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Biomedical subjects

J Pokorny

Publications and source records attributed to J Pokorny.

At least 19 recordsLinked to original sources
PubMed2004

Axosomatic synapses on granule cells are preserved in human non-infiltrating tumour or lesion-related and mesial temporal sclerotic epilepsy, but markedly reduced in tumour-infiltrated dentate gyrus with or without epilepsy.

Granule cells of the human hippocampal dentate gyrus were examined. In controls, granule cells displayed somatic spines and cell nuclei with small infoldings. In addition, the cytoplasm of human granule cells always displayed lipofuscin. Subsurface cisterns of endoplasmic reticulum were frequently observed in the human granule cells. Two types of axosomatic synapses were found; most frequently symmetric and less frequently asymmetric. Many of the axosomatic synapses were isolated by glial processes in tumour or lesion-related epileptic patients, but the ultrastructural characteristics of granule cells were not different from those of the control patients. Large bundles of reactive astroglial fibres appeared regularly in all layers of the dentate gyrus. In tumour infiltrated hippocampi, glial processes dominated the neuropil and the number of perisomatic synapses was markedly reduced. Reduction in the number of perisomatic synapses did not correlate with severity and duration of seizures but did correlate with the malignancy of the tumour. It is suggested that reduction of perisomatic inhibition may not be a characteristic of granule cells in the epileptic human dentate gyrus.

Adolescent↗
PubMed2001

Characterization and use of a digital light projector for vision research.

For creating stimuli in the laboratory, digital light projection (DLP) technology has the potential to overcome the low output luminance, lack of pixel independence, and limited chromaticity gamut of the cathode ray tube (CRT). We built a DLP-based stimulator for projecting patterns on the in vitro primate retina. The DLP produces high light levels and has good contrast. Spatial performance was similar to that of a CRT. Temporal performance was limited by the refresh rate (63 Hz). The chromatic gamut was modestly larger than that of a CRT although the primary spectra varied to a small degree with light output and numerical aperture.

Analog-Digital Conversion↗
PubMed2001

Contrast discrimination deficits in retinitis pigmentosa are greater for stimuli that favor the magnocellular pathway.

Luminance contrast discrimination was measured in 14 patients with retinitis pigmentosa (RP) and 14 control observers with normal vision, using steady-pedestal and pulsed-pedestal paradigms [Pokorny, J., & Smith, V. C. (1997). Psychophysical signatures associated with magnocellular and parvocellular pathway contrast gain. Journal of the Optical Society of America A, 14, 2477-2486] to bias performance toward the magnocellular (MC) or parvocellular (PC) pathway, respectively. The aim was to determine the relative effects of retinal degeneration on MC- and PC-pathway function in RP. For five of the RP patients, contrast discrimination thresholds were within normal limits for both the steady-pedestal and pulsed-pedestal paradigms. The other nine RP patients showed threshold elevations for the steady-pedestal paradigm (presumed magnocellular mediation), whereas their thresholds for the pulsed-pedestal paradigm (presumed parvocellular mediation) were within normal limits for all but the two patients who had the most extreme threshold elevations using the steady-pedestal paradigm. A control experiment on four of the RP patients, using a greater number of pedestal contrasts, verified that the patients' thresholds for the pulsed-pedestal paradigm showed the pattern expected for contrast discrimination mediated by the PC pathway. The higher threshold elevations for the steady-pedestal paradigm than for the pulsed-pedestal paradigm indicate that the retinal degeneration that occurs in RP predominantly disrupts contrast discrimination under stimulus conditions that favor the MC pathway.

Adult↗
PubMed2001

The consequences of opponent rectification: the effect of surround size and luminance on color appearance.

Smith and Pokorny (Vision Res. 36 (1996) 3087.) described conditions under which chromatic contrast induction can reveal a hiatus, a region of chromaticity space which appears neither reddish nor greenish when presented in a chromatic equiluminant surround. The current study investigated the effect of varying the size and the luminance of the inducing surround. The color appearance of test stimuli in chromatic surrounds was assessed by asymmetric color matching to a comparison display. Equiluminant (12 cd/m(2)), 1 degrees square stimuli were generated on a CRT display and presented haploscopically. Ten test fields varied in their L-cone excitation along a constant S-cone line. The chromatic surrounds were of either high (red) or low (green) L-cone excitation on a constant S-cone line. In Experiment 1, surrounds were 1.1 degrees, 1.5 degrees, 2.0 degrees, or 3.0 degrees square (surround widths of 3', 15', 30', 1 degrees ). In Experiment 2, the test and comparison surrounds were at higher (16.7 cd/m(2)) or lower (8.3 cd/m(2)) retinal illuminance than the test field. Contrast induction reached an asymptote for surround widths of 30' or larger. The amount of induction decreased for the surround widths of 15' and 3'. The hiatus was present for the larger surrounds and decreased as surround size decreased. The use of a higher or lower surround luminance did not affect the magnitude of induction or the size of the hiatus.

Adult↗
PubMed2001

The role of spatial frequency in color induction.

Color induction was measured for test and inducing chromaticities presented in spatial square-wave alternation, with spatial frequencies of 0.7, 4.0, 6.0 and 9.0 cpd. Observers matched the test chromaticities to a rectangular matching field using haploscopic presentation. Data were collected and analyzed within the framework of a cone chromaticity space, allowing analysis of spatial frequency effects on post-receptoral spectral opponent pathways. Assimilation, a shift of chromaticity toward the inducing chromaticity, was found at the highest spatial frequency (9.0 cpd). Contrast, a shift of chromaticity away from the inducing chromaticity, occurred at the lowest spatial frequency (0.7 cpd). The spatial frequency at the transition point from assimilation to contrast was near 4 cpd, independent of the cone axis. Assimilation was unaffected by the presence of a neutral surround and could be described by a spread light model. Contrast was reduced in the presence of a neutral surround. The data suggested that retinal contrast signals are important determinants in the perception of chromatic contrast.

Adult↗
PubMed2001

Pulse and steady-pedestal contrast discrimination: effect of spatial parameters.

The goal of this study was to establish the spatial summation properties associated with inferred PC- and MC-pathway mediated psychophysical contrast discrimination. Previous work has established two paradigms that reveal characteristic signatures of these pathways. In the pulse paradigm, a four-square array was pulsed briefly, on a constant background. In the steady-pedestal paradigm, the stimulus array was presented continuously as a steady-pedestal within a constant surround. In both paradigms, one square differed from the others, giving the observer a forced choice spatial discrimination task. Area summation functions derived for the pulse paradigm decreased with area, with a slope of -0.25 on a log-log axis. Area summation functions derived for the steady-pedestal paradigm decreased as a power function of area, approaching an asymptote above one square degree. The latter are consistent with the classical data of threshold spatial summation.

Adult↗
PubMed2001

Primate horizontal cell dynamics: an analysis of sensitivity regulation in the outer retina.

The human cone visual system maintains sensitivity over a broad range of illumination, from below 1 troland to 1,000,000 trolands. While the cone photoreceptors themselves are an important locus for sensitivity regulation-or light adaptation-the degree to which they contribute in primates remains unclear. To determine the range of sensitivity regulation in the outer retina, the temporal dynamics, neural gain control, and response range compression were measured in second-order neurons, the H1 horizontal cells, of the macaque retina. Situated at the first synapse in the retina, H1 cells receive input from a large population of cones. Lee et al. have previously shown that sensitivity regulation in H1 cells is both cone type-specific and spatially restricted. The sensitivity regulation seen in H1 cells at moderate illuminances thus takes place before the summation of cone signals in these cells, and the data establish the H1 cell as a convenient locus for analyzing cone signals. In the present study, cone-driven responses of primate H1 cells to temporally modulated sine-wave stimuli and to increment pulses were measured at steady levels of 1-1,000 trolands. The H1 cell gave a modulated response to sine-wave stimuli and hyperpolarized to increment pulses with overshoots at stimulus onset and offset. The temporal amplitude sensitivity function was primarily low-pass in shape, with a small degree of low-frequency roll off and a resonance shoulder near 40 Hz. A model incorporating a cascade of first-order filters together with an underdamped second-order filter could describe both temporal sinusoidal and pulse hyperpolarizations. Amplitude sensitivity was estimated from both pulse and sine-wave data as a function of the steady adaptation level. Sensitivity at low light levels (1 troland) showed a slowing in temporal dynamics, indicating time-dependent sensitivity regulation. Sensitivity was reduced at light levels above approximately 10 trolands, reflecting both response range compression and neural gain control. Thus the outer retina is a major locus for sensitivity regulation in primates.

Animals↗
PubMed2001

Brightness induction from rods.

Rod modulation of an annular surround can produce brightness contrast in a test field centered at 100 from the fovea. In our research, stimuli originated from a colorimeter that provided 4 primaries in both the circular test and the annular surround fields, and allowed independent modulation of the rods and each of the short (S)-, middle (M)-, and long (L)-wavelength-sensitive cone types. The chromaticity was set so fields had the same appearance as the equal energy spectrum. At 1 photopic troland (td), rod-induced modulation in the test field could be cancelled by either a rod- or a cone-nulling modulation added to the test field. The best cone nulling of rod induction showed residual flicker. Nulling was more effective, though still imperfect, with a cone-nulling stimulus of higher S-cone modulation contrast. Rod induction with square-wave, on-pulse, and off-pulse temporal profiles was closely similar. At higher light levels, 10 and 100 td, rod contrast could not be nulled by rod or cone modulation. The failure to achieve nulls may have been caused by either or both of the following hypotheses: (1) there is a mismatch between the rod and cone temporal waveforms; (2) there is strong rod input to the magnocellular pathway, but negligible rod input to the parvocellular pathway, as shown by single-unit electrophysiological data.

Color Perception↗
PubMed2001

Rod-cone interactions assessed in inferred magnocellular and parvocellular postreceptoral pathways.

Interactions between receptor-isolating rod and long (L)- or middle (M)-wavelength-sensitive cone modulations at 2 Hz and 10 Hz were analyzed in terms of underlying inferred magnocellular (MC) and parvocellular (PC) postreceptoral pathways. Stimuli originated from a colorimeter with 4 primaries in both the center and surround fields. The first experiment employed a phase paradigm in which the thresholds for mixed rod and cone modulations were measured as a function of relative phase. The amplitudes of the rod and cone modulations, equated in threshold units, were varied in tandem. In the second experiment, thresholds for mixed rod and cone modulations were measured as a function of the ratio of the rod and cone modulation amplitudes for 2 fixed phase offsets. Both experiments yielded similar interpretations of rod and L- (or M-) cone interactions. At 1 and 10 troland (td), rod and L- (or M-) cone interactions varied depending on the postreceptoral pathways underlying the detection. When cone thresholds were mediated by the inferred MC pathway, rod and cone thresholds showed almost linear summation. When cone thresholds were mediated by the inferred PC pathway, rod and cone thresholds showed probability summation. Assuming that signals within the same pathway follow linear summation, and signals traveling in different pathways follow probability summation, we concluded that the rod thresholds were mediated by the inferred MC pathway for both the 2-Hz and 10-Hz conditions.

Contrast Sensitivity↗
PubMed2000

Technical advance. Double-stranded RNA interferes with gene function at the single-cell level in cereals.

Double-stranded RNA (dsRNA) has been shown to specifically interfere with gene function in several organisms including tobacco and the model plant Arabidopsis. Here, we report on rapid and sequence-specific interference of dsRNA with gene function in cereals. Delivery of cognate dsRNA into single epidermal cells of maize, barley or wheat by particle bombardment interfered with the function of co-bombarded UidA (GUS) and TaGLP2a:GFP reporter genes. Cognate dsRNA was also found to specifically interfere with the function of the endogenous genes A1 and Ant18 encoding dihydroflavonol-4-reductase in maize and barley, respectively. Dihydroflavonol-4-reductase is an essential enzyme of the anthocyanin biosynthetic pathway in maize and barley. This pathway can be induced by transient expression of the C1- and b-Peru genes that encode transcription factors. In the presence of dsRNA corresponding to the dihydroflavonol-4-reductase gene, C1- and b-Peru-dependent, cell-autonomous accumulation of red anthocyanin pigments in bombarded cells of maize and barley was reduced. dsRNA was also demonstrated to negatively interfere with Mlo, which encodes a negative regulator of race non-specific resistance to the powdery mildew fungus in barley. In the presence of Mlo dsRNA, transformed cells became more resistant, thereby phenocopying plants that carry a heritable loss-of function mlo resistance allele. The results suggest that direct delivery of dsRNA to cereals leads to a rapid and sequence-specific interference with gene function at the single-cell level.

Alcohol Oxidoreductases↗
PubMed1999

Horizontal cells reveal cone type-specific adaptation in primate retina.

The human cone visual system maintains contrast sensitivity over a wide range of ambient illumination, a property known as light adaptation. The first stage in light adaptation is believed to take place at the first neural step in vision, within the long, middle, and short wavelength sensitive cone photoreceptors. To determine the properties of adaptation in primate outer retina, we measured cone signals in second-order interneurons, the horizontal cells, of the macaque monkey. Horizontal cells provide a unique site for studying early adaptational mechanisms; they are but one synapse away from the photoreceptors, and each horizontal cell receives excitatory inputs from many cones. Light adaptation occurred over the entire range of light levels evaluated, a luminance range of 15-1,850 trolands. Adaptation was demonstrated to be independent in each cone type and to be spatially restricted. Thus, in primates, a major source of sensitivity regulation occurs before summation of cone signals in the horizontal cell.

Adaptation, Physiological↗
PubMed1999

Reconstructive surgery in children after meningococcal purpura fulminans.

BACKGROUND/PURPOSE: Purpura fulminans (PF) is a serious, often life-threatening disease. As more children are surviving their disease, surgeons are presented with increasing numbers of multiple and complicated wounds as sequelae of PF. The purpose of this paper is to review the management of nine cases of PF, and present the reconstruction method in treating bilateral exposed elbow and knee joints. METHODS: All cases of pediatric patients with PF and treated by the division of plastic and reconstructive surgery between 1986 and 1998 were reviewed. RESULTS: Seven children (78%) had meningococcal PF, and one (11%) had PF after Haemophilus influenza septicemia. PF developed in one (11%) but with no growth in either blood or cerebrospinal fluid cultures. Five children (56%) required amputation procedures. Two children (22%) required knee disarticulation. Two patients (22%) had free myocutaneous flap transfers for bone coverage. One (11%) had PF involving the face. CONCLUSIONS: Meningococcal PF is a rare, often life-threatening disease generally of childhood. More children are surviving their diseases but with devastating sequelae. Successful reconstructive treatment outcome of these children requires a multidisciplinary team approach involving multiple specialties. The goal is to preserve function, maintain maximal length, and salvage limbs when possible. Flexibility and innovation are necessary in treating these multiple and complicated wounds.

Amputation, Surgical↗
PubMed1998

Red-green chromatic discrimination with variegated and homogeneous stimuli.

Chromatic discrimination thresholds were measured under conditions which yielded fine and degraded discrimination steps. Discrimination was assessed by identification of the location of one of four homogeneous equiluminant stimuli arranged in a square or with pseudoisochromatic (PIC) figures using the stimulus design of Regan, Reffin and Mollon (Vis Res 1994; 34: 1279-1299). Stimuli were presented on CRT monitors and specified in units of cone trolands. They were viewed within a surround metameric to the equal energy spectrum. L troland threshold versus retinal illuminance (TVR) functions were measured by four-alternative spatial forced-choice staircase procedures for (1) a four 1 x 1 degree equiluminant stimuli arranged in a square and (2) 'C' shaped pseudoisochromatic figures in which the observer had to identify the gap location. The 'C' was constructed of spatially discrete patches of varying size and luminance to ensure that the observer's responses depended on chromatic signals. The TVR functions appeared V-shaped and were similar for the two paradigms. The minimum occurred near the L excitation of equal-energy white. The PIC stimuli yielded poorer discrimination with the TVR function being displaced by approximately 0.5 log unit. Discrimination for stimuli degraded by luminance and spatial noise presented within an achromatic appearing surround is sharpest near the chromaticity metameric to the equal energy spectrum.

Adult↗
PubMed1998

Color vision in two observers with highly biased LWS/MWS cone ratios.

Two sisters, heterozygous carriers for congenital X-linked protanopia, were diagnosed as normal trichromats by the Rayleigh match on the anomaloscope. The heterozygous state was established by molecular analysis of their visual pigment genes. The normal color match establishes that the spectral sensitivities of their long-wavelength-sensitive (LWS) and middle-wavelength-sensitive (MWS) cone visual photopigments are within normal variability. Their FM 100-hue test error scores were low, demonstrating superior chromatic discrimination. Heterochromatic flicker photometric (HEP) spectral sensitivities were like those of protanopes. The estimated LWS/MWS cone ratios from the HFP data were 0.09/1 and 0.03/1, compared with ratios in the range of 0.6/1 to 10/1 for typical normal trichromats. Measurements of chromatic grating acuity on chromatically selective backgrounds were performed to study the cone mosaic. The data were consistent with a sparsity of LWS cones. Both protan carriers showed normal spectral sensitivities for all three cone types under cone isolating chromatic adaptation and normal three-peaked curves for increment thresholds on a white pedestal. Hue estimation, run on one carrier was normal. The equilibrium yellow locus was measured in the other carrier and was in the range of normal trichromats. The data indicate that normal color vision can occur even when the LWS/MWS cone ratio is quite abnormal.

Adaptation, Ocular↗
PubMed1998

Color appearance: neutral surrounds and spatial contrast.

The experimental data in this paper show that chromatic bars presented in alternation with equiluminant neutral-appearing bars are seen as more saturated than the same chromaticity presented as a uniform rectangle. This effect was diminished but not eliminated when test and match stimuli were presented within a slightly dimmer neutral surround. The test stimulus was a 2 x 5 degrees rectangle with a 0.7 cpd square wave grating composed of alternating equiluminant chromatic test bars and neutral bars. Asymmetric matching was used to match the test bar appearance to a uniform 2 x 5 degrees comparison rectangle. Test and comparison stimuli were presented to separate eyes in a haploscope and appeared flanking a central fixation target. Data were collected with test and inducing stimuli on the cardinal axes of color space. Test bars separated by neutral bars appeared more saturated than the comparison rectangle for both the l- and s-axis directions. Manipulation of excitation on one cardinal axis did not affect the appearance matches made for the other cardinal axis.

Adult↗
PubMed1997

Rod inputs to macaque ganglion cells.

The strength of rod inputs to ganglion cells was assessed in the macaque retina at retinal positions within 3-15 deg eccentricity. The experimental paradigm used temporally modulated heterochromatic lights whose relative phase was varied. This paradigm provided a sensitive test to detect rod input. In parvocellular (PC) pathway cells, the gain of the cone-driven signal decreased with decrease in luminance. At 2 td a weak rod response, of a few impulses per second for 100% rod modulation, was revealed in about 60% of cells. For blue-on cells, the cone-driven response also decreased with retinal illuminance, but no rod response could be found. In magnocellular (MC) pathway cells, rod input was much more apparent. Responses became rod dominated at and below 20 td; we cannot exclude rod intrusion at higher retinal illuminances. Responsivity was maintained even at low retinal illuminances. Temporal-frequency dependent rod-cone interactions were observed in MC-pathway cells. Rod responses were of longer latency than cone responses, but there was no evidence of any difference in rod latency between parvocellular and magnocellular pathways.

Animals↗