Spectral Sharpening Research Articles

Properties of Correlated Neural Activity Clusters in Cat Auditory Cortex Resemble Those of Neural Assemblies

Spiking activity was recorded from cat auditory cortex using multi-electrode arrays. Cross-correlograms were calculated for spikes recorded on separate microelectrodes. The pair-wise cross-correlation matrix was constructed for the peak values of the correlograms. Hierarchical clustering was performed on the cross-correlation matrix for six stimulus conditions. These were silence, three multi-tone stimulus ensembles with different spectral densities, low-pass amplitude-modulated noise, and Poisson-distributed click trains that each lasted 15 min. The resulting neuron clusters reflect patches in cortex of up to several mm(2) in size that expand and contract in response to different stimuli. Cluster positions and size were very similar for spontaneous activity and multi-tone stimulus-evoked activity but differed between those conditions and the noise and click stimuli. Cluster size was significantly larger in posterior auditory field (PAF) compared with primary auditory cortex (AI), whereas the fraction of common spikes (within a 10-ms window) across all electrode activity participating in a cluster was significantly higher in AI compared with PAF. Clusters crossed area boundaries in <5% of the cases were simultaneous recording were made in AI and PAF. Clusters are therefore similar to but not synonymous with the traditional view of neural assemblies. Common-spike spectrotemporal receptive fields (STRFs) were obtained for common-spike activity and all-spike activity within a cluster. Common-spike STRFs had higher signal-to-noise ratio than all-spike STRFs and showed generally spectral and temporal sharpening. The coincident and noncoincident output of the clusters could potentially act in parallel and may serve different modes of stimulus coding.

Improving the energy resolution of X-ray and electron energy-loss spectra

We discuss some practical problems of improving the resolution of X-ray and electron spectra. Iterative Bayesian methods promise greater resolution enhancement than Fourier techniques but they also give rise to spectral artifacts. Satellite peaks are generated adjacent to strong peaks in the original spectrum and oscillatory artifacts become prominent after a large number of iterations, particularly when the original data contain high noise content. In the case of valence-electron energy-loss spectra, satellite peaks are reduced by removing the zero-loss peak prior to spectral sharpening. Even so, care should be exercised in interpreting low intensity at low energy loss (after sharpening) as evidence for a bandgap in the electronic density of states.

Multispectral processing without spectra.

It is often the case that multiplications of whole spectra, component by component, must be carried out,for example when light reflects from or is transmitted through materials. This leads to particularly taxing calculations, especially in spectrally based ray tracing or radiosity in graphics, making a full-spectrum method prohibitively expensive. Nevertheless, using full spectra is attractive because of the many important phenomena that can be modeled only by using all the physics at hand. We apply to the task of spectral multiplication a method previously used in modeling RGB-based light propagation. We show that we can often multiply spectra without carrying out spectral multiplication. In previous work [J. Opt. Soc. Am. A 11, 1553 (1994)] we developed a method called spectral sharpening, which took camera RGBs to a special sharp basis that was designed to render illuminant change simple to model. Specifically, in the new basis, one can effectively model illuminant change by using a diagonal matrix rather than the 3 x 3 linear transform that results from a three-component finite-dimensional model [G. Healey and D. Slater, J. Opt. Soc. Am. A 11, 3003 (1994)]. We apply this idea of sharpening to the set of principal components vectors derived from a representative set of spectra that might reasonably be encountered in a given application. With respect to the sharp spectral basis, we show that spectral multiplications can be modeled as the multiplication of the basis coefficients. These new product coefficients applied to the sharp basis serve to accurately reconstruct the spectral product. Although the method is quite general, we show how to use spectral modeling by taking advantage of metameric surfaces, ones that match under one light but not another, for tasks such as volume rendering. The use of metamers allows a user to pick out or merge different volume structures in real time simply by changing the lighting.

New techniques in electron energy-loss spectroscopy and energy-filtered imaging

This article is a survey of hardware and software advances that promise to increase the power and sensitivity of electron energy-loss spectroscopy (EELS) and energy-filtered imaging (EFTEM) in a transmission electron microscope. Recent developments include electron-gun monochromators, lens-aberration correctors, and software for spectral sharpening, spectral processing and interpretation of fine structure. Future improvements could include the deployment of new electron sources. The expected enhancements in energy and spatial resolution are compared with fundamental limitations that arise from the natural widths of spectral peaks, the delocalization of inelastic scattering and the problem of electron-irradiation damage.

Spectral sharpening of the PtLedges by high-resolution x-ray emission

The platinum ${2p}_{3/2},$ ${2p}_{1/2},$ and $2s$ x-ray absorption spectra, recorded by monitoring the fluorescence intensity with eV energy resolution, show a spectral broadening that is significantly less than the $2p$ and $2s$ core hole lifetime broadening. The background of such spectral sharpening is discussed. It is shown, experimentally as well as theoretically, that the lifetime broadening of the $2p$ and $2s$ core holes is replaced by a new lifetime broadening. It is demonstrated that from the combination of normal x-ray absorption, selective x-ray absorption, and x-ray emission, the individual lifetimes of all participating core states can be determined. The difference between the ${5d}_{5/2}$ and ${5d}_{3/2}$ densities of states can be obtained from a combination of the ${2p}_{1/2}$ and ${2p}_{3/2}$ x-ray absorption spectra. The present spectra are limited by the experimental resolution. With the prospect of an improved experimental resolution for x-ray excitation and decay, the Pt edge absorption spectra could be obtained with even better resolution, thus providing a high-resolution hard x-ray probe of the empty density of states with important advantages for in-situ and high-pressure studies.

Determination of lysergic acid diethylamide (LSD) by application of online 77 K fluorescence spectroscopy and a sweeping technique in micellar electrokinetic chromatography

The principal advantage of the use of Shopl'skii effect (low temperature spectrum) is that spectral sharpening occurs both in absorption and emission. However, thus far using the technique of capillary electrophoresis/low temperature fluorescence spectroscopy (CE/LTFS) either at 77 or 4.2 K remains difficult to obtain an on-line spectrum, if the analyte is present at low concentration. This paper examines the feasibility of combining the techniques of online concentration and CE/LTFS to identify LSD and related compounds in urine at 77 K. To improve sensitivity, sweeping-micellar electrokinetic chromatography (sweeping-MEKC) and cation-selective exhaustive injection-sweep-micellar electrokinetic chromatography (CSEI-sweep-MEKC) were used for on-line concentration which resulted in detection limits of ∼20 ∼60 ppt, respectively.

Spectral Sharpening: What is it and why is it important?

Spectral Sharpening: What is it and why is it important?

Vertical molecular alignment and spectral sharpening in thin films of thiophene/phenylene co-oligomers

Spectroscopic features of various thiophene/phenylene co-oligomers have been investigated using their thin films with the vertical molecular alignment. A sharpened spectral line clearly shows up in the incident-angle dependent polarised absorption spectra. We seek the origin of this line from the Davydov transition. The magnitude of its splitting is estimated both experimentally and theoretically.

Spectral sharpening with positivity

Spectral sharpening is a method for developing camera or other optical-device sensor functions that are more narrowband than those in hardware, by means of a linear transform of sensor functions. The utility of such a transform is that many computer vision and color-correction algorithms perform better in a sharpened space, and thus such a space can be used as an intermediate representation for carrying out calculations. In this paper we consider how one may sharpen sensor functions such that the transformed sensors are all positive. We show that constrained optimization can be used to produce positive sensors in two fundamentally different ways: by constraining the coefficients in the transform or by constraining the functions directly. In the former method, we prove that convexity can be used to constrain the solution exactly. In a sense, we are continuing the work of MacAdam and of Pearson and Yule, who formed positive combinations of the color-matching functions. However, the advantage of the spectral sharpening approach is that not only can we produce positive curves, but the process is "steerable" in that we can produce positive curves with as good or better properties for sharpening within a given set of sharpening intervals. At base, however, it is positive colors in the transformed space that are the prime objective. Therefore we also carry out sharpening of sensor curves governed not by positivity of the curves themselves but of colors resulting from them. Curves that result have negative lobes but generate positive colors. We find that this type of constrained sharpening generates the best results, which are almost as good as for unconstrained sharpening but without the penalty of negative colors. All methods discussed may be used with any number of sensors.

Performance of a Chromatic Adaptation Transform based on Spectral Sharpening

Performance of a Chromatic Adaptation Transform based on Spectral Sharpening

Spectral sharpening: sensor transformations for improved color constancy.

We develop sensor transformations, collectively called spectral sharpening, that convert a given set of sensor sensitivity functions into a new set that will improve the performance of any color-constancy algorithm that is based on an independent adjustment of the sensor response channels. Independent adjustment of multiplicative coefficients corresponds to the application of a diagonal-matrix transform (DMT) to the sensor response vector and is a common feature of many theories of color constancy. Land's retinex and von Kries adaptation in particular. We set forth three techniques for spectral sharpening. Sensor-based sharpening focuses on the production of new sensors as linear combinations of the given ones such that each new sensor has its spectral sensitivity concentrated as much as possible within a narrow band of wavelengths. Data-based sharpening, on the other hand, extracts new sensors by optimizing the ability of a DMT to account for a given illumination change by examining the sensor response vectors obtained from a set of surfaces under two different illuminants. Finally in perfect sharpening we demonstrate that, if illumination and surface reflectance are described by two- and three-parameter finite-dimensional models, there exists a unique optimal sharpening transform. All three sharpening methods yield similar results. When sharpened cone sensitivities are used as sensors, a DMT models illumination change extremely well. We present simulation results suggesting that in general nondiagonal transforms can do only marginally better. Our sharpening results correlate well with the psychophysical evidence of spectral sharpening in the human visual system.

Spectral sharpness and vowel dissimilarity.

The effect of sharpening or smoothing the spectral envelopes of synthetic vowel-like sounds on the dissimilarities perceived among these sounds was investigated by means of triadic comparisons. When a spectral envelope (dB on a log-frequency scale) is considered the sum of a series of sinusoidal spectral modulations (or ripples) of different densities (the ripple spectrum), spectral sharpening or smoothing can be described as an amplification or attenuation of a part of the original ripple spectrum. For a set of nine sounds comprising different degrees of spectral sharpening of a single vowel, the perceived dissimilarities were found to be dominated by a specific part of the ripple spectrum, i.e., by spectral modulations with a density of about 2 ripples/oct. The possible role of lateral suppression in relation to this dominant region is discussed. For a set of 18 sounds comprising six vowels, each in three different versions (sharpened, normal, or smoothed), the dissimilarities were found to be determined mainly by the global shape of the spectral envelopes, i.e., by spectral modulations up to about 1.5-2 ripples/oct. Details of the spectral envelope (including the region of 2 ripples/oct where lateral suppression is effective) appear to be of minor influence on vowel dissimilarities.

Low-Temperature Photoacoustic Spectroscopy of Polynuclear Aromatic Hydrocarbons in Shpol'skii Matrices at 77 K

Low-temperature studies of organic and inorganic compounds was suggested by Rosencwaig in 1980 as one new area in the future trends of photoacoustics and photoacoustic spectroscopy. At cryogenic temperatures, optical absorption spectra often exhibit much sharper and less complex features than the corresponding room-temperature spectra. The use of piezoelectric detection for high-resolution, laser-induced optoacoustic spectroscopy at cryogenic temperatures was first reported by Shaw and Howell in 1981. A ceramic disc piezoelectric transducer was compressed in a copper clamp along with a fused quartz substrate on which the sample was applied. Spectra of cubic neodymium sesquioxide, Nd2O3, cooled to less than 10 K, in the yellow spectral region (16 400 to 17 400 cm−1) exhibited sharp lines of full width at half maximum 10 cm−1. Such spectral sharpening, and simplification, upon cooling the sample enhance selectivity for analysis of mixtures.

Changes in field spectral sensitivities of red-, green- and blue-sensitive colour mechanisms obtained on small background fields

When field spectral sensitivities of the red- and green-sensitive colour mechanisms are determined in the presence of a small steady background field (auxiliary conditioning field), spatially coincident with the test field, the resulting curves may be found to be narrowed and shifted away from each other on the wavelength axis. These “sharpened” curves tend to peak at wavelengths of 605 nm and 530 nm respectively. This investigation determines spectral sharpening for variations in the intensity, wavelength, and size of the auxiliary conditioning field, and for variations in the wavelength and duration of the test flash. The following results were obtained. (1) The intensity-wavelength dependence of an auxiliary field that produces a constant spectral sharpening of the red-sensitive mechanism is indistinguishable from the sharpened field spectral sensitivity curve of the green-sensitive mechanism. (2) The wavelengths of the test flash at which maximum spectral sharpening of the red- and green-sensitive mechanisms occurs are 610–620 nm and 530 nm respectively. (3) Spectral sharpening is reduced or eliminated when the test flash has short duration. (4) No spectral sharpening is apparent at moderate-to-high intensities for the blue-sensitive mechanism. (5) The variation of spectral sharpening with auxiliary-field size depends strongly on test-field size. This dependence could be made approximately constant when data were scaled according to cortical magnification factor M. (6) Spectral sharpening is abolished, or severely diminished, when the auxiliary field is presented dichoptically. These findings are considered in relation to an opponent-process theory of colour encoding, the site of the coincident auxiliary-field effect and other data on interactions amongst colour mechanisms.

Forward masking, pulsation threshold, and lateral suppression: An overview

Traditionally, auditory spectral resolution is studied psychophysically by masking experiments (i.e., the effect of a masking stimulus on the detection threshold of a test tone with variable frequency). During the last decade there has been increasing experimental evidence of an essential qualitative difference between the results of two classes of such experiments: (a) masking stimulus and test tone being presented simultaneously (direct masking) or (b) nonsimultaneously (forward masking, pulsation threshold). This difference can be interpreted as an effect of lateral suppression (spectral sharpening) which manifests itself only in case of nonsimultaneous presentation of masking stimulus and test tone. An overview of the relevant data is presented (e.g., auditory Mach bands, two‐tone suppression, increased ripple resolution, etc.), with occasional references to related electrophysiological data. The experimental data lead to the notion of a peripheral stage of spectral sharpening of such a nature that it does not affect the detection threshold of a test tone in direct masking.

Spin-label assay for lysozyme

A spin-label assay for lysozyme, which is based on the enzymatic hydrolysis of spin-labeled peptidoglycan, is described. Hydrolysis of this polymer by lysozyme results in sharpening of the esr spectrum. The rate of spectral sharpening is a function of enzyme concentration. When the activities of hen egg-white and human lysozymes are compared by this method, human lysozyme is 3.5 times as active as the hen enzyme. The pH optima for both enzymes are pH 5.0. At this pH, the maximal activity for the hen egg-white lysozyme is observed at an ionic strength of 0.09. This assay is suitable for measuring lysozyme levels in biological fluids. It is a sensitive, continuous assay that measures muramidase activity on a defined substrate.

Pyridine interactions with phenolic groups in water: evidence for hydrogen bonding and hydrophobic association.

Pyridine interactions with phenol, substituted phenol, tyrosine and poly(Glu50,Tyr50) in aqueous solutions have been studied by ultraviolet (UV) difference spectroscopy, spectrophotometric pH titration, circular dichroism (CD) and proton magnetic resonance (PMR) spectroscopy. A red shift and spectral sharpening of the near-UV spectrum of phenol in water was noted at pyridine concentrations greater than 0.25 M. In addition, the spectrophotometric equivalence point for the phenol- or substituted phenol-phenolate equilibrium was increased about 0.5 pH units upon the addition of 1.0 M pyridine. PMR studies were consistent with the formation of a 1 : 1 phenol-pyridine hydrogen bonded complex. The equilibrium constant derived for this interaction, 0.6-0.7 M-1, is greater than the corresponding value for phenol-acetate hydrogen bonding in water. Enhancement of thepyridine hydrogen bond interaction with Tyr within poly(Glu50,Tyr50) was observed at pH greater than 12 due to a hydrophobic microenvironment produced by pyridine molecules intercalating between neighboring tyrosyl residues.

Auditory-filter characteristics derived from direct-masking data and pulsation-threshold data with a rippled-noise masker.

The masker was ’’rippled noise,’’with a power spectrum (intensity on a linear frequency scale) shaped according to a sinusoidal function. The test signal was a pure tone. The masking effectiveness of the rippled noise depends on the position of its peaks and troughs relative to the test-tone frequency, but this dependence decreases for high-ripple densities (thus, for ripples with small peak-to-peak distances along the frequency scale). The results of masking experiments as a function of the position and the density of the ripple around the test-tone frequency allow an estimation of the degree of auditory frequency resolution in terms of a filter characteristic. Several masking paradigms were applied: direct masking, forward masking, and pulsation threshold. It was found that the degree of frequency resolution estimated both from pulsation-threshold data and from forward-masking data is substantially higher than that estimated from direct-masking data. The difference is about a factor of two when expressed in terms of the bandwidth of the auditory filter. This difference is interpreted as reflecting spectral sharpening by lateral suppression which apparently manifests itself only in threshold measurements where masker and test tone are presented nonsimultaneously.