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Biomedical subjects

K R Alexander

Publications and source records attributed to K R Alexander.

At least 73 records · Page 4Linked to original sources
PubMed1987

Abnormal foveal spectral sensitivity in retinitis pigmentosa.

The authors measured dark-adapted foveal spectral sensitivity functions in 30 patients with retinitis pigmentosa or Usher's syndrome, whose Snellen visual acuities were 20/30 or better. An analysis of variance of the spectral sensitivity functions (normalized at 575 nm) indicated a significant reduction in the patients' relative sensitivity at 450 and 475 nm compared with a group of 15 age-similar normal observers, while the patients' relative sensitivity at middle and long wavelengths was normal. A significant negative correlation between the patients' normalized sensitivity at 450 nm and their non-normalized sensitivity at 575 nm indicated that patients with a normal absolute sensitivity at 575 nm had the greatest reduction in relative sensitivity at 450 nm. The characteristics of the short-wavelength sensitivity loss suggest that it results from an increase in the effective optical density of the yellow macular pigment, rather than from a decreased sensitivity of a specific cone mechanism or an increased lens absorption.

Adult↗
PubMed1986

Dark-adapted foveal thresholds and visual acuity in retinitis pigmentosa.

To determine the relationship of foveal absolute thresholds to visual acuity in retinitis pigmentosa, we measured thresholds in 40 patients with various forms of retinitis pigmentosa (including Usher's syndrome) whose Snellen visual acuities were 20/30 or better. At all visual acuity levels, the patients' foveal thresholds were significantly higher than those of 20 similarly aged normal observers; threshold elevations tended to be greater for a 500-nm than for a 655-nm test flash. Foveal cone spatial summation functions were normal (test flash diameter range, 7' to 1.7 degrees), indicating that the patients' threshold elevations did not result from altered summation properties. A significant correlation between foveal cone thresholds and the midpoints of the patients' Rayleigh matches demonstrated that the threshold elevations resulted in part from a decreased cone optical density.

Adolescent↗
PubMed1986

The effect of ultraviolet, visible and infrared radiation on the rabbit blood-aqueous barrier.

We studied the effects of visible (400-750 nm), infrared (750-1100 nm), and ultraviolet (300-400 nm) radiation on the breakdown of the blood-aqueous barrier (BAB) in albino rabbit eyes using fluorophotometry and fluorescein-bound albumin. There was no evidence of BAB breakdown following exposure to visible (158 J cm-2) or infrared (106 J cm-2) radiation. Fluorescence was detected in the anterior chamber 15 min following ultraviolet (UV) (300-400 nm; 37 J cm-2 and 73 J cm-2) and ultraviolet-A (UV-A) (320-400 nm; 45 J cm-2 and 90 J cm-2) irradiation. This fluorescence peaked at 2 hr after irradiation. Reestablishment of the BAB, indicated by no detectable fluorescence, occurred in all animals after 24 hr. There was a significant increase in fluorescence with increasing UV-irradiation dosage in all groups (P less than 0.05). Comparison of BAB breakdown following exposure to ultraviolet 320-400 nm and 300-400 nm in the same dosage (45 J cm-2) indicated that the inclusion of shorter wavelength UV exposure produced a significantly higher degree of BAB breakdown (P less than 0.001).

Animals↗
PubMed1986

Background size and saturation of the rod system.

To determine the effect of background size on saturation of the human rod system, we measured threshold-vs-intensity (tvi) functions for a 24'-diameter, middle-wavelength test stimulus against long-wavelength background fields that were either 2 degrees or 8 degrees in diameter. When thresholds were measured against a series of background intensities presented during a single experimental session (a standard tvi procedure), rod increment thresholds were higher against the smaller background and eventually disappeared above cone increment thresholds at moderate background intensities. This result suggests that background diameter does influence rod system saturation. However, when adaptation to the smaller background was limited to a 5 min exposure to a single background intensity per experimental session, then thresholds against that background were rod- rather than cone-mediated and were comparable to rod thresholds against an 8 degrees background. Therefore, under these stimulus conditions, background size apparently does not influence rod system saturation unless adaptation to the smaller background is prolonged. Based on previous studies, we suggest that the rod-desensitizing effect of small backgrounds under conditions of prolonged adaptation is due to a post-retinal mechanism.

Adaptation, Ocular↗
PubMed1986

Rod influence on cone flicker detection: variation with retinal eccentricity.

Previous studies have shown that cone flicker thresholds are influenced by the adaptation state of the rod system. We have examined how the properties of this rod-cone flicker interaction differ with retinal eccentricity. The threshold for detecting 25 Hz flicker was measured in the dark-adapted eye, against a rod-saturating Ganzfeld background and following a Ganzfeld bleach. The magnitude of the interaction, defined as the difference between light- and dark-adapted flicker thresholds, covaries with changes in rod absolute threshold across the visual field. At a given eccentricity, the magnitude of the interaction is constant for test diameters ranging from 7' to 1.7 degrees, indicating that variations in magnitude with eccentricity do not result from changes in spatial summation properties. The measurement of cone flicker thresholds during bleaching recovery provides evidence that variations in the magnitude of the rod-cone flicker interaction with retinal eccentricity may result from differences in the channel capacity of the pathway carrying the threshold-elevating signal from rods.

Adaptation, Ocular↗
PubMed1985

Rod-cone interaction in flicker perimetry: evidence for a distal retinal locus.

The luminance threshold for the detection of 25 Hz flicker was measured in nine patients with retinal disorders under stimulus conditions that have been shown previously to involve an interaction between rod and cone systems. The disorders studied included congenital stationary nightblindness, X-linked juvenile retinoschisis, hereditary dominant optic atrophy, optic atrophy found in association with neurofibromatosis, retinitis pigmentosa, choroideremia, and an acquired diffuse photoreceptor disorder, all of which involve pathologic changes that are presumed to occur primarily at specific levels of the retina. The results are consistent with a distal (outer) retinal locus for the rod-cone interaction.

Adolescent↗
PubMed1985

Autosomal dominant retinitis pigmentosa. A method of classification.

Four types of autosomal dominant retinitis pigmentosa (RP) were discernible in 84 patients by clinical, electrophysiologic, and psychophysical criteria. Type 1 patients showed diffuse fundus pigmentary changes and nondetectable electroretinographic (ERG) cone and rod functions. Both type 2 and type 3 patients showed fundus pigmentary changes more apparent within the inferior retina. Type 2 patients showed marked loss in rod ERG function, with prolonged cone implicit times, whereas type 3 patients showed substantial rod function and normal cone implicit times. Type 4 patients had a funduscopically and functionally "delimited" disease, with substantial cone and rod ERG amplitudes and normal implicit times. The classification of dominantly inherited RP is important for studies of natural history in disease progression, for patient counseling, and for various laboratory investigations of patients with RP in which patient homogeneity should be maximized.

Adolescent↗
PubMed1984

The influence of cones on rod saturation with flashed backgrounds.

The increment threshold for a middle-wavelength test flash was measured at the onset of a concentric long-wavelength background flash under conditions that have previously been shown to result in rod system saturation. The influence of the cone system on rod saturation under these conditions was assessed using the Stiles-Crawford effect in normal subjects and by measuring rod thresholds in protanopes, who are deficient in long-wavelength cones. When the background flash is made less effective for cones through the Stiles-Crawford effect, the onset of rod saturation occurs at a higher luminance of background flash than normal. Similarly, protanopes do not show the characteristics of rod saturation until a much higher-than-normal luminance of background flash. The results suggest that rod system saturation with flashed backgrounds is strongly influenced by cones.

Adaptation, Ocular↗
PubMed1984

Supernormal scotopic ERG in cone dystrophy.

Three patients with a bull's-eye macular lesion and other signs characteristic of cone dystrophy gave an unusual ERG finding. In response to a white flash of moderate intensity the scotopic b-wave amplitude was considerably larger than normal. One patient had elevated rod thresholds and nyctalopia, while the other 2 had normal rod sensitivity associated with the supernormal scotopic b-wave amplitude. In the latter 2 patients the abnormal ERG pattern was unchanged for 4 years and 7 years respectively. This atypical finding, of a supernormal scotopic b-wave amplitude in response to light of moderate intensity, appears to characterise a subgroup of patients with cone dystrophy, probably of autosomal recessive inheritance. The pathogenesis of the abnormal ERG remains uncertain.

Adaptation, Ocular↗
PubMed1984

Rod-cone interaction in flicker perimetry.

We have assessed the influence of the rod system on cone flicker sensitivity during flicker perimetry. For temporal frequencies above 18 Hz extrafoveal cone-mediated flicker thresholds for a white test stimulus are as much as 1.5 log units lower when measured against a large background light that saturates the rods than when measured in darkness. Following a Ganzfeld bleach extrafoveal cone flicker thresholds are at their minimum once the cones have recovered their sensitivity, but then thresholds rise as the rods begin to recover from the bleach. Our results indicate that the flicker sensitivity of the extrafoveal cone system at high temporal frequencies is influenced by the rods surrounding the flickering test stimulus. The rods reduce flicker sensitivity maximally in the dark adapted state, and their suppressive influence is minimised only by strong rod bleaches or by large backgrounds that saturate the rod system.

Adaptation, Ocular↗
PubMed1984

Prolonged rod dark adaptation in retinitis pigmentosa.

The time course of rod dark adaptation was measured in 12 patients and carriers of retinitis pigmentosa (RP). In contrast to previous studies the rod absolute threshold was determined prior to any exposure to the bleaching light. For seven of the patients and carriers the recovery of rod sensitivity to the prebleach level was prolonged, with the delay ranging from mild to severe. The prolongation appeared to be limited to the late phase of bleaching recovery; the early portion of rod dark adaptation, measured following a weak bleach, was normal. These results suggest a selective impairment in the processes underlying rod dark adaptation in some patients with RP.

Adolescent↗
PubMed1983

Early receptor potential measurements in human ocular siderosis.

There is electrophysiologic evidence that photoreceptors have normal visual pigment density (outer segment length) at a stage of ocular siderosis when the photoreceptor membrane function is defective. The early receptor potentials (ERPs) from two patients showed normal amplitudes, although the electroretinograms (ERGs) had reduced a- and b-waves for both cone and rod ERG responses. Since the ERP is generated by photolysis of visual pigment and requires orientation of the pigment molecules by outer segment disc membranes, the normal ERP amplitudes suggest that the photoreceptor outer segments have normal photopigment density, are of normal length, and are properly oriented. However, the transduction mechanism that converts visual pigment photolysis into membrane electrical potentials (the ERG a wave) is defective at this stage of ocular siderosis.

Adult↗
PubMed1982

Rayleigh discriminations in young human infants.

The capacity of young infants to discriminate 3 x 3 degrees broadband red or 550 nm green squares from a 589 nm yellow surround was tested by means of the forced-choice preferential looking technique. All 3-month olds, about 3/4 of the 2-month olds, and just under half of the one-month-olds could make at least one of these discriminations. Taken together with other known properties of infant color vision, the failures of discrimination shown by the younger infants are more readily modeled as immaturities of neural processing than as an absence of anomaly of LWS or MWS cones.

Adult↗
PubMed1978

The effect of light flashing at 9 Hertz on rivalry suppression.

The translid binocular interactor (TBI), which presents binocularly out-of-phase lights flashing at 9 Hz, was designed to reduce suppression of amblyopes. To determine the effect of such flashing lights on rivalry suppression, subjects who showed strong suppression during binocular rivalry were exposed to a modified form of the TBI for a total of 2 2/3 hr over 4 days. A similar group of subjects was exposed to steady binocular light. The modified TBI had no effect on rivalry suppression; the changes in suppression that did occur were equivalent in the two groups.

Amblyopia↗
PubMed1975

On the nature of accommodative micropsia.

Both an increase in convergence and an increase in accommodation result in a decrease in the apparent size of objects of a fixed retinal image size (micropsia). The present study demonstrates that the accommodative micropsia produced by a minus sphere lens is due to the retinal image minification introduced by the lens and to the change in convergence accompanying accommodation. The accommodative act itself apparently does not result directly in micropsia.

Accommodation, Ocular↗
PubMed1975

Color vision testing in young children: a review.

It is often recommended that children be screened for possible color vision deficiencies as early as possible. This paper examines the validity of commercially-available color vision tests when used with young children (age three to seven years). It is concluded that with the possible exception of the anomaloscope, no test is entirely suited for children, in most cases because the test makes cognitive demands beyond the capability of the young child.

Adult↗