Determination of scotopic and photopic conventional visual acuity and hyperacuity.

PURPOSE: Visual acuity (VA) is an important determinant of visual function. Here we establish procedures and recommendations for VA testing extending beyond the classical VA and thus make them available for future studies of visual function in health and disease. Specifically, we provide reference values for photopic and scotopic conventional uncrowded visual acuity (cVA) and Vernier-hyperacuity (hVA) and assess their reproducibility and dependence on contrast polarity. METHODS: For ten observers with normal vision, we determined photopic ("p"; maximal luminance 220 cd/m2) and scotopic ("s"; maximal luminance 0.004 cd/m2; 40 min of dark adaptation) cVA and hVA, for two contrast polarities i.e. black optotypes on white background and vice versa. To assess intersession effects, two sets of measurements were obtained on different days. RESULTS: Compared to pcVA (1.32 decimal VA; - 0.12 ± 0.02 LogMAR), the phVA (14.45 decimal VA; - 1.16 ± 0.04 LogMAR) scaled (in terms of decimal visual acuity) on average with a factor 11.0, the scVA (0.12 decimal VA; 0.91 ± 0.03 LogMAR) with a factor of 0.1, and the shVA (1.47 decimal VA; - 0.17 ± 0.02 LogMAR) with a factor of 1.1. There were neither significant effects of contrast polarity (p > 0.12), nor of session (p > 0.28). CONCLUSIONS: Our approach optimises integrated photopic and scotopic cVA and hVA measurements for general use and thus encourages the integration of these important measures of scotopic visual function in future studies. The absence of strong intersession effects demonstrates that no dedicated training session is needed to obtain scotopic and hVA measurements. The combined measures of scotopic and photopic VAs open a field of applications to study interplay and plasticity of the retinal photoreceptor systems and cortical processing in health and visual disease. As a rule of thumb, hyperacuity is 10× higher both in the photopic and scotopic range than conventional acuity. Thus, scotopic hyperacuity is close to photopic conventional acuity.

Scotopic multifocal visual evoked potentials.

OBJECTIVE: To investigate the scope of scotopic multifocal visual evoked potentials (mfVEPS) for the assessment of scotopic visual fields. METHODS: Pattern-reversal mfVEP for photopic (mfVEPP) and scotopic conditions (mfVEPS; 0.003 cd/m2) were recorded from 36 visual field locations of a circular checkerboard pattern (25° radius) in 9 participants with normal vision. MfVEPP were recorded with a conventional central fixation cross, mfVEPS were recorded (i) with (mfVEPS+) and (ii) without (mfVEPS-) an additional fixation aid. Latency shifts were determined using cross-correlations, mfVEP magnitudes were analysed in an eccentricity dependent manner using signal-to-noise ratios (SNRs). RESULTS: In comparison to mfVEPP, mfVEPS- and mfVEPS+ were delayed by 101 ms and 97 ms, respectively, and had smaller signal-to-noise-ratios. Both mfVEPS were reduced down to noise level in the center and also severely reduced for the most peripheral stimulus eccentricity used. The visual-field-coverage for the paracentral eccentricities of mfVEPS+ and mfVEPS- was 76% and 65% [4°-9°], respectively, and 79% and 66% [9°-16°]. CONCLUSIONS: MfVEPS were delayed compared to mfVEPP and demonstrated the expected central response drop-out typical for scotopic vision. SIGNIFICANCE: MfVEPS may hold promise of an objective, spatially resolved visual field test which motivates testing it in patients with diseases affecting scotopic vision.

Differential effects of optic media opacities on simultaneous multifocal pattern electroretinograms and visual evoked potentials.

OBJECTIVE: To identify potential confounds in the comparison of simultaneously acquired multifocal electroretinograms (mfPERGs) and visual evoked potentials (mfVEPs) to pattern reversal stimulation. METHODS: With VERIS Science 5.1.10X monocular mfPERGs and mfVEPs were recorded simultaneously to optimised pattern-reversal stimulation for a reference condition and two filter conditions, i.e. blur and 8% luminance transmission, in two separate experiments in participants with normal vision. The impact of the filter conditions on mfPERG amplitude (P50 and N95 peaks), mfVEP-magnitude (root-mean-squares and signal-to-noise-ratios), and on the response timing was assessed. RESULTS: Blur reduced mfPERG P50 and N95 amplitudes to 16%, 21%, and mfVEP magnitude to 82%. Decreasing stimulus luminance to 8% reduced only the mfPERG (P50 to 72% and N95 to 74%), but delayed both mfPERG and mfVEP responses by 5.3 and 4.6ms, respectively. CONCLUSIONS: Comparatively minor stimulus manipulations, mimicking optic media opacities, had a differential effect on mfPERGs and mfVEPs magnitudes. SIGNIFICANCE: Simultaneous mfPERG/mfVEP recordings are a promising approach to compare retinal and cortical function, but caution must be exerted in the interpretation of response differences due to incongruent response characteristics.

Differential effects of optic media opacities on mfERGs and mfVEPs.

OBJECTIVE: To assess different effects of image degradation that could result from optic media opacities on multifocal retinal (mfERG) and cortical responses (mfVEP). METHODS: Monocular flash-mfERGs and pattern-reversal mfVEPs were recorded. MfERG-P1 amplitudes and implicit times and mfVEP root-mean-square values (RMS) and delays were compared for different filter conditions (none, 8% luminance, 50% luminance, 50% luminance plus blur) in a total of ten participants with normal vision. RESULTS: Reducing stimulus luminance down to 50% and 8% reduced mfERG amplitudes to 86% and 42%, respectively, with no significant effect on mfVEP amplitude. Implicit times were increased for mfERGs by 0.9 ms and 6.0 ms, respectively, and for mfVEPs by 1.0 ms and 6.3 ms, respectively. For '50% luminance plus blur' mfERG amplitudes were significantly reduced centrally and enhanced peripherally and delayed by 1.3 ms. MfVEPs were reduced close to noise level independent of eccentricity. CONCLUSIONS: Degradation of the retinal image is a potential source of discrepancies between mfERGs and mfVEPs. Image blur suppresses the mfVEP at all locations and changes mfERG topography, resulting in a selective loss of central responses. SIGNIFICANCE: Considering optic media opacities is of importance for the correct interpretation of mfERG and mfVEP recordings, particularly in elderly patients.

Isolation, characterization and cDNA cloning of nicotianamine synthase from barley. A key enzyme for iron homeostasis in plants.

Basic cellular processes such as electron transport in photosynthesis and respiration require the precise control of iron homeostasis. To mobilize iron, plants have evolved at least two different strategies. The nonproteinogenous amino acid nicotianamine which is synthesized from three molecules of S-adenosyl-L-methionine, is an essential component of both pathways. This compound is missing in the tomato mutant chloronerva, which exhibits severe defects in the regulation of iron metabolism. We report the purification and partial characterization of the nicotianamine synthase from barley roots as well as the cloning of two corresponding gene sequences. The function of the gene sequence has been verified by overexpression in Escherichia coli. Further confirmation comes from reduction of the nicotianamine content and the exhibition of a chloronerva-like phenotype due to the expression of heterologous antisense constructs in transgenic tobacco plants. The native enzyme with an apparent Mr of approximately 105 000 probably represents a trimer of S-adenosyl-L-methionine-binding subunits. A comparison with the recently cloned chloronerva gene of tomato reveals striking sequence homology, providing support for the suggestion that the destruction of the nicotianamine synthase encoding gene is the molecular basis of the tomato mutation.

A root-specific iron-regulated gene of tomato encodes a lysyl-tRNA-synthetase-like protein.

The tomato mutant chloronerva exhibits a defect in iron-uptake regulation. Despite high apoplastic and symplastic iron concentrations, the mutant shows characteristic symptoms of iron deficiency. Using a subtractive-hybridisation approach, we have screened for cDNA clones specific for genes with altered expression in wild-type versus mutant root tissue. Based on this clone collection, we have isolated and characterised a 2075-bp full-length cDNA encoding a lysyl-tRNA-synthetase-like protein. The corresponding gene is localised as a single copy on chromosome 10. Its expression is strongly induced by changes in the iron status of the plant. This iron-dependent regulation is superimposed upon a strict root specificity of gene expression. Possible functions of the gene product other than in protein biosynthesis will be discussed.

Iron and copper nutrition-dependent changes in protein expression in a tomato wild type and the nicotianamine-free mutant chloronerva.

The nicotianamine-deficient mutant chloronerva resembles phenotypically an Fe-deficient plant despite the high accumulation of Fe in the leaves, whereas if suffers from Cu deficiency in the shoot. Two-dimensional electrophoretic separation of proteins from root tips and leaves of wild-type Lycopersicon esculentum Mill. cv Bonner Beste and the mutant grown with and without Fe showed a number of consistent differences. In root tips of the Fe-deficient wild type and the Fe-sufficient as well as the Fe-deficient mutant, the expression of glyceraldehyde-3-phosphate dehydrogenase, formate dehydrogenase, and ascorbate peroxidase was increased. In leaves of the Fe-sufficient and -deficient mutant, Cu-containing chloroplastic and cytosolic superoxide dismutase (Cu-Zn) and plastocyanin (Cu) were nearly absent. This low plastocyanin content could be restored by supplying Cu via the xylem, but the superoxide dismutase levels could not be increased by this treatment. The differences in the protein patterns between wild type and mutant indicate that the apparent Fe deficiency of mutant plants led to an increase in enzymes involved in anaerobic metabolism as well as enzymes involved in stress defense. The biosynthesis of plastocyanin was diminished in mutant leaves, but it was differentially induced by increased Cu content.