On the analysis of nerve signals deduced from metacontrast experiments with human observers. (1/534)

1. This paper reviews Alpern, Rushton & Torii's (1970a-d) derivation of the size of the inhibitory nerve signal arising from after flashes in the metacontrast experiment. 2. Their geometric argument is recast in terms of simple functional equations. This form of argument clearly displays the role of their assumptions in obtaining their main conclusion: nerve signal is linear in intensity over a range of 3-4 log units. 3. Two disadvantages of their approach are discussed. First, it is noted that in the presence of the data the assumption they employ in their analysis is logically equivalent to their conclusion. 4. Secondly, accepting their claim that the nerve signal generated by the after flash is linear over a broad range of intensities, and that this inhibitory signal simply cancels the excitatory signal of the test flash, leads to the conslusion that over this same intensity range the excitatory nerve signal is a power function with an exponent of close to two. This is incompatible with the suggestion that photoreceptor signals have been measured.  (+info)

Reciprocity between light intensity and rhodopsin concentration across the rat retina. (2/534)

1. If a purpose of photostasis - absorption of a constant number of photons by the retina, regardless of incident light levels - is to maintain rods at saturation during the light period, then in retinal regions where light intensity is low, rhodopsin concentration should be high, and vice versa. 2. Our ocular transmission photometric measurements revealed that the distribution of light intensity across the rat retina was not as simple as had been thought and, furthermore, that the local concentration of rhodopsin had a high negative correlation with the light intensity. 3. The reciprocity between these two parameters leads to nearly uniform rates of photon absorption in rods across the retina.  (+info)

Analysis of pharmacologically isolated components of the ERG. (3/534)

An harmonic analysis was applied to the electroretinogram (ERG) measured in intact cat eyes in control conditions and after pharmacological isolation of the components attributed to photoreceptors (PIII) and bipolar neurons (PII). The frequency response curves obtained in various conditions showed that the bandwidth of the PII component extends over a range of stimulus frequencies higher than the bandwidth of PIII. The enhancement of the PII response to stimuli of high temporal frequency suggests the presence of a frequency dependent gain control located either pre- and/or post-synaptically in the transmission line between the phototransductive cascade and bipolar neurons. A possible role of these processes is to enhance relevant visual information whilst selectively attenuating low frequency signals originating in the transductive cascade.  (+info)

Development of spatial and temporal vision during childhood. (4/534)

Using the method of limits, we measured the development of spatial and temporal vision beginning at 4 years of age. Participants were adults, and children aged 4, 5, 6, and 7 years (n = 24 per age). Spatial vision was assessed with vertical sine-wave gratings, and temporal vision was assessed with an unpatterned luminance field sinusoidally modulated over time. Under these testing conditions, spatial contrast sensitivity at every frequency increased by at least 0.5 log units between 4 and 7 years of age, at which point it was adult-like. Grating acuity reached adult values at 6 years of age. Temporal vision was more mature: at 4 years of age temporal contrast sensitivity at higher temporal frequencies (20 and 30 Hz) and critical flicker fusion frequency were already adult-like. Sensitivity at lower temporal frequencies (5 and 10 Hz) increased by 0.25 log units after the age of 4 to reach adult levels at age 7. The results suggest that temporal vision matures more rapidly than spatial vision during childhood. Thus, spatial and temporal vision are likely mediated by different underlying neural mechanisms that mature at different rates.  (+info)

The distribution of sodium, potassium and chloride in the nucleus and cytoplasm of Bufo bufo oocytes measured by electron microprobe analysis. (5/534)

1. Measurements of cytoplasmic and nuclear Na, K and Cl have been made by electron microprobe analysis on freeze-dried sections of oocytes of Bufo bufo, using standards of bovine plasma albumin and gamma-globulin. Concentrations were obtained per kilogram of dry mass, were converted to concentrations per litre of water content using known figures for water and solid concentration of nucleus and cytoplasm, and were then compared with measurements on cells from the same animal obtained by flame photometry. 2. In fresh oocytes concentrations were (mean +/- S.E. of mean in m-mole/l. H2O) in cytoplasm Na 10.9 +/- 1.95, K 70.2 +/- 3.22, Cl 98.8 +/- 11.0, and in nucleus Na 10.4 +/- 1.79, K 266.4 +/- 22.8, Cl 91.3 +/- 9.0. 3. After treatment with Na-free Ringer (Li substituted for Na) for 5 hr, concentrations were in cytoplasm Na 11.1 +/- 2.44, K 64.4 +/- 5.7, Cl 88.7 +/- 8.8, and in nucleus Na 2.4 +/- 0.73, K 141 +/- 13.9, Cl 75.0 +/- 6.7. Na inexchangeable with Li therefore lay in the cytoplasm but not in the nucleus as previously shown by autoradiography. 4. For K electron microscopic analysis measurements agreed well with those obtained by flame photometry but the former measured only 35% of Na measured by flame photometry. This discrepancy may be due either to technical difficulties with the electron microprobe analysis or to localization of Na in the cytoplasm.  (+info)

Distinct temporal profiles of activity-dependent calcium increase in pyramidal neurons of the rat visual cortex. (6/534)

1. Using fluo-3-based fluorometry, we studied variation in depolarization-induced calcium increases in the proximal dendrites or soma of pyramidal neurons in layer II/III of the rat visual cortex. 2. Depolarization for all durations tested (0.1-2 s; 0.5 nA) evoked a train of action potentials and a small increase in calcium signal (mean 26 %) which peaked within 1 s of the onset of depolarization. With depolarization for longer than 1 s, this small increase was often followed by a larger increase (73 %). This later phase of calcium increase occurred without sudden changes in action potential firing. 3. Application of ryanodine, which suppresses intracellular calcium release, abolished the second phase without affecting the early phase in a use-dependent manner. Meanwhile, no major changes were observed in the pattern of action potential firing. 4. In calcium-free medium, both the early and late phases were almost undetectable, although action potential firing was still evoked by injection of depolarizing currents. Since the late phase depended on intracellular calcium release, this effect of calcium-free medium on the late phase is likely to be indirect through an influence on the early phase. 5. This two-phase profile was observed with somatic depolarization or with antidromic action potentials induced by tetanization. Neocortical pyramidal neurons can thus recruit calcium from different sources, even without chemical sensitization, generating temporally diverse profiles of intracellular calcium signal in response to action potential firing. 6. Such variety in the mechanisms of calcium increase may be relevant to the role of calcium as a versatile second messenger for various types of synaptic plasticity.  (+info)

Flicker ERG responses to stimuli parametrically modulated in color space. (7/534)

PURPOSE: To develop methods for recording human electroretinogram (ERG) responses to stimuli that modulate different classes of cones in various ratios, to draw inferences about the combination of cone signal in early retinal processing. METHODS: Subjects viewed large-field temporal modulations presented on a computer-controlled color monitor. A flicker photometric paradigm was used to equate the ERG response elicited by interleaved reference and test modulations. Test modulations were chosen to stimulate the L- and M-cones in various ratios. Results were obtained from color-normal subjects, dichromats, and an anomalous trichromat. RESULTS: Reliable signals were obtained from all subjects to both L- and M-cone-isolating modulations and to intermediate modulations. Signals from color-defective subjects were predominantly determined by the modulation seen by only one cone type, whereas signals from color-normal subjects were sensitive to both L- and M-cone modulations. For most color-normal subjects, the recorded signal was a linear function of the contrasts seen by the L- and M-cones. There was individual variability in how strongly each cone type contributed to the overall signal. CONCLUSIONS: It is straightforward to record signals to color modulations presented on a CRT by using the flicker photometric ERG. For most observers, signals from L- and M-cones combine linearly. The relative contribution of the two cone classes varies across observers, probably because of individual differences in the relative numbers of L- and M-cones.  (+info)

Dietary carbohydrates and fat influence radiographic bone mineral content of growing foals. (8/534)

Hydrolyzable carbohydrate intake in horse diets may become excessive when rapidly growing pastures are supplemented with grain-based concentrates. The substitution of fat and fiber for hydrolyzable carbohydrate in concentrates has been explored in exercising horses but not in young, growing horses. Our objective was to compare bone development in foals that were fed pasture and concentrates rich in sugar and starch (corn, molasses) or fat and fiber (corn oil, beet pulp, soybean hulls, oat straw). Forty foals were examined, 20 each in 1994 and 1995. In each year, 10 mares and their foals were fed a corn and molasses supplement (SS) and 10 others were fed a corn oil and fiber supplement (FF). The concentrates were formulated to be isocaloric and isonitrogenous, and mineral content was balanced to complement the pastures and meet or exceed NRC requirements. Dorsopalmar radiographs were taken of the left third metacarpal monthly from birth to weaning and then every other month until 1 yr of age. Bone density was estimated using imaging software and an aluminum stepwedge. Radiographic examination indicated differences in medial, lateral, and central bone mineral content of the metacarpal III. Bone mineral content increased with age, and a plateau was observed during winter. Bone mineral content was lower in weanlings and yearlings fed the FF supplement than in those fed SS. Subjective clinical leg evaluations indicated differences in physitis, joint effusion, and angular and flexural limb deformities in response to age, and possibly to season. Regression analysis indicated positive relationships between bone mineral content and body weight, age, and body measurements. Nutrient and chemical interactions, such as the binding of calcium by fat and fiber, may alter the availability of elements necessary for bone development.  (+info)