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Biomedical subjects

M Alpern

Publications and source records attributed to M Alpern.

At least 55 records · Page 3Linked to original sources
PubMed1976

Tritanopia.

The color-matching functions of individual tritanopes differ significantly from those predicted with the assumption that the two cone pigments of the tritanope are those which underlie the matches of the standard observer. The differences cannot be explained completely by the abnormal luminosity curve of the standard observer nor by abnormalities in transmissivity of the eye media. Individual tritanopes differ significantly in their color matches in ways not entirely accounted for by eye media differences. The results are consistent with the view that there are a variety of different long- and medium-wave-sensitive cone visual pigments among different tritanopes.

Color Vision Defects↗
PubMed1974

The density and photosensitivity of human rhodopsin in the living retina.

1. The visual pigment in a 5 degrees circular patch of the living human retina 18 degrees temporal from the fovea was studied with the Rushton retinal densitometer. The measuring light (570 nm) was selected to obviate artifacts from colour photoproducts.2. The action spectrum of a 10% bleach agrees well with the action spectrum at absolute threshold for the same patch of retina. The quantized C.I.E. scotopic spectral sensitivity curve is a good description of both spectra. Therefore, the visual pigment studied must be human rhodopsin.3. Its density has been estimated in five different ways. The results are in reasonable agreement. The optical density of human rhodopsin in vivo is about 0.35 (common logarithmic units) at its gamma(max.)4. The photosensitivity of human rhodopsin in vivo was determined by studying its rate of bleaching in response to steps of monochromatic light exposed to the dark adapted eye, by measuring the amount bleached in the steady state by monochromatic lights as well as the amount bleached by 10 sec flashes of white light.5. The results obtained by the different methods are in good agreement with each other and with previous estimates made by others using white light.6. The photosensitivity of human rhodopsin in vivo [epsilongamma(max) = 62,000 to 120,000 l./cm mole] is much higher than expected from in vitro measurements.

Dark Adaptation↗
PubMed1973

Dark adaptation and visual pigment regeneration in human cones.

Foveal threshold elevation and red-green cone pigment regeneration have been studied in the dark after a wide range of bleaches in normal man with a view to probing the limits of the application of the Dowling-Rushton relation: i.e., the direct proportionality between log threshold elevation and fraction of unregenerated pigment. Cone pigment regeneration (and threshold recovery) is much faster after short bleaches than expected from the kinetics of a simple monomolecular reaction. Recovery is faster after a fixed (short) duration bleach the weaker it is. Except for the first 30 s after relatively weak bleaches and the entire recovery after a very brief (<0.001 s) saturating bright flash which bleaches a little more than 50%, the results are accurately fit by the Dowling-Rushton relation over the entire range tested with only one arbitrary constant (the proportionality factor). Theory predicts too low threshold in comparison with what is obtained, for both of these exceptions

Dark Adaptation↗
PubMed1972

Rhodopsin bleaching signals in essential night blindness.

1. The dark-adaptation curves of two subjects with essential night blindness revealed no evidence for functioning rod vision. Cone vision was normal.2. The photopupillomotor dark adaptation, and flash intensity response amplitude curves on one of these subjects confirmed the absence of rod function.3. However, there is the normal amount of rhodopsin in their rods with normal kinetics.4. Cone pigment kinetics are also nearly normal. After a full bleach, log threshold elevation of the foveal cones is linearly related to pigment regeneration. The constant of proportionality is about 3.0 as it is in the normal retina.5. After a full rhodopsin bleach, the contralateral pupil size recovered its full dark value along a curve which followed the regeneration of rhodopsin.6. The results in (5) are identical to those previously found on normal subjects.7. With the exception of a very small response attributed to the contribution of cones, no significant changes in pupil size were evoked by uniform ganzfeld steady backgrounds until the intensity of retinal illuminance was so high that appreciable rhodopsin was bleached. This contrast to the changes evoked by weak steady backgrounds in the normal eye.8. Therefore, rod bleaching signals are normal in such retinas but rod signals evoked by real lights are not functional. This supports Rushton's concept as to how bleaching signals influence retinal sensitivity as opposed to the view of Barlow.9. The defect in essential night blindness very probably involves the rod automatic gain control, but because of (4) the cone gain control must be normal.10. Therefore, rod and cone gain control mechanisms must be independent in these night blind retinas and, by analogy, in the normal retina as well.

Adolescent↗
PubMed1971

Colour vision in blue-cone 'monochromacy'.

1. Atypical (blue cone) monochromats show two action spectra when tested by the increment threshold method of Stiles with ;central' fixation. One spectrum peaks near 450 nm and has the spectral characteristics of normal blue cones. The other resembles rhodopsin (pi(0)) modified slightly by photostable macular pigment.2. Under some circumstances such observers are dichromats. There is a neutral point (matched to Illuminant ;C') in the neighbourhood of 460-470 nm.3. The spectral colour matching functions using two primaries have been measured on three such subjects. They can be fit reasonably well, although imperfectly, by linear combinations of pi(0) and pi(1). The chromaticity co-ordinates have been calculated according to the convention of W. D. Wright and compared to the results predicted from pi(0) and pi(1). The comparison suggests that part of the imperfections of the colour matching function fit is due to prereceptor differences (e.g. macular pigment) between the blue-cone monochromats and the hypothetical pi(0), pi(1) observer.4. Colour matches made at high light levels continue to hold when the intensity of the field is reduced below the cone threshold.5. Therefore one of the visual pigments participating in the colour matches has an action spectrum which is not measurably different from that of the rod pigment rhodopsin.6. Increment threshold measurements show that the mechanism which has the rhodopsin action spectrum has the directional sensitivity of cones, not rods.7. Blue test threshold during dark adaptation after a full bleach follow a bipartite exponential recovery curve. The first exponential has a time constant of 1 min, the second 2 min. By comparing these curves to the increment thresholds it is possible to relate the first to the pi(1), the second to the pi(0) cones.8. Using a broad band blue gelatin filter in the measuring light of the retinal densitometer and studying the same retinal region tested in 7 it is possible to follow the regeneration of a pigment after a full bleach which has an exponential recovery with a time constant of 1.0 min. With a yellow green filter in the measuring light the exponential recovery observed after a full bleach has a time constant of 2.0 min.9. One of the two visual pigments participating in the colour matches resides in receptors which have the action spectrum, the directional sensitivity and probably the dark adaptation curve of normal blue cones.10. The other resides in receptors which have the action spectrum of normal rods but the directional sensitivity and the dark adaptation curve of normal red and green cones.

Color Perception↗
PubMed1971

Rhodopsin kinetics in the human eye.

1. Rhodopsin has been measured by Rushton's method of reflexion densitometry in a retinal region 18 degrees temporal to the fovea, using a wavelength of measuring light (555 nm) so far into the long wave part of the spectrum that possible blue absorbing intermediates (e.g. transient orange) do not interfere.2. Rhodopsin was bleached by a strong light for 10 sec and then held steady by a weaker light. During a 10 sec bleach, no regeneration occurs and the rate of bleaching is proportional to the quantum catch. The proportionality constant is about 10(-7) (td sec)(-1).3. From 2, the rate of photolysis at equilibrium produced by the steady light was calculated. Since conditions were at equilibrium, photolysis matched regeneration. It was found that the rate of generation was proportional to the amount of pigment still bleached. The proportionality constant was about 0.0025 sec(-1).4. It was found by several different methods that the constant in 3 is the same in the light or dark and hence regeneration occurs independently of bleaching.5. Therefore, the results from bleaching and regeneration experiments can be combined to give the general equation [Formula: see text], where p is the fraction of rhodopsin, t is time in sec and I is the retinal illuminance.6. This equation describes the results of partial bleaching and regeneration experiments under a variety of different exposure intensities of moderately long (at least 10 min) exposure durations.7. The dark adaptation curve in a peripheral region of the rod monochromat's retina where there are few cones follows a simple exponential course over nearly 7 log(10) units. Rhodopsin regeneration and log threshold for this region are described by the same curve with a time constant of about 400 sec. Each log unit fal in threshold is accompanied by 0.835% increase in rhodopsin. This time constant is in agreement with Rushton's (1961) finding, but appreciably longer than that reported by Ripps & Weale (1969a).8. The Ripps & Weale result was, however, obtained by bleaching with a very short bright xenon flash (as they did). Under these conditions, blue absorbing intermediate(s) is (are) formed, the time constant of regeneration of rhodopsin is much faster than after long tungsten bleaches, and the kinetic equation is not valid.9. The general equation, together with the relation found in 7, successfully accounts for results previously published by others of the effect of duration and intensity of bleaching on the recovery of rod threshold in the dark, provided only that more than 5% of the rhodopsin was bleached at the beginning of dark adaptation.

Dark Adaptation↗
PubMed1970

The size of rod signals.

1. This investigation is based upon Alpern's (1965) contrast flash observations. The threshold for the test flash lambda (Fig. 2a) is raised if a second flash varphi falls on the annular surround. Moreover, if lambda excites rods at threshold, it is only the rods in the surround that contribute to the threshold rise.2. The possibility that the rise in lambda threshold might be due to light physically scattered from surround to centre we exclude by several different experiments. We conclude (Fig. 1b) that the varphi flash sets up a nerve signal N which is conducted to some place C where it inhibits the signal from the centre.3. If the luminous surround, instead of being a full circle (Fig. 2a) consists only of the sectors shown black in Fig. 2b, that occupy 1/m of the surround area, it is found (in the physiological range) that the light/area on those sectors must be m times as great to produce the same threshold rise at centre, i.e. the total surround illumination must remain the same.4. This result would obviously follow if N, the inhibitory nerve signal, were proportional to the total surround illumination. We have established the converse; the signal must be proportional to the quantum catch.5. Light can be increased indefinitely, nerve signals cannot. When varphi increases sufficiently, N saturates in the same way that S-potentials and receptor potentials saturate, namely according to N = varphi/(varphi + sigma) where sigma, the semi-saturation constant is about 200 td sec, or 800 quanta absorbed per rod per flash.6. Thus the nerve signal N is proportional to the quantum catch over 4 log units in the physiological range, namely from 1 quantum per 100 rods to 100 quanta per rod per flash. Above this for another 2 log units N continues to increase, but now more slowly, after the manner of S-potentials and receptor potentials.

Dark Adaptation↗
PubMed1970

The attenuation of rod signals by backgrounds.

1. The paper which precedes this investigated the nerve interaction between two flashes, lambda at centre (Fig. 1a) and varphi on the surround region (but not on the centre). The size of the inhibitory nerve signal V generated by varphi is given by V = varphi(varphi + sigma), where sigma is the semi-saturation constant.2. A former paper (Alpern & Rushton, 1967) had shown that when the flash varphi falls upon a steady background theta, V suffers attenuation in the G-box (Fig. 1b) down to the fraction theta(D)/(theta(D) + theta) where theta(D) is the eigengrau or receptor noise. Thus, in general, the nerve signal N is given by [Formula: see text].3. This formula had only been established for a moderate range of values. In this paper we use extreme values to explore the limits of its validity. We find the equation to be true over the entire intensity range where N is measurable.4. Six different types of experiment have been performed to test various features of the equation. For instance, if log N is plotted against log varphi for various fixed values of theta, the curve is always the same with simply a vertical shift. And the shift is equal to log(1 + theta/theta(D)) for all values both of theta and of varphi.5. The most interesting curve is the plot of log varphi against log theta for fixed N. This is similar to the Weber-Fechner increment threshold but the criterion is not that varphi be strong enough to be detected, but strong enough to generate an N signal just sufficient to inhibit some fixed lambda flash. These curves (below the onset of saturation) are all the same except for vertical separation, and prove that the condition for flash detection is that a fixed signal, N(0), is generated of size 10(-5) of the maximum signal obtainable (i.e. with varphi large and theta zero).6. With strong backgrounds the curves of (5) above exhibit a marked saturation of the Aguilar & Stiles' type (1954). The family of curves each with a fixed N value shows a remarkable symmetry (Fig. 8) which in fact follows from the equation in (2) above. It has nothing to do with bleached pigment, but follows from the equation in (1) above. V there cannot exceed unity, thus when scaled by the G-box below the criterion level, further increase in varphi will not bring improvement.

Dark Adaptation↗
PubMed1970

The attenuation of rod signals by bleachings.

1. Contrast flash technique allows the rod threshold to be measured even when it lies far above the cone threshold. In this way the rod dark adaptation curve after rhodopsin bleaching can be measured over 6 log units.2. By retinal densitometry the regeneration of rhodopsin can be measured in the same subject. It is found that the log threshold is raised 1.2 units for each 10% of rhodopsin in the bleached state.3. We have tried to discover whether bleaching raises the threshold by desensitizing the rods, or (like backgrounds) by attenuating their signals. Neither suggestion satisfies all conditions.4. All are satisfied by [Formula: see text], where N is the size of rod signal, constant for threshold; theta, theta(D) are steady backgrounds of light and receptor noise; varphi is the threshold flash with sigma a constant of about 2.5 log td sec; B the fraction of pigment in the bleached state.

Dark Adaptation↗