scholarly journals SENSORY EQUILIBRIUM AND DARK ADAPTATION IN MYA ARENARIA

1919 ◽  
Vol 1 (5) ◽  
pp. 545-558 ◽  
Author(s):  
Selig Hecht
Keyword(s):  
Reaction Time ◽  
Latent Period ◽  
Dark Adaptation ◽  
Photochemical Reaction ◽  
Short Interval ◽  
Bimolecular Reaction ◽  
Mya Arenaria ◽  
Rational Explanation ◽  
Order Of Magnitude ◽  
Photosensitive Substance

1. The reaction time of Mya to light is composed of two parts. The first, a sensitization period, is an exceedingly short interval of the order of magnitude associated with photographic processes. The second is a latent period of about 1.3 seconds, during which Mya need not remain exposed to the stimulating light. 2. The process of dark adaptation in Mya is orderly. Its progress may be represented by the formation of a photosensitive substance according to the dynamics of a bimolecular reaction. See PDF for Structure 3. Photosensory equilibrium as represented by the light- and dark-adapted conditions finds a rational explanation in terms of the "stationary state" of a reversible photochemical reaction involving a photosensitive substance and its two precursors. 4. There are two corollaries to this hypothesis. The first requires that the reaction time at sensory equilibrium for a given intensity should vary inversely with the temperature; the second, that the rate of dark adaptation should vary directly with the temperature. Experiments verified both of these requirements.

scholarly journals THE PHOTIC SENSITIVITY OF CIONA INTESTINALIS

1918 ◽  
Vol 1 (2) ◽  
pp. 147-166 ◽  
Author(s):  
Selig Hecht
Keyword(s):  
Reaction Time ◽  
Latent Period ◽  
Dark Adaptation ◽  
Local Reaction ◽  
The Body ◽  
Chemical System ◽  
Intensity Analysis ◽  
Constant Ratio ◽  
Neural Mass ◽  
Photosensitive Substance

1. Ciona possesses two means of responding to an increase in the intensity of illumination. One is by means of a local reaction; the other is by a retraction reflex of the body as a whole. 2. The "ocelli" are not photoreceptors. The photosensitive area is in the intersiphonal region containing the neural mass. This area contains no pigment. 3. The reaction time to light is composed of a sensitization period during which Ciona must be exposed to the light, and of a latent period during which it need not be illuminated in order to react to the stimulus received during the sensitization period. 4. The duration of the reaction time varies inversely as the intensity. Analysis shows the latent period to be constant. The relation between the sensitization period and the intensity follows the Bunsen-Roscoe rule. 5. During dark adaptation the reaction time is at first large, then it decreases until a constant minimum is reached. 6. A photochemical system consisting of a reversible reaction is suggested in order to account for the phenomena observed. This system includes a photosensitive substance and its precursor, the dynamics of the reaction following closely the peculiarities of the photosensitivity of Ciona. 7. It is shown that in order to produce a reaction, a constant ratio must be reached between the amount of sensitive substance broken down by the stimulus and the amount previously broken down. 8. From the chemical system suggested certain experimental predictions were made. The actual experiments verified these predictions exactly. 9. The results obtained with regularly repeated stimulation not only fail to show any basis for a learning process or for the presence of a "higher behavior," but follow the requirements of the photochemical system suggested before.

scholarly journals DARK ADAPTATION AND THE LIGHT-GROWTH RESPONSE OF PHYCOMYCES

1929 ◽  
Vol 12 (3) ◽  
pp. 391-400 ◽  
Author(s):  
E. S. Castle
Keyword(s):  
Latent Period ◽  
Dark Adaptation ◽  
Growth Response ◽  
Light Adaptation ◽  
Exposure Period ◽  
Bimolecular Reaction ◽  
Dark Reaction ◽  
Action Time ◽  
Kinetics Of ◽  
Photochemical Action

1. A single-celled, elongating sporangiophore of Phycomyces responds to a sufficient increase in intensity of illumination by a brief increase in growth rate. This is the "light-growth response" of Blaauw. 2. The reaction time is compound, consisting of an exposure period and a latent period (this comprising both the true latent period resulting from photochemical action and any "action time" necessary for the response). During the latter period the plant may be in darkness, responding nevertheless at the end of the latent period. 3. Both light adaptation and dark adaptation occur in the sporangiophore. The kinetics of dark adaptation can be accounted for on the basis of a bimolecular reaction, perhaps modified by autocatalysis. Attention is called to the bimolecular nature of the "dark" reaction in all other photosensory systems that have been studied, in spite of the diversity of the photosensitive substances themselves and of the different forms of the responses to light.

scholarly journals THE NATURE OF THE LATENT PERIOD IN THE PHOTIC RESPONSE OF MYA ARENARIA

1919 ◽  
Vol 1 (6) ◽  
pp. 657-666 ◽  
Author(s):  
Selig Hecht
Keyword(s):  
Latent Period ◽  
Linear Function ◽  
Chemical Reaction ◽  
Photochemical Reaction ◽  
Exposure Period ◽  
Photochemical Activity ◽  
Mya Arenaria ◽  
Effect Of Temperature ◽  
Photic Response

The latent period in the response of Mya to illumination varies inversely as the duration of the exposure to which it is subjected. The reciprocal of the latent period, measuring the velocity of the process which underlies it, is a linear function of the exposure period. Since the duration of the exposure represents the amount of photochemical activity, it is concluded that the substances formed at that time act to catalyze a chemical reaction which determines the duration of the latent period. This explanation is in accord with the previous work on the photochemical reaction and with the effect of temperature on the latent period. As a result of the combined investigations there is presented a concrete hypothesis for the mechanism of photic reception in Mya.

scholarly journals THE DARK ADAPTATION OF THE HUMAN EYE

1920 ◽  
Vol 2 (5) ◽  
pp. 499-517 ◽  
Author(s):  
Selig Hecht
Keyword(s):  
Dark Adaptation ◽  
Photochemical Reaction ◽  
Quantitative Data ◽  
Dark Reaction ◽  
Human Eye ◽  
Visual Threshold ◽  
Bimolecular Chemical Reaction ◽  
Dim Light ◽  
Visual Reception ◽  
Photosensitive Substance

During the dark adaptation of the human eye, its visual threshold decreases to a small fraction of its original value in the light. An analysis of the quantitative data describing this adaptation shows that it follows the course of a bimolecular chemical reaction. On the basis of these findings it is suggested that visual reception in dim light is conditioned by a reversible photochemical reaction involving a photosensitive substance and its two products of decomposition. Accordingly, dark adaptation depends on the course of the "dark" reaction during which the two products of decomposition reunite to synthesize the original photosensitive substance.

scholarly journals THE EFFECT OF TEMPERATURE ON THE LATENT PERIOD IN THE PHOTIC RESPONSE OF MYA ARENARIA

1919 ◽  
Vol 1 (6) ◽  
pp. 667-685 ◽  
Author(s):  
Selig Hecht
Keyword(s):  
Reaction Time ◽  
Latent Period ◽  
Environmental Temperature ◽  
Arrhenius Equation ◽  
Mya Arenaria ◽  
Effect Of Temperature ◽  
Photochemical Process ◽  
Principal Product ◽  
Different Temperatures ◽  
Photic Response

1. The effect of temperature on the reaction time of Mya to light is mainly confined to the latent period. The sensitization period, representing a photochemical process, is changed comparatively little. 2. The relation between the latent period and the temperature is adequately expressed by the Arrhenius equation, for temperatures below 21°C. Above this temperature, the latent period becomes increasingly longer than is required by the Arrhenius formula when µ = 19,680. 3. These deviations, occurring above the highest environmental temperature of Mya, are explained on the assumption that the principal product formed during the latent period is inactivated by heat. 4. Calculation of the velocity of the hypothetical inactivation reaction at different temperatures shows that it also follows the Arrhenius rule when µ = 48,500. This value of µ corresponds to those generally found for spontaneous inactivations and destructions.

scholarly journals THE NATURE OF FOVEAL DARK ADAPTATION

1921 ◽  
Vol 4 (2) ◽  
pp. 113-139 ◽  
Author(s):  
Selig Hecht
Keyword(s):  
Stationary State ◽  
Dark Adaptation ◽  
Light Adaptation ◽  
Bimolecular Reaction ◽  
Dark Side ◽  
Light Perception ◽  
Foveal Vision ◽  
Dark Reaction ◽  
Initial Event ◽  
Photosensitive Substance

1. After a discussion of the sources of error involved in the study of dark adaptation, an apparatus and a procedure are described which avoid these errors. The method includes a control of the initial light adaptation, a record of the exact beginning of dark adaptation, and an accurate means of measuring the threshold of the fovea after different intervals in the dark. 2. The results show that dark adaptation of the eye as measured by foveal vision proceeds at a very precipitous rate during the first few seconds, that most of the adaptation takes place during the first 30 seconds, and that the process practically ceases after 10 minutes. These findings explain much of the irregularity of the older data. 3. The changes which correspond to those in the fovea alone are secured by correcting the above results in terms of the movements of the pupil during dark adaptation. 4. On the assumption that the photochemical effect of the light is a linear function of the intensity, it is shown that the dark adaptation of the fovea itself follows the course of a bimolecular reaction. This is interpreted to mean that there are two photolytic products in the fovea; that they are disappearing because they are recombining to form anew the photosensitive substance of the fovea; and that the concentration of these products of photolysis in the sense cell must be increased by a definite fraction in order to produce a visual effect. 5. It is then suggested that the basis of the initial event in foveal light perception is some mechanism that involves a reversible photochemical reaction of which the "dark" reaction is bimolecular. Dark adaptation follows the "dark" reaction; sensory equilibrium is represented by the stationary state; and light adaptation by the shifting of the stationary state to a fresh point of equilibrium toward the "dark" side of the reaction.

scholarly journals THE KINETICS OF DARK ADAPTATION

1927 ◽  
Vol 10 (5) ◽  
pp. 781-809 ◽  
Author(s):  
Selig Hecht
Keyword(s):  
Dark Adaptation ◽  
Photochemical Reaction ◽  
Bimolecular Reaction ◽  
Chemical Mechanism ◽  
Constant Intensity ◽  
Photosensitive Material ◽  
Chemical Combination ◽  
Other Substances ◽  
Primary Photochemical Reaction ◽  
Kinetics Of

1. Data are presented for the dark adaptation of four species of animals. They show that during dark adaptation the reaction time of an animal to light of constant intensity decreases at first rapidly, then slowly, until it reaches a constant minimum. 2. On the assumption that at all stages of adaptation a given response to light involves a constant photochemical effect, it is possible to describe the progress of dark adaptation by the equation of a bimolecular reaction. This supposes, therefore, that dark adaptation represents the accumulation within the sense cells of a photosensitive material formed by the chemical combination of two other substances. 3. The chemical nature of the process is further borne out by the fact that the speed of dark adaptation is affected by the temperature. The velocity constant of the bimolecular process describing dark adaptation bears in Mya a relation to the temperature such that the Arrhenius equation expresses it with considerable exactness when µ = 17,400. 4. A chemical mechanism is suggested which can account not only for the data of dark adaptation here presented, but for many other properties of the photosensory process which have already been investigated in these animals. This assumes the existence of a coupled photochemical reaction of which the secondary, "dark" reaction is catalyzed by the products of the primary photochemical reaction proper. This primary photochemical reaction itself is reversible in that its main products combine to form again the photosensitive material, whose concentration controls the behavior of the system during dark adaptation.

Corrosion Tests of Lwr Fuels – Nuclide Release

2002 ◽  
Vol 713 ◽  
Author(s):  
Patricia A. Finn ◽  
Yifen Tsai ◽  
James C. Cunnane
Keyword(s):  
Reaction Time ◽  
Release Rates ◽  
Corrosion Tests ◽  
Cumulative Release ◽  
Order Of Magnitude ◽  
High Burnup ◽  
Fractional Release

ABSTRACTTwo high burnup BWR fuels [64 and 71 (MWd)/kgU], one of which contained 2% Gd, and two low burnup PWR fuels [30 and 45 (MWd)/kgU], are tested by dripping groundwater on the fuel under oxidizing and hydrologically unsaturated conditions for 2.4 and 8.2 yr, respectively, at 90°C. The 99Tc, 129I, 137Cs, 97Mo, and 90Sr releases are presented to show the effects of reaction time and of gadolinium (Gd) on nuclide release. For the PWR fuels, the five nuclides have similar fractional release rates at 8.2 yr. For the BWR fuels, the presence of 2% Gd reduced the 99Tc cumulative release fraction after 2.4 yr by about an order of magnitude from that of a fuel with a similar burnup but without added Gd.

Combining transcranial direct current stimulation with aerobic exercise to optimize cortical priming in stroke

2020 ◽  
Author(s):  
Anjali Sivaramakrishnan ◽  
Sangeetha Madhavan
Keyword(s):  
Reaction Time ◽  
Aerobic Exercise ◽  
Direct Current ◽  
Transcranial Direct Current Stimulation ◽  
Lower Limb ◽  
Synergistic Effects ◽  
Short Interval ◽  
Moderate Intensity ◽  
Direct Current Stimulation ◽  
Transcallosal Inhibition

Aerobic exercise (AE) and transcranial direct current stimulation (tDCS) are priming techniques that have been studied for their potential neuromodulatory effects on corticomotor excitability (CME), however the synergistic effects of AE and tDCS are not explored in stroke. Here we investigated the synergistic effects of AE and tDCS on CME, intracortical and transcallosal inhibition, and motor control for the lower limb in stroke. 26 stroke survivors participated in three sessions - tDCS, AE and AE + tDCS. AE included moderate intensity exercise and tDCS included 1 mA of anodal tDCS to the lower limb motor cortex with or without AE. Outcomes included measures of CME, short interval intracortical inhibition (SICI), ipsilateral silent period (iSP) (an index of transcallosal inhibition) for the tibialis anterior and ankle reaction time. Ipsilesional CME significantly decreased for AE compared to AE + tDCS and tDCS. No differences were noted in SICI, iSP measures or reaction time between all three sessions. Our findings suggest that a combination of exercise and tDCS, and tDCS demonstrate greater excitability of the ipsilesional hemisphere compared to exercise only, however these effects were specific to the descending corticomotor pathways. No additive priming effects of exercise and tDCS over tDCS was observed. Novelty: • An exercise and tDCS paradigm upregulated the descending motor pathways from the ipsilesional lower limb M1 compared to exercise. • Exercise or tDCS administered alone or in combination did not affect intracortical or transcallosal inhibition or reaction time.

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