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N S Peachey

Publications and source records attributed to N S Peachey.

At least 19 recordsLinked to original sources
PubMed1992

Light adaptation and the luminance-response function of the cone electroretinogram.

Cone electroretinograms are typically isolated by presenting stimulus flashes against rod-desensitizing adapting fields. To investigate the manner in which adapting-field luminance affects cone electroretinogram response properties, we measured cone electroretinogram luminance-response functions of two normal subjects, with stimuli presented against adapting fields that ranged in luminance from -1.2 to 2.1 log cd/m2. A flicker rate of 31.1 Hz was used to isolate cone electroretinograms under all adaptation conditions. A hyperbolic equation of the form (R/Rmax) = Ln/(Ln + Kn) was fitted to each luminance-response function by a least-squares criterion. As adapting field luminance increased, the best-fit values of the variables K and n increased, which is in general agreement with results of electrophysiologic studies of light adaptation in retinal neurons. However, Rmax values also increased with adapting field luminance. The change in all three of these variables with adapting field luminance must be considered in the interpretation of cone electroretinogram luminance-response functions from patients with retinal disorders.

Adaptation, Ocular↗
PubMed1992

Rapid and slow changes in the human cone electroretinogram during light and dark adaptation.

Changes in the response characteristics of the human cone electroretinogram (ERG) during light and dark adaptation were studied in two visually normal subjects. Cone ERG responses were isolated under all adaptation conditions through the use of 31 Hz flicker. To determine the time-course of changes in the cone ERG during adaptation, responses to stimuli of constant luminance were measured repeatedly during 15 min of exposure to an adapting field of 2.0 log cd/m2 and during 30 min of dark adaptation following adapting field termination. In addition, luminance-response functions were obtained before and immediately after adapting field onset, as well as before and immediately after adapting field termination. The results indicate that the human cone ERG is influenced by two major processes. One process has a relatively rapid time-course and serves to reposition the luminance-response function along the luminance axis following changes in ambient light levels. The second process, which has a slow time-course, scales response amplitudes during light and dark adaptation by the same proportion at all stimulus luminances. The results provide a framework for predicting the manner in which the cone ERG will change with alterations in the state of retinal adaptation.

Adaptation, Ocular↗
PubMed1992

Light adaptation, rods, and the human cone flicker ERG.

During the course of light adaptation, the amplitude and implicit time of the human cone ERG change systematically. In the present study, the effect of adapting field luminance on these ERG changes was assessed, and the hypothesis that light adaptation of the rod system is the primary determining factor was evaluated. Cone ERG responses, isolated through the use of 31.1-Hz flicker, were obtained from two visually normal subjects, initially under dark-adapted conditions and then repeatedly for 30 min following the onset of each of a series of ganzfeld adapting fields with luminances that ranged from -1.2 to 2.1 log cd/m2. The increase in flicker ERG amplitude and decrease in implicit time during light adaptation were greatest at the highest adapting field luminances. Photopically equivalent achromatic and long-wavelength adapting fields induced comparable increases in flicker ERG amplitude, while scotopically equivalent adapting fields had considerably different effects. This latter finding demonstrates that the rod system is not a major determinant of the adaptation-induced increase in cone ERG amplitude.

Adaptation, Ocular↗
PubMed1992

'On' response defect in paraneoplastic night blindness with cutaneous malignant melanoma.

Response properties of rod and cone systems were assessed in a patient with an acquired form of night blindness associated with a metastatic cutaneous malignant melanoma. The night blindness, a sensation of shimmering lights, and selective reductions in the amplitudes of both rod and cone electroretinographic (ERG) b-waves were present before and after chemotherapy, confirming that this disorder was a paraneoplastic consequence of the melanoma rather than a response to chemotherapy. During ERG testing with flashes of extended duration, the cone b-wave abnormality was found to be a predominant loss of the cone ERG "on" response with relative preservation of the "off" response, similar to that observed in patients with congenital stationary night blindness. An impairment in signal transmission specific for retinal "on" pathways may be a primary defect in both of these forms of night blindness.

Adolescent↗
PubMed1991

Effects of light adaptation on the response characteristics of human oscillatory potentials.

We have examined the response characteristics of the oscillatory potentials (OPs) of the human electroretinogram (ERG) obtained to ganzfeld flash stimuli presented against adapting fields. First, we determined the extent to which the OPs obtained to high luminance flashes change during the course of light adaptation to a cone-isolating adapting field. Regardless of the number of OP wavelets, the last OP wavelet increased in amplitude and decreased in implicit time to a greater extent than did the earlier wavelet(s). In addition, we examined the role of both flash and adapting field luminance in determining the wave form of the OPs. For each adapting field luminance that was tested, the number of OP wavelets increased as flash luminance increased, primarily resulting from the splitting of the last OP into 2 distinct wavelets. While the number of OP wavelets generally decreased as adapting field luminance increased, the amplitude of the last OP became larger. These functional distinctions between the last and the earlier wavelets are consistent with their representing the activity of different retinal generators.

Adult↗
PubMed1991

Intravitreal U75412E: a new free radical scavenger.

Free-radical-mediated tissue damage is thought to be involved in a multitude of ophthalmic pathologies. Therefore, drugs that scavenge free radicals may find wide clinical application. A new chemical class of antiinflammatory agents, the 21-aminosteroids, has been developed as free radical scavengers. These agents are thought to inhibit lipid peroxidation and prevent the release of free arachidonic acid from injured cell membranes. In the present study, we investigated the intraocular toxicity of one of the 21-aminosteroids, U75412E. Intravitreal doses of up to 0.1 mg of U75412E were nontoxic to rabbit ocular tissues.

Animals↗
PubMed1991

Acuity-luminance and foveal increment threshold functions in retinitis pigmentosa.

Acuity-luminance functions and foveal increment threshold functions were measured in 20 subjects with retinitis pigmentosa (RP) who had Snellen acuities of 20/40 or better, minimal or no posterior subcapsular cataracts, and no atrophic-appearing foveal lesions. Compared with the results from ten normal subjects, the visual acuities of the RP subjects were reduced at all luminance levels; the acuity deficits were more pronounced at low luminances. Foveal detection thresholds of the RP subjects showed the greatest elevation at low background luminances and approached normal values at high adapting levels. There was a statistically significant correlation (r = 0.79, P less than 0.01) between the visual acuities and absolute thresholds of the RP subjects. The overall pattern of results cannot be explained by a reduced quantal absorption in foveal cones, but it is consistent with the hypothesis that a reduced cone spatial density is the primary mechanism of foveal visual loss in this group of RP subjects.

Adaptation, Ocular↗
PubMed1991

Visual adaptation and the cone flicker electroretinogram.

This study examined the hypothesis that changes in the response properties of the human cone ERG during light adaptation represent the recovery of cone system responsiveness toward a dark-adapted value after an initial decrease in responsiveness at adapting field onset. The electroretinographic (ERG) responses to 31.1 Hz flicker were obtained under both dark-adapted and light-adapted conditions for stimulus luminances ranging from -1.42(-)+0.82 log cd sec/m2. At low stimulus luminances, flicker ERG amplitudes were larger under dark-adapted than under light-adapted conditions, consistent with the hypothesis. However, at high stimulus luminances, flicker ERG amplitudes obtained under light-adapted conditions were approximately double those recorded from the dark-adapted eye. Therefore, the increase in cone ERG amplitude that occurs during light adaptation at high stimulus luminances does not represent a return toward a dark-adapted level but instead entails a substantial enhancement above the dark-adapted value, by a mechanism that is presently unidentified.

Adult↗
PubMed1990

Correlation of electroretinographic findings and peripheral retinal nonperfusion in patients with sickle cell retinopathy.

This study examined the relationship between the proportion of peripheral retinal nonperfusion and electroretinogram measures in one untreated eye of 44 patients with sickle cell retinopathy. The proportion of peripheral retinal nonperfusion was derived from fluorescein angiographic frames covering 180 degrees of the temporal periphery. The extent of peripheral retinal capillary nonperfusion was greater in patients with peripheral retinal neovascularization compared with those without neovascularization. Reductions in electroretinogram a- and b-wave amplitudes were found in patients with and without neovascularization. However, such reductions were of greater frequency and magnitude in patients with neovascularization. Electroretinogram abnormalities were restricted to reductions in amplitude; implicit times were generally normal. In terms of the variables of the b-wave luminance-response function, abnormalities were restricted to reductions in the amplitude variable Rmax; values of logK and n were generally within the normal range. Across all patients, there was a significant negative correlation between electroretinogram amplitude measures and capillary nonperfusion, indicating that the electroretinogram provides information about the functional consequences of peripheral capillary nonperfusion in sickle cell retinopathy.

Adolescent↗
PubMed1990

Spatial properties of rod-cone interactions in flicker and hue detection.

Rod-cone interactions in flicker and hue detection were compared to examine the hypothesis that they are mediated by mechanisms with different spatial properties. Flicker and hue thresholds for a 1 deg test stimulus (TS) were measured as a function of background luminance and diameter. Flicker thresholds were reduced from their dark-adapted value by an 11 deg diameter background, but not by a 1 deg background. These results, in agreement with previous work, demonstrate that light adaptation of rods surrounding the TS is necessary to eliminate their effect on cone flicker thresholds. In contrast, hue thresholds were reduced from their dark-adapted value to a comparable degree by 1 and 11 deg backgrounds, indicating that light adaptation of the rods stimulated by the TS is sufficient to abolish the rod-cone hue interaction. Our results support the contention that the rod-cone flicker and hue interactions are mediated by different mechanisms. We also demonstrated that changing the detection task while keeping stimulus parameters constant is sufficient to shift between these two types of rod-cone interactions.

Adaptation, Ocular↗
PubMed1990

A form of congenital stationary night blindness with apparent defect of rod phototransduction.

We report findings obtained from an individual with an unusual form of congenital stationary night blindness (CSNB). Although the rhodopsin density difference of this subject was normal, there was no evidence of rod-mediated visual function. Dark-adapted thresholds were cone-mediated, and dark-adapted electroretinograms (ERGs) represented activity of the cone system exclusively. ERG a- and b-waves obtained under light-adapted conditions were normal. The absence of a rod a-wave but the presence of normal rhodopsin density, in combination with normal cone function, indicates that this form of CSNB likely involves a defect of phototransduction that is limited to the rods. In addition, light-adapted b-wave responses to high luminance flashes were larger than dark-adapted responses, whereas a-wave amplitudes were reduced by light adaptation. These ERG results address proposed mechanisms by which light adaptation might enhance cone system responses.

Adolescent↗
PubMed1989

The luminance-response function of the dark-adapted human electroretinogram.

A hyperbolic equation of the form (R/Rmax) = Ln/(Ln + Kn) has been used to describe the b-wave luminance-response function of the dark-adapted human electroretinogram (ERG). However, this equation accurately describes the function only at low to moderate flash luminances. At high flash luminances, a second amplitude increase or "limb" appears in the function. The results of the present study demonstrate that this limb does not represent a cone system response. First, the spectral sensitivity of the entire luminance-response function is rod-determined in normal subjects. Second, the limb was present in a rod monochromat, in whom cone system ERGs were indistinguishable from noise. Instead, the nonmonotonic nature of the luminance-response function may result from a luminance-dependent algebraic summation of the components that underlie the ERG waveform of the rod system.

Color Vision Defects↗
PubMed1989

Electroretinographic findings in human oculocutaneous albinism.

Electroretinograms (ERGs) were obtained under dark- and light-adapted conditions from 15 human oculocutaneous albinos and compared with the results from a group of 30 normally pigmented subjects. Amplitude and implicit time measures of the ERG recordings obtained from six autosomal recessive and three autosomal dominant tyrosinase-positive albinos fell within the normal range. The six tyrosinase-negative albinos examined also had normal ERGs at low-to-moderate flash luminances. However, at the highest flash luminances, the dark-adapted ERGs of five such subjects were at or just above the normal range in amplitude, and were shorter than normal in implicit time. The dark-adapted luminance-response functions of these subjects were positioned normally on the luminance axis; light-adapted functions were shifted slightly to the left of normal. The possibility that anterior parts of the retina responding to light passing through the anterior eyewall may contribute to the ERGs obtained from tyrosinase-negative albinos was examined by recording dark-adapted ERGs with the sclera occluded. Amplitudes were reduced compared with those obtained with the sclera unoccluded, suggesting that responses to transscleral illumination contributed to the ERG in this type of albinism.

Adolescent↗
PubMed1989

Rod-cone dystrophy associated with a rod system electroretinogram obtained under photopic conditions.

A 14-year-old girl with a history of night blindness and poor visual acuity was examined. Cystic-appearing lesions were apparent in the fovea, and pigmentary changes were observed in the midperipheral retina. Electroretinograms (ERGs) recorded from the dark- and light-adapted eye were similar in waveform, implicit time, and approximate amplitude. Results of examination of the spectral characteristics of the light-adapted ERG demonstrated that the recordings represented activity of the rod system, despite the presence of a background field, which eliminates the response of the rod system in normal subjects.

Adaptation, Ocular↗
PubMed1988

Rod and cone dysfunction in carriers of X-linked retinitis pigmentosa.

Carriers of X-linked retinitis pigmentosa were studied using electroretinographic and psychophysical procedures. Under both dark- and light-adapted (cone-isolated) conditions, electroretinogram (ERG) a-waves of carriers were reduced in amplitude but normal in implicit time, whereas b-waves were reduced in amplitude and delayed in implicit time. Reductions in b-wave amplitudes of the carriers as a group were equivalent for the rod and cone systems. Luminance-response functions for both dark-adapted and cone-isolated b-waves were fit by the Naka-Rushton equation and demonstrated a selective reduction of Rmax; the semi-saturation constant (K) and the slope parameter (n) were normal. Electroretinograms recorded using the brightest stimulus flashes were most effective at distinguishing carriers from normals. Absolute thresholds of the carriers were elevated significantly across the central 40 degrees of the visual field. As a group, the threshold elevations of the carriers were approximately equal for the rod and cone systems.

Adult↗
PubMed1987

Psychophysical and electroretinographic findings in X-linked juvenile retinoschisis.

We compared psychophysical and electroretinographic test results of four patients with X-linked juvenile retinoschisis who had clinically apparent lesions isolated to the foveal area. The b-wave of the flash electroretinogram was selectively reduced, while the a-wave was within the normal range. Oscillatory potentials generated by either the rod or cone systems were markedly reduced. Absolute thresholds outside the fovea were either normal or only moderately elevated, indicating that the sensory neural pathways were, by and large, operating with limited dysfunction. Therefore, the oscillatory potentials do not directly reflect, to any appreciable extent, the function of bipolar or ganglion cells. Finally, a sequence of pathologic events is proposed for X-linked juvenile retinoschisis that initiates with Müller cell dysfunction.

Adolescent↗
PubMed1987

Electroretinographic findings in sickle cell retinopathy.

We obtained electroretinograms (ERGs) from normal subjects and from patients with sickle cell disease. The ERG components (a-wave, b-wave, and oscillatory potentials) obtained from normal subjects and patients without peripheral retinal neovascularization did not differ in either amplitude or implicit time. However, ERG components obtained from patients with peripheral retinal neovascularization were reduced in amplitude compared with those obtained from normal subjects or patients without neovascularization. The reduced a-wave, b-wave, and oscillatory potential amplitudes may have been due to photoreceptor dysfunction secondary to choroidal ischemia or possibly increased oxygen demands by the inner retina. Ischemia of the inner retina may also have contributed to the altered b-wave and oscillatory potentials. These results suggest that ERG provides a means of assessing the consequence of peripheral retinal ischemia to retinal cell function and could be of value in monitoring patients with sickle cell disease for the development of clinically significant peripheral retinal neovascularization.

Anemia, Sickle Cell↗