Contrast Sensitivity Deficits in Inferred Magnocellular and Parvocellular Pathways in Retinitis Pigmentosa

PURPOSE

To define the contrast sensitivity deficits of patients with retinitis pigmentosa (RP) under testing conditions designed to emphasize threshold mediation by either the magnocellular (MC) or parvocellular (PC) pathway.

METHOD

Contrast sensitivity was measured with spatially localized, narrow-band test patterns at peak spatial frequencies ranging from 0.25 to 8 cycles per degree (cpd), using a steady-pedestal paradigm (brief presentation of the test stimulus against a continuously presented luminance pedestal) and a pulsed-pedestal paradigm (simultaneous brief presentation of the test stimulus and luminance pedestal) to favor the MC and PC pathways, respectively. The contrast sensitivity functions of 12 patients with RP who had visual acuities ranging between 20/12.5 and 20/40 were compared to those of 10 visually normal, age-equivalent control observers.

RESULTS

Five of the patients with RP who had Snellen visual acuities better than 20/25 had contrast sensitivity functions that were within the normal limits at all spatial frequencies for both testing paradigms. The other seven patients with RP had reduced contrast sensitivities for both paradigms, with the greatest reduction in sensitivity occurring at the highest spatial frequency. Their contrast sensitivity deficits were equivalent for the steady- and pulsed-pedestal paradigms.

CONCLUSIONS

As observed in previous studies, the degree of contrast sensitivity loss shown by the patients with RP was greatest at the highest stimulus spatial frequency. However, in comparison to prior studies of contrast discrimination in patients with RP, there was no evidence of a preferential contrast sensitivity loss within the MC pathway. This apparent discrepancy is attributed to differences in the test targets and psychophysical judgments that were used in the studies, which emphasizes the importance of task characteristics in evaluating relative deficits within the MC and PC processing streams in visual disorders.

The application of chromatic dark-adapted kinetic perimetry to retinal diseases

PURPOSE

To demonstrate the value of a 2-color perimetric procedure for determining cone and rod system contributions to the dark-adapted kinetic visual field (VF).

DESIGN

Prospective evaluation of perimetric testing procedure.

PARTICIPANTS

Five patients with retinal diseases and 6 visually normal individuals.

METHODS

Long- and short-wavelength stimuli were presented under dark-adapted conditions in a Goldmann perimeter. Visual fields were measured for the II and V test target sizes with a long-wavelength filter (cut-on at 600 nm) and a short-wavelength filter (cutoff at 510 nm). Light intensities through these filters were matched scotopically for the rod system by producing equal peripheral boundaries on 6 visually normal individuals. To validate the application of this procedure, we tested a patient with congenital achromatopsia and another patient with congenital stationary night blindness (CSNB). We then tested 2 patients with retinitis pigmentosa (RP) and 1 patient with Usher's syndrome to determine the cone and rod contributions to their VF isopters.

MAIN OUTCOME MEASURES

Isopters for long- and short-wavelength test stimuli, and the appearance of the test stimuli, whether reported as chromatic or achromatic.

RESULTS

The patient with congenital achromatopsia showed superimposed isopters for the 2 stimuli, which were reported as achromatic, demonstrating that the peripheral field boundaries were rod mediated. The patient with CSNB showed an isopter in response to the long-wavelength stimulus that was considerably larger than that in response to the short-wavelength stimulus, both stimuli reported as chromatic, showing that the cone system determined peripheral thresholds for both stimuli. In 2 patients with RP, we observed a mixed pattern of cone or rod system detection of the chromatic stimuli. The peripheral isopters were rod mediated, whereas the cone system determined the central field isopters. In an Usher's syndrome patient, cones mediated both the peripheral and the central field isopters.

CONCLUSIONS

A 2-color dark-adapted Goldmann perimetric procedure was able to determine whether the VF isopters were rod or cone mediated in 5 patients with various forms of retinal disease.

Pharmacological analysis of the rat cone electroretinogram

The electroretinogram (ERG) of the cone system provides a useful noninvasive measure of the activity of the cone pathway. Despite a wide application of the cone ERG in the study of rodent models of human hereditary retinal disease, the cellular origins of the rat cone ERG have not been well defined. Here, we address this issue using a pharmacological approach that has been used previously to derive ERG response components. Agents that impair synaptic transmission at well-defined retinal loci were dissolved in saline and injected into the vitreous of adult Sprague-Dawley rats anesthetized with ketamine/xylazine, and cone ERGs were recorded approximately 2 h later. Analysis of the resulting waveforms indicated that the rat cone ERG includes a relatively small-amplitude component of negative polarity that is derived from the activity of cone photoreceptors, and perhaps retinal glial (Müller) cells. The cone depolarizing bipolar cell pathway contributes a positive potential of large amplitude to the rat cone ERG. In comparison, the contribution of hyperpolarizing bipolar cells is of negative polarity and of much smaller amplitude. The inner retina contributes a negative wave upon which higher frequency oscillations are superimposed. These results provide a foundation for interpreting changes in the waveform of the rat cone ERG that may be observed following genetic alteration or other experimental treatment.

Contrast-processing deficits in melanoma-associated retinopathy

PURPOSE

To evaluate the hypothesis that patients with melanoma-associated retinopathy (MAR) have a selective functional loss within the magnocellular (MC) pathway of the cone system, with sparing of parvocellular (PC) pathway function.

METHODS

Two patients with MAR, ages 57 and 61 years, with normal Snellen visual acuity, participated in the study. Contrast sensitivity was measured at spatial frequencies ranging from 0.25 to 8 cycles per degree (cpd), using two paradigms (steady pedestal and pulsed pedestal) designed to assess the functional integrity of the MC and PC pathways, respectively. Results in patients with MAR were compared with those in 10 visually normal observers, aged 23 to 57 years.

RESULTS

Both patients with MAR showed a loss of contrast sensitivity compared to normal observers, but the pattern of loss differed for the two testing paradigms. For the steady-pedestal paradigm (presumed MC-pathway mediation), the patients' sensitivity loss was greatest at the lowest spatial frequency (0.25 cpd) and the sensitivity loss decreased systematically with increasing spatial frequency. For the pulsed-pedestal paradigm (presumed PC-pathway mediation), the sensitivity loss was greatest at an intermediate spatial frequency of 1 cpd. For both paradigms, the patients' sensitivities were within the normal range at the highest spatial frequency (8 cpd), consistent with their normal visual acuity.

CONCLUSIONS

The contrast sensitivity deficits of patients with MAR under photopic conditions are not specific to the MC pathway, as proposed previously, but instead are related to the spatial frequency of the test target. The overall pattern of contrast sensitivity loss shown by the patients with MAR is consistent with the dysfunction at the level of the retinal bipolar cells that is presumed to underlie the MAR syndrome.

ON-pathway dysfunction and timing properties of the flicker ERG in carriers of X-linked retinitis pigmentosa

PURPOSE

Carriers of X-linked retinitis pigmentosa (XLRP) frequently show prolonged implicit times of the flicker electroretinogram (ERG). This study tested the hypothesis that a preferential response attenuation within the cone depolarizing (ON) bipolar cell (DBC) pathway is a major contributing factor.

METHODS

Light-adapted, full-field ERGs were recorded from 10 XLRP carriers and 12 visually normal control subjects. Fundamental amplitudes and phases of ERG responses to sinusoidally flickering stimuli at temporal frequencies ranging from 8 to 96 Hz were analyzed within the framework of a recent vector summation model of the cone system ERG to test for evidence of a response attenuation within the DBC pathway. In addition, ERG responses to sawtooth flicker were examined for a reduced b- to d-wave amplitude ratio, indicative of ON pathway dysfunction.

RESULTS

The carriers' fundamental response phases at 32 Hz correlated significantly with their log ratios of response amplitudes at 32 versus 12 Hz (r = 0.89, P < 0.001) and with their log b- to d-wave amplitude ratios (r = 0.71, P < 0.05), both of which were used as indices of response attenuation within the DBC pathway. A control experiment demonstrated that a reduced sensitivity of cone phototransduction made at most only a minimal contribution to the timing changes in the carriers' flicker ERG responses.

CONCLUSIONS

The overall pattern of results indicates that a preferential response attenuation within the DBC pathway is the primary source of timing changes in the flicker ERGs of these carriers of XLRP. These findings illustrate the value of analyzing ERG responses to flickering stimuli at multiple temporal frequencies to evaluate mechanisms of disease action in photoreceptor degenerations.

Deficits in temporal integration for contrast processing in retinitis pigmentosa

PURPOSE

The purpose of this study was to evaluate the properties of foveal temporal integration in patients with retinitis pigmentosa (RP) within the framework of contrast processing by the magnocellular (MC) and parvocellular (PC) pathways.

METHODS

Temporal integration functions were measured in eight patients with RP whose visual acuities ranged from 20/25 to 20/63. Contrast thresholds were obtained at durations ranging from 15 to 480 ms, using steady-pedestal and pulsed-pedestal paradigms to bias performance toward the MC and PC pathways, respectively. The patients' results were compared with those of 10 age-similar control observers with normal vision. For both paradigms, contrast thresholds as a function of duration were fit with a two-limbed function to derive the critical duration for temporal integration (t(c)) and the asymptotic threshold at long durations (Delta L(infinity)).

RESULTS

The log t(c)s of the patients with RP were significantly longer than those of the control subjects for the steady-pedestal paradigm (presumed MC-pathway mediation; t = 3.67, P < 0.001), but not for the pulsed-pedestal paradigm (presumed PC-pathway mediation; t = 0.76, P = 0.45). Further, the patients with RP showed a significant correlation between log t(c) and log Delta L(infinity) for the steady-pedestal paradigm (r = 0.72, P < 0.05) but not for the pulsed-pedestal paradigm (r = -0.37, P = 0.36).

CONCLUSIONS

The patients with RP in this study showed greater deficits in contrast sensitivity and a more prolonged critical duration under test conditions that favor the MC rather than the PC pathway. A likely explanation is a high-frequency response attenuation at the level of the cone photoreceptors that has a differential effect on contrast-processing tasks that emphasize different postreceptoral mechanisms.

Comparison of contrast sensitivity, visual acuity, and Humphrey visual field testing in patients with glaucoma

BACKGROUND

Despite their normal or near-normal Snellen visual acuity, patients with glaucoma often complain of "poor" vision.

OBJECTIVE

To investigate the relationship between large-letter contrast sensitivity, high-contrast visual acuity, and visual field defects in patients with glaucoma who have 20/40 or better visual acuity.

DESIGN

Prospective, cross-sectional case series.

PATIENTS AND METHODS

We evaluated 250 eyes of 144 subjects from the Glaucoma Service at the University of Illinois at Chicago College of Medicine. Subjects with a diagnosis of glaucoma, suspected glaucoma, or ocular hypertension who met the 20/40 or better vision requirement were recruited. Visual acuity was measured using the rear-illuminated Lighthouse Visual Acuity Chart at 4 m. Contrast sensitivity was measured using the Pelli-Robson Chart in a front-illuminated box with even luminance across the chart. Visual fields of the patients were measured using the 24-2 full-threshold program on the Humphrey Visual Field Analyzer.

RESULTS

A significant correlation (r = 0.57, P < 0.001, n = 127) was found between the visual field mean deviations and the contrast sensitivity scores. The correlation (r = -0.322, P < 0.001, n = 127) was less between the visual field mean deviation and the log MAR visual acuity values, as was the correlation between the contrast sensitivity scores and log MAR visual acuity values (r = -0.370, P < 0.001, n = 127). In the subgroup of patients with chronic open-angle glaucoma, the correlation between the mean visual field deviation and the contrast sensitivity score was higher at 0.689 (P < or = 0.001, n = 62).

CONCLUSIONS

Reduced contrast sensitivity is significantly correlated with visual field losses in patients with glaucoma and a visual acuity of 20/40 or better. The study data support the conclusion that, compared with visual acuity, the disease process preferentially affects contrast sensitivity. In our previous work, contrast sensitivity was shown to be more related than visual acuity to real-world function in patients with early glaucomatous changes.

Nature of the cone ON-pathway dysfunction in melanoma-associated retinopathy

PURPOSE

To establish the basis for an ON-pathway abnormality of the cone system in melanoma-associated retinopathy (MAR) through analysis of the electroretinogram (ERG) and visual evoked potential (VEP).

METHODS

Two patients with MAR syndrome whose sera produced immunolabeling of retinal bipolar cells participated in the study. Full-field ERGs were recorded in response to brief flashes, to rapid-on and rapid-off sawtooth stimuli at a temporal frequency of 8 Hz, and to sine-wave stimuli at temporal frequencies ranging from 8 to 96 Hz. Fundamental responses to the sine-wave stimuli were evaluated within the context of a vector-summation model of the depolarizing bipolar cell (DBC) and hyperpolarizing bipolar cell (HBC) contributions to the response fundamental. VEPs were recorded to the onset of luminance increments and decrements that had contrasts of 10%, 20%, and 50%. The patients' results were compared with those of age-similar control subjects.

RESULTS

The patients with MAR showed abnormal ERG responses to luminance increments, consisting of a marked attenuation of the initial portion of the b-wave, but their ERG responses to luminance decrements were normal in amplitude and timing. The ERG temporal response functions of the patients with MAR had normal amplitudes at frequencies of 32 Hz and higher, with a constant phase lag across these frequencies, but larger-than-normal amplitudes at frequencies below 32 Hz, and a phase lead at 8 Hz. Their VEP responses showed a marked delay to increments but only a minimal delay to decrements.

CONCLUSIONS

Within the context of the vector-summation model, the ERG findings in the patients with MAR are more consistent with an attenuation of the DBC contribution to the ERG response than with a DBC response delay. The delayed VEP responses of the patients with MAR to luminance increments may represent a late response of the OFF system to increment onset.

A distinctive form of congenital stationary night blindness with cone ON-pathway dysfunction

PURPOSE

To characterize a distinctive form of congenital stationary night blindness (CSNB).

DESIGN

Observational case report.

PARTICIPANTS

A 30-year-old male with a history of night blindness, several members of his family, a patient with "complete" congenital stationary night blindness (CSNB1), and groups of age-similar control subjects.

METHODS

Rod-system function was evaluated by measuring psychophysical dark-adapted thresholds, by recording dark-adapted electroretinograms (ERGs), and by fundus reflectometry. Cone-system function was evaluated by recording light-adapted ERGs, including those to sawtooth flicker, and by recording light-adapted visually evoked potentials (VEPs) to luminance increments and decrements.

MAIN OUTCOME MEASURES

Dark-adapted thresholds, ERGs, rhodopsin double densities, Goldmann visual fields, and VEPs.

RESULTS

The patient's visual acuity, visual fields, and color vision were normal. His peripheral dark-adapted thresholds were rod-mediated but elevated by approximately 3 log units above normal. Rhodopsin double density and bleaching recovery were normal. His dark-adapted maximal-flash ERG showed a "negative" waveform, in which the b-wave was more reduced in amplitude than the a-wave, although the a-wave amplitude was also reduced. The rod photoreceptors contributed to the patient's dark-adapted ERGs, as illustrated by the unequal responses to cone-matched stimuli. The patient's cone-mediated thresholds for long-wavelength stimuli were within the normal range. However, his light-adapted brief-flash b-wave was abnormal in amplitude and implicit time. Selective abnormalities of the ON responses of the cone system were apparent in the patient's reduced b-wave amplitude to rapid-on flicker with a normal response to rapid-off flicker, and his prolonged VEP latencies to increments but not to decrements.

CONCLUSIONS

The overall pattern of findings distinguishes this patient from previously described forms of CSNB. The results suggest that two factors likely contribute to the patient's night blindness: (1) a rod phototransduction defect and (2) a postreceptoral defect. The results also indicate dysfunction within the cone ON pathway.

Comparison of intraocular light scatter in carriers of choroideremia and X-linked retinitis pigmentosa

OBJECTIVE

To compare the extent of intraocular straylight in carriers of choroideremia (CHM) and X-linked recessive retinitis pigmentosa (XLRP) to clarify further the relationship between photoreceptor cell degeneration and intraocular light scatter in retinal diseases.

DESIGN

Prospective case-control study.

PARTICIPANTS

Six obligate carriers of CHM, 12 obligate carriers of XLRP, and 30 age-similar control subjects with normal vision. The controls had no posterior subcapsular (PSC) lens opacities, and the carriers had minimal or no PSC lens opacities, as assessed by slit-lamp biomicroscopy.

MAIN OUTCOME MEASURES

Straylight levels were measured using a van den Berg Straylightmeter. Visual acuity and Goldmann visual field area using a II/4e target were assessed for both eyes of each carrier. Electroretinogram (ERG) data were available on four of six carriers of CHM and all carriers of XLRP. The extent of retinal pigment epithelial degenerative changes was evaluated by fundus examination.

RESULTS

All six carriers of CHM had typical fundus abnormalities with normal visual fields. Five of the six carriers of CHM had age-normal levels of intraocular light scatter, and one showed minimally elevated intraocular light scatter. The 12 carriers of XLRP had a spectrum of fundus abnormalities and varying severity of functional impairment as derived from visual field areas and ERG amplitudes. Seven of the 12 carriers of XLRP showed an elevated level of intraocular light scatter in at least one eye. The degree of straylight elevation above the normal mean age value was correlated significantly with both visual field area and amplitude of the maximal-intensity, dark-adapted ERG b-wave.

CONCLUSIONS

The carriers of XLRP who had evidence of photoreceptor cell dysfunction (as determined by visual field loss and reduced ERG amplitudes) had increased levels of intraocular straylight, whereas the carriers of CHM, who showed fundus abnormalities alone, in the absence of demonstrable photoreceptor cell dysfunction, had normal or minimally elevated levels. This finding supports the hypothesis that the increased levels of intraocular light scatter observed in some patients with hereditary retinal degenerations result from subclinical changes in the PSC region of the lens as a consequence of photoreceptor cell degeneration.

Temporal properties of the mouse cone electroretinogram

To determine the temporal response characteristics of the mouse cone electroretinogram (ERG), we recorded responses to high contrast sinusoidal stimuli ranging from 2 to 52 Hz. The largest response amplitudes obtained from wild-type (WT) mice occurred at stimulus frequencies below 10 Hz, and cone ERG amplitude declined progressively with increasing stimulus frequency above that level. In comparison, human responses recorded under the same stimulus and recording conditions displayed maximal responses to stimulus frequencies near 4 and 40 Hz, and a pronounced dip at 12 Hz. Responses were also obtained from nob (no b-wave) mice, which lack ERG contributions from depolarizing bipolar cells (DBCs). At low temporal frequencies, nob cone ERGs were smaller than those of WT mice and had a different waveform. As temporal frequency increased, nob and WT responses became more similar and came into register at the highest temporal frequencies. To evaluate the contribution of the DBC pathway to the mouse cone ERG, nob responses were vector-subtracted from those of WT mice. The derived DBC response was maximal at low stimulus frequencies and fell sharply as stimulus frequency increased. These results indicate that the mouse cone ERG is more linear than the primate response and that the temporal response of the mouse outer retina is tuned to much lower frequencies than that of primate.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.