Showing papers on "Foveal published in 1967"

Blue-blindness in the normal fovea

Abstract: An area at the center of the human fovea, subtending a visual angle of only 7–8 min and hence hardly larger than the fixation area, is blue-blind in the sense of almost or entirely lacking blue-sensitive cones. This is a matter of foveal topography, not size of field, for in fields of this size elsewhere in the fovea or in the parafovea, blue-sensitive cones are well represented. The blue-cone system falls in sensitivity from the border of the photopic zone—the functionally all-cone area—to a minimum, usually to extinction, at its center. Other features of foveal topography oppose this trend: the density of cones rises and the macular pigmentation thins out toward the center of the fovea. Also the red- and green-cone systems display the opposite gradient; their sensitivities decline regularly from the center toward the borders of the fovea and beyond.Tritanopia, though the rarest form of congenital color-blindness, is thus a regular feature of the center of the normal fovea. The existence of two neutral points in this condition, in the yellow and violet, has its basis in the observation that the luminosity curves of the red- and green-sensitive cones, drawn so as to cross in the yellow, cross again or fuse in the violet region. It is suggested that the blue-blindness of the fixation area is a final step in the general withdrawal of image vision from the short wavelengths of the spectrum, for which the chromatic aberration of the eye is greatest. The blue-blindness of the fixation area, taken together with the red-green blindness of more-or-less concentric zones of the near periphery, and the total colorblindness of the far periphery, raises the possibility that various zones of the normal retina display all the major forms of colorblindness. Trichromic vision is normal only in the broad, central annulus of the retina, which alone is ordinarily tested. Some instances of defective color vision may be similarly localized. The problems of both normal and defective color vision involve not only the presence or absence of certain visual pigments and types of cone, but their spatial distributions on the retinal surface, and their neural connections.

Blue-Blindness in the Normal Fovea*

Abstract: An area at the center of the human fovea, subtending a visual angle of only 7–8 min and hence hardly larger than the fixation area, is blue-blind in the sense of almost or entirely lacking blue-sensitive cones. This is a matter of foveal topography, not size of field, for in fields of this size elsewhere in the fovea or in the parafovea, blue-sensitive cones are well represented. The blue-cone system falls in sensitivity from the border of the photopic zone—the functionally all-cone area—to a minimum, usually to extinction, at its center. Other features of foveal topography oppose this trend: the density of cones rises and the macular pigmentation thins out toward the center of the fovea. Also the red- and green-cone systems display the opposite gradient; their sensitivities decline regularly from the center toward the borders of the fovea and beyond.Tritanopia, though the rarest form of congenital color-blindness, is thus a regular feature of the center of the normal fovea. The existence of two neutral points in this condition, in the yellow and violet, has its basis in the observation that the luminosity curves of the red- and green-sensitive cones, drawn so as to cross in the yellow, cross again or fuse in the violet region. It is suggested that the blue-blindness of the fixation area is a final step in the general withdrawal of image vision from the short wavelengths of the spectrum, for which the chromatic aberration of the eye is greatest. The blue-blindness of the fixation area, taken together with the red-green blindness of more-or-less concentric zones of the near periphery, and the total colorblindness of the far periphery, raises the possibility that various zones of the normal retina display all the major forms of colorblindness. Trichromic vision is normal only in the broad, central annulus of the retina, which alone is ordinarily tested. Some instances of defective color vision may be similarly localized. The problems of both normal and defective color vision involve not only the presence or absence of certain visual pigments and types of cone, but their spatial distributions on the retinal surface, and their neural connections.

Dependence of the magnitude of the Stiles—Crawford effect on retinal location

Abstract: 1. The directional sensitivity (Stiles—Crawford effect) of retinal cones is supposed to be associated with their shape, but only extrafoveal cones have a cone-like shape; cones in the central fovea are elongated and look like rods. 2. To determine whether the directional sensitivity of cones depends on their shape, the Stiles—Crawford effect was measured both in the central fovea and in the parafovea of the human eye. 2. To ensure that the cone population tested was homogeneous, a small brief test flash, brought into the eye through the centre of the pupil, was placed at threshold by varying the intensity of a large adapting field. The directional sensitivity of the cones was determined by finding the efficiency of light to act as an adapting background as a function of position of entry in the pupil. 4. Central foveal cones have a less pronounced directional sensitivity than parafoveal cones and this lends support to the conclusion that the Stiles—Crawford effect is connected with the shape of the retinal receptors.

The electroretinogram evoked by the excitation of human foveal cones.

Abstract: 1. A 2° test stimulus foveally fixed and viewed against a blue background (40° in extent and producing 2·0 × 104 scotopic td of retinal illuminance) evokes a small voltage which can be recorded from the human eye with a conventional contact lens electrode if the test stimulus is flashed at a rate of 15 c/s, and the responses to at least several hundred flashes are averaged. 2. The action spectrum of the response obtained in this way agrees reasonably well with the observer's psychophysical foveal luminosity curve. 3. For the peripheral retina, the action spectrum is similar to that of the fovea when allowance is made for differences in screening macular pigment. 4. Such responses diminish when the test stimulus is focused on to the peripheral retina and disappear when the test light is focused on the blind spot. 5. Therefore, the response to the test light fixated centrally is the result of the excitation only of cones mainly, if not exclusively, in the fovea. 6. When the intensity of the background is reduced by a factor of 10, the action spectrum shows evidence of the effect of excitation of rods in the blue part of the spectrum and of cones in the red. These red and blue responses add linearly when combined together, provided they are adjusted to coincide in phase.

Appearance of Color for Small, Brief, Spectral Stimuli, in the Central Fovea*

Abstract: While color-vision characteristics of tritanopia (blue–yellow deficiency) are well known to occur with small fields in the central fovea, the possibility of similar confusions as a function of brief duration has previously only been suggested The problem has been investigated in this study by determining the color names given by nine color-normal and two deuteranopic observers to spectral stimuli from 565 to 590 mμ and to a white light The test stimuli, all presented foveally, subtended diameters of 54, 21, and 11 min at durations of 200 and 20 msec For stimuli presented at small subtense and short duration, green was sometimes seen as blue or blue–green, a neutral band was found in the yellow–green (570–580 mμ), and no confusion was found between reds and greens The degree of tritanopic-like color confusions in the fovea is related to both the exposure time and the size of the test area The results are discussed in relation to the foveal vs small-field characteristics of apparent tritanopia

The perception of multiple simultaneously presented forms as a function of foveal spacing)

Abstract: Using five capital letters as the form stimuli and tachistoscopic presentation, an exposure duration was determined for each S that yielded 80% identification accuracy when single letter displays were presented. Then the increment in exposure duration necessary for a correct identification of all letters on a display on 80% of the trials was determined for 2, 3, and 4 letter displays. In view of evidence that perceptual independence breaks down when stimuli are spaced much closer than 1° apart in the fovea, the effect of different foveal spacing of the form stimuli in the display was studied. Spacings of 1/2, 3/4, and 1° of angle were employed. Less than a 30% increment in exposure duration was necessary to recognize 2 form displays at the same accuracy level as single form. But no further increase in exposure duration was necessary to recognize 3 and 4 form displays at the same accuracy criterion. Evidence for positive correlation of sensory perceptual error for forms spaced less than 1° apart in the fovea was found.

The physical stimulus, the quality of the retinal image and foveal brightness discrimination in one amblyopic and two normal eyes

Abstract: Many attempts have been made to analyse and describe the nature of the functional disturbance in amblyopia. There is little doubt that amblyopia cannot be more than a collective term, covering a multitude of entities with distinct and different functional behaviour. In fact, if consideration is given to the highly complex anatomy of the eye and its superordinated central nervous system, it would be astonishing if visual acuity in particular and the light sense in general, could not be impaired at many levels of the retino-cortical pathway. Having this in mind, it appears logical not to treat amblyopia as an entity but rather to analyse the phenomenology of single cases in subsequent steps, attempting to set up distinct amblyopic sub-groups; here, the localisation of the level of interference is of primary importance. Amblyopia can be subdivided into : a) disturbances of the imaging apparatus, b) disturbances of the receptors or their orientation and c) pathology of the neurological apparatus which may be retinal or central. Accordingly, an analysis of amblyopia would start in the periphery. The first step in this direction was accomplished by ENOCH (1957) who demonstrated that amblyopia may be causally related to receptor disorientation. He also found amblyopes with normal receptor orientation, where the disturbance obviously was of another kind. His findings demonstrated that the classification, structural versus functional, is largely a matter of the refinement of our instrumentation and examination methods.

The Relationship of Near-Vision Peripheral Acuity and Far-Vision Search Performance1

Abstract: Thirty-five subjects who did not wear glasses or contact lenses and with foveal acuity of 20/30 or better monocular and binocular far and near vision were given a near-vision peripheral acuity test...