M.A. Bouman

The mechanism of dark-adaptation

An analysis is given of the various possible components of the dark-adaptation process. A distinction is made between: Photochemical component (ƒ1), neural receptor component (ƒ2), spatial summation component (D), temporal summation component (T) and a quantum coincidence component (k). These various components partly reveal themselves in different ways in dark-adaptation curves obtained by sensitivity measurements as a function of time for different target-sizes, different target-colours and different retinal locations. Experiments were carried out in order to study the relative contribution of each component for foveal as well as for peripheral vision. Foveal adaptation appears to be governed by the component ƒ1 and perhaps also ƒ2. In phase I of peripheral adaptation a small D-component is added and in phase II of peripheral adaptation only the k-component is active. The behaviour of the resting potential of the human eye during dark-adaptation does not bear a simple relation to any of the neural components mentioned in the present paper.

Adaptation and the electrical excitability of the eye.

SummaryRecent discoveries on the ommatidium mosaic structure of the human retina for sharp vision in colour are shortly reviewed and discussed in terms of new experimental results of electric phosphene threshold measurements with sinusoidal and pulse-shaped current.RésuméLes dernières découvertes sur la disposition en mosaique des ommatidies dans la rétine humaine sont passées en revue et leur importance pour une bonne vision des couleurs est soulignée. Elles sont discutées étant donné que de nouveaux résultats expérimentaux portant sur des mesures du seuil de phosphènes électriques, provoqués par un courant sénusoidal ou puisé, ont été obtenus.ZusammenfassungNach einem kurzen Überblick über die letzten Beobachtungen der Ommatidien-Mosaikstruktur der menschlichen Retina für scharfes Farbsehen werden diese im Hinblick auf experimentelle Resultate für elektrische Phosphenschwellenmessungen mit sinusförmigem und pulsförmigem Strom diskutiert.

The Mechanism of Dark Adaptation

Many investigators have tried to explain the phenomena of human dark adaptation solely on the basis of photochemical theories. All their attempts have failed. In the first place, there is no doubt but that the increase of the concentration of photopigments during dark adaptation is insufficient for a complete explanation of the phenomena; in the second place psycho-physical and electro-physiological evidence have made it clear that apart from a “photochemical” mechanism, “neural” mechanisms are active during dark adaptation. The present paper has as its purpose an experimental evaluation of the relative importance of the various components of dark adaptation. On theoretical and experimental grounds the following list of mechanisms of dark adaptation is suggested: fΓcomponent. This is the photochemical component discussed above. f.2-component. The neural receptor component. In the dark adapted eye a certain amount of decomposition of visual pigment causes the receptor to answer with a spike response whereas the same amount of decomposition does not have this effect in the light adapted eye. D-component. The spatial integration component. In the dark adapted eye the diameter (D) of the retinal area within which complete integration occurs is larger than in the light adapted eye. T-component. The temporal integration component. In the dark adapted eye the duration within which complete integration occurs is larger than in the light adapted eye. In the experiments which will be studied in the present paper this component was eliminated by the use of light flashes of very short duration. k-component. The coincidence component. Bouman and υ. d. Velden have shown that for the fully dark adapted eye two “coincident” quanta (i.e., two quanta incident within a critical retinal area with diameter D and within a critical duration T) are sufficient to elicit a visual response. Bouman and ten Doesschate have shown that for the light adapted eye this number k > 2. The resulting adaplational effect is called coincidence or k-component. Subsequently il is shown that some of the components mentioned above can be separated experimentally by studying the behaviour of dark adaptation as a function of the diameter (d) of the test field. Such experiments can thus be made for the centre of the retina (exclusively foveal cones) and for the Ophthalmologica, Vol. 144, No. 4 (Oktober 1962) 21 256 Ten Doesschate and B o u m a n retinal periphery (rods and cones) and further for both red light (that predominantly stimulates the cones) and green light (that stimulates both rods and cones). The results of such experimental analyses are described.

Photochemical and Nervous Components of Visual Adaptation

Measurements of the chance of observation of brief light-flashes as a function of the diameter of the testfield for the dark and light adapted eye, combined with theoretical considerations based on van der Velden’s 2 quantum hypothesis lead to the conclusion that the process of visual adaptation is not only governed by a change in concentration of the visual purple (photochemical component) but also by a change in the switching of the neurons (nervous component). Theoretically it is possible to separate both components in cases of pathological disturbance of dark adaptation. A more extensive paper on the same subject has been published in Oph-thalmologica 126, 222– 230, 1953. Discussion. Jonkers: It would be interesting to know if your conclusions would prove applicable to varying conditions of illumination of the testfield, i.e. in varying degrees of dark adaptation. Does not the so-called “retinal grey” cause disturbances in more advanced dark adaptation? ten Doesschate answers: I refer Mr. Jonkers to the papers of Bouman on the chance of observation of intensity differences. The presence of the adapting field causes complications which we tried to avoid by the present experimental conditions. The “retinal grey” did not disturb the well-trained subject.