100 development of the duplicity theory from 1930–1966

The systematic evaluation of the duplicity theory by Saugstad and Saugstad could leave little doubt that there was ample room for improvement and further development of the duplicity theory. In particular, they stressed the lack of knowledge with regard to rod-cone interaction under mesopic conditions. The available evidence strongly suggested that rod and cone impulses interacted and produced a combined effect under a variety of mesopic test conditions, but the essential characteristics of the interaction processes were still largely unknown.

9.3  Ivar Lie: interactions between rod and cone functions at mesopic intensity

9.3.1  Psychophysical experiments

Only a few years later, however, the question of how rods and cones interact under mesopic conditions was successfully addressed by Lie (1963). In an extensive psychophysical, empirical investigation, using a Hecth & Schlaer adaptometer (Hecth & Schlaer, 1938) he provided conclusive evidence that the achromatic rod and the chromatic cone components may interact in a kind of colour-mixing process.

In his investigation, Lie took advantage of the generally accepted assumption that cones dark adapt much faster than rods. Thus, it had long been known that following substantial bleaches, cones reach their maximum sensitivity after a few minutes of dark adaptation, while the rods may need more than 40 minutes to approach their final level. Hence, the effect of rod activity upon cone functions may be measured at mesopic intensity levels after the cones have reached their dark-adapted state and the rods still increase their sensitivity.

Loeser (1904) appears to be the first to utilize this ideal method under adequately controlled conditions. In this study he investigated how rod signals affected cone-mediated colour sensation when they intruded during long-term dark adaptation. During the first phase of the dark adaptation period, he found the absolute and the specific-hue thresholds (i.e. the intensity levels where the subject first becomes

102 development of the duplicity theory from 1930–1966

These important findings of Loeser (1904) did not receive much attention from the scientific community until Lie (1963), nearly 60 years later, pointed out their significance. He confirmed the basic results of Loeser (1904) and showed that the rise in thespecific-hue threshold and the fall in the absolute rod threshold obtained during the rod phase of dark adaptation were closely related (Fig. 9.1). Also, in accord with the assumption that the rise in the specific-hue threshold was due to rod activity, he found little or no rise in the specific-hue threshold when the test light was confined to the rod-free fovea or when deep-red test light was used.

Yet, the mathematical relationship between the fall of the absolute threshold and the rise of the specific-hue threshold during the rod phase of dark adaptation was not a simple one. Thus, Lie (1963) showed that the specific-hue threshold level became gradually less influenced by the drop in rod threshold as the rod sensitivity increased. Indeed, when the rod threshold had surpassed the cone threshold by about 1 log unit, the further fall in rod threshold had little or no effect on the specific hue-threshold level. Furthermore, Lie (1963) found the relationship between the two values to be a joint function of a number of test variables.

In spite of these complications, Lie (1963) concluded, in accord with the explanation given previously by Loeser (1904), that the rise of the specific-hue threshold was an effect of the increase of the rod activity.

In a separate experiment (Experiment 7), he compared the ­saturation and brightness of a green monochromatic test colour observed extrafoveally at intensities above the specific hue-threshold, during the cone-plateau period and in a dark-adapted state. He found that the test field observed in the dark-adapted state appeared more desaturated and bright. As the test intensity was increased, however, the difference in saturation and brightness decreased until, at about 1 log unit above the specific-hue threshold, he found saturation and brightness obtained during the cone-plateau period and in a dark-adapted state to be the same.

duplicity theory of willmer et al. 103

9.3.2  The colour-mixing hypothesis

Presuming that the sensitivity of cones stayed essentially constant, while the sensitivity of rods increased during the rod phase of dark adaptation, Lie (1963) explained his results (the rise of the specif- ic-hue threshold during the rod phase of dark adaptation and the effects on saturation and brightness of dark adaptation obtained with the green test light, Experiment 7) by a colour-mixture hypothesis, where the cone and rod mechanisms contributed a chromatic and an achromatic component, respectively.

Presuming that the relative strength of the achromatic rod and the chromatic cone components varied with changes in the test conditions, he could also explain the changing relationship obtained between the fall in rod threshold and the rise in the specific-hue threshold when test conditions were varied.

9.3.3  An alternative explanatory model

Although Lie (1963) held that his simple colour-mixture hypothesis could account reasonably well for his results, he presented a more sophisticated, alternative explanatory model where he combined the colour-mixture hypothesis with a hypothesis of antagonistic retinal interaction between the rod and cone activities. In defence of this antagonistic interaction he argued that rod and cone impulses when activating the same nerve fibre simultaneously must be mutually exclusive since they mediate qualitatively different colour sensations.

With the alternative model he could explain change in colour with intensity observed in a dark-adapted state as follows:

When the eye is test stimulated with a monochromatic light between the absolute threshold and the cone-plateau level in a dark-adapted state, there is no antagonistic rod-cone interaction, since only the rod system is activated. The subject will therefore observe the test flash as achromatic.

When the test intensity reaches the cone-plateau level, fibres connected to both rods and cones will still be occupied with rod activity, since the cone impulses are inhibited by the stronger rod

104development of the duplicity theory from 1930–1966

impulses. Nerve fibres connected solely to cones, on the other hand, become occupied with cone activity. However, the achromatic rod component in the colour-mixture process will be dominant so that the subject will not detect the small chromatic component involved.

As the test intensity increases further, the relative strength of the cone impulses gradually increases resulting in an increasing number of cone-occupied nerve fibres. Eventually, the subject will detect the so-called ‘correct’ hue of the test light. It should be noted that the change-over from rod to cone occupied nerve fibres was assumed to take place at different intensity levels for different fibres, depending on the sensitivity of the individual receptors and the number of cones relative to rods connected to each fibre.

When the test intensity increases above the specific­ -hue threshold level, the saturation of the test colour will increase as the number of cone-occupied nerve fibres increases.

At the highest intensity levels only fibres connected exclusively to rods will be occupied with rod activity. However, as indicated by his results obtained with the green test light (Experiment 7), the subjects would not be able to detect this small achromatic rod component in the colour-mixture process.

Although both his theoretical models could explain the data reasonably well, Lie (1963) pointed out that several important questions remained, and that his investigation should be expanded. In particular, he found it of interest to explore how the specific-hue threshold level changed with light adaptation. He predicted that if the mechanisms of light and dark adaptation were the same, the ­specific-hue threshold should be obtained at the same intensity level under mesopic test conditions when the absolute rod thresholds for dark and light adaptation under scotopic test conditions were the same. He held that this prediction should be subjected to an empirical test, but never engaged in such an investigation.

Lie (1963) also pointed out the lack of knowledge with regard to variation in the relative strength of the achromatic rod and chromatic cone component with test parameters.