upon test stimulation at a
stage in the visual chain central to the phototransduction process.
Several research workers have later confirmed and extended
Wright's findings . A particularly important and revealing experiment
in this tradition was performed by Mollon and Poldon at Cambridge.
They found that the sensitivity of a violet test flash on a steady blue
background field
increased
0.3-0.4 log units when yellow light was
added
to the blue field so as to yield a composite background field
that appeared white (Mollon & Polden,
1979
; Polden & Mollon,
1980
).
To explain this remarkable observation, they suggested that the blue
background field reduced sensitivity both by reducing the gain of
the short-wave receptors and by polarizing a putative, chromatically
