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Biomedical subjects

J Pokorny

Publications and source records attributed to J Pokorny.

At least 19 recordsLinked to original sources
PubMed1992

Sawtooth contrast sensitivity: effects of mean illuminance and low temporal frequencies.

Temporal contrast sensitivity was measured for mirror-image sawtooth (rapid-on and rapid-off) and sine waveforms for a 1.8 deg foveal target. In one experiment, contrast sensitivity was measured for 2-26 Hz stimuli at target mean illuminance levels of 5-1260 td. At 5 td, contrast sensitivity functions for sawtooth and sine waveforms, expressed in terms of the Fourier fundamental amplitude, are equivalent. At higher light levels, sawtooth sensitivity increasingly exceeds sine sensitivity and rapid-off (decremental) sawtooths show progressively greater sensitivity than rapid-on (incremental) sawtooths. This pattern of results was obtained for two color-normal observers and for a deuteranopic observer. In a second experiment, sawtooth and sine sensitivity was tested at 500 td with an extended low-frequency range, to 0.5 Hz. Rapid-off and rapid-on sensitivities declined only slightly at low temporal frequencies in contrast with sine sensitivity. To interpret our data, we evaluate two single-pathway models (last-stage asymmetric detector and compressive response-intensity non-linearity) and a dual-pathway model in which incremental and decremental waveforms are detected by separate ON and OFF visual mechanisms.

Adaptation, Ocular↗
PubMed1992

Responses of macaque ganglion cells to the relative phase of heterochromatically modulated lights.

1. We measured the response of macaque ganglion cells to sinusoidally modulated red and green lights as the relative phase, theta, of the lights was varied. 2. At low frequencies, red-green ganglion cells of the parvocellular (PC-) pathway with opponent inputs from middle-wavelength sensitive (M-) and long-wavelength sensitive (L-) cones were minimally sensitive to luminance modulation (theta = 0 deg) and maximally sensitive to chromatic modulation (theta = 180 deg). With increasing frequency, the phase, theta, of minimal amplitude gradually changed, in opposite directions for cells with M- and L-cone centres. 3. At high frequencies (at and above 20 Hz), phasic cells of the magnocellular (MC-) pathway were maximally responsive when theta approximately 0 deg and minimally responsive when theta approximately 180 deg, as expected from an achromatic mechanism. At lower frequencies, the phase of minimal response shifted, for both on- and off-centre cells, to values of theta intermediate between 0 and 180 deg. This phase asymmetry was absent if the centre alone was stimulated with a small field. 4. For PC-pathway cells, it was possible to provide an account of response phase as a function of theta, using a model involving three parameters; phases of the L- and M-cone mechanisms and a L/M cone weighting term. For red-green cells, the phase parameters were monotonically related to temporal frequency and revealed a centre-surround phase difference. The phase difference was linear with a slope of 1-3 deg Hz-1. If this represents a latency difference, it would be 3-8 ms. Otherwise, temporal properties of the M- and L-cones appeared similar if not identical. By addition of a scaling term, the model could be extended to give an adequate account of the amplitude of responses. 5. We were able to activate selectively the surrounds of cells with short-wavelength (S-) cone input to their centres, and so were able to assess L/M cone weighting to the surround. M- and L-cone inputs added linearly for most cells. On average, the weighting corresponded to the Judd modification of the luminosity function although there was considerable inter-cell variability. 6. To account for results from MC-pathway cells, it was necessary to postulate a cone-opponent, chromatic input to their surrounds. We developed a receptive field model with linear summation of M- and L-cones to centre and surround, and with an additional M,L-cone opponent input to the surround.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Full-spectrum cone sensitivity functions for X-chromosome-linked anomalous trichromats.

We derived the cone fundamentals for X-chromosome-linked anomalous trichromats for the wavelength range of 400-700 nm. Pigment templates were constructed from the cone fundamentals of normal trichromats after correction for ocular media absorption. The resultant retinal-level sensitivities had small irregularities in the short-wavelength region that were smoothed. The pigment templates, expressed as quantal sensitivities, were then shifted on a frequency abscissa to solve for the lambda max of the pigments of anomalous trichromats needed to predict average anomaloscope matching data. We found that the protanomalous M- and L'-cone pigments are separated by 10 nm and the deuteranomalous M'- and L-cone pigments are separated by 6 nm (rounded to the nearest nanometer), where M and L indicate middle- and long-wavelength sensitive, respectively. The triads of peak wavelengths for the corneal energy-based sensitivities were as follows: normal: 440, 543, and 566 nm; protanomalous: 440, 543, and 553 nm; and deuteranomalous: 440, 560, and 566 nm.

Adult↗
PubMed1991

Foveal cone detection statistics in color-normals and dichromats.

We measured for six male observers, the psychometric functions for the detection of two simultaneously presented points of light. The test stimuli were two 1 min point sources separated by 17 min arc, and pulsed for 0.5 msec. The stimuli varied in wavelength from 500 to 620 nm. The psychometric functions were fit with a model that assumes ideal detection and the following properties: (1) Poisson-distributed quantal absorptions; (2) binomial sampling of foveal long-wavelength-sensitive (LWS) and middle-wavelength-sensitive (MWS) cones; (3) independent responses of the LWS and MWS cones; and (4) the Smith-Pokorny fundamentals for cone spectral sensitivities. Based on chi 2 fits to the psychometric functions for detecting neither, one or both of the two-point stimuli presented, estimates were derived for the minimum quantal catch by a single cone for detection (C), the number of effective cones illuminated by a point stimulus at threshold (N) and the proportion of central foveal cones of the LWS type (PL). Three observers were color-normal, two were protanopes and one was a deuteranope. A second deuteranope was included in the design but his data were too unreliable for an unambiguous solution. The estimated quantal requirement C was consistently near 5(4-6), and the effective number of illuminated cones always was 1 or 2. The plausible range of PL (98% confidence interval) for the color-normal observers was 0.48-0.68 (observer YY), 0.76-0.90 (observer MW) and 0.77-0.95 (observer DF). The best fitting PL solution for these observers were 0.61, 0.82 and 0.88, respectively. These were comparable to the values obtained from flicker photometric data. The best PL value for each of the protanopes was 0.00 and for the deuteranope the best PL value was 0.98.

Color Perception↗
PubMed1991

Human heart sarcolemmal sheath antibodies in children with non-suppurative sequelae of group A streptococcal infections: a follow up study.

The kinetics of the human heart sarcolemmal sheath antibody were studied in children with acute rheumatic fever who had no carditis, children with acute rheumatic fever who had carditis and developed rheumatic heart disease, and in children with acute poststreptococcal glomerulonephritis. The children with rheumatic fever and those who developed valvular heart disease were given continuous secondary antistreptococcal prophylaxis. The titre of antibody at onset was significantly higher than that of the controls in children with acute rheumatic fever and carditis and in children with acute poststreptococcal nephritis. The difference in the antibody titre between children with rheumatic fever who had no carditis and controls was not statistically significant. After a mean follow up of three years, however, a high titre was only maintained in children with rheumatic fever who developed valvular heart disease.

Acute Disease↗
PubMed1991

[Serum reactivity of antigen fractions of atypical mycobacteria in patients with lung diseases].

In connection with studies on the pathogenesis of sarcoidosis antigen fractions were isolated from 8 mycobacteria species, three out of each strain. These fractions were tested for their reactivity to serum antibodies by means of RIA-technique, using 40 selected sera from controls, and patients with sarcoidosis, tuberculosis and asthma. Comparing the results (average titer steps) sera from asthmatics showed the lowest and those from sarcoidosis patients the highest reactivities to the mycobacterial antigen fractions. The reactivities clearly differed in dependence on the mycobacteria species. The highest mean reactivity in sarcoidosis patients was found with the HIP-antigen fraction of M. xenopi. It was 8 times higher compared to the control sera as well as the tuberculosis sera and 32 times higher than that of the asthma sera. There were also clear differences in the reactivities within the sarcoidosis sera tested. In sera from patients with clinically inactive sarcoidosis there were found nearly the same or only slightly higher titer steps than in control sera as well as tuberculosis sera, however in clinically active sarcoidosis the titer steps were clearly elevated. The findings are seen in connection with the role of atypical mycobacteria (MOTT) in the pathogenesis of sarcoidosis. The potential applications of the HIP- and Triton X-100 antigen fractions for in vitro diagnostics are discussed.

Antibodies, Bacterial↗
PubMed1990

Genetic studies of variation in Rayleigh and photometric matches in normal trichromats.

The inheritance of Rayleigh match midpoints and photometric matches (551-667 nm) was studied in observers with normal color vision. An analysis was performed to evaluate whether the measured interobserver variations in these two traits were consistent with single gene allelic variation, polygenic variation, or environmental factors. A bipartite 2 deg field and a computerized tracking method were used to obtain Rayleigh matches; a new photometric technique, termed heterochromatic modulation photometry (HMP), was used to obtain photometric matches. Data were collected from 72 nonrelated males to determine distribution characteristics for the normal population. The distributions were analyzed for evidence of multimodality and the results indicated that the distributions for Rayleigh match midpoints and HMP matches obtained in this study were unimodal and symmetrical. Data from 52 observers from 5 families were used to study the transmission of the two traits in pedigrees. Statistical analysis of the pedigree data suggested that the major source of variations for Rayleigh match midpoints and for HMP matches were each due to allelic variation at single gene loci; that is, each may be determined by a single gene. Results were inconclusive as to whether variation in the two traits could be determined by the same gene.

Adolescent↗
PubMed1990

Luminance and chromatic modulation sensitivity of macaque ganglion cells and human observers.

We measured the sensitivity of macaque ganglion cells to luminance and chromatic sinusoidal modulation. Phasic ganglion cells of the magnocellular pathway (M-pathway) were the more sensitive to luminance modulation, and tonic ganglion cells of the parvocellular pathway (P-pathway) were more sensitive to chromatic modulation. With decreasing retinal illuminance, phasic ganglion cells' temporal sensitivity to luminance modulation changed in a manner that paralleled psychophysical data. The same was true for tonic cells and chromatic modulation. Taken together, the data suggest strongly that the cells of the M-pathway form the physiological substrate for detection of luminance modulation and the cells of the P-pathway the substrate for detection of chromatic modulation. However, at high light levels, intrusion of a so-called luminance mechanism near 10 Hz in psychophysical detection of chromatic modulation is probably due to responses in the M-pathway, arising primarily from a nonlinearity of cone summation. Both phasic and tonic ganglion cells responded to frequencies higher than can be psychophysically detected. This suggests that central mechanisms, acting as low-pass filters, modify these cells' signals, though the corner frequency is lower for the P-pathway than for the M-pathway. For both cell types, the response phase at different frequencies was consistent with the cells' description as linear filters with a fixed time delay.

Animals↗
PubMed1989

Evaluation and limitations of the lipid-associated sialic acid test for the detection of human cancer.

Previous studies and those of the authors have reported about raised levels of lipid-bound sialic acid (LSA) in sera of patients with various neoplasms. Most authors have used the method of Katopodis and Stock for isolating serum LSA. Because of the discrepancy of the amount of extracted LSA with the content of pure gangliosides in sera reported by other groups the authors analyzed the composition of the extracted LSA by immunochemical approaches. They were able to isolate considerable amounts of glycoproteins containing sialic acid as acid alpha-1-glycoprotein, antitrypsin, haptoglobin, antichymotrypsin, and immunoglobulins from the so-called "LSA" fraction. There is a very strong correlation between the raised LSA levels and the content of acid-alpha-1-glycoprotein in the sera of patients with malignancy. Therefore the term lipid-bound sialic acid applied to this test is misleading, since glycoproteins containing sialic acid are mostly responsible for the high "LSA" levels.

Blood Proteins↗
PubMed1989

Autosomal dominantly inherited macular dystrophy with preferential short-wavelength sensitive cone involvement.

We found an apparently inherited tritan-like color vision defect in five members of a family, spanning three generations. The defect was associated with mild macular pigmentary changes, poor foveolar reflexes, or slightly reduced visual acuity in four of the affected individuals. The inheritance pattern appeared to be autosomal dominant. Results of various color vision tests indicated preferential involvement of the short-wavelength sensitive cone system, with relative preservation of the middle- and long-wavelength sensitive cone systems. Both anomaloscope testing with larger (8-degree) fields and short-wavelength sensitive electroretinography indicated some short-wavelength sensitive cone system involvement beyond the central macula in the three affected individuals on whom testing was performed. The condition appeared to be a familial macular dystrophy with preferential short-wavelength sensitive cone involvement. The abnormal macular findings and mild reduction in visual acuity distinguish this condition from congenital tritanopia; the normal optic disks distinguish it from autosomal dominant optic atrophy.

Adolescent↗
PubMed1989

Sawtooth contrast sensitivity: decrements have the edge.

Mirror-image sawtooth waveforms (rapid-on and rapid-off) were used to test for differences in sensitivity to incremental and decremental stimuli. Temporal contrast sensitivity functions were measured for rapid-on and rapid-off sawtooths and for sine wave stimuli (for 2-26 Hz, mean retinal illuminance of 500 td, circular target of 1.8 deg, foveal). Rapid-off sawtooths yielded higher sensitivity than rapid-on sawtooths at low and middle frequencies. This advantage for detection of decremental lights was confirmed in an experiment in which contrast sensitivity was determined for the sum of a rapid-on and a rapid-off waveform added over the full range of phase angles. Our data, based on periodic rather than pulsed stimuli, broaden and strengthen the evidence that the visual system is more sensitive to decrements than to increments in light level.

Contrast Sensitivity↗
PubMed1989

Foveal cone thresholds.

The method of constant stimuli was used to estimate the psychometric functions for detection of one or two flashes when two light pulses were presented. The test stimulus consisted of two simultaneous 0.5 msec, 1' pulses separated by 17'. Observers reported seeing 0, 1 or 2 flashes. A computer-controlled direct-view apparatus allowed sampling of slightly different foveal locations on each trial. The data were analyzed assuming a binomial probability for sampling of L and M cones and Poisson distributed quantal fluctuation. Under these assumptions, the measurements imply that detection requires a minimum of 5-7 quanta absorbed per cone, and that the effective number of cones illuminated by the 1', 0.5 msec pulse is two. The estimated L/M cone ratio was 1.6 for one observer and 4.0 for the other; each observer's ratio was in general agreement with the value estimated independently by heterochromatic flicker photometry.

Adult↗
PubMed1989

Heterochromatic modulation photometry.

Heterochromatic modulation photometry is a method for obtaining equiluminance for a pair of heterochromatic lights presented in temporal alternation. A series of fixed standard luminance/test luminance ratios are presented, and at each ratio the modulation depth of the pair is reduced in tandem until the observer reports that flicker disappears. The data can be described by a luminance contrast template that appears V shaped when plotted on log-log coordinates. In the fitting of individual data, a free vertical scaling factor reflects the observer's sensitivity to luminance modulation and a free horizontal scaling factor reflects the observer's similarity in spectral sensitivity to the CIE standard observer. Data for red/green flicker photometric matches demonstrate the technique. Heterochromatic modulation photometry offers several advantages over flicker photometry: (1) a single fixed perceptual transition occurs on each trial series, namely, the transition from flicker to steady, and (2) luminance matches can be obtained at fixed frequencies at a number of luminance levels. The same procedure can be applied to the measurement of the minimally distinct border and to the identification of tritan pairs (stimulus pairs that differ only in their stimulation of short-wavelength-sensitive cones).

Adult↗
PubMed1989

Retinal densitometry in acute posterior multifocal placoid pigment epitheliopathy.

Cone photopigment kinetics were investigated by retinal densitometry in six patients with acute posterior multifocal placoid pigment epitheliopathy (APMPPE). Our goal was to document the course of cone impairment during a period of at least 12 months following the onset of the disease process. During the active stage, we found that the amount of pigment measurable by densitometry (the density difference) was reduced and that the time constant of pigment regeneration was unmeasurable. Following resolution of the fundus lesions, the densitometric parameters gradually improved in eight of ten eyes. In patients who maintained foveal fixation (five eyes), the density difference and the time constant of pigment regeneration improved simultaneously, though with individual differences in time course. Photopigment kinetics returned to normal in all these patients but the density difference remained lower than normal in four of the five eyes. In patients with parafoveal fixation (five eyes), photopigment kinetics were slow, possibly reflecting mixed cone and rod contributions. With time, the density difference improved to a level comparable to that measured in normal observers at a similar retinal location, but the photopigment regeneration time constant remained abnormal or unmeasurable. Recovery was variable, with one eye changing from parafoveal to foveal fixation, and no improvement noted in two eyes with extreme parafoveal fixation. Our findings demonstrate a large interpatient variation associated with APMPPE.

Color Perception↗
PubMed1989

The effect of background luminance on cone sensitivity functions.

Implementations of the Wald-Marré technique have employed fixed luminance backgrounds to isolate cone sensitivity mechanisms. We evaluated the effect of varying the adaptation level on the relative isolation of the different cone types. For MWS and LWS cone isolation we used a 15 Hz flickering test light to isolate the achromatic channel, and we modelled the resulting spectral sensitivity functions as a linear sum of LWS and MWS input. We found only mild improvement in relative cone isolation with increasing adaptation level. The LWS and MWS cone mechanisms showed decreasing sensitivity with adaptation level and reached limiting Weber behavior above 1000 Td. SWS cones were isolated with a 2.4 Hz flickering light. SWS cone isolation improved with adapting level, reaching a plateau above 1000 Td. The SWS cone mechanism showed decreasing sensitivity with adaptation level but did not reach a limiting Weber region. Our data indicate that the use of fixed high adaptation levels has different effects on the cone mechanisms. Absolute sensitivity loss for LWS or MWS mechanism will not be revealed. LWS and MWS thresholds will appear normal unless there is an adaptation abnormality. On the other hand, the SWS cone thresholds would be sensitive to both absolute and increment sensitivity loss. More than one adaptation condition is needed to separate different types of sensitivity loss characteristic of eye disease.

Adaptation, Physiological↗