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G Wald

Publications and source records attributed to G Wald.

At least 19 recordsLinked to original sources
PubMed1973

Color-vision mechanisms in the peripheral retinas of normal and dichromatic observers.

It is possible that so-called normal trichromatic vision occurs only between the central blue-blind fixation area and about 30 degrees peripherally. Beyond about 30 degrees vision has been alleged to become dichromatic (red-green blind), and beyond about 60 degrees , monochromatic. Hence every form of color blindness may characterize various zones of the normal retina. We have studied mechanisms of peripheral color vision, mainly by measuring the spectral sensitivities of the blue-, green-, and red-sensitive systems, isolated by differential color adaptation. In normal observers the sensitivity of the blue-mechanism falls off about 2 log units by 80 degrees out. The green- and red-sensitive systems decline only about 0.7 log unit over the same range. Protanopes, deuteranopes, and tritanopes exhibit comparable changes. We have not found any color mechanism present centrally to be wholly lost peripherally. Nor, for dichromats, have we found any mechanism missing centrally to be present peripherally. Whatever evidences of peripheral color blindness have been observed appear to involve other mechanisms than failure of receptors, probably including some fusion of neural pathways from receptors to centers.

Color Perception↗
PubMed1973

Orientation of intermediates in the bleaching of shear-oriented rhodopsin.

Cattle rhodopsin can be highly oriented by shearing a wet paste of digitonin micelles of this visual pigment between two quartz slides. This orients the rhodopsin micelles so that their chromophores lie mainly parallel to the direction of shear. In such preparations the orientation of rhodopsin and intermediates of its bleaching by light have been measured with plane-polarized light from -195 degrees C to room temperature. The chromophore maintains essentially the same orientation as in rhodopsin in all the intermediates of bleaching: bathorhodopsin (prelumirhodopsin), lumirhodopsin, and metarhodopsins I and II. When, however, the retinaldehyde chromophore is hydrolyzed from opsin in the presence of hydroxylamine, the retinaldehyde oxime that results rotates so as to lie mainly across the direction of shear. That is, the retinal oxime, though free, orients itself upon the oriented matrix of the opsin-digitonin micelles. These experiments show the rhodopsin-digitonin micelle to be markedly asymmetric, with the chromophore lying parallel to its long axis. The asymmetry could originate in the formation of the micelle, in rhodopsin itself, or by its linear polymerization under the conditions of the experiment. If rhodopsin itself is markedly asymmetric, for which there is some evidence, then, since in the rod outer segments its chromophores lie parallel to the disk membranes, the molecules themselves must lie with their long axes parallel to the membranes.

Animals↗
PubMed1972

The orientation of rhodopsin and other pigments in dry films.

When rhodopsin in a gelatin film is dried, the rhodopsin chromophores orient primarily in the plane of the film. When the film is wetted, the chromophores disorient. These changes are reversible. When rhodopsin in a wet film. is bleached in the presence of hydroxylamine and redried, the retinal oxime which results is oriented more perpendicularly to the plane of the film. These orientations in dry gelatin films resemble those in the disc membranes of rod outer segments. A variety of other proteins are similarly oriented in dry gelatin films: methemoglobin, cytochrome c, phycocyanin. Films of methemoglobin and cytochrome c display prominently the high Soret band near 410 nm when measured with unpolarized light passing through the face of the fim, but display no Soret band at all with light passing through the edge of the film. All of these orientations imply a large asymmetry of the protein micelles, perhaps conferred upon them by linear polymerization in the course of drying. Such asymmetry can be demonstrated directly with rhodopsin. A wet paste of rhodopsin-digitonin micelles, sheared between glass slides, becomes highly oriented, the rhodopsin chromophores lining up in the direction of shear, the retinal oxime produced by bleaching orienting more perpendicularly to the shear.

Acetates↗
PubMed1971

Rhodopsin and porphyropsin fields in the adult bullfrog retina.

Though it had been supposed earlier that the bullfrog undergoes a virtually complete metamorphosis of visual systems from vitamin A(2) and porphyropsin in the tadpole to vitamin A(1) and rhodopsin in the adult, the present observations show that the retina of the adult frog may contain as much as 30-40% porphyropsin, all of it segregated in the dorsal zone. The most dorsal quarter of the adult retina may contain 81-89% porphyropsin mixed with a minor amount of rhodopsin; the ventral half contains only rhodopsin. Further, the dorsal zone contains a two to three times higher concentration of visual pigments than the ventral retina. The pigment epithelium underlying the retina contains a corresponding distribution of vitamins A(1) and A(2), predominantly vitamin A(2) in the dorsal pigment epithelium, exclusively vitamin A(1) in the ventral zone. The retina accepts whatever vitamin A the pigment epithelium provides it with, and turns it into the corresponding visual pigment. Thus, a piece of light-adapted dorsal retina laid back on ventral pigment epithelium regenerates rhodopsin, whereas a piece of light-adapted ventral retina laid back on dorsal pigment epithelium regenerates predominantly porphyropsin. Vitamin A(2) must be made from vitamin A(1), by dehydrogenation at the 3,4-bond in the ring. This conversion must occur in the pigment epithelium, presumably through the action of a vitamin A-3,4-dehydrogenase. The essential change at metamorphosis is to make much less of this dehydrogenase, and to sequester it in the dorsal pigment epithelium. Some adult bullfrogs, perhaps characteristically taken in the summer, contain very little porphyropsin-only perhaps 5%-still sequestered in the dorsal retina. The gradient of light over the retinal surface has little if any effect on this distribution. The greater density of visual pigments in the dorsal retina, and perhaps also-although this is less clear-the presence of porphyropsin in this zone, has some ecological importance in increasing the retinal sensitivity to the dimmer and, on occasion, redder light received from below.

Adaptation, Ocular↗
PubMed1968

Single and multiple visual systems in arthropods.

Extraction of two visual pigments from crayfish eyes prompted an electrophysiological examination of the role of visual pigments in the compound eyes of six arthropods. The intact animals were used; in crayfishes isolated eyestalks also. Thresholds were measured in terms of the absolute or relative numbers of photons per flash at various wavelengths needed to evoke a constant amplitude of electroretinogram, usually 50 microv. Two species of crayfish, as well as the green crab, possess blue- and red-sensitive receptors apparently arranged for color discrimination. In the northern crayfish, Orconectes virilis, the spectral sensitivity of the dark-adapted eye is maximal at about 550 mmicro, and on adaptation to bright red or blue lights breaks into two functions with lambda(max) respectively at about 435 and 565 mmicro, apparently emanating from different receptors. The swamp crayfish, Procambarus clarkii, displays a maximum sensitivity when dark-adapted at about 570 mmicro, that breaks on color adaptation into blue- and red-sensitive functions with lambda(max) about 450 and 575 mmicro, again involving different receptors. Similarly the green crab, Carcinides maenas, presents a dark-adapted sensitivity maximal at about 510 mmicro that divides on color adaptation into sensitivity curves maximal near 425 and 565 mmicro. Each of these organisms thus possesses an apparatus adequate for at least two-color vision, resembling that of human green-blinds (deuteranopes). The visual pigments of the red-sensitive systems have been extracted from the crayfish eyes. The horse-shoe crab, Limulus, and the lobster each possesses a single visual system, with lambda(max) respectively at 520 and 525 mmicro. Each of these is invariant with color adaptation. In each case the visual pigment had already been identified in extracts. The spider crab, Libinia emarginata, presents another variation. It possesses two visual systems apparently differentiated, not for color discrimination but for use in dim and bright light, like vertebrate rods and cones. The spectral sensitivity of the dark-adapted eye is maximal at about 490 mmicro and on light adaptation, whether to blue, red, or white light, is displaced toward shorter wavelengths in what is essentially a reverse Purkinje shift. In all these animals dark adaptation appears to involve two phases: a rapid, hyperbolic fall of log threshold associated probably with visual pigment regeneration, followed by a slow, almost linear fall of log threshold that may be associated with pigment migration.

Adaptation, Biological↗
PubMed1968

Oscillations of potential in the electroretinogram of the lobster.

The electroretinogram (ERG) evoked in the lobster by a short flash of light consists of a highly damped, slow oscillation of potential, triggered apparently by a single excitatory process. Near the threshold, only one wave may be evident; but as the intensity of stimulus rises, a prior wave appears, and grows so much more rapidly as to become dominant. Simultaneously third and later waves appear, so that at high intensities the response may include five to seven waves. Dark adaptation favors the second and later waves relative to the first; light adaptation tends to suppress them. On turning on a steady light the oscillations are superimposed on the early stages of development of a maintained, steady-state potential (on-response). Turning off the light causes a rapid fluctuation of potential followed by a similarly damped slow oscillation (off-response). These phenomena resemble in part oscillations recently observed in the b wave of the ERG of many vertebrates including man.

Adaptation, Biological↗
PubMed1968

Spectral sensitivity of the common prawn, Palaemonetes vulgaris.

The vision of Palaemonetes is of particular interest in view of extensive studies of the responses of its chromatophore systems and eye pigments to light. The spectral sensitivity is here examined under conditions of dark adaptation and adaptation to bright colored lights. In each case the relative number of photons per one-fiftieth sec flash needed to evoke a constant peak amplitude (usually 25 or 50 microv) in the electroretinogram (ERG) was measured at various wavelengths throughout the spectrum. The sensitivity is the reciprocal of this number. In dark-adapted animals the spectral sensitivity curve consists of a broad, almost symmetrical band, maximal at about 540 mmicro, with a shoulder near 390 mmicro. Adaptation to bright red or blue light, left on continuously throughout the measurements, depresses the 540 mmicro peak without notably changing its shape or position, implying that only one visual pigment operates in this region. Adaptation to red light, however, spares a violet-sensitive system, so that a high, narrow peak at 390 mmicro now dominates the spectral sensitivity function. The 540 and 390 mmicro peaks are apparently associated with different visual pigments; and these seem to be segregated in different receptor systems, since the associated ERG's have markedly different time constants. It is suggested that these two sensitivity bands may represent the red- and violet-sensitive components of an apparatus for color differentiation.

Animals↗