Dark adaptation and the photochromatic interval

Summary

An experimental as well as a theoretical analysis of the photochromatic interval as a function of dark adaptation has been undertaken. It is commonly assumed that the variation of the photochromatic interval with dark adaptation bears the following simple relation to the two-step dark adaptation curve. During the first step of dark adaptation the specific threshold coincides with the absolute threshold, and the photochromatic interval should not appear until this period of dark adaptation is passed. During further stay in the dark, the specific threshold remains unchanged from its value at the break in the dark adaptation curve. The intensity area covered by the second step of the dark adaptation curve should, therefore, be a representation of the changing value of the photochromatic interval as dark adaptation proceeds beyond the first step.

An attempt was made to summarize the available empirical evidence relevant to this assumption. It was concluded that the assumed relation between the photochromatic interval and the dark adaptation curve has not been put to a direct empirical test. Other evidence having some bearing upon the question was found to be far from conclusive.

The first group of experiments was set up to decide whether the photochromatic interval can be derived (according to the prevailing assumptions) from the dark adaptation curve. The courses for both the absolute and the specific threshold curves were determined with red, orange, yellow, and blue light. As far as the first step of dark adaptation is concerned, the results from these experiments show that sometimes the two thresholds coincide, at other times, a small photochromatic interval is observed. It was concluded that to some extent this inconsistency may reasonably be ascribed to an inconsistent use of the criteria adopted for determination of the two thresholds. Concerning the second step of dark adaptation, the specific threshold starts rising in a negatively accelerated way at the break in the dark adaptation curve. Moreover, the specific threshold appears to rise to much the same extent as the absolute threshold drops. The rise of the specific threshold at a given moment and the corresponding drop of the absolute threshold were labelledb anda, respectively. The question was discussed as to whetherb can be expressed as a function ofa, independent of wave-length. It was concluded that although inter-colour differences exist with respect to this function, thea-b relation appears to represent an adequate quantitative expression of a lawful relationship between the dark adaptation curves for the absolute and the specific thresholds.

A second group of experiments was conducted to investigate further the extent of invariance in thea-b relation. The courses of the two thresholds during dark adaptation were determined with regard to variations in the following conditions: the location of the retinal area stimulated, the size of the retinal area stimulated, the duration of the preceding light adaptation, and the duration of the test-exposure. It was concluded from this second group of experiments that, except, perhaps, from duration of pre-adaptation, thea-b relation cannot be specified independent of the experimental conditions. However, variations in the factors investigated were generally found to produce systematic variations in thea-b relation.

A final experiment was carried out to investigate some questions concerning the variation in saturation with a variation of intensity above the specific threshold.

The result has been interpreted within the frame-work of the duplicity theory. This theory appears to be the only one which provides a satisfactory explanation of both the two-step dark adaptation curve and the photochromatic interval. However, the duplicity theory does not allow for a prediction of the course of the specific threshold curve and its relation to the dark adaptation curve. An evaluation of the explicit statements of the theory reveals that it falls short on the following critical points - which directly bear on the prediction of the specific threshold: Do cones make hue discrimination possible whenever they function ? How and where does the transition from rod to cone activity take place ? Do rods and cones function independently of each other ? What is the psychological effect of a possible simultaneous activity of rods and cones ? In view of these shortcomings the main implication of the present results is that in the dark-adapted eye rods and cones function and influence colour vision simultaneously within a wide range of intensities.

Résumé

Le but des expérimentations qui ont été entreprises était d'effectuer un test en vue d'établir la plausibilité de la supposition ordinaire selon laquelle la proportion quantitative entre l'intervalle photochromatique et l'état d'adaptation de l'oeil peut être constatée directement de la courbe d'adaptation à l'obscurité.

En se basant sur les résultats obtenues antérieurement on a supposé que l'intervalle photochromatique n'apparaît qu'au moment où l'oeil s'est adapté à un niveau correspondant au point de brisement dans la courbe d'adaptation à l'obscurité. On a de plus et sans base d'expérimentation, supposé qu'à partir du point de brisement le seuil spécifique reste inchangé, nonobstant les variations de l'état d'adaptation.

A la lumière de ces suppositions, les résultats principaux peuvent être résumés dans les deux points suivants:

• 1) Dans la plupart des conditions qui provoquent une courbe d'adaptation à l'obscurité bi-sectionnée, une petite intervalle photochromatique pourra être observée même au cours de l'espace de temps correspondant à la première partie de la courbe d'adaptation à l'obscurité.

• 2) A partir du moment correspondant au point de brisement dans la courbe d'adaptation à l'obscurité, et dans la plupart des conditions, le seuil spécifique augmente à chaque moment donné approximativement au même degré que le seuil absolu baisse.

Les résultats impliquent que dans l'oeil adapté à l'obscurité fonctionnent à la fois des bâtonnets et des cônes sur un domaine d'intensité étendu.

Zusammenfassung

Der Zweck der vorliegenden Experimente war die Haltbarkeit der gewöhnlichen Vermutung zu erproben, dass das quantitative Verhältnis zwischen dem photochromatischen Intervall und dem Adaptations-zustand des Auges direkt von der Dunkeladaptierungskurve abgeleitet werden kann.

Unterstützt durch frühere Ergebnisse ist angenommen worden, dass das photochromatische Intervall erst dann auftritt, wenn das Auge an ein Niveau adaptiert ist, welches dem Knickpunkt der Dunkeladaptierungskurve entspricht. Im weiteren ist, ohne irgendwelche experimentelle Unterstützung, angenommen worden, dass vom Knickpunkt aus die spezifische Schwelle ungeändert bleibt, ungeachtet der Variationen im Adaptationszustand.

Im Lichte dieser Vermutungen können die Hauptergebnisse in folgenden zwei Punkten zusammengefasst werden.

• 1) Unter den meisten Bedingungen, die eine zwei-geteilte Dunkel-adaptierungskurve hervorbringen, wird ein kleines photochromatisches Intervall beobachtet werden, auch in dem Zeitraum, der dem ersten Teil der Dunkeladaptierungskurve entspricht.

• 2) Von dem Zeitpunkt an, welcher dem Knickpunkt in der Dunkel-adaptierungskurve entspricht, steigt die spezifische Schwelle unter den meisten Bedingungen, zu jedem Zeitpunkt annäherungsweise im gleichen Grade wie die absolute Schwelle sinkt.

Die Ergebnisse implizieren, dass in dem dunkeladaptierten Auge sowohl Stäbchen wie Zapfen gleichzeitig über ein weites Intensitätsgebiet wirksam sind.