Simulated bipolar cells in fovea of human retina

Fourier analysis is used to study resolution of images processed by the matrix of simulated red-center (BCR) and green-center (BCG) bipolar cells (BC) of the human central fovea. Simulated achromatic and chromatic sine and square waves, and a two-bar stimulus are used to activate the BCs. Due to the "honeycomb" packing of the cones and BC matrices Fourier transforms are computed row by row using a one-dimensional FFT. Resolution computed by the Fourier transform is compared with the resolution index (RI), which is a method for determining resolution based on two-point discrimination in the space domain. In general the harmonic with the maximum amplitude gives the best correlation with RI for the three stimuli. Amplitudes at all spatial frequencies are enhanced by increasing the number of cycles in the sine and square wave gratings. Results with simulated BCs compare favorably with human and macaque psychophysics measuring contrast sensitivity. Square wave gratings are better than sine wave greetings for studying resolution.     

Simulated bipolar cells in fovea of human retina

A computer model of simulated bipolar cells (BC) in the human retina is used to study wavelength discrimination (lambda delta). lambda delta curves are obtained for the two C-types of BCs in the central fovea and the three C-type BCs of the parafovea under various conditions. For the parafovea algebraic addition of the three C-type BCs with proper weighting of the blue-center BC, such that the unique orange locus = 600 nm, gives a combined channel whose lambda delta curve agrees remarkably well with those found in the literature based on human and primate psychophysics. Other studies include effects of chromatic adaptation and dispersion on lambda delta. From this and earlier studies it can be concluded that the center/surround organization of the BCs optimize resolution in the presence of natural occurring dispersion; in addition a specific BC receptive field organization could be picked as being optimal.     

INTERMITTENT STIMULATION BY LIGHT : III. THE RELATION BETWEEN INTENSITY AND CRITICAL FUSION FREQUENCY FOR DIFFERENT RETINAL LOCATIONS

When measurements of the critical fusion frequency for white light over a large range of intensities are made with the rod-free area of the fovea, the relation between critical frequency and log I is given by a single sigmoid curve, the middle portion of which approximates a straight line whose slope is 11.0. This single relation must be a function of the foveal cones. When the measurements are made with a retinal area placed 5° from the fovea, and therefore containing both rods and cones, the relation between critical frequency and log I shows two clearly separated sections. At the lower intensities the relation is sigmoid and reaches an upper level at about 10 cycles per second, which is maintained for 1.25 log units, and is followed by another sigmoid relationship at the higher intensities similar to the one given by the rod-free area alone. These two parts of the data are obviously separate functions of the rods at low intensities and of the cones at high intensities. This is further borne out by similar measurements made with retinal areas 15° and 20° from the fovea where the ratio of rods to cones is anatomically greater than at 5°. The two sections of the data come out farther apart on the intensity scale, the rod portion being at lower intensities and the cone portion at higher intensities than at 5°. The general form of the relation between critical frequency and intensity is therefore determined by the relative predominance of the cones and the rods in the retinal area used for the measurements.     

Simulated bipolar cells in fovea of human retina

The static model developed in Part I is used to study spectral responses of C-type bipolar cells (BC). Once unique loci are adjusted to their proper wavelengths, and with a specified set of absorption spectra for cones, spectral responses of C-type BCs are dependent on only the balance between BC receptive field center and surround responses. This is true regardless of cone mosaic or BC receptive field organization. The unique yellow loci for the r-g channel is set at 576.7 nm while the unique green locus for the blue-center BC is set at 517.7 nm. A unique orange locus for a combined r-g and bl-y channels is set at 600 nm by multiplying the blue-center BC response spectra by a factor of six before adding to the r-g channel.     

Simulated bipolar cells in fovea of human retina

The ability of simulated bipolar cells (BC) of the human central fovea to resolve images is studied with a two-bar stimulus using a resolution index (RI) as a measure of resolvability. RIs are determined for intensity and chromatic contrasts using all combinations of white, black, red, yellow, green, and blue lights. arious cone matrixes and BC receptive field organizations are studied for orientation preference by using two-bars oriented either 0, 45, 90, or 135 degrees to the horizontal axis of the retina. Nonpreference for orientation, i.e. RI does not change with bar orientation, varies with matrix type, and receptive field organization. For a given orientation RI increases asymptotically as bar width or length, or gap between the bars increases. Systematic changes in RI occur with systematic changes in contrast. For most color pairs there are residual RIs at isoluminance.The major part of this work was done while the author was a Senior Research Associate of the National Research Council, USA     

Electrical coupling between mammalian cones

BACKGROUND: Cone photoreceptors are noisy because of random fluctuations of photon absorption, signaling molecules, and ion channels. However, each cone's noise is independent of the others, whereas their signals are partially shared. Therefore, electrically coupling the synaptic terminals prior to forward transmission and subsequent nonlinear processing can appreciably reduce noise relative to the signal. This signal-processing strategy has been demonstrated in lower vertebrates with rather coarse vision, but its occurrence in mammals with fine acuity has been doubted (even though gap junctions are present) because coupling would blur the neural image. RESULTS: In ground squirrel retina, whose triangular cone lattice resembles the human fovea, paired electrical recordings from adjacent cones demonstrated electrical coupling with an average conductance of approximately 320 pS. Blur caused by this degree of coupling had a space constant of approximately 0.5 cone diameters. Psychophysical measurements employing laser interferometry to bypass the eye's optics suggest that human foveal cones experience a similar degree of neural blur and that it is invariant with light intensity. This neural blur is narrower than the eye's optical blur, and we calculate that it should improve the signal-to-noise ratio at the cone terminal by about 77%. CONCLUSIONS: We conclude that the gap junctions observed between mammalian cones, including those in the human fovea, represent genuine electrical coupling. Because the space constant of the resulting neural blur is less than that of the optical blur, the signal-to-noise ratio can be markedly improved before the nonlinear stages with little compromise to visual acuity.     

An Isolated Retinal Preparation to Record Light Response from Genetically Labeled Retinal Ganglion Cells

The first steps in vertebrate vision take place when light stimulates the rod and cone photoreceptors of the retina ¹. This information is then segregated into what are known as the ON and OFF pathways. The photoreceptors signal light information to the bipolar cells (BCs), which depolarize in response to increases (On BCs) or decreases (Off BCs) in light intensity. This segregation of light information is maintained at the level of the retinal ganglion cells (RGCs), which have dendrites stratifying in either the Off sublamina of the inner plexiform layer (IPL), where they receive direct excitatory input from Off BCs, or stratifying in the On sublamina of the IPL, where they receive direct excitatory input from On BCs. This segregation of information regarding increases or decreases in illumination (the On and Off pathways) is conserved and signaled to the brain in parallel.The RGCs are the output cells of the retina, and are thus an important cell to study in order to understand how light information is signaled to visual nuclei in the brain. Advances in mouse genetics over recent decades have resulted in a variety of fluorescent reporter mouse lines where specific RGC populations are labeled with a fluorescent protein to allow for identification of RGC subtypes ^{2 3 4} and specific targeting for electrophysiological recording. Here, we present a method for recording light responses from fluorescently labeled ganglion cells in an intact, isolated retinal preparation. This isolated retinal preparation allows for recordings from RGCs where the dendritic arbor is intact and the inputs across the entire RGC dendritic arbor are preserved. This method is applicable across a variety of ganglion cell subtypes and is amenable to a wide variety of single-cell physiological techniques.Download video file.^{(77M, mov)}     

Effect of Soil pH Increase by Biochar on NO,N2O and N2 Production during Denitrification in Acid Soils

Biochar (BC) application to soil suppresses emission of nitrous- (N₂O) and nitric oxide (NO), but the mechanisms are unclear. One of the most prominent features of BC is its alkalizing effect in soils, which may affect denitrification and its product stoichiometry directly or indirectly. We conducted laboratory experiments with anoxic slurries of acid Acrisols from Indonesia and Zambia and two contrasting BCs produced locally from rice husk and cacao shell. Dose-dependent responses of denitrification and gaseous products (NO, N₂O and N₂) were assessed by high-resolution gas kinetics and related to the alkalizing effect of the BCs. To delineate the pH effect from other BC effects, we removed part of the alkalinity by leaching the BCs with water and acid prior to incubation. Uncharred cacao shell and sodium hydroxide (NaOH) were also included in the study. The untreated BCs suppressed N₂O and NO and increased N₂ production during denitrification, irrespective of the effect on denitrification rate. The extent of N₂O and NO suppression was dose-dependent and increased with the alkalizing effect of the two BC types, which was strongest for cacao shell BC. Acid leaching of BC, which decreased its alkalizing effect, reduced or eliminated the ability of BC to suppress N₂O and NO net production. Just like untreated BCs, NaOH reduced net production of N₂O and NO while increasing that of N₂. This confirms the importance of altered soil pH for denitrification product stoichiometry. Addition of uncharred cacao shell stimulated denitrification strongly due to availability of labile carbon but only minor effects on the product stoichiometry of denitrification were found, in accordance with its modest effect on soil pH. Our study indicates that stimulation of denitrification was mainly due to increases in labile carbon whereas change in product stoichiometry was mainly due to a change in soil pH.     

Effect of sodium fluoride on the root apex border cells in one-day-old wheat seedlings

The number of border (scaled off) cells (BCs) was determined in the root apex of 1-day-old wheat (Triticum aestivum L.) seedlings. Microscopic examination of cytological root preparations showed that in 24 h the number of BCs in the gel sheath of the root apex was 30–40 per root. When the gel sheath was preparatively removed, their number per root increased twice. It is assumed that the subpopulation of BCs directly associated with the root apex differs from the subpopulation of BCs freely accommodated in the gel sheath. The number of BCs was the same in the roots with low and high natural growth rates. NaF (1–20 mM) suppressed growth of wheat seedling roots; the viscosity of the gel sheath increased (by 3–5 times), and the number of BC rose with the most pronounced increment in the size of the BC subpopulation directly associated with the root apex. Published in Russian in Fiziologiya Rastenii, 2009, Vol. 56, No. 4, pp. 530–538.     

Modelling of the vertebrate visual system 3. topological analysis of the cone mosaic

Spatial organization of the cone mosaic of the generalized vertebrate retina consists of rows of red and green cones alternating with rows of blue and blank cones. Cone inputs to retinal elements are defined spatially by red and green unit hexagons. Topological analysis entails determining for each cone in the mosaic the number of each cone type present in the unit hexagon which the activated cone can influence via electrical coupling between cones and/or stray light. Only weighted inputs in one-half of a sextant of the unit hexagon need be designated, since all other weighted inputs can be determined by rules giving systematic transformations of all cone types from one sextant to another: these rules arise from symmetries of the cone mosaic. Four retinal types are possible depending on replacement of blank cones by specific cone types; three cone-dominant retinas, where all blank cones are replaced by a specific cone type, and two forms of a trichromatic retina, where blank cones are replaced by equal numbers of red and green cones. The weighted input is the sum of individual cone type contributions and depends on the number of each cone type in the unit hexagon which can influence the cone in question. Weighted inputs for cone-dominant retinas are readily found by replacing blank cones with the proper cone type, while weighted inputs for trichromatic retinas require use of a specified cone mosaic to determine extra red and green cones. Receptive field size of post-cone elements as well as overlap of the center and surround fields of annular organized receptive fields of retinal elements increased with increasing values for attenuation factors.     

The effect of light adaptation on the dynamic properties of phototransduction in the fly,Phormia regina

The static and dynamic characteristics of phototransduction were studied in photoreceptors of the compound eye of the fly Phormia regina (Calliphoridae) using a green light emitting diode driven by a controlled current source. The LED provides sufficiently intense light to investigate the behaviour of the receptors over about half of the dark adapted range of the response versus log intensity curve. The effects of constant adapting light intensities upon the step response and upon the frequency response and coherence functions were examined. Using both methods the effect of light adaptation upon receptor sensitivity can be closely approximated by a similar linear dependence of log sensitivity upon log adapting intensity. However, there was no reliably detectable effect of light adaptation upon the time constant of the response over the range of adapting intensities used.Abbreviation LED Light Emitting Diode     

Human wavelength discrimination of monochromatic light explained by optimal wavelength decoding of light of unknown intensity

We show that human ability to discriminate the wavelength of monochromatic light can be understood as maximum likelihood decoding of the cone absorptions, with a signal processing efficiency that is independent of the wavelength. This work is built on the framework of ideal observer analysis of visual discrimination used in many previous works. A distinctive aspect of our work is that we highlight a perceptual confound that observers should confuse a change in input light wavelength with a change in input intensity. Hence a simple ideal observer model which assumes that an observer has a full knowledge of input intensity should over-estimate human ability in discriminating wavelengths of two inputs of unequal intensity. This confound also makes it difficult to consistently measure human ability in wavelength discrimination by asking observers to distinguish two input colors while matching their brightness. We argue that the best experimental method for reliable measurement of discrimination thresholds is the one of Pokorny and Smith, in which observers only need to distinguish two inputs, regardless of whether they differ in hue or brightness. We mathematically formulate wavelength discrimination under this wavelength-intensity confound and show a good agreement between our theoretical prediction and the behavioral data. Our analysis explains why the discrimination threshold varies with the input wavelength, and shows how sensitively the threshold depends on the relative densities of the three types of cones in the retina (and in particular predict discriminations in dichromats). Our mathematical formulation and solution can be applied to general problems of sensory discrimination when there is a perceptual confound from other sensory feature dimensions.     

A role for the chaperone Hsp70 in the regulation of border cell migration in the Drosophila ovary

Unravelling the molecular mechanisms that govern cell migration is of great importance towards understanding both normal embryogenesis and physiological and pathological processes occurring in the adult. Migration of border cells (BCs) during Drosophila oogenesis provides a simple and attractive model in which to address this problem. Here, we show that the molecular chaperone Hsp70 is required for BC migration. Thus, BCs lacking all Hsp70 genes present in the fly genome fail to reorganize their actin cytoskeleton, resulting in migration defects. Similar defects are found when the Hsp70 co-chaperone DnaJ-1, the Drosophila homolog of the human Hsp40, is overexpressed specifically in BCs. In addition, we provide biochemical and genetic evidence for an interaction between DnaJ-1 and PDGF/VEGF receptor (PVR), which is also required for actin-mediated BC migration. Furthermore, our results showing that PVR also interacts genetically with Hsp70 suggest that a mechanism by which the DnaJ-1/Hsp70 chaperone complex regulates BC migration is by modulating PVR function.     

Viability and cellulose synthesizing ability of Gluconacetobacter xylinus cells under high-hydrostatic pressure

The effect of pressure on viability and the synthesis of bacterial cellulose (BC) by Gluconacetobacter xylinus ATCC53582 were investigated. G. xylinus was statically cultivated in a pressurized vessel under 0.1, 30, 60, and 100 MPa at 25°C for 6 days. G. xylinus cells remained viable and retained cellulose producing ability under all the conditions tested, though the production of cellulose decreased with increasing the pressure. The BCs produced at each pressure condition were analyzed by field emission scanning electron microscopy (FE-SEM) and Fourier Transform Infrared (FT-IR). FE-SEM revealed that the widths of BC fibers produced under high pressure decreased as compared with those produced under the atmospheric pressure. By FT-IR, all the BCs were found to be of Cellulose type I, as the same as typical native cellulose. Our findings evidently showed that G. xylinus possessed a piezotolerant (barotolerant) feature adapting to 100 MPa without losing its BC producing ability. This was the first attempt in synthesizing BC with G. xylinus under elevated pressure of 100 MPa, which corresponded to the deep sea at 10,000 m.     

Are T cells at the origin of B cell lymphomas?

Lymphoma pathogenesis is at least in some cases related to transformed B cells (BCs) arising from germinal centre reactions (GCRs). In this article possible deregulations of GCRs are investigated using in silico simulations. It is found that the final differentiation of BCs as regulated by helper T cells (TCs) is the best candidate mechanism for such a deregulation. This shifts the paradigm of BC lymphoma pathogenesis from BC transformations to an emphasized role of TC-BC interactions.     

Light-induced changes of sensitivity in Limulus ventral photoreceptors

The responses of Limulus ventral photoreceptors to brief test flashes and to longer adapting lights were measured under voltage clamp conditions. When the cell was dark adapted, there was a range of energy of the test flashes over which the peak amplitude of the responses (light-induced currents) was directly proportional to the flash energy. This was also true when test flashes were superposed on adapting stimuli but the proportionality constant (termed peak currently/photon) was reduced. The peak current/photon was attenuated more by brighter adapting stimuli than by less bright adapting stimuli. The peak current/photon is a measure of the sensitivity of the conductance-increase mechanism underlying the light response of the photo-receptor. The response elicited by an adapting stimulus had a large initial transient which declined to a smaller plateau. The peak current/photon decreased sharply during the declining phase of the transient and was relatively stable during the plateau. This indicates that the onset of light adaptation is delayed with respect to the onset of the response to the adapting stimulus. If the adaptational state just before the onset of each of a series of adapting stimuli was constant, the amplitude of the transient was a nearly linear function of intensity. When the total intensity was rapidly doubled (or halved) during a plateau response, the total current approximately doubled (or halved). We argue that the transition from transient to plateau, light-elicited changes of threshold, and the nonlinear function relating the plateau response to stimulus intensity all reflect changes of the responsiveness of the conductance-increase mechanism.     

Human eye tracking as a sequential input adaptive process

Sequential input adaptive system theory is applied to human eye tracking of targets with one-dimensional dimensional motion defined by sine functions, sums of sine functions and triangle functions. The sequential theory describes the mean eye tracking movements of two subjects for these inputs. The theory is predictive for irregular inputs in that when its parameters for a subject are identified and adjusted for a reference input, it then predicts mean eye tracking behavior for that subject for inputs of comparable complexity.This research was initiated while D. E. Greene was at the Biodynamics and Bioengineering Division, Aerospace Medical Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio     

Adaptation of the visual system

We recorded the total pulse response of the optic nerve in frogs to varying degrees of increase and decrease of light from the original adapting level. On the basis of these data, we plotted curves of dependence of the magnitude of response on the logarithm of relative value of increase and decrease of light (the amplitude characteristic — AC). The AC is steepest in the zone of adapting background and sloped on either side of it. It follows that under stationary conditions of illumination, the eye is capable of finely differentiating light intensity only within a narrow range (one logarithmic unit). After adaptation to a new level of illumination, the AC shifts along the scale of light intensity in such a way that the steepest portion corresponds to the adapting brightness. Increase in steepness of the AC occurs precisely during the process of adaptation. The contrast sensitivity of the human visual system is greatest near the adapting level and declines on either side of it. It follows that in man steepness of the visual system AC is greatest in the zone of the adapting background. Both increase and decrease of intensity of the adapting background are accompanied by a decline of contrast sensitivity, which rises again during the process of adaptation to a new level. Thanks to adaptive shift of the steep portion of the AC along the scale of light intensity, a visual system having a high contrast sensitivity only within a narrow "working" range is capable of finely differentiating light intensity in significantly changing conditions of illumination.Institute of Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 1, No. 1, pp. 81–89, July–August, 1969.     

A simulated human fovea: the L-type cells of the magnocellular pathway

A static model of the human fovea is used to study the properties of L-type amacrine cells (L-AC) that link the cones with the magnocellular pathway. Sine and square wave gratings are used to obtain response spectra of L-ACs and C-type bipolar cells (C-BC); these two types of cells are compared in both central fovea, where there are no blue-sensitive cones and parafovea, where the blue-sensitive cones represent 12% of the population. Three dispersion conditions are used: no, aberration-free, and chromatic dispersions. The abilities of L- and C-type cells to resolve a twobar image are also compared. The findings are consistent with the magnocellular pathway having higher contrast luminance and chromatic sensitivity gains than those of the parvocellular pathways, but under specified conditions. And under specified conditions the findings are also consistent with both pathways being involved in the detection of chromatic and achromatic signals. Nevertheless when all factors are considered the parvocellular pathway appears to be involved with fine spatial and chromatic tuning while the magnocellular pathway appears to deal with coarser tuning.