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K R Alexander

Publications and source records attributed to K R Alexander.

At least 19 recordsLinked to original sources
PubMed2001

Contrast discrimination deficits in retinitis pigmentosa are greater for stimuli that favor the magnocellular pathway.

Luminance contrast discrimination was measured in 14 patients with retinitis pigmentosa (RP) and 14 control observers with normal vision, using steady-pedestal and pulsed-pedestal paradigms [Pokorny, J., & Smith, V. C. (1997). Psychophysical signatures associated with magnocellular and parvocellular pathway contrast gain. Journal of the Optical Society of America A, 14, 2477-2486] to bias performance toward the magnocellular (MC) or parvocellular (PC) pathway, respectively. The aim was to determine the relative effects of retinal degeneration on MC- and PC-pathway function in RP. For five of the RP patients, contrast discrimination thresholds were within normal limits for both the steady-pedestal and pulsed-pedestal paradigms. The other nine RP patients showed threshold elevations for the steady-pedestal paradigm (presumed magnocellular mediation), whereas their thresholds for the pulsed-pedestal paradigm (presumed parvocellular mediation) were within normal limits for all but the two patients who had the most extreme threshold elevations using the steady-pedestal paradigm. A control experiment on four of the RP patients, using a greater number of pedestal contrasts, verified that the patients' thresholds for the pulsed-pedestal paradigm showed the pattern expected for contrast discrimination mediated by the PC pathway. The higher threshold elevations for the steady-pedestal paradigm than for the pulsed-pedestal paradigm indicate that the retinal degeneration that occurs in RP predominantly disrupts contrast discrimination under stimulus conditions that favor the MC pathway.

Adult↗
PubMed2001

Perceived and actual performance of daily tasks: relationship to visual function tests in individuals with retinitis pigmentosa.

PURPOSE: To investigate the functioning in daily task performance of individuals with retinitis pigmentosa (RP). Goals were (1) to quantify the relationships among clinical tests of vision, self-reports, and evaluations of actual task performance to predict difficulty in these tasks; and (2) to validate self-report questionnaire data about daily task performance with observations and measurements of actual task performance conducted by a certified low-vision specialist. DESIGN: A cross-sectional study and survey. PARTICIPANTS: Sixty-two individuals with RP (mean age, 37 years) participated in the study. METHODS: We obtained data about task performance from subjects' reports about their daily performance as assessed by a 53-item questionnaire and from a specialist's rating about actual ability on a 64-item battery of tasks, including ones similar to those assessed with the questionnaire. MAIN OUTCOME MEASURES: Clinical measures of vision included visual acuity, visual fields using Goldmann perimetry, letter contrast sensitivity, and cone and rod electroretinogram (ERG) function. The questionnaire and functional tasks were clustered into three categories: "reading," "mobility," and "peripheral detection." RESULTS: Self-report was correlated significantly with actual task performance. Task performance was correlated significantly with clinical test performance. Moderate or worse difficulty in performance was observed only for visual acuity worse than 20/40; log contrast sensitivity less than 1.4; a visual field area smaller than 2000 deg(2) (area equivalent to a 50-degree diameter of visual field to the Goldmann II-4-e target); and ERG amplitudes less than 10 microvolts for 32-Hz light-adapted white flicker. CONCLUSIONS: Despite the significant correlations, there remains variability in task performance that is unaccounted for in some individuals with low levels of clinical test performance. The assessment of actual task performance validated the use of self-reports in individuals with RP.

Activities of Daily Living↗
PubMed2001

Origin of deficits in the flicker electroretinogram of the cone system in X-linked retinoschisis as derived from response nonlinearities.

The aim of this study was to identify the origin of a high-frequency attenuation in the flicker electroretinogram (ERG) of patients with X-linked retinoschisis (XLRS) through an analysis of nonlinearities in the ERG response. The ERGs of six patients with XLRS and six age-similar control subjects were recorded in response to stimuli that consisted of pairs of sinusoids that had varying temporal frequencies and that differed by either 8 or 16 Hz. Compared with the control subjects, the patients with XLRS showed a significant reduction in the amplitude of the difference frequency to high-frequency stimuli that paralleled the high-frequency attenuation of their ERG response fundamental. This result indicates that a response attenuation at an initial linear filter, most likely photoreceptoral, was a major determinant of the reduced ERG amplitude of the XLRS patients at high temporal frequencies. Additional analyses of nonlinearities in the ERG responses provided evidence of a postreceptoral component to the flicker ERG deficits of the XLRS patients, as well.

Adult↗
PubMed2001

On-response deficit in the electroretinogram of the cone system in X-linked retinoschisis.

PURPOSE: The purpose of this study was to evaluate the hypothesis that the reduced b-wave to a-wave ratio of the brief-flash electroretinogram (ERG) of the cone system typically observed in X-linked retinoschisis (XLRS) represents a relatively greater deficit in the ON response (response to light onset) than the OFF response (response to light offset). A second purpose was to investigate the use of sawtooth flicker as a stimulus for eliciting ERG ON and OFF responses. METHODS: Light-adapted, full-field ERGs were recorded in six patients with XLRS and six age-similar control subjects in response to 8-Hz rapid-on and rapid-off sawtooth flicker to emphasize ON and OFF responses, respectively. ERG responses were analyzed in terms of the amplitudes and implicit times of the a-wave, b-wave, and d-wave components. RESULTS: There was no significant difference between the patients with XLRS and the control subjects for either the amplitude of the a-wave of the ON response or the amplitude of the d-wave of the OFF response. However, the amplitude of the b-wave of the ON response was reduced significantly in the patients with XLRS, resulting in a significantly reduced b-wave to d-wave ratio. The patients' implicit times were increased significantly for all waveform components. CONCLUSIONS: The reduced b-wave to d-wave ratio of the ERG of the cone system in these patients with XLRS is consistent with a relative dysfunction of the cone ON bipolar cell pathway in this disorder. The results show further that sawtooth flicker is a promising stimulus for eliciting well-defined ERG waveforms that can provide a quantitative assessment of the properties of ON and OFF responses in retinal disease.

Adaptation, Ocular↗
PubMed2001

High-frequency attenuation of the cone ERG and ON-response deficits in X-linked retinoschisis.

PURPOSE: Individuals with X-linked retinoschisis (XLRS) show a comparatively greater reduction of the ON response than the OFF response of the electroretinogram (ERG) of the cone system. At high temporal frequencies, they also show a marked attenuation of the flicker ERG that has been attributed to an abnormal cone photoreceptor response. The purpose of this study was to determine whether the high-frequency response attenuation contributes to the abnormal ERG ON response in XLRS. METHODS: Light-adapted ERGs were recorded from three patients with XLRS and from three control subjects, by using rapid-on and rapid-off sawtooth flicker to emphasize ON and OFF responses, respectively, and by using low-pass sawtooth flicker, from which the high temporal frequencies had been removed to mimic the high-frequency attenuation in XLRS. RESULTS: For the control subjects, removing the high stimulus frequencies reduced the amplitude of the b-wave component of the ON response but had little effect on the amplitude of the d-wave component of the OFF response. In the patients with XLRS, the b-wave component of the ON response was already diminished using the full sawtooth stimulus, and removing the higher stimulus frequencies had no further effect. Patients' ERG responses to the 16-Hz stimulus fundamental alone were also abnormal, in that an initial response component normally present in the ERG was absent. CONCLUSIONS: The overall pattern of findings indicates that two factors contribute to the preferential ON-response deficit in XLRS: first, a high-frequency attenuation of the cone photoreceptor response that effectively produces a low-pass stimulus for the postreceptoral pathway and that affects the ON response more than the OFF response and, second, a relatively greater attenuation of the ON- than of the OFF-bipolar cell response that is evident in the aberrant response to the sawtooth fundamental.

Adaptation, Ocular↗
PubMed2000

Temporal frequency deficits in the electroretinogram of the cone system in X-linked retinoschisis.

We investigated the extent of, and basis for, abnormalities in the flicker electroretinogram (ERG) of the cone system of patients with X-linked retinoschisis (XLRS), a form of hereditary vitreoretinal degeneration. ERGs were recorded from six patients with XLRS and from six visually normal subjects using high-contrast sinusoidal flicker that ranged in temporal frequency from 8 to 96 Hz, and that was presented against a rod-desensitizing adapting field. Compared to the control subjects, the patients with XLRS showed a significant reduction in the amplitude of the ERG response fundamental at temporal frequencies of 32 Hz and higher. In addition, their response phases were at or below the lower limits of normal (representing a phase lag) for temporal frequencies greater than 8 Hz. The higher harmonics of the patients' ERG responses to a low frequency stimulus were attenuated over the same temporal frequency range as was the response fundamental. This finding indicates that a major component of the abnormal temporal filtering responsible for the ERG abnormalities in XLRS occurs beyond the level of the early retinal nonlinearity that generates the harmonic components of the ERG response, and therefore is most likely postreceptoral in origin. Consistent with this interpretation, the ERG waveforms of the XLRS patients showed a significant attenuation of the ON-response component, with a normal OFF response. The overall pattern of results suggests that the marked reduction of ERG response amplitudes and the phase lag at the higher temporal frequencies in XLRS stem, at least in part, from a predominant attenuation of the ON-bipolar cell contribution to the flicker ERG.

Adolescent↗
PubMed1999

Coherence and the judgment of spatial displacements in retinitis pigmentosa.

We used a motion coherence paradigm to test the hypothesis that patients with retinitis pigmentosa (RP) have difficulty discriminating the direction of spatial displacements because of a random loss of motion-sensitive units owing to cone photoreceptor dropout. Minimum (Dmin) and maximum (Dmax) displacement thresholds of patients with typical RP or Usher syndrome were compared with those of age-similar, visually normal subjects. Two-frame random dot cinematograms were used, in which a group of target dots, which comprised 40-100% of the dot array in steps of 20%, were displaced in one of four directions, whereas the non-target dots were randomly repositioned between frames. Reducing the dot coherence in this way increased Dmin and reduced Dmax for both the RP patients and control subjects. Furthermore, the displacement thresholds of the RP patients were displaced laterally from normal along a log coherence axis, consistent with the hypothesis that the patients had a reduced effective (intrinsic) coherence. However, the displacement thresholds of control subjects, when measured at a reduced coherence, did not mimic those of RP patients at full coherence when both groups were tested with a range of dot contrasts and dot areas. These apparently discrepant findings can be reconciled if it is assumed that the patients' effective coherence varies with stimulus visibility.

Adult↗
PubMed1998

Discrimination of spatial displacements by patients with retinitis pigmentosa.

We compared maximum displacement thresholds (Dmax) with minimum displacement thresholds (Dmin) in patients with retinitis pigmentosa (RP) in order to characterize the nature of their visual disability, as well as to assess possible models of foveal vision loss. Thresholds for discriminating the direction of the spatial displacement of random dot patterns were measured in a group of 20 patients with typical RP or Usher syndrome whose visual acuities were 20/40 or better and who had minimal or no clinical evidence of changes in the ocular media. Findings were compared with those from an age-similar group of 15 visually normal subjects. Displacement thresholds were measured using a two-frame random dot cinematogram and a four-alternative forced-choice procedure. Measurements were made at each of three dot contrasts and three dot sizes. For the patients with RP, reducing either the dot contrast or dot size increased Dmin and decreased Dmax such that the range of discriminable displacements became considerably restricted, even at modest reductions in dot contrast or size. This restriction in the displacement thresholds of the patients with RP was correlated significantly with their visual acuity. By comparison, the control subjects showed little change in either Dmin or Dmax under these conditions. These results indicate that patients with RP who have only relatively minor reductions in their visual acuity can have severely compromised motion perception. The pattern of findings suggests that an abnormal contrast response of the foveal cone system is a major determinant of the impaired displacement thresholds of these patients with RP.

Adult↗
PubMed1998

X-linked retinitis pigmentosa in two families with a missense mutation in the RPGR gene and putative change of glycine to valine at codon 60.

OBJECTIVE: This study describes the ophthalmic findings in two unrelated white families with X-linked retinitis pigmentosa (XLRP) caused by a missense mutation in the retinitis pigmentosa GTPase regulator (RPGR) gene. DESIGN: Genetic screening and clinical correlation. PARTICIPANTS: Thirty-six families with XLRP seen by the authors were screened for a possible mutation in the RPGR gene to identify three affected hemizygotes with retinitis pigmentosa and four heterozygote carriers in one family and one hemizygote and one carrier in a second family. INTERVENTION: All nine patients underwent a routine ocular examination, including slit-lamp biomicroscopy and a dilated fundus examination. Goldmann visual field kinetic perimetry, static threshold perimetry, and electroretinography also were obtained. The DNA screening was performed on the three affected male patients and four obligate carriers examined from one family and the two examined patients, plus an additional male and obligate carrier, from the second family to determine the presence of any causative mutation in the RPGR gene. MAIN OUTCOME MEASURES: Findings on fundus examination, static threshold and kinetic perimetry, and electroretinography testing were the main outcome measures. RESULTS: A G-->T nucleotide change at position 238 in exon 3 of the RPGR gene resulting in a putative substitute of Gly-->Val at codon 60 was shown to segregate with RP in affected males and the carrier state in female heterozygotes in these two families. The ophthalmologic findings in hemizygotes as well as the carriers in this family were within the spectrum of findings characteristically noted in XLRP families. A tapetal-like reflex was not observed in any of the five female carriers. Psychophysical and electrophysiologic testing on the carriers indicated that cone and rod functions were impaired equivalently. When present in the carriers, visual field restriction was most apparent in, or limited to, the superotemporal quadrant, which corresponded to the retinal pigmentary changes that tended to occur in the inferonasal retina. CONCLUSIONS: A mutation in exon 3 of the RPGR gene, which would result in a putative glycine to valine substitution at codon 60, is associated with a severe clinical phenotype in male patients and a patchy retinopathy without a tapetal-like reflex in carrier females. In these families, heterozygote carriers showed equivalent impairment of their cone and rod function.

Adolescent↗
PubMed1998

Intraocular light scatter in patients with choroideremia.

OBJECTIVE: This study aimed to evaluate the extent of intraocular light scatter in patients with choroideremia. DESIGN: Prospective case-control study. PARTICIPANTS: Twelve male patients with choroideremia who had predominantly minimal or no posterior subcapsular cataract (PSC) lens opacities and visual acuities of 20/40 or better and 30 age-similar control subjects with normal vision and no lens opacities were studied. INTERVENTION: Intraocular light scatter was measured using a van den Berg Straylightmeter. MAIN OUTCOME MEASURES: Visual acuities, letter contrast sensitivities, Goldmann visual fields using a II4e target, and straylight parameters were obtained for each patient. Lenses were assessed by slit-lamp biomicroscopy to determine whether there were PSC opacities. The degree of retinal pigment epithelial and choroidal degenerative changes was evaluated from color fundus photographs. RESULTS: Three of the patients with choroideremia who had clinically apparent PSC lens opacities showed an increase in intraocular light scatter. More notable was the fact that seven of the remaining nine patients who did not have any clinically apparent changes in the lens also had a considerable increase in the intraocular light scatter as compared to the control subjects. The relative elevations of the log straylight parameters of the patients with choroideremia, as compared to age-similar control subjects, were correlated significantly with their log visual field areas (r = -0.69, P < 0.05). CONCLUSIONS: Intraocular light scatter may be increased in patients with choroideremia, even in the absence of clinically observable PSC opacities. It is hypothesized that the increase in light scatter may be caused by changes in the posterior subcapsular region of the lens before the formation of frank PSC cataracts. The increased straylight could, at least in part, account for the disability glare reported by these patients.

Adult↗
PubMed1998

ON-pathway dysfunction in a patient with acquired unilateral night blindness.

PURPOSE: To investigate possible functional correlates of an apparent ON-pathway defect observed in the cone electroretinogram of a patient with acquired unilateral night blindness. METHOD: Visual evoked potentials were recorded to the onset of a grid pattern consisting of either incremental or decremental squares. Saccadic eye movements were measured to luminance increments and decrements presented 5 degrees from fixation. The patient's results were compared with normative data. RESULTS: Visual evoked potential latencies were prolonged to incremental stimulation of the patient's affected left eye but were within normal limits for the other three conditions (increments and decrements, right eye; decrements, left eye). A similar pattern of asymmetry between latencies to incremental and decremental stimulation of the affected eye was observed for saccadic eye movements. CONCLUSIONS: The observed predominant delay in response to luminance increments supports the hypothesis of an ON-pathway dysfunction in this patient with acquired unilateral night blindness.

Adult↗
PubMed1998

Effect of noise contrast polarity and temporal asynchrony on visual sensitivity.

We evaluated the effect of substitutive noise on contrast sensitivity within the context of linear (Fourier) and nonlinear (non-Fourier) visual processes. Orientation judgments for D6 (sixth spatial derivative of Gaussian) patterns were obtained from three visually normal subjects when random regions of the target and background were occluded by small (1.7 arc min) pixel arrays that were either all of the same contrast polarity or a mixture of equal percentages of negative and positive contrast. The target was presented either synchronously or asynchronously with the occluding elements. Our results indicate that the manipulation of noise characteristics in this way can bias performance either toward a nonlinear process that is insensitive to noise contrast polarity but sensitive to temporal asynchrony or toward a quasi-linear process that is sensitive to noise contrast polarity but insensitive to temporal asynchrony. These findings have relevance to models of the effect of spatial sampling on the visual performance of persons with retinal disease.

Adult↗
PubMed1997

Relationship between difficulty in performing daily activities and clinical measures of visual function in patients with retinitis pigmentosa.

OBJECTIVES: To assess the level of perceived difficulty experienced by patients with retinitis pigmentosa (RP) in the performance of everyday activities and to determine the correlation between patients' self-reported difficulty and clinical measures of visual function. METHODS: One hundred sixty-seven patients with typical RP and Usher syndrome type 2, with a wide range of disease severity, rated their difficulty in the performance of 33 activities. We obtained data on visual acuity and visual field area for all patients, and electroretinogram (ERG) recordings on a subgroup of 49 of these patients. Results from the questionnaire were analyzed with factor analysis, and patients' self-reports were compared with their clinical data using correlational analyses and multiple regression. RESULTS: The patients' questionnaire responses clustered into 6 factors: activities involving central vision, miscellaneous activities (no discernible common factor), activities related to mobility, driving, negotiating steps, and eating meals. Of the clinical tests, visual acuity was most strongly related to the patients' ratings of their difficulty in performance. Visual field area also was related to patients' self-assessments but not as strongly as visual acuity. Because visual field area and the ERG measures were correlated, adding ERG information did not improve predictability. CONCLUSIONS: In patients with RP, perceived difficulty in performing common tasks was most strongly related to level of visual acuity and visual fields. Although certain ERG amplitude measures did show positive correlations with some self-reported activities, overall, the ERG amplitude measures showed the least relationship with patients' self-reports. Our results provide insight into RP patients' perceived difficulties in performing everyday activities and the clinical measures of visual function that most highly correlate with these difficulties.

Activities of Daily Living↗
PubMed1997

Visual acuity and contrast sensitivity for individual Sloan letters.

Sloan letter optotypes are used frequently to evaluate visual impairment, and scoring procedures have been developed that are based on the numbers of letters that are identified correctly. However, previous studies have presented conflicting evidence regarding the relative identifiability of the individual Sloan letters. To investigate this issue further, we measured psychometric functions for the identification of each of the 10 Sloan letters, with individual letters presented in random order on the gray-scale display of a Macintosh computer-based testing system. Data were obtained from three visually normal subjects under each of three conditions: (1) as a function of log contrast at a relatively large letter size; (2) as a function of log contrast at a letter size near the acuity limit; and (3) as a function of log MAR (minimum angle of resolution) at maximum letter contrast. Estimates of threshold log contrast and threshold log MAR were derived from best-fitting Weibull functions. Threshold log contrast for small letters showed the greatest interletter variability. There was relatively little interletter variability in either threshold log contrast for large letters or threshold log MAR for high-contrast letters. However, due to the relatively steep psychometric functions under these latter two conditions, the different Sloan letters had considerably different percent correct values near threshold. The overall pattern of results suggests that the contrast sensitivity functions for individual Sloan letters are displaced laterally along a log MAR axis, while their vertical positions are essentially equivalent.

Adult↗
PubMed1997

Rate of visual field loss in retinitis pigmentosa.

PURPOSE: The authors quantitate the rate of visual field loss in patients with retinitis pigmentosa as it relates to different clinical field phenotypes. PATIENTS AND METHODS: Goldmann visual fields were obtained with target V4e in 77 patients and with target II4e in 71 patients who had either isolated or various genetic types of retinitis pigmentosa and who met certain entrance criteria. The visual fields were categorized into five distinct clinical field phenotypes on the basis of their pattern of field loss. Mixed-model methods for the analysis of longitudinal data were used to model the natural logarithm of the visual field area as a function of patient age and clinical field phenotype. The average half-life (time over which half of the remaining field area would be lost) of the visual field area for each phenotype was computed from the results of this analysis. Visual field data were not analyzed for patients with a normal clinical field phenotype (type 1). RESULTS: Independent of the field phenotype, average half-life values were 7.3 years for target V4e and 6.8 years for target II4e, which were not statistically different (P = 0.16). Visual fields with partial or complete midperipheral ring scotomas (type 2) and those with only a residual central field (type 4) had a half-life of 9.5 and 9.4 years, respectively, for target V4e, and 8.9 and 8.0 years, respectively, for target II4e. Patients with partial peripheral restriction (type 5) lost visual fields with a half-life of 9.5 years for target V4e and 7.3 years for target II4e. None of these differences in the half-lives between the different phenotypes were statistically significant for either targets V4e or II4e. Fields with a residual central area and remaining temporal and/or nasal islands (type 3) had a half-life of 4.8 years for target V4e and 6.0 years for target II4e. The differences in half-lives between type 3 and each of the other field phenotypes were statistically significant for the V4e target, but not for the II4e target. CONCLUSIONS: The results of this study can be useful for counseling patients with retinitis pigmentosa and various visual field phenotypes as to their potential rate of visual field loss.

Half-Life↗
PubMed1997

Serine-27-phenylalanine mutation within the peripherin/RDS gene in a family with cone dystrophy.

PURPOSE: To evaluate the clinical and electrophysiologic findings in a family with two heterozygous sequence changes in the peripherin-retinal degeneration slow (RDS) gene. METHODS: A family study was done of a pedigree obtained by screening for rhodopsin, peripherin/RDS, or rom-1 gene mutations in probands from families with hereditary retinal diseases. The patients consisted of three affected and four unaffected members from a family with cone dystrophy. Ophthalmoscopy, visual field testing, electroretinography, and DNA analysis were performed. RESULTS: Denaturing gradient gel electrophoresis showed the presence of two different sequence changes in the RDS genes of this family. In three members with a retinal disease, the authors observed the substitution of phenylalanine for serine in codon 27 (serine-27-phenylalanine). The clinical and functional findings in these three patients were most consistent with autosomal-dominant cone dystrophy. Three other family members, unaffected with retinal disease, were found to show a substitution of serine for cysteine in codon 72 of the peripherin protein. CONCLUSION: A peripherin/RDS sequence change may produce a cone dystrophy with minimal ophthalmoscopic changes in the macula and limited peripheral degenerative changes. Caution is warranted to avoid ascribing nondisease-causing sequence polymorphisms in candidate genes as responsible for determining the development of a retinal disease phenotype.

Adult↗
PubMed1996

Intraocular light scatter in patients with retinitis pigmentosa.

We evaluated the level of intraocular light scatter in a group of patients with retinitis pigmentosa (RP) who had minimal or no lens opacities, since such patients not infrequently complain of photoaversion. Intraocular light scatter was measured in 20 patients with RP who were < 60 years of age and who had no more than a trace of posterior subcapsular (PSC) lens opacity by slit-lamp evaluation. Measurements of intraocular straylight were made using a van den Berg Straylightmeter. Results from the patients with RP were compared with those of a control group of 30 subjects with normal vision whose ages were similar to those of the patients with RP. Seventeen of the 20 patients with RP had straylight levels that were above the range of age-similar normal control subjects. In some patients, the straylight parameter was increased by a factor of 2.5 above the normal mean for the patient's age and by as much as four to five times the normal mean for 20-yr-old subjects. There was a statistically significant correlation (r = -0.73, P < 0.01) between the patients' log relative elevation in the straylight parameter and their log visual field areas. Our findings indicate that patients with RP can have increased levels of intraocular light scatter despite minimal or no clinically observable PSC lens opacities. The increased intraocular straylight, which is likely due at least in part to subclinical abnormalities in lens morphology, can accentuate the visual disability of patients with RP in the presence of glare sources.

Adult↗