Residual Visual Function Research Articles

Behavioural and electrophysiological chromatic and achromatic contrast sensitivity in an achromatopsic patient.

In cases of incomplete achromatopsia it is unclear whether residual visual function is mediated by intact striate cortex or results from incomplete lesions to extrastriate cortical visual areas. A patient with complete cerebral achromatopsia was tested to establish the nature of his residual vision and to determine the integrity of striate cortex function. Behavioural contrast sensitivity, using the method of adjustment, and averaged visually evoked cortical potentials were measured to sinusoidally modulated chromatic and achromatic gratings in an achromatopsic patient and a normal observer. Eye movements were measured in the patient using a Skalar infrared monitoring system. The patient's chromatic contrast sensitivity was normal, indicating that despite his dense colour blindness his occipital cortex still processed information about spatial variations in hue. His sensitivity to achromatic gratings was depressed particularly at high spatial frequencies, possibly because of his jerk nystagmus. These behavioural results were reinforced by the nature of visually evoked responses to chromatic and achromatic gratings, in which total colour blindness coexisted with an almost normal cortical potential to isoluminant chromatic gratings. The results show that information about chromatic contrast is present in some cortical areas, and coded in a colour-opponent fashion, in the absence of any perceptual experience of colour.

Chapter 28 Neural bases of residual vision in hemicorticectomized monkeys

In this series of studies, we have attempted to characterize anatomically the organization of the retinofugal pathways in monkeys that underwent the surgical removal in infancy of the entire left cerebral hemisphere. Hemidecordication in baby monkeys produced a transneuronal retrograde degeneration of the retinal ganglion cells (RGCs) that affected mainly the foveal rim. Although the density of RGCs in this region was drastically diminished, the soma sizes of the surviving cells remained normal. The lateral geniculate nucleus (dLGN) ipsilateral to the removed cortex was dramatically reduced in size although it still showed normal layering. There was a marked reduction in the number of neurons in both the parvocellular and magnocellular layers and a heavy gliosis. By contrast, the superior colliculus ipsilateral to the lesion was remarkably well preserved: although slightly reduced in volume, it showed little gliosis and a metabolic activity, as revealed by cytochrome oxidase histochemistry, similar to the superior colliculus contralateral to the lesion. Behavioral perimetry indicated a partial sparing of vision up to 45 degrees in the 'blind' hemifield. We argue that the preservation of the retino-tectal pathway mediates most of the residual visual functions found in the 'blind field' of hemispherectomized human subjects.

Clinical uses of the scanning laser ophthalmoscope

SummaryThe scanning laser ophthalmoscope (SLO), designed in the early 1980s, has many advantages over conventional imaging techniques. Continuous, real‐time images are displayed on a TV monitor and usually recorded on video‐tape. Images can be obtained without pupil dilation and it is often possible to image through cataract and other media opacities. The lasers used for retinal imaging, helium‐neon, argon and infra‐red diode, are low powered and the light levels used in the SLO are about 1000 times less than in conventional fundus photography. Fluorescein and indocyanine green angiography can be performed and the dynamic sequences recorded on video‐tape.Several systems are available, for example Rodenstock, Zeiss and Heidelberg, and each have specific features. Typically the field of view is 20 or 40 degrees. Instruments can work in a non‐confocal, confocal or indirect mode. While the non‐confocal mode is used for general retinal imaging, in the confocal mode, thin slices of a structure can be imaged, e.g. the optic nerve head, and by means of a computer three‐dimensional reconstruction is possible. In the indirect mode, the image is formed from structures that scatter light, e.g. the choroidal vessels.The SLO also has the ability to perform psychophysical measurement. Any graphics which can be displayed on a TV monitor can be encoded into the scanning laser beam. The stimuli can be altered in character, size, contrast and velocity. The tests that allow assessment of visual function include visual acuity, retinal perimetry and measurements of fixation stability. Scrolled text can be used to allow precise evaluation of reading ability at different retinal locations.Using these tests we have investigated residual vision in patients with macular disease. Microperimetry was able to define accurately the size and location of retinal lesions and in some patients residual visual function was detected in areas that appeared to be non‐functioning. Some patients had fixation stability which was exceptionally poor whilst in others it was normal. The ability to recognize scrolling text declined with increasing retinal eccentricity and decreasing text contrast. The imaging and psychophysical abilities of SLOs are providing valuable information on patients with retinal diseases.

Clinical uses of the scanning laser ophthalmoscope

Summary The scanning laser ophthalmoscope (SLO), designed in the early 1980s, has many advantages over conventional imaging techniques. Continuous, real-time images are displayed on a TV monitor and usually recorded on video-tape. Images can be obtained without pupil dilation and it is often possible to image through cataract and other media opacities. The lasers used for retinal imaging, helium-neon, argon and infra-red diode, are low powered and the light levels used in the SLO are about 1000 times less than in conventional fundus photography. Fluorescein and indocyanine green angiography can be performed and the dynamic sequences recorded on video-tape. Several systems are available, for example Rodenstock, Zeiss and Heidelberg, and each have specific features. Typically the field of view is 20 or 40 degrees. Instruments can work in a non-confocal, confocal or indirect mode. While the non-confocal mode is used for general retinal imaging, in the confocal mode, thin slices of a structure can be imaged, e.g. the optic nerve head, and by means of a computer three-dimensional reconstruction is possible. In the indirect mode, the image is formed from structures that scatter light, e.g. the choroidal vessels. The SLO also has the ability to perform psychophysical measurement. Any graphics which can be displayed on a TV monitor can be encoded into the scanning laser beam. The stimuli can be altered in character, size, contrast and velocity. The tests that allow assessment of visual function include visual acuity, retinal perimetry and measurements of fixation stability. Scrolled text can be used to allow precise evaluation of reading ability at different retinal locations. Using these tests we have investigated residual vision in patients with macular disease. Microperimetry was able to define accurately the size and location of retinal lesions and in some patients residual visual function was detected in areas that appeared to be non-functioning. Some patients had fixation stability which was exceptionally poor whilst in others it was normal. The ability to recognize scrolling text declined with increasing retinal eccentricity and decreasing text contrast. The imaging and psychophysical abilities of SLOs are providing valuable information on patients with retinal diseases.

Defensive responses to looming visual stimuli in monkeys with unilateral striate cortex ablation

A number of residual visual functions including detection, localization and discrimination of visual stimuli, have been demonstrated in the “blind” fields of monkeys and human patients following damage to striate cortex. We report here that avoidance movements of the head can also be elicited from monkeys with unilateral striate cortex ablations when a “looming” stimulus is presented within the hemianopic and presumably “blind” field. The possible role of the retinofugal projection to superior colliculus in the mediation of these defensive head movements is discussed.

An optometric approach to low vision services

This paper was presented to Welsh social workers from all parts of the principality at the Wales Council for the Blind National Conference, University College, Aberystwyth, 29 July 1992. It was part of a wider training seminar on current clinical low vision and medical practice. The general concept of a multi-disciplinary approach to low vision services is discussed, with particular refer ence to the Kooyong Low Vision Clinic in Melbourne. The deficiencies found in the UK in this regard are highlighted, together with suggestions for improvement. It is claimed that better client- centred liaison is required and that this should be two-way and perhaps formalised. A protocol for the optometric assessment of residual visual function and the assessment for low vision aids is outlined. Statistics of epidemiology, LVA provision and success rates are described.

Towards a classification of visual impairment.

For the rehabilitation of people with impaired vision, it is essential to have adequate (preferably quantitative) information about their residual visual functions. Special attention is given to the extra information provided by the results of measurement of the contrast sensitivity, especially in combination with the results of other measurements, such as the visual field, the amount of intraocular straylight and the visual acuity. The value of the contrast sensitivity function as a predictor of the extent of dysfunctioning in the visual activities of everyday life, such as outdoor vision, reading and recognition of faces, will be discussed. As far as this is concerned, a comparison is made between the value of visual acuity and contrast sensitivity as measures of the extent of vision and visual dysfunctioning.

Wavelength sensitivity in blindsight.

Blindsight--the residual visual functions observed in visualfield defects resulting from destruction of part of the primary visual cortex (striate cortex) even though visual stimuli presented in the field defect are not consciously perceived--has generated new insights into the nature of consciousness and the role of the extrastriate pathways in visual processing. Some patients can detect and localize unseen stimuli when they are required to guess. Discrimination of movement, flicker and orientation may also be present, but residual colour discrimination is controversial. Negative results imply that only the pathways from eye to striate cortex can transmit information about colour in primates. By measuring sensitivity to light of different wavelengths in patients with blindsight we show that spectral sensitivity in the blind fields is surprisingly high, with a reduction of only 1 log unit or less across the visible spectrum. It is also essentially normal in form, whether the patients are adapted to light or dark. The shift in peak sensitivity from medium to shorter wavelengths in adaptation to the dark (the Purkinje shift) and the presence of discontinuities in the light-adapted curve together show that blindsight involves both rod and cone contributions, and that some colour opponency remains. As colour opponency requires input from primate beta retinal ganglion cells, two-thirds of which degenerate transneurally after a striate cortical lesion in juvenile monkeys, our results show that the surviving subpopulation of primate beta cells is functional.

Dissociation of two residual visual functions in areas of blindness due to postgeniculate lesions

Se sabe que la estimulación mediante acupuntura del punto E 36 (Zusanli) produce efectos beneficiosos en patologías inflamatorias tanto del tracto digestivo como del resto del cuerpo a través de unos mecanismos no del todo bien conocidos. La estimulación de E 36 (Zusanli) es capaz de reducir citocinas proinflamatorias. Por otra parte, la estimulación del nervio vago es capaz también de reducir estas citocinas a través de la “vía parasimpática antiinflamatoria”. Artículos recientes demuestran que la electroacupuntura de E 36 (Zusanli) vehiculizada a través de fibras del nervio ciático es capaz de activar centros troncoencefálicos y hacer descender información hasta las glándulas suprarrenales a través de fibras del vago. El efecto antiinflamatorio así conseguido se basa en la liberación del neurotransmisor dopamina y activación de su receptor tipo D1 en las suprarrenales. El conocimiento de este mecanismo abre las puertas a la utilización potencial de la estimulación de puntos concretos del cuerpo para controlar un proceso inflamatorio.Electrical stimulation of the E 36 (Zusanli) acupuncture point has shown beneficial effects for several inflammatory pathologies throughout the body and in the digestive system in particular. However, the mechanisms underlying this phenomenon are not well understood. Stimulation of E 36 (Zusanli) can result in a reduction in the level of proinflammatory cytokines. Similarly, stimulation of the vagus nerve is also able to reduce cytokine levels via the so-called “parasympathetic anti-inflammatory pathway”. Recent studies have shown that electroacupuncture of E 36 (Zusanli) leads to activation of the brainstem via signaling along the sciatic nerve and subsequent signaling to the adrenal glands via the vagus nerve. The ensuing anti-inflammatory effects are achieved via liberation of the dopamine neurotransmitter and subsequent activation of its D1 dopamine receptors in the adrenal glands. A deeper scientific understanding of the underlying mechanisms and the therapeutic effects of acupuncture confers greater confidence in this non-invasive, non-pharmacological, cost-effective method of electro-stimulating specific areas of the body to control inflammatory processes.

Neuroanatomy of the human visual system: Part II Retinal projections to the superior colliculus and pulvinar

Experimental animal investigations have shown the superior colliculus (SC) to be a primary visual nucleus of great significance for visual function. Despite this evidence, the SC has been largely disregarded as a primary visual nucleus in man. Most reports of total cortical blindness in humans described a loss of all visual responses. Several reports, however, demonstrated that a small, but significant, residual visual function remains in patients with cortical lesions. Such clinical evidence suggests the existence of multiple retinofugal pathways.Direct histological examination of the human visual system awaited the development of a technique applicable to postmortem brain tissue. Using the PPD method, we have demonstrated retinofugal projections to both the SC and the pulvinar.

Pseudomonas Aeruginosa Scleritis

In two patients Pseudomonas aeruginosa scleral infection led to ocular perforation. In one patient, a scleral abscess was identified anteriorly. A scleral perforation occurred at a more posterior focus, but the eye was salvaged with minimal residual visual function. In the other patient, perforation at the corneoscleral limbus occurred after initial corneal improvement with antibiotic therapy; histopathologic examination of the enucleated globe disclosed an abscess extending from the corneoscleral limbus to the equator superiorly.

Leter: Residual visual function after brain wounds involving the central visual pathways in man.

IT is generally believed that total destruction of visual cortex and optic radiations in man should lead to blindness, and that regional (subtotal) destruction should correspondingly produce circumscribed areas of blindness (“scotomata”) in the visual field1. Such areas of blindness are defined by a patient's inability to detect and report visual stimuli projected into the affected region. Standard methods of visual field testing (perimetry) suggest that such scotomata may be “absolute”, that is the patient seems to be unable to distinguish between the presence and absence of visually presented targets whenever they are presented in the scotomatous area. Suspecting that the response of the visual system may depend on the task requirements, we used a technique which requires a localizing response from the patient2, in addition to clinical perimetry. As a result, we have found evidence for the processing of information about the locus of light stimuli presented in areas of the visual field which are “blind” by the traditional definition.

Unit analysis of the pretectal nuclear group in the rat

AbstractUnit responses to visual stimuli in the pretectal region of the rat were investigated with steel microelectrodes. Most units could be classified into two broad categories; “phasic” (brief on and off discharges) and “tonic‐on” (sustained discharge during the visual stimulus). The responsive region appears to be coextensive with the n. praetectalis anterior (Pta) which can be divided into a dorsal portion containing “phasic” units and a ventral portion containing “tonic‐on” units. The projection of the contralateral visual field is a mirror reversal of the field projection onto the superior colliculus. “Tonic‐on” units may suserve total luminous flux detection necessary for the pupillary reflex. The topographical organization of units within Pta, as well as their diverse properties may account for residual visual function in animals with extensive lesions of the main visual pathways.

Wide angle high speed lens

To investigate the efficacy of a new surgical and rehabilitative procedure designed to improve vision in patients with central scotoma due to macular diseases.Case series of 40 consecutive surgical and rehabilitative procedures.Forty eyes of 35 consecutive patients with a stable central scotoma due to macular disease underwent phacoemulsification cataract surgery with the implant of the IOL-Vip System.The IOL-Vip System consists of a biconcave high minus-power intraocular lens (IOL) in the capsular bag and a biconvex high plus-power IOL in the anterior chamber, reproducing an intraocular Galilean telescope with ×1.3 magnification for distance. Selection of the candidate patients was carried out by means of a low-vision diagnostic and rehabilitative program (IOL-Vip software) that evaluates the residual visual function of patients and prognosis for visual improvement based on simulation of the postoperative condition. The software also designs the rehabilitation strategies based on preoperative and postoperative training of the preferred retinal locus.Best-corrected visual acuity (BCVA), evaluated by means of the Early Treatment Diabetic Retinopathy Study charts and procedure; reading magnification; and reading distance.All patients showed an improvement of visual acuity (VA) due to the surgical and rehabilitative procedure, confirming or exceeding the preoperative expected results. Mean postoperative BCVA was 0.77 (logarithm of the minimum angle of resolution), compared with 1.28 preoperatively. The mean postoperative best reading magnification gain was ×6.2, and the mean postoperative reading distance gain was 7.66 cm. No cases of intraoperative or postoperative complications were detected, and the implant was subjectively well tolerated in both monocular and binocular procedures.In this pilot study, the IOL-Vip System was shown to be effective and apparently well tolerated in improving the vision of patients with macular disease. Best-corrected VA, reading magnification, and reading distance improved in all cases of this low-vision patient series.