Biology Reference
In-Depth Information
Svaetichin (1953), using electrophysiological measurements, found
evidence of a retinal origin. Thus, he obtained responses from cells
in a fish retina that hyperpolarized for short and depolarized for
long wavelengths. It was first assumed that these opponent colour
responses were due exclusively to the cone system. Mitarai
et al
.
(
1961
), however, provided strong evidence indicating that the rod
system also contributed to the antagonistic colour responses.
The relation between the colour-related opponent responses
of the retina and the pattern of discharge in the fibres of the optic
nerve cells was later revealed by De Valois (1965). Recording from the
lateral geniculate nucleus of the thalamus of the macaque monkey,
they found cells which increased or decreased their rate of firing,
depending on the wavelength. Maxima of excitation and inhibition
were obtained in the red, green, yellow and blue regions of the
spectrum in accordance with Hering's colour theory (De Valois
et al
.,
1966
; Abramov,
1968
). As with the finding of Mitarai
et al
. (
1961
),
both De Valois (
1965
) and Wiesel and Hubel (
1966
) found to their
surprise that the colour opponent cells also received inputs from rods
under scotopic conditions.
With these remarkable findings and the evidence indicating that
hue
sensations were determined by the relative activity rate of the
different types of spectrally opponent cell (De Valois
et al
.,
1966
), the
scotopic hue of successive contrast could be explained in a straight-
forward manner based on the following assumptions:
1.
Hue, both in scotopic and photopic vision, is encoded in the visual
pathway by the relative activity rates of the different types of spectrally
opponent cells.
2.
The opponent cells are activated to about the same degree producing an
achromatic sensation when the eye, in a completely dark-adapted state,
is stimulated at scotopic intensity levels.
3.
Following complete dark adaptation, selectively chromatic pre-
stimulation of the three types of cone reduces the sensitivity of the
opponent cells in proportion as they are activated.
4.
Upon test stimulation at scotopic intensity levels, the change in
sensitivity of the opponent cells produced by the pre-stimulation affects
the impulse pattern initiated in the rods and thus changes the relative
