Changes to colour vision on exposure to high altitude.

OBJECTIVES: Several studies have shown deterioration in colour vision at altitudes above 3,000m. These studies have been conducted in photopic (bright daylight) conditions, whereas many military operations take place in mesopic (dim light) conditions. Data suggests that the tritan colour vision axis (blue cones, TA) are more susceptible to hypoxic insult than protan axis (red cones, PA). The objective of this study was to examine colour vision at high altitude, in mesopic conditions, and using a novel method of assessment to discriminate between the tritan and protan axis. METHODS: We examined 42 eyes (21 subjects, mean age 44, range 22-71), at sea level and within 12-36 hours of exposure to 3300m. This was done in a darkened room, with refractive error correction. Colour vision was studied using ChromaTest, a software programme that analyzes colour contrast threshold (CCT) of both TA and PA. We planned to repeat CCT measurement at 4,392m, but technology failure prevented this. Non-parametric paired data was examined using the Wilcoxon signed rank test. RESULTS: There was found to be no change to either the PA (p = 0.409) or the TA (p = 0.871) upon ascent. Within the PA 16 eyes had a lower CCT at high altitude, whilst 26 were higher. In the TA 20 eyes had a lower CCT and 22 were higher. At sea level, mean CCT for PA was 4.21 (SD 2.29) TA was 7.06 (SD 1.77). At 3,300m mean CCT for PA was 4.36 (SD 2.86) and TA was 6.93 (SD 2.39). CONCLUSIONS: This experiment revealed no changes to colour vision with exposure to 3,300m. This may be below the threshold altitude for cone dysfunction, alternatively colour vision deterioration may be less significant in mesopic conditions.

Core-shell magnetic morphology of structurally uniform magnetite nanoparticles.

A new development in small-angle neutron scattering with polarization analysis allows us to directly extract the average spatial distributions of magnetic moments and their correlations with three-dimensional directional sensitivity in any magnetic field. Applied to a collection of spherical magnetite nanoparticles 9.0 nm in diameter, this enhanced method reveals uniformly canted, magnetically active shells in a nominally saturating field of 1.2 T. The shell thickness depends on temperature, and it disappears altogether when the external field is removed, confirming that these canted nanoparticle shells are magnetic, rather than structural, in origin.

The incidence of negative ERG in clinical practice.

INTRODUCTION: A negative electroretinogram (ERG) is one in which there is a selective reduction in amplitude of the b-wave, such that it does not exceed that of the a-wave. The purpose of this study was to determine the incidence and clinical causes of negative ERGs at a tertiary referral centre. In addition, interesting and previously unreported aetiologies are described. PATIENTS AND METHODS: Retrospective review of all ERGs done at Moorfields Eye Hospital from November 1995 to December 1998 under ISCEV standard conditions. Many patients had photopic ON- and OFF-response recording in addition to conventional ISCEV Standard ganzfeld ERG. RESULTS: A total of 2,640 ERGs were performed during the study period. 128 cases (4.8%) showed a negative ERG. The causes, where a firm clinical diagnosis was possible, include X-linked juvenile retinoschisis, congenital stationary night blindness, central retinal artery occlusion, birdshot chorioretinopathy and melanoma-associated retinopathy (MAR). Unilateral negative ERG waveforms with normal fundal appearances were seen in 7 patients. Photopic ON- responses could be selectively affected. CONCLUSIONS: The incidence of negative ERGs over a 34-month period presenting to a large tertiary centre was almost 5%. The presence of a negative ERG may be instrumental in demonstrating the site of visual dysfunction, with many cases showing minimal or no fundus abnormality. ON- and OFF-response recording yielded additional information regarding photopic post-receptoral/phototransduction function.

S-cone ERGs elicited by a simple technique in normals and in tritanopes.

PURPOSE: To measure changes in the relative spectral sensitivities of the dark adapted and light adapted ERG and thus to establish the possible contribution of rods to the 'blue cone' ERG elicited by flashes of blue light. BACKGROUND: Short wavelength stimuli in the light-adapted eye evoke small rounded b-waves which have been considered to be S-cone responses. We have recorded such responses from tritanopes, which called the assumptions into question. METHODS: Small ERGs were recorded to blue and green flashes. The stimulus was a Ganzfeld which employed light emitting diodes. ERGs were obtained in both the dark-adapted eye and after light adaptation to intense orange light (peak wavelength 610 nm). The change in sensitivity with light adaptation and the relative spectral sensitivity was determined from the voltage/log light intensity functions, using a 10 microV criterion. RESULTS: (1) peak times and changes in sensitivity did not help distinguish light-adapted rod from possible S-cone responses; (2) analysis of the change in the ratio of blue:green sensitivity from darkness to 4.4 log Td. 610 nm background suggests that in seven normal subjects, 90% or more of the ERG evoked by 440 nm flashes is generated by S-cones; (3) three tritanopes have insignificantly reduced S-cone responses. CONCLUSIONS: (1) clinical techniques used to isolate S-cone ERGs are appropriate; (2) there are at least two types of tritanope and in those we investigated, functional S-cones are probably displaced into the retinal periphery.

Colour contrast sensitivity in patients with age-related Bruch's membrane changes.

Patients with bilateral drusen as a manifestation of early age-related macular degeneration (AMD) may have minor psychophysically detectable visual defects in the presence of normal visual acuity. In a variety of retinal diseases, one of the earliest changes in visual processing is an impairment of normal colour vision. This study was undertaken to evaluate colour vision deficits in patients with macular drusen and to determine whether changes in colour contrast sensitivity may occur over time. In a prospective study, colour vision in 84 eyes of 84 patients aged 55-84 years (mean, 68.89 +/- 6.23 years) with macular drusen and clear media was tested using a computer graphics technique. A total of 47 patients were reviewed annually for up to 2 years and measurements were obtained at annual intervals. Colour contrasts sensitivity along protan, deutan and tritan colour confusion lines was determined at a foveal and a parafoveal region. The sensitivity to all stimuli showed large variations between patients. The thresholds for foveal blue-colour contrast sensitivity were elevated and increased during the review period. In contrast, there was no significant change in sensitivity with time for red and green at the foveal or parafoveal region. Tritan threshold changes suggest that the SW cone-receptor population is more susceptible to damage associated with early age-related macular disease than are red or green cones. The results indicate that blue colour contrast sensitivity determined over time may serve as a measure to assess the progression of age-related maculopathy prior to the manifestation of atrophic or exudative macular lesions associated with visual loss.

Gold foil electrodes: a two-center study of electrode reliability.

Using the routine techniques employed at our two centers for recording pattern electroretinograms and a variety of methods, we investigated the reliability of the gold foil electrode over repeated uses. We found no significant difference between the pattern electroretinograms recorded with new electrodes and those that had had such extensive use in patients that they were believed unsuitable for further clinical use. We therefore continue to recommend the gold foil electrode for repeated use.

A survey of color discrimination in German ophthalmologists. Changes associated with the use of lasers and operating microscopes.

Color vision tests were performed on 211 German ophthalmologists during their annual meeting at Essen. The subjects also answered detailed questionnaires about their use of lasers and operating microscopes, and their ocular and general health. It was found that 33% of doctors who use lasers or operating microscopes have decreased color discrimination for colors in a tritan color-confusion axis (greater than 2 standard deviations above normal). There is a relationship between number of patients treated and the degree of threshold elevation. Thirty hours of using the operating microscope produces an increase in tritan threshold equivalent to one panretinal photocoagulation.

Nyctalopia with normal rod function: a suppression of cones by rods.

Twenty-nine patients with exaggerated rod-cone interaction are described. All were referred because they appeared to suffer from night blindness. ERG and EOGs were performed but were normal. However, investigation with a modified dark-adaptometry technique showed that in these patients cone flicker thresholds rise considerably more during dark adaptation than is normal, and this is sufficient to explain the symptoms. In one case, the condition appears familial. Many patients report their symptoms begin in early adult life and slowly get worse, but we have no objective evidence of progression.

Separable evoked retinal and cortical potentials from each major visual pathway: preliminary results.

Single cell experiments in primates show that there are two major parallel pathways named after the lamination in the lateral geniculate nucleus. Each of these systems can be preferentially excited by appropriate stimuli. Here we report that in man the polarity of the evoked potentials both in retina and in cortex depends on which of these pathways is stimulated. The identification of the resulting waveforms is thereby simplified--a matter of practical importance. The fact that at retina and cortex there are characteristic potentials may reflect the different cell biology of the two pathways.

A new cause for difficulty in seeing at night.

A syndrome is described in which patients complain of inability to see at night, although their rod and cone thresholds are normal. In such persons the rods, when dark-adapted, elevate cone flicker thresholds by 2 log units or more, and this condition is the basis of their complaint. Patients are often thought to be hysterics or to have congenital stationary night blindness.

Rod-cone interactions and analysis of retinal disease.

Cone flicker threshold rises as the rods dark adapt, though the cone threshold to continuous light remains constant. The rise is normally about 1 log unit, but in certain patients who complain of night blindness it may be as great as 2.5 log units. In these persons the kinetics of the rod-cone interaction are those of the recovery of rod sensitivity. The rods impose a low-pass filter on the cones. This effect is absent in congenital nyctalopia and X-linked retinoschisis. We suggest that cone flicker is maintained through a feedback system involving horizontal cells, and when the rod dark current returns in dark adaptation this feedback is altered. Rod cone interaction thus tests rod dark current, and cases of abnormal interaction in patients with retinitis pigmentosa occur, which indicate that the transduction mechanism and the membrane dark current may be differentially affected.

Visual evoked responses to light emitting diode (LED) photostimulation in newborn infants.

The use of a Xenon discharge stroboscope to elicit VERs in a neonatal intensive care unit has several disadvantages. To overcome these a photostimulator has been developed which consists of an array of 25 red light emitting diodes (LEDs). The LEDs are driven by pulses of fixed length and current and the output intensity is controlled by the pulse repetition rate. Such a photostimulator is compact and can be hand-held inside a cot or incubator. Using this method VERs have been recorded in healthy full-term and pre-term infants which are comparable in wave form and latency to those elicited by a stroboscope.

Rod and cone activity in patients with dominantly inherited retinitis pigmentosa: comparisons between psychophysical and electroretinographic measurements.

Extended electroretinographic (ERG) testing has been carried out in a series of patients with retinitis pigmentosa, dominantly inherited. In 36 of 57 cases only cone b waves were evoked. In 20 of these, psychophysical tests showed only cones mediated vision (Massof class I), while in 16 statis scotopic perimetry demonstrated residual rod function (class II). In the remaining cases, where rod ERGs were seen, the light intensities required to evoke responses were not greatly elevated. A computer model was constructed to relate psychophysical threshold measurements to ERG data. This analysis of the results suggests that in one subgroup of patients the scotopic ERG is smaller than expected from the losses of visual field and that in another the psychophysical elevation of rod visual threshold is greater than expected from ERG measurements.

A modified ERG technique and the results obtained in X-linked retinitis pigmentosa.

An electroretinographic (ERG) technique is described in which the relationship between scotopic b wave amplitude and stimulus light intensity is determined. The relative amplitude of scotopic to photopic responses is assessed by means of red light and flicker. The method is applied to the detection of ERG abnormalities in heterozygotes for X-linked retinitis pigmentosa. These have been found in only a proportion of cases. The ERG results can be used to suggest the nature of the retinal abnormality.

An automated statis perimeter/adaptometer using light emitting diodes.

An automated static perimeter/adaptometer is described which measures thresholds with lights of 2 wavelengths. The instrument uses light-emitting diodes to produce the stimuli and is controlled by a small computer, making it very suitable for clinical testing of large numbers of patients. The use of 2 LEDs with different peak emission wavelengths (530 and 660 nm) permits an assessment of the relative state of rod and cone mechanisms in a particular region of the retina either during dark adaptation or when the eye is fully dark adapted.

Clinical and experimental evidence that the pattern electroretinogram (PERG) is generated in more proximal retinal layers than the focal electroretinogram (FERG).

A TV monitor was used to evoke either a pattern ERG to a contrast-reversing checkerboard (PERG), or a focal ERG to alternate increases and decreases of luminance of the blank screen within a bright surround (FERG). Both responses are small (approx 2 microV) and fast (approx 50 msec to peak) and are similar in several other properties. However, they differ in timing and respond differently to changes in contrast. Each frame of a TV picture evokes a "raster ERG," even though the screen is blank. The response is focal and specific to a small central strip of the screen. It is simpler to record than the FERG, where the whole screen is flashing. Because the FERG summation area is about 4 deg, small squares (checks) reversing in contrast produce little luminance response. In 5 of 7 cases where the PERG is unilaterally reduced, the FERGs or raster responses were not affected. Thus clinical evidence also suggests that the PERG may be a separate phenomenon to the FERG and produced at a different site. Toxic, traumatic, congenital, and degenerative diseases of the optic nerve reduce the PERG. The comparison is most easily made in unilateral disease. Ten weeks after an optic nerve insult, the PERG becomes reduced in the affected eye as if retrograde degeneration was occurring. In 27 amblyopes of various types, the PERG was reduced in 23 where orthoptic treatment had failed. In 4 patients responding to treatment, PERGs of the amblyopic eye were as large as, or larger than, those of the fellow eye. The loss is greater with smaller checks. Retinal changes do occur after age 4 but so slowly that responses in heavily occluded eyes are not reduced. An additional level in the visual pathway is thus accessible to evoked potential investigation.

Pattern ERGs are abnormal in many amblyopes.

We have used a pattern-reversing chequerboard (1 degree squares, or smaller: 84 per cent contrast, or less: 2 to 16 reversals/sec: field size 22 degrees x 16 degrees: mean luminance 50 cd/m2) to evoke responses from the eyes of young adults and children. ERGs were recorded with gold foil electrodes, which did not interfere with the normal optics. All patients were affected accurately. The fixation point was adjusted so that squinters received the stimulus on corresponding areas of each retina. The normal ERG obtained is 1.5 to 3 mu V in amplitude, and in thirteen normal subjects right and left eyes gave equal responses. The extreme asymmetry detected was c. 10 per cent. In all amblyopes, including children of 6 years old and upwards, satisfactory recordings were obtained. In most amblyopes, the response from the affected eye was 50 per cent or less of the response from the fellow eye. In all cases of failure to respond to treatment, the asymmetry was highly significant. In adults, occluding the screen area corresponding to the area of amblyopic suppression, reduces the ERG in the fellow eye, but does not reduce the ERG in the amblyopic eye. We conclude that amblyopic retina produces very little electrical activity that we can record. The timing, response to frequency of stimulation, and other parameters in the pattern ERG are dissimilar from those in the ERG produced by small changes of luminance. We have little evidence about the class of cell generating the response we observe, but evidently there is peripheral abnormality in the visual system even of amblyopic children.