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PubMed1995

Contributions of neural pathways to age-related losses in chromatic discrimination.

Chromatic-discrimination thresholds were measured for light mixtures lying along individually determined tritan axes and an axis of constant short-wavelength-sensitive- (S-) cone stimulation for 30 color-normal observers (age range 22-77 years). The stimulus was a foveally viewed 2 degrees, circular bipartite field consisting of a standard and a test light. Heterochromatic flicker photometry was used to equate the retinal illuminance of the stimuli at 120 Td for all observers. All stimuli were presented in Maxwellian view. Age-related losses in chromatic discrimination depended on the level of cone stimulation. At relatively lower levels of S- and long-wavelength-sensitive (L-) cone stimulation, discrimination thresholds were elevated for older relative to younger observers. As the level of simulation increased for these two cone types, thresholds converged, on average, for all observers. Application of a model of chromatic discrimination mediated by an S-cone pathway suggests that there is no significant age-related change in Weber fractions and that age-related losses in chromatic discrimination are due, at least in part, to spontaneous neural noise arising in the pathway and/or neural changes that multiplicatively scale all incident light.

Adult↗
PubMed1997

Spectral mechanisms of spatially induced blackness: data and quantitative model.

Spectral efficiency functions and tests of additivity were obtained with three observers to identify possible chromatic contributions to spatially induced blackness. Stimuli consisted of a series of monochromatic (400-700 nm; 10-nm steps), 52-arcmin circular test lights surrounded by broadband (x = 0.31, y = 0.37), 63-138-arcmin annuli of fixed retinal illuminance. The stimuli were imaged on the fovea in Maxwellian view as 500-ms flashes with 10-s interstimulus intervals. Observers decreased the intensity of the test center until it was first perceived as completely black. Action spectra determined for two surround levels [2.5 and 3.5 log trolands] had three sensitivity peaks (at approximately 440, 540, and 600 nm), However, when monochromatic surrounds were adjusted to induce blackness in a broadband center, action spectra were unimodal and identical to functions obtained by heterochromatic flicker photometry. Tests of additivity revealed that when blackness is induced by broadband surround into a bichromatic center, there is an additivity failure of the cancellation type. This additivity failure indicates that blackness induction is influenced, in part, by signals from opponent-chromatic pathways. A quantitative model is presented to account for these data. This model assumes that blackness induction is determined by the ratio of responses to the stimulus center and the annulus, and while signals form the annulus are based only on achromatic information, responses from the center are based on both chromatic and achromatic properties of the stimulus.

Color Perception↗
PubMed1999

Aging and the saturation of colors. 1. Colorimetric purity discrimination.

Colorimetric purity (Pc) discrimination functions were measured for 21 color-normal observers (11 younger and 10 older observers with mean ages of 30 and 74 years, respectively). On each two-alternative-forced-choice trial, observers saw two flashes of light, a broadband white light [CIE(x, y) = (0.33, 0.35)] and a mixture of broadband and monochromatic light (420-680 nm). The observer's task was to choose the flash that had a chromatic component. Foveally viewed, circular, 1.2 degrees-diameter stimuli were presented as 1.5-s flashes with 3-s interstimulus intervals in Maxwellian view. Stimuli [250 trolands (td) and 10 td] were equated on the basis of individual heterochromatic flicker photometry functions. Measured Pc discrimination sensitivity was lower in the older group than in the younger group at both light levels, and the performance difference between the age groups was approximately constant across the spectrum. The difference between discrimination at 10 and 250 td was relatively small for the younger group but larger for the older group, indicating a selective performance decrement for older observers at low light levels. The data were modeled as a sum of differential responses from S-cone and L/M-cone chromatic channels. The model and the data indicate similar age-related losses of sensitivity in the two channels, perhaps secondary to receptorial sensitivity losses.

Adult↗
PubMed1999

Aging and the saturation of colors. 2. Scaling of color appearance.

Saturation of two sets of stimuli was scaled by 21 color-normal observers (ten younger and 11 older observers, mean ages: 30 and 73 years). Circular fields, 1.2 degrees in diameter, were presented in Maxwellian view as 1.5-s flashes with 3-s interstimulus intervals. Stimuli were mixtures of broadband light [CIE(x, y) = (0.35, 0.39), 200 trolands (td)] and monochromatic light (420-700 nm, 50 td). Monochromatic lights were equated by the 1978 2 degrees fundamental observer's luminosity function in one set of stimuli [J. J. Vos, Color Res. Appl. 3, 125 (1978) and by each observer's heterochromatic flicker photometry function in the other set of stimuli. Comparing the two age groups reveals no sizable differences in saturation for either set of stimuli, neither supporting nor refuting neural compensation for age-related increases in ocular media density (OMD). Examining short-wavelength saturation as a continuous function of estimated OMD reveals a more complicated pattern of results, however, suggesting substantial compensation over a certain range of OMD values but incomplete compensation for observers with the highest OMD values.

Adult↗
PubMed2000

Relative number of long- and middle-wavelength-sensitive cones in the human fovea.

Flicker photometric measurements yield spectral sensitivity curves that are well fitted by sums of the spectral sensitivity curves of long-wavelength-sensitive (L) cones and middle-wavelength-sensitive (M) cones if the L cones are given twice the weight of the M cones. This result has been interpreted as implying that L cones are more numerous than M cones but is also consistent with a different numerical ratio, say, 1:1, and with the assignment of greater weight to the L cone input than to the M cone input by the mechanism subserving flicker photometry. Measurements of temporal sensitivity are presented for lights that modulate the inputs of either only the L cones or only the M cones. Sensitivity to modulation of the L cones is approximately twice that of modulation of the M cones at approximately 30 Hz, but that advantage disappears at approximately 2 Hz. Thus flicker sensitivity is equivocal with regard to cone numerosity. Electrophysiological, anatomical, and psychophysical evidence is reviewed, with particular weight placed on the statistics of color appearance of small, brief, monochromatic lights and on increment thresholds measured on the same observers. It is concluded that, in the central fovea, the ratio of L:M cone numbers is close to unity and may not be so variable as is usually supposed.

Fovea Centralis↗
PubMed2000

Stochastic independence of color-vision mechanisms confirmed by a subthreshold summation paradigm.

We have used a subthreshold summation protocol to analyze spatial color-color interaction. By means of a CRT color monitor, we measured the threshold contours for a spatial frequency of 0.5 cycles/degree. Heterochromatic flicker photometry was used to obtain isoluminance. The results suggest that the blue-yellow (b-y) and red-green (r-g) contrast thresholds remained unchanged by the addition of fixed r-g and b-y subthreshold pedestals. Our subthreshold summation data then support the stochastic independence of colorvision mechanisms derived from Mullen and Sankeralli's work [Vision Res. 39, 733 (1999)] despite the differences that exist between the two experimental methods.

Color Perception↗
PubMed2005

Effective contrast of colored stimuli in the mesopic range: a metric for perceived contrast based on achromatic luminance contrast.

Little is known about how color signals and cone- and rod-based luminance signals contribute to perceived contrast in the mesopic range. In this study the perceived contrast of colored, mesopic stimuli was matched with that of spatially equivalent achromatic stimuli. The objective was to develop a metric for perceived contrast in the mesopic range in terms of an equivalent achromatic luminance contrast, referred to here as effective contrast. Stimulus photopic luminance contrast, scotopic luminance contrast, and chromatic difference from the background all contributed to effective contrast over the mid-mesopic range, but their contributions were not independent and varied markedly with background luminance. Surprisingly, color made a significant contribution to effective contrast from 10 to approximately 0.003 cd m(-2). A model describing this relationship is introduced (R2 = 0.89) and compared with predictions of mesopic luminance contrast obtained from a number of models proposed as systems of mesopic photometry.

Adult↗
PubMed2005

Polymorphic New World monkeys with more than three M/L cone types.

Most New World (platyrrhine) monkeys have M/L cone photopigment polymorphisms that map directly into individual variations in visual sensitivity and color vision. We used electroretinogram flicker photometry to examine M/L cone photopigments in the New World monkey Callicebus moloch (the dusky Titi). Like other New World monkeys, this species has an M/L cone photopigment polymorphism that reflects the presence of X-chromosome opsin gene alleles. However, unlike other platyrrhines in which three M/L photopigments are typical, Callicebus has a total of five M/L cone photopigments. The peak sensitivity values for these pigments extend across the range from 530 to 562 nm. The result is an enhanced array of potential color vision phenotypes in this species.

Animals↗
PubMed1986

Spectral luminous efficiency functions in the mesopic range.

Spectral luminous efficiency functions for mesopic vision were measured extensively to establish a basic data set for the mesopic photometry system. These functions were obtained by the direct brightness matching for a 10 degrees field at nine retinal illuminance levels from 100 to 0.01 Td in 0.5-log-unit steps. Data for 12 subjects were collected. In the mesopic range, the spectral luminous efficiency function, averaged over 12 subjects, gradually changes form with retinal illuminance. This suggests that the mesopic function should be established for all the levels in that range. An attempt was made to find an empirical formula that expresses the mesopic function as a simple combination of the photopic and the scotopic brightness functions.

Humans↗
PubMed1987

Short-wavelength-sensitive cones do not contribute to mesopic luminosity.

It has been suggested that the short-wavelength-sensitive cones (S cones) play a significant role in the transition from scotopic to photopic vision (the Purkinje shift). We address this issue directly over a 5-log10-unit range of light levels covering scotopic, mesopic, and photopic vision. At each light level we make flicker matches to two reference stimuli by 2-Hz flicker photometry. The two reference lights (441 and 481 nm) differ only in their stimulation of S This novel technique utilizes the different magnitudes of the rod and cone Stiles--Crawford effects. Despite the large difference in S-cone stimulation by the two reference lights (more than 1 log10 unit), the pairs of luminosity functions are indistinguishable at each light level tested. The results indicate that S cones do not contribute to either photopic or mesopic luminosity.

Humans↗
PubMed1987

Temporal modulation sensitivity and pulse-detection thresholds for chromatic and luminance perturbations.

We studied temporal processing of chromatic and luminance perturbations of a 600-nm field, measuring both modulation sensitivity (sinusoidal frequencies from 0.25 to 40 Hz) and pulse-detection thresholds (pulse durations from 5 to 2560 msec) for mean luminances of 0.9 to 900 Td and field sizes of 0.5 degrees to 8 degrees. Chromatic stimuli were produced by antiphase modulation of lights matched by heterochromatic flicker photometry. Both mean luminance and field size affected sensitivity, and the magnitude of field-size effects increased with mean luminance. We derived both luminance and chromatic impulse response functions for each set of experimental conditions, using the modulation-sensitivity data. At high mean luminances and large field sizes the chromatic impulse response functions are complex, suggesting contributions from both chromatic and luminance mechanisms. Pulse-detection data were fitted by a peak detector model based on these impulse response functions.

Color↗
PubMed1990

Additivity of spatially induced blackness.

Tests of additivity of the postreceptoral pathways that mediate the perception of blackness were conducted under conditions of spatial contrast. Observers increased the radiance of a surrounding annulus until a broadband (white) test center appeared completely black. Additivity tests with heterochromatic flicker photometry (HFP) and direct brightness matching were also conducted for each observer. The results indicated that the luminance level of the annulus required to induce blackness did not change with variations in spectral composition. Results consistent with additivity were also obtained for HFP, but the results from brightness matching were not consistent with additivity. The data support the view that the perception of blackness is mediated by neural mechanisms that additively combine the input of middle- and long-wave photoreceptors.

Adult↗
PubMed2006

Tuning D* with modified thermal detectors.

We report on the fabrication and characterization of a modified thermopile detector that has a spectral detectivity, D*, primarily determined by the absorbance of a polymer film. This was done by coating the detector with a metal mirror, followed by the polymer film, so that the film absorbances are responsible for most thermal conversion. The detector is designed to tailor the spectral response of optical systems more specifically to analytes in order to improve precision in methods such as multivariate optical computing and simple photometry. Interference effects in the thin-film response are eliminated by the textured surface of the silicon thermopile, which makes the spectral response relatively simple. The maximum detectivity due to a 1-micrometer-thick film is found to be 20% of the detectivity of the original wide-band detector at 10 Hz modulation frequency. We estimate the thermal diffusion length in the polymer at 10 Hz to be 40 micrometers. We also suggest that the detectivity of the modified detector can be approximated as the product of the D* of the underlying thermal detector and the absorbance of the modifying film, provided the modulation frequency is low and interference effects are defeated.

Journal Article↗
PubMed1998

Sensitivity of aerosol bolus behavior to methacholine-induced bronchoconstriction.

STUDY OBJECTIVES: Airway narrowing causes alterations in the shape of an exhaled aerosol bolus that can serve as indexes of airway changes during bronchoprovocation. We compared the sensitivities of aerosol bolus behavior and specific airway conductance (SGaw) during bronchoprovocation in normal subjects. DESIGN AND PARTICIPANTS: Fifteen normal, nonsmoking subjects were studied. Doubling methacholine (MCh) concentrations were delivered during tidal breathing. After each dose, SGaw was determined followed by inhalation of narrow pulses of 1-microm particles introduced into 1-L breaths. Inhaled and exhaled particle concentrations were measured with light scattering photometry. Using plots of concentration vs volume, the exhaled bolus was compared with the inhaled bolus for measurements of volumetric change in mode location (modal shift), particle deposition, and dispersion. To determine baseline intrasubject variability, sham studies using buffer solution were performed on five subjects. RESULTS: MCh caused a proximal modal shift, and increased dispersion and deposition of the exhaled bolus. At most doses, a greater percentage of subjects showed significant change (p<0.05) from baseline for modal shift and deposition than for SGaw. Aerosol bolus behavior displayed less intrasubject variability than did SGaw during sham studies. CONCLUSION: Aerosol bolus behavior is at least as sensitive as SGaw in detecting MCh-induced airway constriction in normal subjects and exhibits less intrasubject variability.

Administration, Inhalation↗
PubMed2004

Fluorescence resonance energy transfer-based HIV-1 virion fusion assay.

The fluorescence resonance energy transfer (FRET)-based HIV-1 virion fusion assay exploits the incorporation of beta-lactamase-Vpr chimeric proteins into HIV-1 virions and their subsequent delivery into the cytoplasm of target cells as a marker of fusion. This transfer can be monitored by the enzymatic cleavage of the CCF2-AM dye, a fluorescent substrate of beta-lactamase (BlaM), loaded into the target cells. BlaM cleavage of the beta-lactam ring in CCF2-AM prevents the FRET between the coumarin and fluorescein moieties of the dye. This cleavage changes the fluorescence emission spectrum of CCF2-AM from green (520 nm) to blue (447 nm), and thus permits detection of fusion by fluorescence microscopy, flow cytometry, or UV photometry. This assay is simple and rapid to perform, and exhibits high sensitivity and specificity. Importantly, it can be applied to study HIV-1 virion fusion in primary cells and can be combined with immunostaining for subset discrimination in heterogeneous target cell populations. Finally, the assay can also be adapted to study fusion mediated by the envelope proteins from other viruses through the construction of HIV-1 pseudotypes.

Flow Cytometry↗
PubMed2004

Covalent coupling of DNA oligonucleotides and streptavidin.

Semisynthetic DNA-protein conjugates are synthesized by covalent coupling of thiol-modified DNA oligonucleotides and streptavidin. The resulting conjugates have a binding capacity for four equivalents of biotin and one nucleic acid of complementary sequence. The conjugates are purified to homogeneity by ultrafiltration and chromatography and characterized by photometry and gel electrophoresis. Subsequently, the conjugates are applied as molecular linkers in the DNA-directed immobilization of a biotinylated enzyme on a microplate, containing complementary capture oligonucleotides.

Cross-Linking Reagents↗
PubMed1980

[Catabolism of carnitine: products of carnitine decarboxylase and carnitine dehydrogenase in vivo].

1) Rats and mice were given large oral or subcutaneous doses of (-)-L-, (+)-D- and DL-carnitine (5 mg/g body weight). The carnitine metabolites, beta-methylcholine and acetonyltrimethylammonium, were isolated from the urine by special methods, and determined as their characteristic derivatives (2,4-dinitrophenylhydrazone and butyric ester) by thin-layer chromatography or photometry. 2) beta-Methylcholine, the product of carnitine decarboxylase, was not excreted, even when animals were heavily dosed with both carnitine isomers, with or without starvation. 3) After the administration of (+)-D- and DL-carnitine, both species excreted acetonyltrimethylammonium, which is already known as the spontaneous decarboxylation product of dehydrocarnitine (product of carnitine dehydrogenase) in bacteria. Injection of 0.71 mmol (+)-D-carnitine resulted in the excretion of 5.0 mumol (average) acetonyltrimethylammonium per mouse during the 48 h post injection. Under the same conditions, rats produced up to 40 mumol acetonyltrimethylammonium. The ratio of excreted acetonyltrimethylammonium to injected (+)-D-carnitine depended on the method of administration and the dose. 4) Production of the pharmacologically active (+)-acetyl-L-beta-methylcholine is not to be expected, following high exogenous doses of (-)-L-carnitine or (-)-acetyl-L-carnitine. The chief metabolites are trimethylamine, trimethylamine oxide and gamma-butyrobetaine (this journal 361, 1059), and both the (-)-L-carnitine pool and exogenous (-)-L-carnitine are dehydrogenated or decarboxylated only to a very small extent, if at all. When DL-carnitine is used therapeutically, the formation of acetonyltrimethylammonium must be taken into account.

Administration, Oral↗
PubMed1976

Evaluation of the Abbott Bichromatic Analyzer 100 (A proposal for an evaluation scheme).

This report summarizes a one year evaluation of Abbott's ABA 100, with respect to mechanical parts (syringe plates, precision and linearity of photometry, band width of several filters, multicuvet precision, temperature control) and the reliability of several methods (endpoint procedures: determination of the glucose concentration with hexokinase- and the glucose dehydrogenase method, and of the protein concentration; enzyme activities: alanine and aspartate aminotransferase, creatine kinase, alkaline phosphatase). The critical batch size was estimated as an indicator of economy (about 40 samples per day for the glucose concentration). Various aspects of the instrument are discussed with respect to its use in clinical chemistry.

Autoanalysis↗