THE PHOTOCHEMISTRY OF THE
SENSITIVITY OF ANIMALS
TO LIGHT1

I

AN analysis of sensory stimulation, in order to be objective, must take its data from the relations between the properties of the stimulating agent and those of the responses of the animal. If the analysis is to be quantitative as well as objective, not only should the response be a qualitatively invariable reflex but, together with the source of stimulation, it should be capable of precise and easy control.

There are a number of animals which possess such characteristic responses. Typical of these are the ascidian, Ciona intestinalis and the long-neck clam, Mya arenaria. Both of these animals, when exposed to light, respond by a vigorous retraction of the siphons. It has therefore been possible to investigate quantitatively the properties of their photic sensitivity, and as a result to propose an hypothesis which accounts for this type of irritability in terms of an underlying photochemical mechanism.

I propose now to describe briefly the evidence which has been accumulated in this connection, and to present the outstanding features of the proposed hypothetical mechanism.

II

The photosensory activities of these animals possess four striking and important properties. (1) When exposed to light, the animal 1 Delivered at the Symposium on General Physiology held by the American Society of Naturalists on December 30, 1920, at its Chicago meetings. The paper was illustrated with a number of charts which are not reproduced here. They may be found, together with the data on which this summary is based, in a series of articles in the Journal of General Physiology from 1918 to the present time.

responds only after a measurable interval, which is usually longer than one and a half seconds. This interval is called the reaction time. (2) The animals will respond to light only when there has been a decided increase in its intensity. (3) Once a response has been secured to a given illumination, the continued application of the same intensity fails to produce any additional effect. (4) If, following this, the animal is placed in the dark, it very soon recovers its sensitivity to the light which had previously become ineffective.

It is apparent that these four characteristics are not confined merely to these two species of animals. They belong generally to all animals which are sensitive to increased illumination. Their analysis is therefore of more than immediate interest. Their presence and their quantitative aspects have determined the nature of the hypothesis proposed, and they in turn find their explanation in terms of the hypothesis. It will therefore be well to consider these four outstanding characteristics in greater detail.

III

The reaction time is the interval from the beginning of the exposure to the beginning of the response. In Ciona this may vary from 2 to 10 seconds, and in Mya from 1.5 to 4 seconds, depending on circumstances such as temperature, intensity of light, and duration of exposure. If these are kept constant, the reaction time is constant.

Fortunately this reaction time is made up almost entirely of the time lost in the sense organ. For example, mechanical stimulation produces the same reflex as illumination. Yet the retraction of the siphons occurs so rapidly that it is not possible to measure it with a stop watch. The adjustor and effector processes, therefore, take almost no measurable time, and the reaction time is confined to the processes which take place in the receptor. This is, to say the least, highly convenient.

The reaction time, however, is not a simple interval. The total exposure to light, which

it represents, is not necessary. If the animal is exposed for, say, half the reaction time, it will still respond in the dark at the end of the usual reaction time. By proper methods it is possible to reduce the exposure and at the same time to measure the reaction time. It is found that for each intensity of light there is a minimum exposure which will cause a response at the end of the usual reaction time. This short exposure is the sensitization period. Exposures longer than the sensitization period make no change in the duration of the reaction time; exposures shorter than the sensitization period prolong the reaction time, as will presently be described. That portion of the reaction time during which the animal is in the dark, or during which the exposure to light is not necessary, is called the latent period. Normally, therefore, the reaction time is composed of two parts: a sensitization period and a latent period.

The whole matter is strikingly illustrated with Mya. Here the sensitization period is extremely short. With a strong light it is only a few hundredths of a second long, whereas the latent period comprises most of the reaction time, which in such a case is about one and a half seconds.

The sensitization period varies with the intensity. The latent period however, provided certain conditions are maintained, is constant for all intensities. At room temperatures the latent period for Ciona is 1.76 seconds; for Mya it is 1.31 seconds. Since it is our purpose to study the quantitative aspects of this photic sensitivity, it is apparent that the analysis of the reaction time into its two constituents is of first rate significance. The composition of the reaction time was first discovered with Ciona, and it immediately opened a hitherto inaccessible field of investigation.

IV

The second characteristic of the sensitivity of these animals is the fact that they will respond to light only when it is increased. This initial action of the light must be on a photosensitive substance contained in the

sense organ. It is necessary to determine whether this action of the light on the sensory process possesses the ordinarily welldemonstrated characteristics of photochemical reactions. Photosensitive chemical reactions have been studied extensively, and certain of their properties have been found to be commonly distributed. One of these is that a definite quantity of radiant energy is associated with a definite photochemical effect. This is the well known Bunsen-Roscoe law, which states that to produce a given effect the product of the intensity and the time of exposure of the light is a constant.

Tested by this standard, the action of light in the sensory responses of Ciona and Mya is photochemical in nature. With Ciona, in the production of a response, the sensitization period varies inversely with the intensity, and their product is constant and equal to 4,746 meter-candle-seconds. The same is true for Mya. To produce the minimum stimulating effect the intensity must vary inversely as the exposure, the product of the two being in this case only 5.62 meter-candle-seconds.

Another common property of photochemical reactions is that they possess a low temperature coefficient. Whereas ordinary chemical reactions are markedly accelerated by an increase in temperature, photochemical reactions proceed at pretty much the same rate over wide ranges of temperature. Experiments show that the temperature coefficient for the action of light on the sensory activity of Mya is 1.06 for a rise of 10° C. This value is so characteristic for endo-energetic photochemical reactions that, combined with the applicability of the Bunsen-Roscoe law, it can lead to but one conclusion. That is that the initial effect of the light in photic stimulation is a rather simple photochemical phenomenon. These results further indicate that in order to produce a photosensory effect a definite amount of a photosensitive substance must be broken down by the light.

V

The third point which was made with regard to the sensory responses of these ani

mals is that the continued application of the light fails to elicit any additional effect. This has been tested with intense sunlight and with artificial light of over 10,000 metercandles intensity. The result is always the same. After the first retraction of the siphons, the animal comes into sensory equilibrium with the light. The siphons are slowly extended, and the animal appears to act as if there were no light present.

This brings us to the fourth characteristic of photic sensitivity-the one which has served as the key to the whole situation. This is the fact that when an animal has come into sensory equilibrium with a bright light, it may be made to recover its sensitivity to light by being placed for some time in the dark. The rate at which this recovery takes place is of significance, and has been carefully investigated in the case of Mya.

An animal is exposed to an intense light for an hour. It is then suddenly darkened, and at regular intervals its sensitivity is determined by measuring the reaction time to a light of low intensity. What one finds is this. For about three minutes the animal is still insensitive to the particular intensity used. On the fourth minute its first response appears. The reaction time when measured at this time is nearly twice as long as usual. Measured at regular intervals, the reaction time is found to decrease continuously during the next thirty-five minutes. At first this decrease is rapid, then slow, until after thirtyfive minutes or so it ceases entirely, and the reaction time is at its minimum for that intensity.

The course of dark adaptation is very orderly. It is similar in the case of Ciona, except that it is much slower, requiring about three hours for completion.

What is the significance of these regular changes? Physically they mean that during dark adaptation the quantity of light required for a response is much greater than normal, and that this quantity decreases at first rapidly, then more slowly. The effect of the light we have shown to be the photochemical decomposition of a sensitive sub