128 chromatic rod vision: a historical account

complementary colours. Assuming that rod signals may feed into these cortical cells, the colour after-effect seen in scotopic illumination could be accounted for.

14.4  Contribution of S. Buck and co-workers

A more recent and systematic study of the effect of rod activity on colour perception has been performed by Steven Buck and colleagues. In a series of experiments, they attempted to single out multiple rod influences on hue (Buck et al., 1998; Buck et al., 2000; Buck, 2001; Knight & Buck, 2002). In the study, wavelength of a circular 8° test field centred 7° extrafoveally was varied between 420 and 630 nm in 10-nm steps. For each wavelength tested, the observers described the appearance of the test stimulus by using two of the four basic hues (red, green, blue and yellow) assigning a percentage to the relative strength of each component hue, so that the sum of the percentage equalled 100% for each trial. Following the determination of the hue percentages, the observers also assigned a percentage to the relative strength of the saturation of the test stimulus.

In order to measure the influence of rod signals on colour appearance, the colour scaling was obtained both in a completely dark-adapted eye and during the cone-plateau period of the long-term dark-adaptation curve, i.e. under conditions that, respectively, maximized and minimized rod contribution. The test intensity was mesopic, so that both rods and cones could be activated by the test stimulus when in the dark-adapted state.

Also, they measured time-dependent changes in rod influence on hue. A scotopic background field was flashed on for 2 or 6 s, while a mesopic test flash of 30 ms was presented at 1 or 5 s after the onset of the scotopic background flash (Knight & Buck, 2002).

Most importantly, the experiments performed provided strong evidence for a variety of different time patterns of rod influences on hue. Evidence was obtained for a so-called ‘fast’ rod influence that enhanced green relative to red and a so-called ‘slow’ rod influence that enhanced short-wavelength red relative to green and also blue relative to yellow.

further exploration of chromatic rod vision 129

Presupposing that rod signals had access to all three cone pathways of normal trichromatic colour processing, and that rod and cone signals acted with the same sign on shared neurons, they attempted to explain these different rod-hue biases on the assumption that the green rod-hue bias resulted from a stronger rod influence on M-cone pathways than on L-cone pathways, while the blue and short-wave red rod-hue biases resulted from a relatively strong rod input into S-cone pathways, i.e. pathways to small bistratified ganglion cells (Buck, 2004).

The basic assumption that rod and cone signals may mimic and reinforce each other was found to be consistent with a wide variety of psychophysical studies and also to be consistent with more recent anatomical and neurophysiological evidence indicating that rod signals, both through the primary and secondary pathway, are transferred to the cone pathway via gap junctions (see Fig. 10.1). Since gap junctions are sign-conserving synapses, signals from rods will tend to enhance cone signals both through the primary and secondary rod pathway (Daw et al., 1990).

Interestingly, the finding that the rod signals are transferred to cone pathways provides strong evidence that the cone receptor systems preceded the rod system in the evolutionary process and that the rod system simply utilized the already existing cone pathway in order to convey their information to the brain.

14.5.  Contribution of J. L. Nerger and co-workers

In a series of papers at the turn of the twentieth century, Nerger, Volbrecht and co-workers investigated the effect of rod activity on the unique hues of the spectrum; the pure blue, green and yellow (see Nerger et al., 1995, 1998; Volbrecht et al., 2000). The measurements were made both during the cone plateau period when presumably only cones were activated and in a dark-adapted state when both rods and cones were activated.

The results showed that the added rod activity in the dark-adapted state may change the spectrum locus of all three unique

130chromatic rod vision: a historical account

hues. Also, they found the rod effect to be most marked with small test fields.

To explain their results, they suggested that rods might contribute to both the R-G and Y-B opponent channels, and that there was a reciprocal antagonistic interaction between the rod and cone receptor signals. The reduced influence of the rods with size of the test field, for example, was explained by the assumption that the cones became progressively more effective in suppressing the rod signals as the size of the test field increased, in a similar way to their suppression when test intensity increased above cone threshold. They also attempted to explain the change in the spectrum loci of unique blue and green towards shorter wavelengths with rod intrusion on the assumption that rod signals tended to inhibit the short-wave cone input to the R-G and Y-B opponent channels (Nerger et al., 1995).

In summary, the study of chromatic rod vision made it clear that the two basic assumptions of the orthodox duplicity theory, that rods and cones function independently of each other and that rods mediate achromatic vision only, were wrong. Yet, it must be admitted that our understanding of the underlying mechanisms of chromatic rod vision is still in a rudimentary state.