Frequency spectrum and amplitude analysis of dark- and light-adapted oscillatory potentials in albino mouse,rat and rabbit

We studied frequency spectrum, implicit time and amplitude of oscillatory potentials (OPs) in albino mice, rats, and rabbits. Oscillatory potentials were extracted digitally from dark- and light-adapted electroretinograms (ERGs) recorded with a protocol commonly used in our laboratory. The frequency spectra of OPs were analyzed by using Fast Fourier Transform (FFT). Oscillatory potential amplitudes were calculated via numerically integrating the power spectrum. Oscillatory potential frequency spectra vary among species and are light-intensity dependent. In dark-adapted ERG, mouse and rat OPs have one major component with a frequency peak at approximately 100 Hz. Rabbits show multiple frequency peaks with a low frequency peak around 75 Hz. In all the three species, the implicit time of light-adapted OP is longer than that of the dark-adapted OPs. At a given intensity, mice have the highest OP responses. Our data suggest that the commonly used bandpass of 75 Hz (or even 100 Hz) to 300 Hz for OP extraction is insufficient in these animals. In order to acquire the complete OP responses from the ERG signals, it is necessary to determine the OP frequency spectrum. In this study, the lower end cutoff frequency was set at 40 Hz in mice, 65 Hz in rats and rabbits.     

Attenuation of oscillatory potentials in <Emphasis Type="Italic">nob2</Emphasis> mice

Purpose To examine changes in inner retinal function of nob2 mice, expressing a null mutation in Cacna1f encoding the Ca_V1.4 subunit of voltage-dependent calcium channels. CACNA1F mutations underlie one form of incomplete X-linked congenital stationary night blindness (CSNB2). In addition to a loss of dark-adapted (rod-driven) visual sensitivity, electroretinogram (ERG) b-waves and oscillatory potentials (OPs) are decreased in CSNB2 patients. Methods ERGs were recorded under dark-and light-adapted conditions from the corneal surface of nob2 mice, WT littermates and nob4 mice. ERG frequency spectra were calculated by fast Fourier transform (FFT). A FFT-based high-pass filter was used to derive OP waveforms. Results Under dark-adapted conditions, the dominant frequency of the OPs varied between 90 to 120 Hz in WT mice. In WT mice, OP frequency first increased with flash intensity and then decreased at the highest flash levels while overall OP amplitude increased monotonically with increasing flash intensity. In response to low stimulus flashes, reliable OPs were not obtained from nob2 mice. OPs were only seen at stimulus intensities at or above −1.8 log cd s/m², where they occurred at a lower frequency range (70–90 Hz) than for WT mice. When flash stimuli were superimposed against a steady rod-desensitizing adapting field, the amplitude and frequency of WT OPs increased with flash intensity above 0.4 log cd s/m². In comparison to WT results, cone-mediated OPs obtained from nob2 mice were smaller in amplitude, of lower frequency and had delayed implicit times. We compared the extent to which OPs and the b-wave were reduced in nob2 mice, by normalizing to the results obtained from WT mice. In comparison to the b-wave, the OPs were relatively spared, under both dark- and light-adapted conditions. Conclusions In nob2 mice, rod- and cone-driven OPs are reduced in amplitude and occur at a lower frequency range. Since Ca_V1.4 is expressed in both the inner and outer plexiform layers, these changes are likely to reflect reduced transmission from photoreceptors to bipolar cells as well as alterations in inner retinal function. That the OPs were better preserved than b-waves suggests that inner retinal pathways may be reorganized in response to the decreased bipolar cell response in nob2 mice.     

The electroretinogram of the rhodopsin knockout mouse.

The electroretinogram (ERG) of the rhodopsin knockout (rho-/-) mouse of Humphries et al. (1997) (Humphries et al., 1997) was studied for evidence of light-evoked rod activity and to describe the cone function. The rho-/- retina develops normal numbers of rod and cone nuclei, but the rods have no outer segments, and no rhodopsin is found by immunohistochemistry. The dark-adapted ERG threshold was elevated 4.7 log units above wild-type (WT) control mice, indicating that any residual rod responses were reduced >50,000-fold, consistent with a complete functional knockout. The dark-adapted rho-/- ERG had a cone waveform, and the spectral sensitivity peaked near 510 nm for both dark-adapted and light-adapted conditions, without evidence of a Purkinje shift. The light-adapted ERG b-wave amplitude of young rho-/- mice was the same as WT. The amplitude remained steady up to postnatal day P47, but thereafter it declined to only 1-2% by P80 when no cone outer segments remained. Cone b-wave threshold of dark-adapted rho-/- mice was -1.07 +/- 0.39 log cd-s/m2 (n = 17), which is 1.27 log units more sensitive than light-adapted thresholds against a rod-suppressing Ganzfeld background of 1.61 log scotopic cd/m2. This indicates that dark-adapted WT responses to still dimmer stimuli are exclusively rod driven with minimal cone intrusion. Above this cone threshold intensity, the dark-adapted b-wave of WT will be a summation of rod and cone responses. Threshold versus intensity (TVI) studies gave no evidence of a rod influence on the mouse cone b-wave.     

Rod-/L-cone and rod-/M-cone interactions in electroretinograms at different temporal frequencies

We recorded electroretinograms (ERGs) under stimulus conditions that only modulated one cone type: either the L- or the M-cones. In these conditions the rods were also modulated. We measured the ERG responses at different temporal frequencies. A simple model that assumes that the first harmonic components of the responses are the result of a vector addition of rod- and cone-driven ERG responses can explain the data satisfactorily for temporal frequencies at and above 6 Hz. From fits of the model to the data, estimates of the gains and phases of the rod- and cone-driven responses can be obtained. At 6 Hz, the fundamental responses originate exclusively in the rods. The gains of the cone-driven responses are substantial at the other temporal frequencies, are maximal at 12 or 18 Hz, and then decrease with increasing temporal frequencies. The gains of the rod-driven responses decrease more steeply with increasing temporal frequencies than the cone gains. Furthermore, the rod and cone phases decrease approximately linearly with temporal frequency, suggesting that they are mainly determined by a response delay. The response delay in the rod-driven ERGs is larger than in the cone-driven ERGs.     

Ultraviolet light-induced and green light-induced transient pupillary light reflex in mice

PURPOSE: To study UV light-induced and green light-induced pupillary light reflex (PLR) in mice and to measure the illumination thresholds of rod-mediated and cone-mediated PLR responses. METHODS: We measured dark-adapted transient PLR- in C57BL/6 mice elicited with ultraviolet and green light over an intensity range of 9 log units. To assist in isolating the responses mediated by rods and cones, we studied the PLRs in mouse models presenting pure cone and pure rod functions. We also characterized ERG signals in these mice under the same experimental conditions. RESULTS: The UV light-induced transient PLR has identical intensity-response curves as green light-induced PLR in all the three mice strains. The threshold (5% PLR) of rod-driven PLR is 107 approximately 108 photons cm2 s-1, which is 1 approximately 2 log units lower than the dark-adapted ERG b-wave. The threshold of cone-driven PLR is approximately 1012.5 photons cm-2 s-1 and is similar to that of the cone ERG. CONCLUSIONS: We demonstrated that mice have PLR responses under UV stimulation. The cone-elicited PLRs have a threshold that is approximately 5 log units higher than that of rod-elicited PLR at both UV and green wavelengths. We observed a divergence between the spectra responses in PLR and ERG. However, the mechanism and implications of this phenomenon are yet to be identified.     

Isoflurane is an effective alternative to ketamine/xylazine/acepromazine as an anesthetic agent for the mouse electroretinogram

The electroretinogram (ERG) is an essential measure of retinal function for studying mouse models of retinal disease. Ketamine, in combination with xylazine and/or acepromazine, is the most commonly used anesthetic agent. Although it works well in most situations, some fragile mouse strains have high mortality rates with this ketamine cocktail. We compared isoflurane with the ketamine cocktail in a longitudinal study of light-adapted and dark-adapted ERGs in C57BL/6J mice. Waveforms were averaged, oscillatory potentials (OPs) were extracted by digital filtration, and key ERG parameters were analyzed. The ERG waveforms were qualitatively similar with both anesthetics, and the male and female ERG parameters did not show significant differences. For light-adapted ERGs, b-wave amplitude and implicit time, and wavelet index were decreased under isoflurane anesthesia, whereas for dark-adapted ERGs, a- and b-wave implicit times were decreased and wavelet index was increased. The dark-adapted b-wave amplitude showed a significant inverse correlation with animal weight and age. Rod phototransduction gain and the Naka–Rushton n and R _max parameters were the same for both anesthetics, and only the Naka–Rushton log k parameter was significantly elevated for isoflurane anesthesia. We propose that isoflurane is a satisfactory alternative to the ketamine cocktail for anesthesia in the mouse ERG. Precise quantitative comparisons, however, should only employ study designs using isoflurane versus isoflurane, or ketamine versus ketamine. Moreover, in light of the effects of both isoflurane and the ketamine cocktail on blood glucose levels, it would be prudent to control the fasting state of the animals in quantitative ERG studies.     

Oscillatory potentials of X-linked carriers of congenital stationary night blindness

ERG oscillatory potentials (OPs) were recorded from obligate carriers of CSNB and from age-matched normal subjects. The OPs were recorded under four stimulus conditions and were analyzed in the time and frequency domains. The results, first of all, provide confirmation of the previous report that the OP amplitude is reduced in carriers. Second, the results show that, of four stimulus conditions examined, the best condition for discriminating the carriers and normal subjects was when the flash was blue and the eye was dark-adapted. Third, the results show that, in the frequency domain, optimal discrimination occurs when examining the power content of the OP at a center frequency of about 130 Hz using a 70 Hz bandwidth window. In the time domain, optimal discrimination occurs when examining the amplitude of the third peak of the response.     

Effect of experimental glaucoma in primates on oscillatory potentials of the slow-sequence mfERG

PURPOSE: To determine the effect of experimental glaucoma in macaque monkeys on oscillatory potentials (OPs) in the slow-sequence multifocal electroretinogram (mfERG). METHODS: Photopic slow-sequence mfERGs were recorded from anesthetized adult macaque monkeys and normal human subjects. The stimulus consisted of 103 equal-sized hexagons within 17 degrees of the fovea. The m-sequence was slowed, with 14 blank frames, approximately 200 ms, interleaved between flashes for monkeys and 7 blank frames, approximately 100 ms, for humans, to produce waveforms similar to the photopic full-field flash ERG. Recordings were made under control conditions (24 monkey eyes, 7 human) and after laser-induced experimental glaucoma in monkeys (n = 8). A Fourier fast transform [FFT] was used to determine the frequency ranges of the major OPs. OP amplitudes were quantified by using root mean square (RMS) for two-frequency bands in five horizontal and four vertical locations. Visual field defects were assessed using behavioral static perimetry. Full-field photopic flash ERGs also were recorded. RESULTS: OPs in two distinct frequency bands were discriminated in the monkey mfERG: fast OPs, with a peak frequency of 143 +/- 20 Hz, and slow OPs, with a peak at 77 +/- 8 Hz. There were similar findings in humans and with the flash ERG in monkeys. The fast OP RMS in monkey control eyes was significantly larger in temporal than nasal retina (P < 0.01) and in superior versus inferior retina (P < 0.05) as reported previously. The slow OP RMS was largest in the foveal region. Experimental glaucoma reduced fast OP RMS in all locations studied, even when visual field defects were moderate (MD = -5 to -10 dB; P < 0.05), whereas the slow OP RMS was reduced significantly primarily in the foveal region when field defects were severe (MD < -10 dB; P < 0.05). The fast OP RMS showed a moderate correlation with local visual field sensitivity and with local ganglion cell density (calculated from visual field sensitivity). For the slow OPs the correlation was much poorer. Consistent with previous studies, the photopic negative response (PhNR) amplitude was significantly reduced when the visual sensitivity was minimally affected. CONCLUSIONS: OPs in the ERG of primates fall in two frequency bands: fast OPs with a peak frequency around 143 Hz and slow OPs, with a peak frequency around 77 Hz. The fast OPs, which rely more on the integrity of retinal ganglion cells and their axons than do the slow OPs, have potential utility for monitoring the progression of glaucoma and the effects of treatment.     

Rod- versus cone-driven ERGs at different stimulus sizes in normal subjects and retinitis pigmentosa patients

Purpose

To study how rod- and cone-driven responses depend on stimulus size in normal subjects and patients with retinitis pigmentosa (RP), and to show that comparisons between responses to full-field (FF) and smaller stimuli can be useful in diagnosing and monitoring disorders of the peripheral retina without the need for lengthy dark adaptation periods.

Method

The triple silent substitution technique was used to isolate L-cone-, M-cone- and rod-driven ERGs with 19, 18 and 33% photoreceptor contrasts, respectively, under identical mean luminance conditions. Experiments were conducted on five normal subjects and three RP patients. ERGs on control subjects were recorded at nine different temporal frequencies (between 2 and 60 Hz) for five different stimulus sizes: FF, 70°, 60°, 50° and 40° diameter circular stimuli. Experiments on RP patients involved rod- and L-cone-driven ERG measurements with FF and 40° stimuli at 8 and 48 Hz. Response amplitudes were defined as those of the first harmonic component after Fourier analysis.

Results

In normal subjects, rod-driven responses displayed a fundamentally different behavior than cone-driven responses, particularly at low temporal frequencies. At low and intermediate temporal frequencies (≤ 12 Hz), rod-driven signals increased by a factor of about four when measured with smaller stimuli. In contrast, L- and M-cone-driven responses in this frequency region did not change substantially with stimulus size. At high temporal frequencies (≥ 24 Hz), both rod- and cone-driven response amplitudes decreased with decreasing stimulus size. Signals obtained from rod-isolating stimuli under these conditions are likely artefactual. Interestingly, in RP patients, both rod-driven and L-cone-driven ERGs were similar using 40° and FF stimuli.

Conclusion

The increased responses with smaller stimuli in normal subjects to rod-isolating stimuli indicate that a fundamentally different mechanism drives the ERGs in comparison with the cone-driven responses. We propose that the increased responses are caused by stray light stimulating the peripheral retina, thereby allowing peripheral rod-driven function to be studied using the triple silent substitution technique at photopic luminances. The method is effective in studying impaired peripheral rod- and cone- function in RP patients.

     

利用锥体细胞失功能大鼠和先天性静止性夜盲大鼠对视锥与视杆通路振荡电位的分离研究

背景 振荡电位(OPs)是评估视网膜缺血缺氧性疾病视网膜功能变化的重要工具,利用视网膜退行性病变动物模型对视锥、视杆通路起源的OPs特点进行研究非常重要. 目的 在两种自发性视网膜退行性病变模型大鼠中分离视锥、视杆通路,对比分析视杆、视锥通路起源的OPs波的特点. 方法 采用雄性SD大鼠、锥体细胞失功能(RCD)大鼠、先天性静止性夜盲(CSNB)大鼠各6只,以RETI-scan视觉生理记录系统分别在暗适应(12h)和明适应(10 min)条件下,用不同强度的刺激光(-35、-25、-15、-5、0、5 db)进行刺激,记录各组大鼠的闪光视网膜电图(FERG),通过Matlab 7.0的Butterworth滤波提取OPs,采用快速傅里叶变换(FFT)对所得OPs进行频谱分析.结果 暗适应条件下SD大鼠和RCD大鼠的ERG均可见a波和b波,但CSNB大鼠b波阙如;明适应条件下,SD大鼠和CSNB大鼠可见b波,但RCD大鼠各波阙如.暗适应较高刺激光强度下,SD大鼠和RCD大鼠均有低频(主频)和高频(次频)两个明显的频峰,分别为75 ～ 110 Hz、90～120 Hz和90～ 120 Hz、110 ～ 135 Hz;不同刺激光强度下,CSNB大鼠只有一个频峰,为70～100 Hz.而明适应不同刺激光强度下,SD大鼠和CSNB大鼠均只有一个频峰,分别为75～95 Hz和70～85 Hz.明适应条件下与SD大鼠比较,CSNB大鼠b波隐含时延长,b波振幅明显下降,差异均有统计学意义(P＜0.05);暗适应条件下,RCD大鼠b波隐含时和振幅与SD大鼠比较,差异无统计学意义(P＞0.05);与SD大鼠比较,RCD和CSNB大鼠OPs波振幅下降,隐含时延长,差异均有统计学意义(P＜0.05);明适应条件下不同刺激光强度下CSNB大鼠OPs波的隐含时明显长于SD大鼠,振幅明显低于SD大鼠,差异均有统计学意义(P＜0.05). 结论 视锥、视杆通路起源的OPs有不同特性,自发性视网膜退行性改变大鼠的视杆OPs有两个频峰,正常情况下,视杆通路对OPs的贡献比视锥通路大.     

The oscillatory potentials of the dark-adapted electroretinogram in retinopathy of prematurity

PURPOSE: To study the development of the electroretinographic (ERG) oscillatory potentials (OPs) in two rat models of ROP and in human subjects with a history of ROP. METHODS: Sprague-Dawley rats (n = 36) were studied longitudinally. Rat models of ROP were induced, either by exposure to alternating 50%/10% oxygen (50/10 model) from postnatal day (P) 0 to P14 or by exposure to 75% oxygen (75 model) from P7 to P14. Control rats were reared in room air. Infant and adult human subjects with and without a history of ROP (n = 91) were also studied. Dark-adapted ERGs were recorded and filtered to demonstrate the OPs. Discreet Fourier transform (DFT) allowed evaluation of the OP power spectrum. OP energy (E), dominant frequency (F(peak)), and sensitivity (log i(1/2)) were evaluated. RESULTS: In 50/10 model rats, E was low compared with that in the 75 model rats and control animals. F(peak) (approximately 95 Hz) did not vary with age or group. Intriguingly, log i(1/2) in 75 model rats was greater than that in controls or 50/10 model rats. Human adults with a history of ROP had lower-energy OPs than did the control adults, but infants with a history of ROP had higher-energy OPs than did the control infants. F(peak) was lower (approximately 120 Hz) in infants than in adults (approximately 130 Hz). ROP did not affect log i(1/2) in humans. CONCLUSIONS: Differences between OPs in healthy rats and healthy humans were substantial, suggesting that OPs in rat models of ROP are unlikely to provide insight into the effects of ROP on human OPs. Indeed, neither ROP model studied showed a pattern of effects similar to that in human ROP.     

Cone properties of the light-adapted murine ERG

Purpose: Because the mouse lacks a typical Purkinje shift, we have examined its light-adapted ERG to determine whether there was other evidence in addition to tolerance to background light, that could be used to identify cone function in the ERG. Methods: Full field comeal ERGs to white flashes, double flashes and flash trains were examined in the presence of a strong full field light adaptation and compared with the human cone ERG. Results: The following cone-like properties could be identified. (1) The light-adapted murine ERG increases in amplitude gradually during the first 10 minutes of light-adaptation; (2) It is capable of responding to a 50 Hz stimulus, although its overall frequency response is slower than that of the human cone ERG; (3) A corneal positive d-wave occurs to the termination of a flash train; (4) The response increases linearly with light intensity. Conclusion: The light-adapted murine ERG has several properties of cones but it has a slower response than the human cone ERG. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synaptic lamellae of the photoreceptors of pearl and wild-type mice

The retina of the pearl mutant mouse, C57BL/6J pe/pe, exhibits reduced light sensitivity in the dark-adapted condition (Balkema and Pinto, J Neurophysiol 48:968, 1982). The authors searched for an anatomic correlate in the retina which could relate to the functional deficit. Electron microscopic mosaics of the outer plexiform layer of light- and dark-adapted pearl and wild-type mice were analyzed. The numerical density and length of the photoreceptor synaptic lamellae showed these parameters to be indistinguishable in wild-type and pearl retinas under conditions of both light- and dark-adaptation. Light-adapted pearl retinas exhibited some rod spherules that contained structurally modified synaptic lamellae with bulbous thickenings and adjacent electron-dense bodies. These lamellar modifications were neither apparent in the light-adapted, wild-type retinas, nor in the dark-adapted retinas of either genotype. Pronase application to ultrathin sections hydrolyzed synaptic lamellae, bulbous thickenings and electron dense bodies.     

Light- and dark-adapted electroretinograms (ERGs) and ocular pigmentation: comparison of brown- and blue-eyed cohorts

This study characterizes differences in human ERGs based on ocular pigmentation. Light- and dark-adapted luminance-response (LR) series for a-, b- and i-waves and light-adapted oscillatory potentials (OPs) were recorded in 14 healthy volunteers (7 blue-eyed Caucasians; 7 brown-eyed Asians, aged 20–22 years). Amplitude interpolations were by logistic growth (Naka-Rushton), Gaussian or the combined ‘photopic hill’ functions. Implicit times (IT) for dark-adapted a- and b-waves, and for light-adapted a-, b- and i-waves were earlier in the blue-eyed group than in the brown-eyed group across all flash strengths (P < 0.05). For dark-adapted ERGs, saturated a-wave amplitude was larger for blue eyes (397 vs. 318 μV, P < 0.05) as was the a-wave to strong flash (10 cd·s/m²; 357 vs. 293 μV, P < 0.05) and the b-wave to ISCEV standard 0.01 (354 vs. 238 μV, P < 0.05). Light-adapted b-waves for midrange flash stimuli were much larger for the blue-eyed group (photopic hill, Gaussian peak: 155 vs. 82 μV, P < 0.001) with no difference in saturated amplitudes. Similarly, interpolated i-wave amplitudes were larger (48 vs. 18 μV, P < 0.01). For a light-adapted 2.6 stimulus, a- and b-waves were larger for the blue-eyed group (52 vs. 39 μV; 209 vs. 133 μV, P < 0.01) as were OP4 and OP5 (37.2 vs. 15.6 μV; 47.5 vs. 22.2 μV, P < 0.01), but OP1-OP3 did not differ. ERGs have shorter ITs in people with blue irides than in those with dark pigmentation. Amplitude differences are highly non-linear and substantially larger from eyes with light pigmentation for components thought to be associated with the OFF retinal pathways.     

Syndromic retinitis pigmentosa: ERG and phenotypic changes

PURPOSE: Our aim was to review the phenotype and extent of ERG changes in syndromic RP (SRP). PATIENTS AND METHODS: A retrospective review of charts of 82 patients seen over the last 20 years with SRP was carried out. Clinical data were compared with changes in ERG. Full-field ERGs comprised selective rod-driven, maximal dark-adapted mixed responses, and isolated cone-driven signals. Occasionally, ERGs were recorded under brief general anaesthesia. ERG changes were classified as normal, reduced or extinguished. RESULTS: Syndromic RP was diagnosed for the following entities: Usher (45 patients), Laurence-Moon-Bardet-Biedl (LMBB, 17 patients), Kearns-Sayre (10), Batten (6), Refsum (3), Senior-Loken (1). ERG changes varied in every subgroup and were therefore not specific for the syndromes. CONCLUSION: There was wide variation of clinical presentation in SRP, much as seen in isolated RP, often without obvious ophthalmoscopic changes. ERG testing is a prerequisite for differential diagnosis as well as for early detection of multiple handicaps.     

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.     

Age-related changes in <Emphasis Type="Italic">Cngb1</Emphasis>-X1 knockout mice: prolonged cone survival

The rod photoreceptor cGMP-gated cation channel has an essential role in phototransduction functioning as the primary point for calcium and sodium entry into the rod outer segment. The channel consists of two subunits, α and β. The α-subunit can function in isolation as an ion channel, and the β-subunit modulates channel activity and has a structural role. We previously reported that a mouse knockout (KO) of the β-subunit and related glutamic acid–rich proteins (GARPs) attenuates rod function and causes structural alterations and slowly progressive retinal degeneration. Here, we have extended our functional analyses of the KO mice evaluating rod and cone function using the electroretinogram in mice up to 4 months of age. Retinal stratification is preserved in the knockout mice at 3 months, and a significant number of cones remain up to 7 months based on PNA staining of cone sheaths. Electroretinography of KO mice at 1 month old revealed a diminished dark-adapted b-wave and normal light-adapted b-wave compared to wild-type mice. Over the next 3 months, both dark- and light-adapted b-wave amplitudes declined, but the reduction was greater for dark-adapted b-wave amplitudes. In one-month-old mice, the critical flicker frequency (CFF) was substantially lower for the KO mice at scotopic intensities, but normal at photopic intensities. CFF values remained stable in the KO mice as the b-wave amplitudes decreased with age. Declining b-wave amplitudes confirm an RP phenotype of rod followed by cone degeneration. Flicker responses show that the cone circuits function normally at threshold despite significant losses in the maximum light-adapted b-wave amplitude. These results confirm that rods are marginally functional in the absence of the β-subunit and in addition show that CFF may be a more sensitive measure of remaining functional cone vision in animal models of RP undergoing progressive rod–cone degeneration.     

Longitudinal changes in photopic OPs occurring with vigabatrin treatment

PURPOSE: Vigabatrin (gamma-vinyl-GABA) is an antiepileptic drug successful in the management of infantile spasms. Photopic ERGs were tested in children followed longitudinally before and during vigabatrin treatment. METHODS: Subjects were 26 infants (age range 1.5-24 months, median 7.6 months) on vigabatrin treatment who had been tested on multiple visits (two to four visits; mean, three visits). Eighteen of these were assessed initially before starting vigabatrin therapy and eight were assessed within 1 week of initiation of the drug. ERGs were recorded at 6-month intervals. Standard ISCEV protocol with Burian-Allen bipolar contact-lens electrodes (standard flash 2.0 cd.s/m2) was used. Although ISCEV standards were followed, a higher flash intensity (set at 3.6 cd.s/m2) was chosen for single-flash cone assessment to provide a better definition of OPs. Photopic OPs were divided into categories of early OPs and late OP (OP4). Responses were compared with age corrected limits extrapolated from our lab control database. RESULTS: Results showed differential effects of vigabatrin on the summed early OP amplitudes versus the late OP (OP4) and cone b-wave amplitude. The early OPs showed significant decrease (p = 0.0005, repeated measures analysis of variance) after 6 months and remained decreased for the duration of treatment. There was no significant change seen in the late OP. The cone b-wave amplitude showed initial increase (p = 0.04) after 6 months, followed by a decrease after 18 months; a trend similar to that of the late OP. CONCLUSION: Early photopic OPs were disrupted more than the late OP, suggesting relative deficit in the ON (depolarizing) retinal pathways.     

Human oscillatory potentials: intensity-dependence of timing and amplitude

Oscillatory potentials (OPs) have been described as reduced in amplitude or delayed in diabetic retinopathy, glaucoma, and vascular occlusions. Although OPs are thought to have useful diagnostic applications, some of their basic physiologic properties remain to be fully described. In the present study, we examined the relationship between the timing and amplitude of OPs and stimulus intensity. Five normal volunteers had one eye anesthetized and dilated. Dark-adapted full-field ERGs were recorded to white stimuli of 0.0125–40 cd s/m². The timing of the OPs was measured as the sum of the time to the peak (TTP) of four peaks beginning at 15 ms after the stimulus. The amplitude was taken as the sum of the amplitudes of those same peaks. As an alternative value, OP strength was represented by the area under the OP curve or power around 150 Hz (±30 Hz) in the frequency domain. The OP timing, as measured by TTP, was found to be inversely related to stimulus intensity. OP-amplitudes grew with intensity, but then declined for stimulus intensities above about 4 cd s/m². At bright light intensities, the TTP continued to shorten, yet amplitudes, power, and area all declined. Individual OPs behaved similarly and reflected the overall response pattern of the group as a whole. Brighter stimuli produced larger, faster OPs for stimulus strengths up to the intensity standard used to produce OPs (3.5 cd s/m²). We have extended the range of stimuli to some 10-fold higher than the ISCEV standard for producing OPs and found that the timing continued to accelerate but that OP-amplitudes, OP-area, and OP-power all decline at higher stimulus intensities. These alternative measures of OP energy are easily measured and may be useful for further studies.