13  Rod-cone interactions in mesopic vision

13.1  Rod-cone interactions under mesopic conditions in a chromatically neutral state of adaptation

In the period from Schultze (1866) to Lie (1963) it had been generally agreed that rods and cones interacted in a kind of colour-­mixture process in mesopic vision. Rods and cones were assumed to contribute an achromatic and a chromatic component, respectively. The most advanced attempt to further characterize the rod-cone interaction under mesopic conditions had been made by Granit (1938, 1947) and Lie (1963). They both suggested that rod and cone activities ­antagonized each other at the retinal level in that the most sensitive receptor system tended to suppress the other. The rise of the­specific-hue threshold obtained when rods intruded during long-term dark adaptation was compelling evidence in favour of this antagonistic interaction. There could be little doubt that rod signals completely suppressed the chromatic cone signals within the intensity interval between the cone-plateau level and the specific-hue threshold.

Yet, in apparent opposition to the psychophysical data and the hypothesis of rod-cone antagonism proposed by Lie (1963), the histological­ and electrophysiological evidence obtained in the late 1980s and early 1990s indicated that rods and cones added their responses of the same polarity both at the receptor and bipolar cell levels (Daw et al., 1990; Schneeweis & Schnapf, 1995).

Fortunately, this apparent conflict between the psychophysical and the electrophysiological data could easily be resolved. Thus, the desaturation effect of the rods could be explained simply by suggesting that rod signals in a dark-adapted and chromatically neutral state of adaptation activated the different types of spectrally opponent cell

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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 thespecific-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, 1971a, 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 monochromatic deep-red spectral light at intensities somewhat below absolute

122chromatic rod vision: a historical account

cone threshold, the light quanta might excite a few of the ‘red’ cone receptors and, thereby, increase the responsiveness of the redand yellow-related opponent cells. If in this state rod activity were added, the relative activity of these opponent cells may be enhanced, and facilitate the specific-hue threshold. It should be noted that this facilitation of the chromatic cone component by rod activity could only be found under conditions where the chromatic cone component was relatively strong and the achromatic rod component relatively weak. Nevertheless, the conclusion had to be drawn that rod activity in a chromatically neutral state of adaptation was capable not only of desaturating chromatic cone activity, as previously assumed, but also of facilitating it.

13.2  Rod-cone interactions under mesopic conditions in a chromatic state of adaptation

Ever since Schultze (1866) published his duplicity theory, scientists had inquired into how the rod and cone systems interacted in mesopic vision to produce a combined effect. As we have seen, the prevailing hypothesis had been that rods and cones interacted in a kind of colourmixture process in which the cone and rod systems contributed­ a chromatic and an achromatic component, respectively.

The finding that rod signals also may give rise to chromatic colour sensations immediately raised the question of how rods and cones interacted when they both contributed a chromatic component.

A series of experiments was performed to explore this kind of rod-cone interaction. Three different procedures were employed: the change in colour of a test field was measured when (1) test condition changed from photopic to mesopic during long-term dark adaptation,

(2) the rod stimulus was simply superimposed on the cone stimulus, and (3) light intensity of the test field was increased from scotopic to photopic intensity levels. Under all these conditions both receptor types contributed a chromatic component (see Stabell, 1967b; Stabell & Stabell, 1973b, 1974, 1998).

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The results of the experiments indicated that rod activity under mesopic test conditions may contribute with all kinds of colour, and that the relative contributions of rods and cones in the colour-­mixture process may change with test intensity, in that rods dominate at low and cones at high levels. Although this change in relative contribution was found regardless of colour quality of the rod and cone components, it could most simply be demonstrated when the rod and cone components represented a pair of opponent colours. For instance, when a blue rod and a yellow cone component were mixed and the intensity of the test field gradually increased within the mesopic intensity interval, the saturation of the blue rod component was seen to decrease gradually until only the achromatic component remained. Thereafter, the yellow cone component emerged with increasing saturation. Apparently, the rod and cone opponent colour processes antagonized each other to the extent that only the chromatic rod component could be observed at low and the chromatic cone component at high mesopic intensity levels. The inhibition of rods was completed when the test intensity reached photopic levels, where no trace of the rod component could be observed.

It should be noted that this inhibition of the rod influence has an obvious survival value. Thus, by taking over the control of the relative activity rate of the spectrally opponent cells at high mesopic intensities, cones may prevent rods from confusing the coded information­ about hue (Stabell & Stabell, 1973b).

Another fundamental difference between cone-cone and rod-conemixturedatawasalsorevealedbyexperimental­investigation. Thus, in sharp contrast to the cone-cone colour-mixture data, gross failures of the additive colour-mixture law were found when rod and cone colours were mixed. The extent of the deviation was found to vary both with colour quality of the rod and cone components, and also with test intensity (Stabell & Stabell, 1998).