Latency of the mfVEP to diagnose glaucoma?
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Diagnostic Techniques, Ophthalmological↗
Biomedical subjects
M Bach
Publications and source records attributed to M Bach.
Large-scale homogeneous molecular templates for femtosecond time-resolved studies of the guest-host interaction.
Self-assembled monolayer films based on iodobenzoyloxy-functionalized resorc[4]arenes were prepared on gold substrates to serve as model systems for future time-resolved studies of molecular recognition, a mechanism of outstanding importance in bioorganic systems. The film properties were tested using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and imaging ellipsometry. An apparatus for time-resolved electron spectroscopy utilizing femtosecond soft X-ray pulses is capable of detecting iodine core-level photolines and the photoinduced dissociation after ultraviolet illumination. The developed technique holds promise for tracking the temporal evolution of chemical shifts of atomic markers as local probes for the dynamics of the guest-host interaction.
Benzoates↗
Groundwater-borne nitrate intakes into surface waters in Germany.
The nitrogen loads entering the surface waters in Germany via the groundwater path were quantified. For this purpose, the results of a nitrogen balance model (Bach et al., 2000), which considers the most important N-inputs to the soil (manure, inorganic fertiliser, atmospheric deposition) and N-removals from the soil through crop harvest, were combined with the groundwater residence time/denitrification model WEKU (Kunkel and Wendland, 1997; Wendland et al., 2001). The modelled groundwater-borne nitrogen inputs into surface waters were validated using results from the MONERIS model (Behrendt et al., 2000) concerning riverine nitrogen retention, nitrogen inputs from point sources as well as nitrogen inputs through direct run-off (drainage etc.). In the vicinity of surface waters and in solid rock areas, the groundwater borne nitrogen inputs into surface waters are considerably high compared to the inputs into the aquifer due to predominantly unfavourable de-nitrification conditions and short residence times of groundwater. In the North German lowlands, however, the groundwater-borne nitrate inputs into surface waters are considerably low compared to the inputs into the aquifer. There, the residence time of groundwater in the aquifer is high and the groundwater is predominantly oxygen free and contains pyrite and/or organic carbon compounds, allowing a halving of the nitrate loads in the groundwater within a period of 1 to 4 years (see Wendland and Kunkel, 1999).
Carbon↗
DRIPS--a decision support system estimating the quantity of diffuse pesticide pollution in German river basins.
The GIS based decision support system DRIPS--Drainage Spraydrift and Runoff Input of Pesticides in Surface Waters--has been developed to estimate the predicted environmental concentration (PECsw) of pesticides in surface waters resulting from diffuse sources. PECsw can be calculated on a catchment scale by quantifying the expected mean daily inputs of pesticides via surface runoff, tile drainage and spraydrift for various types of river basins characterized by their daily discharges. DRIPS is fitted with a Graphical User Interface (GUI) to provide easy access to the model algorithms. Model parameters like dose rate, DT50, Koc and date of pesticides application, etc. can be modified by the user in order to generate customized scenarios predicting PECsw for a choice of field crops, orchards and vineyards. Results are available as grid cell maps for the territory of Germany with high temporal and spatial resolution featuring distinct PECsw values for approximately 400 catchments.
Algorithms↗
[Comparison between dissociated and associated heterophoria].
BACKGROUND: Since Ogle (1949) dissociated heterophoria is distinguished from associated heterophoria. Dissociated heterophoria is defined as a deviation from the orthovergence position that occurs when no fusional contours are provided. Associated heterophoria is a deviation that occurs under prism correction of fixation disparity, i.e. under a viewing condition in which fusional contours are absent only in the centre of the visual field. Both dissociated and associated heterophoria have been used as indications for prismatic correction. It has been controversially discussed to what extent dissociated and associated heterophoria are comparable. METHODS: A total of 43 subjects were investigated using the white Maddox rod test for horizontal dissociated heterophoria and the cross test of Hans-Joachim Haase (a test similar to the fixation disparity test described by Ogle 1949) for horizontal associated heterophoria. In both tests subjects were asked to continually correct any deviation using Herschel prisms. Readings were taken after 1 min. RESULTS: The correlation between the results obtained with the white Maddox rod test and the cross test was r = 0.89 (p < 0.0001) and the slope of the regression line was 0.92 (95% confidence interval from 0.77 to 1.07). One subject was unable to reach a match in the cross test because she had an "obligate" fixation disparity due to a staphyloma posticum. CONCLUSIONS: Our study revealed a high correlation between dissociated and associated heterophoria. This result appears plausible since the feedback loop for fusion is disconnected in both cases, although in a different manner: When dissociated heterophoria is measured, fusionable contours are absent. When associated heterophoria is measured, any disparity of peripheral contours that could serve as an error signal is nullified by instantaneous prism correction. The high concordance of dissociated and associated heterophoria suggests that both measures are of equal value as an indication for prismatic spectacle correction. In rare cases of an "obligate" fixation disparity, however, associated heterophoria cannot be determined so that dissociated heterophoria remains as the only indication.
Adult↗
Coupling of neural activity and BOLD fMRI response: new insights by combination of fMRI and VEP experiments in transition from single events to continuous stimulation.
Functional magnetic resonance imaging (fMRI) measures the correlation between the fMRI response and stimulus properties. A linear relationship between neural activity and fMRI response is commonly assumed. However, the response to repetitive stimulation cannot be explained by a simple superposition of single-event responses. This might be due to neural adaptation or the hemodynamic changes underlying the fMRI BOLD response. To assess the influence of adaptation, the BOLD responses and visual evoked potentials (VEPs) to identical stimuli were recorded. To achieve different adaptation levels, 2-s stimulus epochs alternated with different interstimulus intervals (ISI = 0.0, 0.4, 0.8, 2.0, and 12 s) were presented. Neural adaptation during the checkerboard reversal paradigm used for fMRI measurements is demonstrated. Even if the measured VEP amplitude is used as the weighting function for a linear model, the measured BOLD fMRI signal time-course is not adequately predicted.
Adult↗
Steady-state electroretinograms and pattern electroretinograms in pigs.
BACKGROUND: Electroretinograms (ERG) or pattern-electroretinograms (PERG) could be valuable for the quantification of potential damage to the pig retina by experimental erbium:YAG laser treatment. We therefore performed a normative study of ERGs and PERGs in pigs. METHODS: We recorded ERGs and PERGs under general anaesthesia in two experiments. In experiment 1 we examined eight eyes from six pigs of 20-25 kg body weight; in experiment 2 we examined four eyes from four pigs of 40-45 kg body weight. We used flash and checkerboard stimuli. In experiment 1, the stimulus parameters were mean luminance 48.3 cd/m2 for checkerboard stimuli, 96.6 cd/m2 for ERG, check sizes of 4 degrees, 8 degrees, and 16 degrees, temporal frequencies were 16 Hz for ERG and 8 rev/s for PERG. Three measurements were repeated after two weeks. Stimulus parameters for experiment 2 were luminance 175 (350) cd/m2, check sizes 1.6 degrees, 3.2 degrees, 6.7 degrees, and 16 degrees, temporal frequencies 6.3 Hz for ERG and 8 rev/s for PERG. Recordings were subjected to Fourier analysis. RESULTS: In experiment 1 the mean ERG amplitude was 1.02 +/- 0.89 microV with a coefficient of variation of 42% for repeat sessions. The mean PERG amplitudes were 0.53 +/- 0.25 microV for 16 degrees checks, 0.36 +/- 0.21 microV for 8 degrees, and 0.25 +/- 0.17 microV for 4 degrees. The mean coefficient of variation between two measurements was 103% for 16 degrees checks, 24% for 8 degrees, and 116% for 4 degrees. In experiment 2 the mean ERG amplitude was 9.72 +/- 3.96 microV. The mean PERG amplitudes were 0.77 +/- 0.50 microV for 16 degrees checks, 0.09 +/- 0.16 microV for 6.7 degrees, 0.07 +/- 0.13 microV for 3.2 degrees, and 0.08 +/- 0.09 microV for 1.6 degrees. CONCLUSIONS: It was possible to record ERGs and PERGs in pigs. However, the ERG amplitudes were small; PERG amplitudes were even smaller in both groups and cannot be reliably recorded. A problem for both ERG and PERG was the high intra-individual and interindividual variability. Therefore, only very extensive damage to the retina by vitrectomy or Er:YAG laser treatment might lead to a significant change in the ERG or PERG amplitudes.
Animals↗
[Multifocal electroretinography (mfERG)].
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Algorithms↗
Heterophoria measured with white, dark-grey and dark-red Maddox rods.
BACKGROUND: Heterophoria is defined as a deviation from orthovergence position when no fusional contours are provided. It has been suggested that darkening one eye effects the phoric angle between the two eyes. We tested this assumption by comparing the phoric angles with white, dark-grey and dark-red Maddox rods. METHODS: In a first group of 14 subjects horizontal heterophoria was measured with white, dark-grey and dark-red Maddox rods. In a second group of 43 different subjects horizontal heterophoria was measured with white and dark-red Maddox rods. Subjects were asked to continually correct any perceived deviation, using Herschel prisms. Readings were taken after 1 min. RESULTS: Group 1 (n=14): No significant difference between white, dark-grey and dark-red Maddox rods (Friedmann test). Correlation between white and dark-grey Maddox rods r=0.97 (P<0.0001), slope of regression line 0.92 (95% confidence interval 0.78-1.06). Correlation between white and dark-red Maddox rods r=0.94 (P<0.0001), slope of regression line 0.67 (95% confidence interval 0.53-0.82). Group 2 (n=43): No significant difference between white and dark-red Maddox rods (Wilcoxon signed rank tests). Correlation r=0.96 (P<0.0001), slope of regression line 0.95 (95% confidence interval 0.87-1.03). CONCLUSION: Our study revealed no significant difference and a high correlation among the phoric angles obtained with white, dark-grey and dark-red Maddox rods. The close match suggests that darkening one eye does not effect the phoric angle between the two eyes.
Diagnostic Techniques, Ophthalmological↗
The Freiburg Stereoacuity Test: automatic measurement of stereo threshold.
BACKGROUND: There is a need for a stereotest with the following properties: (1) Natural viewing conditions, i.e. stimulus contours visible for each eye alone, but no or hardly any cue for monocular detection, and (2) suitability for threshold determination over a wide range of disparities. To comply with these requirements, we developed the Freiburg Stereoacuity Test. METHOD: The stimulus configuration is shown on a visual display unit (VDU) using phase-difference haploscopy with ferromagnetic liquid crystal shutters. The stereo target consists of a vertical bar that can be presented "in front of' or "behind" a frame. The sizes of the bar and the frame are kept constant relative to the stereo disparity. Anti-aliasing allows for disparities finer than the pixel raster. To mask monocular cues the bar is displaced randomly to the right or left. The stereo threshold was determined in two observers with normal eyes, using first the method of constant stimuli and then the best PEST. Both procedures were repeated with observers wearing scatter transparencies that reduced their visual acuity to about 1/10. In addition, the two observers with insight into the test design and two strabismic patients performed the best PEST procedure with one eye only. RESULTS: With constant stimuli both observers achieved a stereoacuity of 2.6 arcsec and 3.1 arcsec, respectively, taking a hit rate of 75% as the threshold. The best PEST revealed a stereoacuity of 2.5 arcsec and 3.0 arcsec, respectively. The scatter transparencies raised the threshold to 261 and 257, respectively. With one eye only, the two observers with insight into the test design exploited the subtle position cue and reached a coarse pseudostereopsis. The two strabismic patients did not utilise the position cue. CONCLUSION: The Freiburg Stereoacuity Test allows determination of stereoacuity over a wide range of disparities (1-1000 arcsec). Although the stimuli can be seen with each eye alone, monocular depth cues are sufficiently masked. The Freiburg Stereoacuity Test is available at http://www.ukl.uni-freiburg.de/aug/bach/fst/.
Adolescent↗
Directional tuning of human motion adaptation as reflected by the motion VEP.
Motion onset evoked visual potentials are dominated by a negativity (N2) at occipital electrodes and a positivity (P2) at the vertex. The degree of true motion processing reflected by N2 and P2 was estimated from the direction specificity of motion adaptation. Adapting stimuli moved to the right and test stimuli (random dot patterns of 26 degrees diameter; 10% contrast; 10.5 degrees /s velocity) moved in one of eight directions, which differed by 45 degrees. VEPs were recorded from occipito/temporal and central sites in eight subjects. Two adaptation effects were observed for N2 (P<0.01): a global amplitude reduction by 47% and a direction-specific reduction by a further 28%. For P2, only the global effect (54%; P<0.01) was observed. The global adaptation effect could also be induced by pattern reversal and pattern-onset adaptation, i.e. stimuli containing ambiguous or very little motion energy, respectively. We conclude that at least 28% of the N2 amplitude reflects the activity of direction-specific elements, whereas P2 does not at all.
Adaptation, Physiological↗
Adaptation dynamics in pattern-reversal visual evoked potentials.
Recording a VEP usually involves prolonged repetitions of the stimulus, but the influence of adaptation is rarely discussed in this context. Two experiments were performed. In Experiment 1 the time course of the response amplitude during steady-state stimulation was assessed. During the first seconds of stimulation we found an increase in amplitude, followed by a continuous exponential decline. This confirmed earlier results. There is considerable inter-subject variability concerning all aspects of the time course in our 19 subjects. Experiment 2 used two types of transient pattern reversal stimuli: one regular stimulus as used in standard clinical applications and one with a pause in between each reversal. N1 and P1 amplitudes did not show significant differential effects. N2 amplitude was reduced by 73% in the standard condition whereas P1 peak time increased slightly but significantly (3.2 ms).
Adaptation, Ocular↗
Post-transcriptional regulation of rat carnitine octanoyltransferase.
Carnitine octanoyltransferase (COT) produces three different transcripts in rat through cis- and trans-splicing reactions, which can lead to the synthesis of two proteins. The occurrence of the three COT transcripts in rat has been found in all tissues examined and does not depend on sex, fat feeding, peroxisome proliferators or hyperinsulinaemia. Rat COT exon 2 contains a putative exonic splicing enhancer (ESE) sequence. Mutation of this ESE (GAAGAAG) to AAAAAAA decreased trans-splicing in vitro, from which it is deduced that this ESE sequence is partly responsible for the formation of the three transcripts. The protein encoded by cis-spliced mRNA of rat COT is inhibited by malonyl-CoA and etomoxir. cDNA species encoding full-length wild-type COT and one double mutant COT were expressed in Saccharomyces cerevisiae. The recombinant enzymes showed full activity towards both substrates, carnitine and decanoyl-CoA. The activity of the doubly mutated H131A/H340A enzyme was similar to that of the rat peroxisomal enzyme but was completely insensitive to malonyl-CoA and etomoxir. These results indicate that the histidine residues His-131 and His-340 are the sites responsible for the interaction of these two inhibitors, which inhibit COT by interacting with the same sites.
Acyl Coenzyme A↗
Optimized linker sequences for the expression of monomeric and dimeric bispecific single-chain diabodies.
Bispecific single-chain diabodies (scDb) consist of the variable heavy and light chain domains of two antibodies connected by three linkers. The structure of an scDb in the V(H)-V(L) orientation is V(H)A-linkerA-V(L)B-linkerM-V(H)B-linkerB-V(L)A, with linkers A and B routinely chosen to be 5-6 residues and linker M 15-20 residues. Here, we applied display of scDb on filamentous phage to analyse the composition of optimal linker sequences. The three linkers were randomized in length and sequence using degenerated triplets coding for only six hydrophilic or aliphatic amino acids (Thr, Ser, Asp, Asn, Gly, Ala). Antigen-binding clones were then isolated by one to two rounds of selection on the two different antigens recognized by the bispecific scDb. Using an scDb directed against carcinoembryonic antigen (CEA) and beta-galactosidase (Gal), we found that monomeric scDb had a preferred length of 15 or more amino acid residues for the middle linker M and of 3-6 residues for the linkers A and B. No obvious bias towards a preferred linker sequence was observed. Reduction of the middle linker below 13 residues led to the formation of dimeric scDb, which most likely results from interchain pairing between all the V(H) and V(L) domains. Dimeric scDb were also formed by fragments possessing a long linker M and linkers A and B of 0 or 1 residue. We assume that these dimeric scDb are formed by intrachain pairing of the central variable domains and interchain pairing of the flanking variable domains. Thus, the latter molecules represent a novel format of bispecific and tetravalent molecules. The described strategy allows for the isolation of both optimized and minimal linker sequences for the assembly of monomeric or dimeric single-chain diabodies.
Antibodies, Bispecific↗
Input pathways and river load of pesticides in Germany--a national scale modeling assessment.
A model has been developed which estimates the magnitude and the spatial distribution of pesticide losses from diffuse sources (surface run-off, tile drains and spraydrift) into surface waters for the entire area of Germany. The cumulative annual losses of 42 active ingredients applied to 11 field crops, orchards and vineyards are calculated for river basins in Germany based on grid maps with a resolution of 1 x 1 km2. The model validation showed a sufficient degree of accuracy of the model results compared to measured pesticide loads in 13 small catchments. According to the model results the pesticide input from diffuse sources into surface waters amounted to 13.8 t in 1994 aggregated for the entire area of Germany. Input via surface runoff contributed 9.1 t while tile drainage was 1.4 t and spraydrift 3.4 t respectively. Alongside the model calculations empirical data of the pesticide load of rivers in Germany are presented. A comparison of the measured river loads with the modeled inputs from non-point sources leads to the conclusion that in most regions of Germany the largest portion of the load is due to the input from farm effluents.
Agriculture↗
Contrast adaptation in human retina and cortex.
PURPOSE: Although cortical contrast adaptation has been extensively studied with both psychophysical and electrophysiological techniques, little is known about retinal contrast adaptation in humans. METHODS: Retinal and cortical long-term contrast adaptation was assessed with simultaneous measurement of pattern electroretinogram (PERG) and cortical visual evoked potentials (VEPs). This study involved three approaches: sampling of the contrast transfer function from 2.7% to 98% with adaptation to high (98%) and low (7.3%) contrasts, linearity of adaptation effects, and transfer of contrast adaptation between parallel and orthogonal grating orientations. RESULTS: Contrast adaptation affected retinal and cortical recordings quite differently. The VEP showed a sigmoid contrast transfer function, which was shifted toward higher contrasts (by a factor of 1.9), whereas amplitudes at higher test contrasts were enhanced to 127%. The PERG decreased in amplitude to approximately 90%, and the latency was significantly reduced by 4 to 6 msec (P < 0.05). All measured effects were linear with adaptation contrast. Orientation played no role in the PERG results, whereas the VEP was enhanced to 125% when tested parallel and to 150% when tested orthogonal to adaptation. CONCLUSIONS: VEP results confirm and extend previous findings and fit well with single-cell recordings. The PERG findings suggest that retinal contrast adaptation occurs and mainly operates in the temporal domain, comparable to rapid gain-control findings in cats and primates.
Adaptation, Ocular↗
Modeling pesticide losses from diffuse sources in Germany.
A GIS-based model estimates the losses from diffuse sources in surface waters in Germany for 42 active ingredients applied to 11 field crops, vineyards and orchards. For the following pathways of entry: tile drainage, runoff and spray drift, the calculated mean pesticide input amounts to 1490 kg/year, 9060 kg/year and 3350 kg/year, respectively, in 1994. The model results are highly sensitive to the model parameters, primarily the chemical properties of the active ingredients. The modeled water inputs were compared with measured pesticide loads in smaller catchments and large river basins to validate model results. Both datasets agree as to the order of magnitude, nevertheless due to the scale of the study the results should be addressed mainly to comparative interpretations with the focus on the proportions between different active ingredients, soil regions, climates and application periods.
Agriculture↗
[Reading speed in movement in the peripheral visual field].
BACKGROUND: Moving and distracting stimuli often occur in the peripheral visual field during visual tasks in daily life. Do they influence reading performance? METHOD: In 30 normal subjects with a normal visual acuity we investigated eye movements by electronystagmography (ENG) during silent reading of a standardized text. The 17 degrees-wide text was presented at a distance of 1 m, ENG was recorded bitemporally. Blinks were monitored via vertical ENG electrodes. For surrounding stimulation we presented a bar stimulus moving at 30 degrees/s. Three experimental conditions were compared: [1] no peripheral motion, [2] peripheral motion to the right, and [3] peripheral motion to the left. We quantitatively analyzed reading speed and blink frequency. RESULTS: Reading speed and blink frequency was nearly identical in the three experimental conditions, they differed by 1-8%. CONCLUSIONS: Although the full periphery was stimulated, and the moving stimulus was noticed by our subjects, reading was not affected by surrounding optokinetic stimulation. Attentive reading does not appear to be affected by visual motion in the periphery.
Adult↗