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

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.     

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     

Simulated bipolar cells in fovea of human retina

This static bipolar cell (BC) model of the human fovea is based on a number of reasonable assumptions. The human fovea is directly responsible for visual acuity and color vision. The fovea can be considered as having two parts; a central fovea with only red- and green-sensitive cones and a parafovea with blue-sensitive cones added to the other two. A cone mosaic can be precisely organized spatially into unit hexagons that specify inputs to horizontal cells (HC) and BCs. The retina up to and including BCs is piece-wise linear, i.e. at a given steady-state adapting light intensity BC outputs are linear functions of the physical image. BC centers receive inputs directly from weighted cones, while antagonistic surrounds receive inverted inputs from HCs. Appropriate optical and chromatic filtering due to the eye that are taken from human data are incorporated into the model. Chromatic aberrations are simulated by three separate point spread functions that also are taken from human data. Automatic gain control of cones is a function of intensity and wavelength of the steady adapting light.The major part of this work was done while the author was a Senior Research Associate of the National Research Council, USA     

Simulated fovea of the human retina: psychophysical data confirming the model's ability to accurately predict resolution

A series of psychophysical tests were designed to determine whether a computer simulation of the human retina could accurately predict the geometry of various stimuli that were optimally resolved for human foveal vision. Stimuli were used that were of the order of the grain of the cone mosaic, i.e., of the order of 2 × 2. In the first set of experiments, resolution was tested using a two-bar stimulus. In one experimental series the gap between the two bars was varied, and in a second series the gap was kept constant and the width of the bars varied. In a second set of experiments, various block letters and a number of series for each letter were used; in each experimental series a single parameter was systematically varied. The same stimuli were also used as inputs for the computer simulation. When proper controls were used, the psychophysical data and computer simulation gave remarkably comparable results. Care was taken to differentiate between simple detection of a pattern, and resolution, which involved proper identification of the image.     

Pale and dark bipolar cells in the chicken retina

Ultrastructurally, two different bipolar cell types can be distinguished in the retina of the chick embryo: one pale or electron-lucent and the other dark or electron-dense. The different electron density was seen over the whole cell, from its enclave in the outer limiting membrane to its termination in the inner plexiform layer. These observations prompted us to study the content and cytoplasmic variations of both cell types. The pale bipolar cell has a higher vacuole, vesicle and endoplasmic reticulum content and a lower number of microtubules and glycogen than the dark bipolar cell. The presence of these two cell types is probably due to a characteristic physiologic state, and the difference between the pale and dark bipolar cells can be attributed to the diverse number of gap unions which they establish with A II amacrine cells.     

Quantitative morphology of the central fovea in the primate retina

Electron micrograph composites of tangenital sections of the fovea centralis of three cynomolgus monkeys (Macaca irus) and one baboon (Papio anubis) were used to determine the spatial density of the principal retinal cells. In the center of the foveola, the density of cones ranged from 113,000 to 230,000/mm2, and pigment epithelial cells from 4,900 to 7,000/mm2. At a distance of 500 microns from the foveolar center the density of the cone cell pedicles ranged from 29,000 to 36,300/mm2, and the density of horizontal cells ranged from 19,000 to 25,100/mm2. Densities of bipolar, Müller, and amacrine cells were determined in only two monkeys and in the baboon. The fact that the cone cell pedicles have a larger diameter than the foveolar cones explains the geometry of the fovea. The morphology of the junction between foveolar cone outer segments and the pigment epithelium reflects the complex metabolism of this functional unit. The comparison with the peripheral primate retina suggests that the densities of horizontal and bipolar cells, but not of amacrine and Müller cells, are correlated with the density of cone cell pedicles.     

Localized morphological differentiation in the bipolar cells of the chick retina

On a morphological and ultrastructural level, we studied a thickening which appears on the ascending prolongation of bipolar cells in the chick retina. We first observed this thickening on day 10 of incubation and it remains unchanged throughout the postnatal life of the chick. Its presence seems to be related to the synaptic activity at a dendritic level in certain bipolar cells.     

The cone synapses of cone bipolar cells of primate retina

One each of bipolar cell types DB2 and DB4, together with a flat and an invaginating midget bipolar cell, were taken from a Golgi-stained rhesus macaque retina; then serially sectioned for EM examination of their synapses with cone pedicles. The cone input to the dendrites of the DB2 cell was exclusively at basal junctions; it had a characteristic distribution. Fifty per cent of the basal synapses were with cone pedicle membrane immediately adjacent to the dendrite of a bipolar cell invaginating to end opposite the ribbon of a cone triad (this, therefore, is called triad-associated). The remainder were one or more synapses distant from the triad-associated position (and, therefore, non-triad associated). The DB4 cell had both basal (predominantly in the triad-associated position) and ribbon-related synaptic input. But the basal to invaginating ratio differed from that of our previously published cell; 56% basal, 43% invaginating, as compared with 31% basal and 69% invaginating. Like foveal IMB cells the synapses of the mid-peripheral invaginating midget bipolar cell were exclusively invaginating; but were about 25% more numerous. The flat midget bipolar cell made exclusively basal synapses. These were 2.5 times more numerous than those of foveal flat midget bipolar cells, and 3.5 times the number of invaginating midget bipolar synapses at equivalent eccentricity. The synapses between cones and diffuse and midget bipolar cells are characteristic for each particular bipolar cell type, but the details depend on a cell’s distance from the fovea (eccentricity). A rather constant number of cone pedicle synaptic ribbons 38.6±2.5 (n±:60) was found across mid-peripheral macaque and vervet monkey retinae. The smaller mean number for vervet monkey, 27.4±3.5 (n ±:23), suggests there can also be generic differences in synaptic detail at cone bipolar cell synapses.     

视网膜双极细胞的突触传递

双极细胞是视网膜中处于信号传递的直接通路中的中间神经元,以带型突触传递由分级电位编码的视觉信号。近年来,应用膜电容测定、钙光成像等技术,对双极细胞递质释放的特点及动力学特性进行了细致的研究,不仅有助于阐明这些细胞独特的生理功能,也对了解以带型突触传递信号的感觉神经元的功能特性提供了启示。     

甘氨酸对鲫鱼视网膜ERG b-波和ON型双极细胞活动的调制

本工作在分离灌流的鲫鱼视网膜上研究了甘氨酸对明,暗视视网膜电图（ＥＲＧ）ｂ－波和胞内记录的ＯＮ型双极细胞反应的作用。结果表明,甘氨酸能明显压抑ＥＲＧ ｂ－波和ＯＮ型双极细胞的反应,其作用能为士的宁所翻转;甘氨酸对用谷氨酸分离的ＥＲＧ ＰⅢ成分（光感受器电位）无明显影响。这些结果提示,甘氨酸可能直接作用于双极细胞的受体,从而调节视网膜ＯＮ通路的活动。     

Dynamics of L-type bipolar and phasic amacrine cells in the vertebrate cone retina

The model of the cone-L-HC circuit of the catfish retina (Siminoff 1985a) is extended to Luminosity bipolar cells (BC) and non-linear phasic amacrine cells (AC), but now applicable to the generalized vertebrate cone retina that involves only one cone type. Two types of BC's are simulated by linear transformation of 2 antagonistic inputs of differing time courses; the faster center field hyperpolarization from the cone and the slower surround field depolarization from the L-HC. The phasic AC was made non-linear by various methods: full- or half-wave rectification using either both or only one of the BC's as the inputs with rectification first and then summation or summation first and then rectification. A method is described using Laplace transforms in conjunction with the convolution theorem to obtain the impulse responses of BC's and AC's, in spite of the non-linearities of the AC even when used as feedback to the BC's. Since the input to the BC consists of 2 antagonistic inputs, feedback from the AC reeinforces one input and attenuates the other.     

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.     

Electronic simulation of cones,horizontal cells and bipolar cells of generalized vertebrate cone retina

Electronic analogue of my theoretical model of generalized vertebrate cone retina [Siminoff: J. Theor. Biol. 86, 763 (1980)] is presented. Cone mosaic is simulated by 25x21 grid of phototransistors that have colored filters mounted in front of then to produce red-, green-, and blue-sensitive cones arranged in a trichromatic retina. Each retinal element is simulated by Summator-Integrator and unit gain voltage invertes are used to give correct polarities to output voltages. Dynamic properties of retinal elements are developed solely by temporal interplay of antagonistic input voltages with differing time courses, and spatial organization of receptive fields is developed by unit hexagons that precisely define cone input voltages to subsequent elements. Electronic model contains both color- and non-colorcoded channels. Negative feedback from L-horizontal cells to cones, electrical coupling of like-cones, and electrical coupling of like-horizontal cells are simulated by feedfoward circuits. Stray light is present due to light scattering properties of colored filters used to simulate color selectivety of cones. Stationary and moving spots of white and colored lights of varied sizes and intensities are used to study characteristics of electronic analogue. Results demonstrate practicality of electronic simulation to function analogous to real cone retinas to process visual stimuli and give information to higher centers as to size, shape, color and motion of objects in visual world.     

The lateral spread of signal between bipolar cells of the tiger salamander retina

When mapped with a small spot of light, the central receptive fields of bipolar cells in the salamander retina are much larger than the extent of bipolar cell dendrites. Furthermore responses of bipolar cells to distant spots of light are considerably delayed relative to proximal spots. Using quantitative modelling, electrical coupling between bipolar cells is examined and rejected as a sufficient explanation of the data. An active process appears to shape signal waveform as signals spread laterally in the bipolar cell layer. Chemical synaptic coupling between bipolar cells is considered and shown to be inconsistent with the data. It is suggested that local, transient negative feedback from amacrine cells is involved in shaping bipolar cell signals.