A frequency view of colour: measuring the human sensitivity to square-wave spectral power distributions.

We have measured the chromatic threshold sensitivity to stimuli with spectral composition determined by a periodic function of energy over wavelength. This approach is analogous to frequency studies of spatial vision for the study of colour. A device was constructed permitting the synthesis of illuminants over the entire visible range (400-700 nm) in which phase, frequency and amplitude can be independently controlled. We have used 12 frequencies of square-wave functions (from 0.5 to 3.6 cycles/300 nm) and seven values of phase (between 0 degrees and 180 degrees) to obtain the contrast sensitivity function of the chromatic system in three normal trichromats. The results show maximum sensitivity around 1.5 cycles/300 nm and a high-frequency cut-off at 3.6 cycles/300 nm. These empirical values are compared with the predictions obtained from three current psychophysical models of opponent-colour process.     

Probing the electronic structure of transition metal ion centres in proteins by coherent Raman-detected electron paramagnetic resonance spectroscopy

The simultaneous excitation of a paramagnetic sample with optical (laser) and microwave radiation can cause an amplitude or phase modulation of the transmitted light at the microwave frequency. The detection of this modulation indicates the presence of coupled optical and electron paramagnetic resonance (EPR) transitions in the sample. Here we report the first application of this technique to a biomolecule: the blue copper centre of Pseudomonas aeruginosa azurin. Using optical excitation at 686 nm, in the thiol to copper(II) charge transfer band, we measure a coherent Raman-detected EPR spectrum of a frozen aqueous solution. Its lineshape is characteristic of the magnetic circular dichroism along each principal g-value axis. This information allows electronic and structural models of transition metal ion centres in proteins to be tested.     

Individual identity coding depends on call type in the South Polar skua Catharacta maccormicki

In colonial birds, acoustic communication is essential for mate recognition. The South Polar skua (Catharacta maccormicki) lives in loose colonies and is highly territorial for feeding and breeding. We studied the potential of individual identity coding in the three main calls of the South Polar skua repertoire: the courtship, the contact and the alarm calls. We investigated parameters in both temporal and frequencial domains, i.e. amplitude modulation, frequency modulation and power spectrum density. For each parameter, the intra- and inter-individual variabilities were calculated. The ratio between these values represents the potential of individuality coding (PIC) of the considered feature. Low values of PICs for amplitude and frequency modulations show that both parameters may not be used for individual recognition. In contrast, high values of PIC for the power spectrum density indicate that the energy distribution among the frequency spectrum is likely to be an individual marker. PIC also varies according to the call type. Both courtship and contact calls have a higher potentiality of individual identity coding than the alarm call. The two former calls may allow individual recognition whereas the latter may not, and this last result can be extrapolated to many other species.     

Amplitude modulation is a major marker of individual signature in lamb bleats

The source-filter theory of vocal production supports the idea that acoustic signatures are preferentially coded by the fundamental frequency (source-induced variability) and the distribution of energy among the frequency spectrum (filter-induced variability). By investigating the acoustic parameters supporting individuality in lamb bleats, a vocalization which mediates recognition by ewes, here we show that amplitude modulation – an acoustic feature largely independent of the shape of the acoustic tract – can also be an important cue defining an individual vocal signature. Female sheep (Ovis aries) show an acoustic preference for their own lamb. Although playback experiments have shown that this preference is established soon after birth and relies on a unique vocal signature contained in the bleats of the lamb, the physical parameters that encode this individual identity remained poorly identified. We recorded 152 bleats from 13 fifteen-day-old lambs and analyzed their acoustic structure with four complementary statistical methods (ANOVA, potential for individual identity coding PIC, entropy calculation 2^Hs, discriminant function analysis DFA). Although there were slight differences in the acoustic parameters identified by the four methods, it remains that the individual signature relies on both the temporal and frequency domains. The coding of the identity is thus multi-parametric and integrates modulation of amplitude and energy parameters. Specifically, the contribution of the amplitude modulation is important, together with the fundamental frequency F ₀ and the distribution of energy in the frequency spectrum.     

A retrieval algorithm to evaluate the Photosystem I and Photosystem II spectral contributions to leaf chlorophyll fluorescence at physiological temperatures

A new computational procedure to resolve the contribution of Photosystem I (PSI) and Photosystem II (PSII) to the leaf chlorophyll fluorescence emission spectra at room temperature has been developed. It is based on the Principal Component Analysis (PCA) of the leaf fluorescence emission spectra measured during the OI photochemical phase of fluorescence induction kinetics. During this phase, we can assume that only two spectral components are present, one of which is constant (PSI) and the other variable in intensity (PSII). Application of the PCA method to the measured fluorescence emission spectra of Ficus benjamina L. evidences that the temporal variation in the spectra can be ascribed to a single spectral component (the first principal component extracted by PCA), which can be considered to be a good approximation of the PSII fluorescence emission spectrum. The PSI fluorescence emission spectrum was deduced by difference between measured spectra and the first principal component. A single-band spectrum for the PSI fluorescence emission, peaked at about 735?nm, and a 2-band spectrum with maxima at 685 and 740?nm for the PSII were obtained. A linear combination of only these two spectral shapes produced a good fit for any measured emission spectrum of the leaf under investigation and can be used to obtain the fluorescence emission contributions of photosystems under different conditions. With the use of our approach, the dynamics of energy distribution between the two photosystems, such as state transition, can be monitored in vivo, directly at physiological temperatures. Separation of the PSI and PSII emission components can improve the understanding of the fluorescence signal changes induced by environmental factors or stress conditions on plants.     

Extending the HKB model of coordinated movement to oscillators with different eigenfrequencies

We study the dynamics of a system of coupled nonlinear oscillators that has been used to model coordinated human movement behavior. In contrast to earlier work we examine the case where the two component oscillators have different eigenfrequencies. Problems related to the decomposition of a time series (from an experiment) into amplitude and phase are discussed. We show that oscillations at multiples of the main frequency of the oscillator system may occur in the phase and amplitude due to the choice of a coordinate system and how these oscillations can be eliminated. We derive an explicit equation for the dynamics of the relative phase of the oscillator system in phase space that enables a direct comparison between theory and experiment.     

精胺和Mg~（2＋）对tRNA~（Ile）圆二向色谱的影响

由于精胺（ｓｐｅｒｍｉｎｅ）能特异地刺激哺乳动物ｔＲＮＡ~（Ｉｌｅ）的氨基酰化,本文用纯化的牛肝ｔＲＮＡ~（Ｉｌｅ）观察了精胺和Ｍｇ（２＋）对ｔＲＮＡ~（Ｉｌｅ）ＣＤ光谱的影响。结果显示：Ｍｇ（２＋）可使牛肝ｔＲＮＡ~（Ｉｌｅ）ＣＤ光谱峰向短波方向偏移２ｎｍ,波峰为２６３ｎｍ,峰值被增大约１０％,ΔθＭｇ（２＋）＝２．３×１０３ｄｅｇ·ｃｍ２／ｄｍｏｌ;而精胺使牛肝ｔＲＮＡ~（Ｉｌｅ）ＣＤ光谱峰减少４０％,Δθｓｐｅｒｍｉｎｅ＝１×１０（－４）ｄｅｇ·ｃｍ２／ｄｍｏｌ;精胺和Ｍｇ（２＋）对肝ｔＲＮＡ~（Ｉｌｅ）－ＩｌｅＲＳ复合物或ＩｌｅＲＳ的ＣＤ光谱基本无影响。表明Ｍｇ（２＋）和精胺可影响牛肝ｔＲＮＡ~（Ｉｌｅ）的构象。实验同时以酵母ｔＲＮＡ（Ｐｈｅ）和Ｅ·ｃｏｌｉｔＲＮＡ~（Ｉｌｅ）作为对照。     

Extending the HKB model of coordinated movement to oscillators with different eigenfrequencies

 We study the dynamics of a system of coupled nonlinear oscillators that has been used to model coordinated human movement behavior. In contrast to earlier work we examine the case where the two component oscillators have different eigenfrequencies. Problems related to the decomposition of a time series (from an experiment) into amplitude and phase are discussed. We show that oscillations at multiples of the main frequency of the oscillator system may occur in the phase and amplitude due to the choice of a coordinate system and how these oscillations can be eliminated. We derive an explicit equation for the dynamics of the relative phase of the oscillator system in phase space that enables a direct comparison between theory and experiment. Received: 30 December 1994/Accepted in revised form: 27 June 1995     

IRRADIATION ULTRA-VIOLETTE DES ORGANITES CELLULAIRES AVEC OBSERVATION CONTINUE EN CONTRASTE DE PHASE

The apparatus described is designed to permit the observation of the smallest cellular components with the phase contrast microscope (which requires an objective of a high numerical aperture) while a part of the cell is irradiated by a very small spot of ultraviolet radiation. Theoretical considerations show a good distribution of the energy for a diameter of the irradiation spot as small as 0.2 microns. Indications are given about the limitations of the brightness caused by diffraction at the spot diaphragm and the energy absorption by volume-unit of the irradiated object. The practical applications of the apparatus are discussed.     

Dynamic Characteristics of Retinal Ganglion Cell Responses in Goldfish

A cross-correlation technique has been applied to quantify the dependence of the dynamic characteristics of retinal ganglion cell responses in goldfish on intensity, wavelength, spatial configuration, and spot size. Both theoretical and experimental evidence justify the use of the cross-correlation procedure which allows the completion of rather extensive measurements in a relatively short time. The findings indicate the following. (a) The shape of the amplitude characteristics depends on the energy per unit of time (power) falling within the center of a receptive field rather than on the intensity of the stimulus spot. For spot diameters of up to 1 mm, identical amplitude characteristics can be obtained by interchanging area and intensity. Therefore the receptor processes do not contribute to the change in the amplitude characteristics as a function of the power of the stimulus light. (b) For high frequencies the amplitude characteristics obtained as a function of power join together in a common envelope if plotted on an absolute sensitivity scale. For spontaneous ganglion cells this envelope holds over a range of three log units and the shape is identical for central and peripheral processes. (c) The amplitude characteristics of the central and peripheral processes converging to a ganglion cell are identical, irrespective of the sign (on or off) and the spectral coding of the response. Therefore we have no evidence for interneurons in the goldfish retina unique to the periphery of the receptive field.     

Effects of an opioid antagonist on pulsatile luteinizing hormone secretion in the ewe vary with changes in steroid negative feedback

In ewes during the breeding season, estradiol (E) and progesterone (P) synergistically regulate pulsatile luteinizing hormone (LH) secretion. E primarily inhibits LH pulse amplitude and P inhibits LH pulse frequency. To determine if endogenous opioid peptides (EOP) mediate these negative feedback effects, we administered the long-acting opioid antagonist WIN 44,441-3 (WIN) to intact ewes during the luteal and follicular phases of the estrous cycle and to ovariectomized ewes treated with no steroids, E, P, or E plus P. Steroid levels were maintained at levels seen during the estrous cycle by Silastic implants placed shortly after surgery. WIN increased LH pulse frequency, but not amplitude, in luteal phase ewes. In contrast, during the follicular phase, LH pulse amplitude was increased by WIN and pulse frequency was unchanged. Neither LH pulse frequency nor pulse amplitude was affected by WIN in long-term ovariectomized ewes untreated with steroids. In contrast, WIN slightly increased LH pulse frequency in short-term ovariectomized ewes. WIN also increased LH pulse frequency in ovariectomized ewes treated with P or E plus P. WIN did not affect pulse frequency but did increase LH pulse amplitude in E-treated ewes. These results support the hypothesis that EOP participate in the negative feedback effects of E and P on pulsatile LH secretion during the breeding season and that the inhibitory effects of EOP may persist for some time after ovariectomy.     

Electric-field-induced luminescence emission spectra of Photosystem I and Photosystem II from chloroplasts

The electroluminescence induced by external electric fields in blebs prepared from chloroplasts consists of two kinetically different phases, rapid (R) and slow (S), which were shown to be linked to Photosystem I (PS I) and Photosystem II (PS II) activities, respectively (Symons, M., Korenstein, R. and Malkin, S. (1985) Biochim. Biophys. Acta 806, 305–310). In this report we describe conditions involving heat treatment of broken chloroplasts, which make it possible to observe R phase electroluminescence essentially devoid of any contribution by the S phase. This allowed the precise measurement of the emission spectrum of PS I electroluminescence. The emission spectrum of PS II electroluminescence was obtained using regular broken chloroplasts, which show only S-type emission. The latter emission spectrum is identical to the one obtained for ordinary prompt fluorescence, peaking at 685 nm with a bandwidth of about 25 nm. The PS I emission spectrum is symmetric around 705 nm and is much broader, about 60 nm.     

Electron energy filtering significantly improves amplitude contrast of frozen-hydrated protein at 300kV

The amplitude contrast of frozen-hydrated biological samples was measured using the bacterial flagellar filament embedded in vitreous ice at an accelerating voltage of 300kV. From the mean radial amplitude spectra of overfocused images, amplitude contrast was estimated to be 6.9+/-1.9% and 2.7+/-1.0% of the whole contrast at the low spatial frequency range with and without energy filtering, respectively, and that of the carbon film to be 9.5+/-2.0% and 5.8+/-1.8%. Energy filtering effectively doubled the signal-to-noise ratio in the images of frozen-hydrated filaments, and substantially improved intensity data statistics of layer lines up to at least approximately 25A resolution in their Fourier transforms. It also markedly improved inter-particle fitting phase residuals of averaged data at resolutions up to approximately 15A. Using the energy filtered data recorded on a new high-performance, lens-coupled CCD camera the three-dimensional map of the flagellar filament was calculated at 8A by applying the amplitude contrast of 6.9%. The map and its mean radial density distribution validated the obtained value of the amplitude contrast.     

Photocycles of bacteriorhodopsin in light- and dark-adapted purple membrane studied by time-resolved absorption spectroscopy.

Nanosecond time-resolved absorption spectra have been measured throughout the photocycle of bacteriorhodopsin in both light-adapted and dark-adapted purple membrane (PM). The data from dark-adapted samples are interpretable as the superposition of two photocycles arising independently from the all-trans and 13-cis retinal isomers that coexist in the dark-adapted state. The presence of a photocycle in dark-adapted PM which is indistinguishable from that observed for light-adapted PM under the same experimental conditions is demonstrated by the observation of the same five relaxation rates associated with essentially identical changes in the photoproduct spectra. This cycle is attributed to the all-trans component. The cycle of the 13-cis component is revealed by scaling the data measured for the light-adapted sample and subtracting it from the data on the dark-adapted mixture. At times less than 1 ms, the resulting difference spectra are nearly time-independent. The peak of the difference spectrum is near 600 nm, although there appears to be a slight (approximately 2 nm) blue-shift in the first few microseconds. Subsequently the amplitude of this spectrum decays and the peak of the difference spectrum shifts in two relaxations. Most of the amplitude of the photoproduct difference spectrum (approximately 80%) decays in a single relaxation having a time constant of approximately 35 ms. The difference spectrum remaining after this relaxation peaks at approximately 590 nm and is indistinguishable from the classical light-dark difference spectrum, which we find, in experiments performed on a much longer time scale, to peak at 588 nm. The decay of this remaining photo-product is not resolvable in the nanosecond kinetic experiments, but dark adaptation of a completely light-adapted sample is found to occur exponentially with a relaxation time of approximately 2,000 s under the conditions of our experiments.     

Linear Tuning of Gamma Amplitude and Frequency to Luminance Contrast: Evidence from a Continuous Mapping Paradigm

Individual differences in the visual gamma (30–100Hz) response and their potential as trait markers of underlying physiology (particularly related to GABAergic inhibition) have become a matter of increasing interest in recent years. There is growing evidence, however, that properties of the gamma response (e.g., its amplitude and frequency) are highly stimulus dependent, and that individual differences in the gamma response may reflect individual differences in the stimulus tuning functions of gamma oscillations. Here, we measured the tuning functions of gamma amplitude and frequency to luminance contrast in eighteen participants using MEG. We used a grating stimulus in which stimulus contrast was modulated continuously over time. We found that both gamma amplitude and frequency were linearly modulated by stimulus contrast, but that the gain of this modulation (as reflected in the linear gradient) varied across individuals. We additionally observed a stimulus-induced response in the beta frequency range (10–25Hz), but neither the amplitude nor the frequency of this response was consistently modulated by the stimulus over time. Importantly, we did not find a correlation between the gain of the gamma-band amplitude and frequency tuning functions across individuals, suggesting that these may be independent traits driven by distinct neurophysiological processes.     

Thermal and Spectral Sensitivities of Discrete Slow Potentials in Limulus Eye

The discrete, subthreshold, slow potential fluctuations (SPF's) which can be recorded intracellularly in Limulus ommatidia are sensitive to temperature and light wavelength. SPF frequency increases with increasing temperature (Q₁₀ about 3.5) and light intensity. The effects are additive. SPF rise and decay time decrease with increasing temperature (Q₁₀ between 2 and 3). There is a peak, near 520 nm, in the spectral sensitivity of SPF frequency. This peak may correspond to the wavelength of maximum absorption by rhodopsin in the ommatidia. Hydroxylamine produces a rapid, irreversible reduction of SPF frequency and amplitude perhaps owing to its action on the photopigment. The cornea and crystalline cones fluoresce (peak about 445 nm) when excited by near-ultraviolet energy (380 nm peak) and this fluorescence may influence SPF spectral sensitivity measurements. These findings suggest that the SPF's are the results of photolytic and thermolytic reactions occurring in the ommatidial visual pigments and that they have a role in the mechanisms which transduce light to electrical activity in the visual receptors.     

Designing a High Performance Phase Gradient Metasurface Using Optical Patch Antennas with Different Patch Thicknesses

Patch-based metasurfaces as generic structures of the reflective flat optical devices, such as flat mirrors, waveplates, polarizer, and holograms, should fulfill two basic requirements of covering 0 to 2π phase shift range and providing a sufficiently high reflection amplitude. Under the current design paradigm, the design process has been based only on the width and length of the patch elements of the metasurfaces. The present study will exploit the potentials of the thickness of the patch elements as a design parameter. While for a metasurface based on patch elements with thickness of 50 nm, a phase shift coverage near 270°, corresponding to 90° phase steps, and reflection amplitude of 0.8 in the wavelength 775 nm are achievable, using just one additional value of 30 nm for thicknesses of the patches will increase the phase shift coverage to 320°, corresponding to 40° phase steps, with reflection amplitude higher than 0.85 in the same wavelength. In this way, the phase steps could be much smaller which means more closely approximating a targeted phase pattern. This would be evidently a remarkable performance improvement, which in the case of a polarization beam splitter, as shown, means reflecting more amount of energy in the desired angles.     

Temporal tuning and nonlinearity of intraretinal pathways in turtle: Effects of temperature,stimulus intensity,and size

Flash responses, amplitude and phase transfer functions, and nonlinearities were measured in turtle retina for pathways with photoreceptor inputs and outputs from horizontal (HC), hyperpolarizing bipolar (HBC), sustained amacrine (AC), and on-off ganglion (GC) cells. Flash responses slowed and attenuated in all cells as temperature decreased. Whitenoise transfer properties of sustained-type cells (HC, HBC, AC) were of low- or bandpass type; highfrequency cut-off (f _c) and phase crossover frequency decreased with temperature. f _c increased as spot diameter was increased. Nonlinearity of these sustained-response pathways (distortion product frequencies in response to a sum-of-sinusoids input probe) increases with intensity and may depend on amplitude saturation limiting. On/off GC synaptic and spike activity increased as spot diameter decreased and intensity increased. Amplitude transfer functions had a low-frequency peak (PSP activity) and monotonically decreasing amplitude vs. frequency shape (spikes and transient PSP activity). Nonlinearity increased with stimulus intensity; it was maximal with 1 mm spot size, less with smaller (500 m) and larger (5 mm) spots. It may depend on the functional equivalent of full-wave rectification (on-off response).This work was supported by NEI grant R01 EY03383     

Investigating the effect of Zn doping and temperature on the photoluminescence behaviour of CuLaSe2 quantum dots

In this study, CuLaSe₂ and ZnCuLaSe₂ quantum dots (QDs) with a mean size of ~4 nm were synthesized and characterized, and their temperature-dependent photoluminescence (PL) properties were studied in the temperature range from 90 to 300 K for the first time. The results show that the obtained QDs were spherical and revealed excitonic band gaps. The PL intensity for both types of materials decreased when increasing the temperature to 300 K, which was attributed to the nonradiative relaxation and thermal escape mechanisms. As the temperature was increased, the PL linewidths broadened, and PL peak energies were red shifted for both types of QDs due to the exciton–phonon coupling and lattice deformation potential mechanisms. In addition, we found that as the temperature was decreased, the PL spectrum of ZnCuLaSe₂ QDs contained two extra components, which could be attributed to the shallow defect sites (low energy peak) and the crystal phase transition process (high energy peak). The spectrum of CuLaSe₂ QDs contained one extra component, which could be attributed to the crystal phase transition process.