Biology Reference
In-Depth Information
to about the same degree. Hence, the differences in ongoing activity
rates of the different types of spectrally opponent cells would become
levelled down to some extent when light signals from rods intruded
during the second phase of long-term dark adaptation (Stabell and
Stabell,
1996
). The rise of the specific-hue threshold obtained during
the rod phase of the long-term dark-adaptation curve could thus
be explained without invoking the retinal rod-cone antagonism
hypothesis offered by Lie (
1963
).
This hypothesis of rod-cone antagonism was also challenged
from another angle. Thus, quite unexpectedly, it was found that the
specific-hue threshold for all spectral lights may fall well below the
cone-plateau level when rod impulses intrude during long-term dark
adaptation. Similarly, the specific-hue threshold of a monochromatic
deep-red spectral light (700 nm) was found to fall markedly when an
achromatic rod component was added to the 700 nm light in a completely
dark-adapted state (Stabell & Stabell,
1971
a, b,
1976
).
Clearly, neither the hypothesis of rod-cone colour mixture
nor the hypothesis of rod-cone antagonism proposed by Lie (
1963
)
could account for the rod-cone facilitation found. Based on these
hypotheses the specific-hue threshold level should never fall below
the cone-plateau level when rod signals were added.
The unexpected rod-cone facilitation observed could, however,
be explained by the following three assumptions (Stabell & Stabell,
1976
):
1.
Hue is encoded in the visual pathway by the relative activity rates of the
different types of spectrally opponent cells (De Valois,
1965
; De Valois
et al
.,
1966
).
2.
Stimulation at intensities somewhat below the absolute cone threshold
may excite a few of the cone receptors and, thereby, change the relative
responsiveness of the spectrally opponent cells.
3.
Rods feed spectrally opponent as well as non-opponent cells (De Valois,
1965
; Wiesel & Hubel,
1966
).
Thus, when, for example, the eye is stimulated with a monochro-
matic deep-red spectral light at intensities somewhat below absolute
