Function Of Flash Intensity Research Articles

Recognition of letters displayed as briefly flashed dot patterns.

Complex shapes can be identified (named) when only the outer boundary of the shape is represented by discrete dots and with the dots being displayed for a duration lasting only a few microseconds (μs). This line of work is extended here to include recognition of letters with 10 μs flashes as a means to study visible persistence and information persistence. The first two studies were designed to assess visible persistence. Models were derived that quantified how recognition changed as a function of flash intensity. Then each letter was displayed twice, each at a near-threshold level of intensity and varying the interval between flashes. The second flash was able to boost the influence of the first flash for about 100 ms. This corresponds to the duration that a brief flash will remain visible, so these conditions likely were producing visible persistence. Information persistence was studied by manipulating dot density of the letter patterns. Recognition declined as the density of dots in each letter pattern was reduced. When two complimentary low-density samples were flashed, there was summation of their influence that declined to an asymptote in about 200 ms, and then remained above the one-flash control out to the maximum test interval of 1 s. The summation of high-salience, low-density dot patterns over such a long interval likely reflects both iconic memory persistence and access to working memory.

Impact of aging and age-related maculopathy on activation of the a-wave of the rod-mediated electroretinogram.

To examine the impact of aging and age-related maculopathy (ARM) on the activation of phototransduction in rod photoreceptors by measuring the a-wave of the flash, full-field electroretinogram (ERG). Enrollees consisted of older adults (> or = 60 years of age) in normal retinal health (n = 41) and those with early (n = 39) or late ARM (n = 7), in whom disease presence and severity were defined based on grading of stereoscopic color fundus photographs according to the Wisconsin Age-Related Maculopathy grading system. Young adults (ages 16-30 years; n = 27) were enrolled for comparison purposes. Previously established procedures were used to estimate the ERG response to two families of flash intensities. By computer subtraction of responses, the isolated rod response was identified. Each participant's ensemble rod responses were fit with the following equation to describe the response (R) as function of flash intensity (I), and time (t): R(I,t) = [1 - exp[-I x S x (t - td)2]] x RmP3, where S is sensitivity, td is the delay before onset of the a-wave, and Rm(P3) is the maximum amplitude. In analyses of older adults, there was no impact of early ARM presence or severity on log S, Rm(P3), or td after adjustment for age and intraocular lens presence. Differences between young and old normal subjects in log S, RmP3, and td disappeared when analyses were limited to older adults with intraocular lenses. When the light absorption of the aged lens is taken into account and reliable definitions of normal retinal aging and ARM are used, the activation of the a-wave as measured by the rod-mediated full-field ERG is not affected by early ARM, nor is it impacted by normal retinal aging.

Dissociating sensory and cognitive contributions to visual persistence I. Photoreceptor response duration as a function of flash intensity, adaptation state, and candidate code

Numerous careful behavioral studies of visual persistence have reported a variety of apparently contradictory effects. Variation of flash intensity has particularly been found to have both direct and inverse effects on subjective duration. This conflict has been addressed by theories which contain both sensory and cognitive components. Depending on the weight given to these components, one obtains theories which emphasize intensity dependence or task dependence. Few comparably detailed physiological studies of persistence exist. To clarify the issues raised by these theories, we examined the responses evoked in the model photoreceptor of the Limulus lateral eye. To explore the role of sensory variables, we manipulated adaptation state and flash intensity. To explore cognitive variables, the durations of the photoreceptor potentials (RPs) evoked in this model system were assessed by a mutually complementary and complete set of candidate sensory codes. Accordingly, sharp microelectrodes were used to record RPs intracellularly from single photoreceptor cells in response to 40-ms flashes whose intensity was varied over at least 3.6 log units. Two light adaptation states were used which differed in sensitivity by 3.5 log units. This model system made it possible to employ stringent objective assessments of data quality which ensured that only cells which had remained stable for several hours contributed to the present data. A variety of code-dependent trends were found: direct, inverse, invariant, and U-shaped trends related flash intensity to RP duration, while adaptation state interacted with some of these trends. Only some of the expectations which had generated this research were qualitatively corroborated and numerous quantitative discrepancies were found between data and theory. While caution is necessary when generalizing from neural responses to perception, these data indicate that two major gaps now exist in this field. First, both task and stimulus variables need to be exhaustively explored in more complete behavioral experiments. The present data make it more likely that sensory models and cognitive models simply address different aspects of the same phenomenon. Second, similarly detailed questions need to be posed to more central neural loci, particularly to those in the various visual cortices.

The general kinetic model of electron transfer in photosynthetic reaction centers activated by multiple flashes.

A new general kinetic model for the functioning of photosynthetic reaction centers (RC) of purple bacteria, under multiple flash activation, has been developed. The model includes the primary electron donor (P870) as well as the primary (QA) and secondary (QB) acceptor quinones. The new features of this general model include: (1) consideration of four different states of the QB binding site (vacant, occupied by QB, by QB- and by QBH2), (2) incorporation of the dark relaxation of the RC between flashes, (3) the assumption of fast exchange of quinones between the RC and quinone pool in detergent micelles or chromatophore membrane, (4) description of the kinetics of electron transfer in both oxidized (no donor for P870+) and reduced (in the presence of donor for P870+) conditions simultaneously, (5) the consideration of both single and multiple flash activation of the RC of purple bacteria and (6) consideration of the cumulative effects of all previous flashes of the series in the response induced by the current flash. This model is used to calculate and predict (1) flash-induced binary oscillations of the secondary acceptor semiquinone (QB-), (2) flash-induced behavior of P870+ in the presence and absence of electron donor and (3) the apparent equilibrium constant of electron transfer between QA and QB and others. Different characteristics of RC are analyzed as a function of flash intensity, time between flashes, concentration of electron donor, redox-potential of the medium, concentration of pool quinone and quinol, association and dissociation equilibrium constants for quinone and quinol at the QB binding site, equilibrium constants of electron transfer between QA- and QB and between QA- and QB-, as well as the rate constants of oxidation of QA- and QB- by redox mediators. The proposed model can be used as a basis for assays of kinetic behavior of native and mutant RC of purple bacteria and for determination of the factors influencing the release of QH2 from RC. The latter is needed for analysis of factors controlling light-activated electron transport in the cytochrome bc1 complexes of purple bacteria by quinol molecules released from RC. The developed general approach for parallel consideration of flash-induced transitions of RC and its following dark relaxation between flashes can also be used for kinetic description of photosynthetic RC of oxygenic photosynthesis.

Electroretinographic and Psychophysical Findings during Early and Late Stages of Human Immunodeficiency Virus Infection and Cytomegalovirus Retinitis

The authors examined electrophysiologic and psychophysical measures of retinal function in patients infected with human immunodeficiency virus (HIV) at different stages of infection, including patients with cytomegalovirus retinitis (CMVR). All patients had complete ophthalmologic examinations. Rod-mediated psychophysical thresholds were measured using a modified two-color dark-adapted perimetry technique. Rod-dominated full field flash electroretinograms (ERGs) were obtained as a function of flash intensity, followed by cone-dominated ERGs. The 26 patients infected with HIV (26 eyes) were categorized into three groups. Six patients were infected with HIV but had not progressed to acquired immunodeficiency syndrome (AIDS), and 14 had AIDS. Six patients had CMVR with less than 10% of the retina involved. The data were compared with results from age-similar control subjects. Psychophysical thresholds as a function of retinal eccentricity were elevated for each of the three stages of HIV infection. The group of patients with CMVR had the greatest amount of threshold elevation and threshold elevation increased with retinal eccentricity. In addition, all three patient groups had abnormal electroretinographic findings. Patients with CMVR were affected more severely on all measures than were the other HIV-infected groups. Results reveal that a diffuse functional retinal pathology exists in eyes with the funduscopic appearance of localized peripheral CMVR. Additionally, patients infected with HIV, including those without cotton wool spots, may have abnormal retinal function.

The locus of origin of augmenting and reducing of visual evoked potentials in rat brain

Humans who are high sensation seekers and cats who demonstrate comparable behavioral traits show increasing amplitudes of the early components of the cortical visual evoked potential (VEP) to increasing intensities of light flash; low sensation seekers show VEP reducing. Roman high-avoidance (RHA) and Roman low-avoidance (RLA) rats have behavioral traits comparable to human and cat high and low sensation seekers, respectively. Previously, we showed that RHA and RLA rats are cortical VEP augmenters and reducers, respectively. The goal of this study was to determine if augmenting-reducing is in fact a property of the visual cortex or if it originates at the lateral geniculate nucleus and is merely reflected in recordings from the cortex. VEPs to five flash intensities were recorded from the visual cortex and dorsal lateral geniculate of RHA and RLA rats. As in the previous study, the slope of the first cortical component as a function of flash intensity was greater in the RHA than in the RLA rats. The amplitude of the geniculate component that has a latency shorter than the first cortical component was no different in the two lines of rats. The finding from the cortex confirms the earlier finding of augmenting and reducing in RHA and RLA rats, respectively. The major new finding is that the augmenting-reducing difference recorded at the cortex does not occur at the thalamus, indicating that it is truly a cortical phenomenon.

Augmenting and reducing of visual evoked potentials in Roman high- and low-avoidance rats

Human and cat high sensation seekers tend to show increasing amplitudes (augmenting) of the P 1 and N 1 components of the visual evoked potential (VEP) to increasing intensities of light flash, whereas low sensation seekers show VEP reducing. Roman high-avoidance (RHA) and Roman low-avoidance (RLA) rats have behavioral traits comparable to human and cat high and low sensation seekers, respectively. RHA rats show greater exploration, activity, and aggression than do RLA rats. Rats of each Roman line and Wistar rats were anesthetized with chloral hydrate and maintained at a stable moderate anesthetic level. VEPs to each of five flash intensities were computer averaged for each rat. The slopes of P 1 amplitudes as a function of flash intensity were significantly greater in the RHA than the RLA rats. RHA rats were clear augmenters; RLA rats had almost flat amplitude-intensity functions. The Wistar rats had slope functions that were similar to those of the RLA rats. This study shows that the relationship between sensation-seeking behavior and VEP augmenting and reducing has a heritable base and extends across species from human, cat, and rat. In addition, we demonstrate a rat model of this relationship that yields advantages of genetic homogeneity and a short generational time, and provides access to a wealth of behavioral data and experimental manipulations available for the rat.

Light saturation curves show competence of the water splitting complex in inactive Photosystem II reaction centers

Photosystem II complexes of higher plants are structurally and functionally heterogeneous. While the only clearly defined structural difference is that Photosystem II reaction centers are served by two distinct antenna sizes, several types of functional heterogeneity have been demonstrated. Among these is the observation that in dark-adapted leaves of spinach and pea, over 30% of the Photosystem II reaction centers are unable to reduce plastoquinone to plastoquinol at physiologically meaningful rates. Several lines of evidence show that the impaired reaction centers are effectively inactive, because the rate of oxidation of the primary quinone acceptor, QA, is 1000 times slower than in normally active reaction centers. However, there are conflicting opinions and data over whether inactive Photosystem II complexes are capable of oxidizing water in the presence of certain artificial electron acceptors. In the present study we investigated whether inactive Photosystem II complexes have a functional water oxidizing system in spinach thylakoid membranes by measuring the flash yield of water oxidation products as a function of flash intensity. At low flash energies (less that 10% saturation), selected to minimize double turnovers of reaction centers, we found that in the presence of the artificial quinone acceptor, dichlorobenzoquinone (DCBQ), the yield of proton release was enhanced 20±2% over that observed in the presence of dimethylbenzoquinone (DMBQ). We argue that the extra proton release is from the normally inactive Photosystem II reaction centers that have been activated in the presence of DCBQ, demonstrating their capacity to oxidize water in repetitive flashes, as concluded by Graan and Ort (Biochim Biophys Acta (1986) 852: 320-330). The light saturation curves indicate that the effective antenna size of inactive reaction centers is 55±12% the size of active Photosystem II centers. Comparison of the light saturation dependence of steady state oxygen evolution in the presence of DCBQ or DMBQ support the conclusion that inactive Photosystem II complexes have a functional water oxidation system.

Kinetic study on the equilibrium between membrane-bound and free photoreceptor G-protein.

Formation of the complex between photoreceptor G-protein (G) and photoactivated rhodopsin (RM) leads to a change in the light scattering of the disk membranes (binding signal or signal P). The signal measured on isolated disks (so-called PD signal) is exactly stoichiometric in its final level to bound G-protein but its kinetics are much slower than the RMG binding reaction. In this study on isolated disks, recombined with G-protein, we analyzed the PD-signal level and kinetics as a function of flash intensity and compared it to the RMG-complex formation monitored spectroscopically (by extra metarhodopsin II). The basic observation is that the initial slopes of the PD signals decrease with flash intensity when the signals are normalized to the same final level. This finding prevents an explanation of the scattering signal by a slow postponed reaction of the RMG complex. We propose to interpret the scattering change as a redistribution of G-protein between a membrane-bound and a solved state. The process is driven by the complexation of membrane-bound G to flash-activated rhodopsin (RM). The experimental evidence for this two-state model is the following: The intensity dependence of the initial rate of the PD signal is explained by the model. Under the assumption of a bimolecular reaction of free G with sites at the membrane, equal to rhodopsin in their concentration, the measured rates yield a KD of 10(-5) M. Evaluation of the extra MII kinetics yields a biphasic rise at saturating flashes. The measured rates fit to the supply of free and membrane-bound G-protein for the reaction with RM. Quantitative estimation of the expected scattering intensity changes gives a comprehensive description of binding signal and dissociation signal by the gain and loss of G-protein scattering mass. The temperature dependence of the PD-signal rate leads to an activation energy of the membrane-association process of E alpha = 44 kJ/mol.

Horizontal cell signal is smaller with texture-like nonuniform patterns than with uniform fields of the same space-average illuminance

While measurement of the pertinent response as a function of flash intensity has been a standard procedure in vision research, we ask here what happens to horizontal cells in the vertebrate retina if the flash energy is varied through the density of a large number of small light spots rather than through the intensity of uniform illumination. The experiment described here demonstrates that the electrical response of horizontal cells in the turtle retina is consistently smaller with the present dot-density modulation than with the usual intensity modulation, even though the comparison is made when the mean irradiance per photoreceptor is equal in the two methods of modulation. Thus, the spatial summation, an important retinal function often thought to provide a base signal level for contrast detection, is affected significantly by how the visual pattern is structured at a level of microscopic dimension far smaller than the receptive field. The present finding, which seems to be a new form of area-intensity effect, can be explained if the dendritic membrane conductance of horizontal cells at each synaptic site increases at a progressively higher rate with decreases of the corresponding local illuminance. This possibility is discussed in the light of relevant photoreceptor response data, the presumed sigmoidal trend of the postsynaptic chemosensitivity and a simple electrical analog of contiguous horizontal cells.

Acid-induced double hits in glutaraldehyde-treated Chlorella

Glutaraldehyde-treated Chlorella cells show pH-dependent oxygen evolution characteristics. 1. 1. The oxygen yield on the second flash ( Y 2) of a sequence of 2 μs flashes is pH-dependent; the rest of the sequence is unchanged. 2. 2. This yield on the second flash increases as the dark-time between sequences is increased. 3. 3. The deactivation times for the S 2 and S 3 states are not significantly changed at different pH values. 4. 4. Using a short flash (0.5 μs) for the first flash, no oxygen was detected on the second flash, indicating that there is no S 2 remaining in the dark. 5. 5. When the intensity of the first flash is varied, a sigmoidal curve is obtained for Y 2 as a function of flash intensity at pH 6.0 but not at pH 7.5. This is indicative of a double hit process. 6. 6. At pH 6.0 the turnover of the Photosystem II centers after the first flash has a fast rise ( t 1 2 < 25 μ s ) followed by a slow phase. This behaviour is not seen at pH 7.5, nor after two flashes. 7. 7. We propose a scheme in which an auxiliary acceptor is in series with Q 1 and in competition with B. The redox potential of the auxiliary acceptor is pH dependent.

Retinal mechanisms of visual latency

The time-course of light-induced potential changes was measured transretinally as a function of flash intensity in the eyecup preparation of dark-adapted Bufo marinus. The electroretinogram (ERG) as indexed by the peak amplitude of the a-wave. and the latency of the a-wave onset yielded action spectra matching that of the red rod pigment (λ max at 502 nm) at low intensities near threshold, but at higher intensities there was evidence of intrusion from the single and principal short-latency, longwave cones (λ max at 575 nm). With uniform illumination of the retina with monochromatic (λ = 502 nm) flashes to isolate the red rod system over a range of 5 log units from threshold to saturation of the a-wave. the latency of the b-wave onset and the latency of the a-wave peak varied linearly with the latency of the a-wave onset. All three measures could be described by linear functions of the inverse cube root of flash intensity, or equivalently by the equation: t min/ t= I 1/3( I 1/3 + σ 1/3). For the mass receptor potential (MRP) isolated with excess Mg 2+. the initial segment and functional form of the time-course did not differ from that obtained for the a-wave at the same saturating intensity. The results are consistent with the hypothesis that the dominant mechanism controlling visual latency lies in the photoreceptors and that subsequent proximal transformations of the time-course are linear.

Relation of albinism and drugs to the visual evoked potential of the mouse.

The individual and combined influences of pentobarbital and chlorprothixene on the early components of the cortical visual evoked potential (VEP) were examined in the C57BL/6 mouse. Pentobarbital produced a large increase in latency, and chlorprothixene resulted in a smaller latency increase. When these agents were combined, their effects on the VEP latency were antagonistic. The effects of pentobarbital on VEP amplitude varied as a function of flash intensity, and this barbiturate anesthetic also produced qualitative VEP waveform changes. When the congenic albino (c/c) C57BL/6 was compared with the black (+/+) C57BL/6 mouse; pentobarbital anesthesia produced a decrease of latency with increasing stimulus intensity only in the c/c genotype, whereas VEP amplitudes were similarly affected by pentobarbital in both genotypes.

Cl+HBr Pulsed Chemical Laser: A Theoretical and Experimental Study

The most powerful and efficient pulsed chemical lasers produced to date have resulted from the simple exothermic reactions between halogen atoms and hydrogen halides. These reactions have been initiated by flash photolysis. One such reaction is Cl+HBr→HCl+Br, − ΔH = 15.5 kcal/mole. A kinetic model has been developed for this laser system which calculates the intensity profile of the laser pulse as a function of time. The input data for the model include estimates of the reaction rate into the zeroth and first vibrational level of HCl and cross sections for collisional deactivation by the different chemical species in the laser medium. The rate of reaction into the second vibrational level of HCl is assumed to be zero. Any small finite rate into the second level would have a negligible effect on the calculated performance of the laser which operates only on the V(1 → 0) fundamental transition of HCl. The theoretical model also requires as input data the photolysis flash intensity profile, the cavity geometry, and the losses. A discussion of the model is given, and a comparison is made between experiment and theory. The model is able to predict quantitative variations of the laser pulse characteristics as a function of flash intensity, cavity losses, and reagent partial pressures. The quantitative calculations are in reasonable agreement with experiment. It is concluded that (a) the most important single contribution to deactivation in the system is HCl (V = 1) + HBr (V = 0) → HCl (V = 0) + HBr (V = 1), and (b) the rate of reaction to produce HCl (V = 1) is at least as large as that to produce HCl (V = 0).

Kinetic measurements on rhodopsin solutions during intense flashes

Kinetic spectroscopy is used to investigate the rate at which metarhodopsin II (meta II) accumulates during (and, in some cases, after) flash photolysis of rhodopsin solutions. This process is studied as a function of flash intensity, temperature, and extracting medium. It is shown that the rate at which meta II accumulates is photo-limited at high temperatures but thermo-limited at low ones. Photoequilibria are shown to exist, under certain circumstances, during the course of a flash. The quantum efficiency of meta II production is shown to be time dependent when intense flashes are used but not when weak flashes are used. It is suggested that kinetic spectroscopy may be useful in investigating the chemical basis of the early receptor potential.

Visually evoked cortical potentials and reaction time in relation to site of retinal stimulation

Using an averaging computer, visually evoked cortical potentials and reaction time were studied as a function of flash intensity, wave-length (red vs. blue) and site of retinal stimulation. All evoked potentials were comprised of periodic, sinusoidal deflections having a frequency equal to S's alpha rhythm, and which grew to a peak value then attenuated. Evoked potential latency and reaction time changed similarly as the eye was stimulated progressively more peripherally. Both increased out to 5°, decreased out to 10–20°, and thereafter progressively increased. The changes corresponded closely to Oesterberg's rod-cone density function and to known differences in retinal sensitivity. Changes in evoked potential amplitude with retinal site differed for the two colors.

Nouvelle mise en évidence de la réaction de Mehler chez chlorella

Glutaraldehyde-treated Chlorella cells show pH-dependent oxygen evolution characteristics. 1.1. The oxygen yield on the second flash (Y2) of a sequence of 2 μs flashes is pH-dependent; the rest of the sequence is unchanged.2.2. This yield on the second flash increases as the dark-time between sequences is increased.3.3. The deactivation times for the S2 and S3 states are not significantly changed at different pH values.4.4. Using a short flash (0.5 μs) for the first flash, no oxygen was detected on the second flash, indicating that there is no S2 remaining in the dark.5.5. When the intensity of the first flash is varied, a sigmoidal curve is obtained for Y2 as a function of flash intensity at pH 6.0 but not at pH 7.5. This is indicative of a double hit process.6.6. At pH 6.0 the turnover of the Photosystem II centers after the first flash has a fast rise (t12 < 25 μs) followed by a slow phase. This behaviour is not seen at pH 7.5, nor after two flashes.7.7. We propose a scheme in which an auxiliary acceptor is in series with Q1 and in competition with B. The redox potential of the auxiliary acceptor is pH dependent.