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From the Department of Ophthalmology and Visual Sciences, University of Michigan, Ann Arbor.
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Abstract |
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METHODS. Monkey photopic flicker ERGs were elicited with sine wave stimuli (mean luminance, 2.66 log cd/m2; 80% modulation depth, on a 40 cd/m2 white background) and were recorded for stimulus frequencies of 4 Hz to 64 Hz, before and after intravitreal injection of DL-2-amino-4-phosphonobutyric acid (APB) and cis-2,3-piperidinedicarboxylic acid (PDA) that block ON- and OFF-bipolar activity, respectively. The amplitude and phase of the fundamental component were analyzed.
RESULTS. The flicker response amplitudes increased after APB, for frequencies of 6 Hz to 32 Hz. The further addition of PDA to isolate the photoreceptor component resulted in a relatively small residual response that decreased monotonically from 4 Hz to 32 Hz. The postsynaptic APB (ON-) and PDA (OFF-) sensitive components were isolated by subtraction and were characterized by amplitude and phase vectors. The ON- and OFF-components were larger than the initial control responses for stimuli of 8 Hz to 40 Hz. These two components had a frequency-dependent phase difference of 160° to 230°; normally, they interfere with each other and reduce their net contribution. The phase difference between ON- and OFF-components was nearly 180° for a 10-Hz stimulus, and the phase cancellation caused a prominent dip in amplitude at this frequency.
CONCLUSIONS. These results indicate that postreceptoral ON- and OFF-components contribute substantially to the sine-wave flicker ERG, especially at higher stimulus frequencies. Because of phase cancellation, they mask each other in the net response in a frequency dependent fashion. The photoreceptor contribution is greater than the net postsynaptic component only for frequencies of approximately less than or equal to 10 Hz. These results can be summarized by a vector model that may be useful for interpreting changes resulting from retinal disease.
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionsReferences |
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It is known that the retinal response to sine-wave flicker stimuli is dominated by a component at the same temporal frequency as the stimulus, termed the fundamental component. However, under some conditions, a component is seen at twice the stimulus frequency (i.e., a second harmonic component).10 11 12 13 Current source density analysis14 suggested that the fundamental component was dominated by photoreceptor activity, yet the second harmonic component originated from multiple retinal sources and included contributions from the middle and inner retina. A systems analytic approach in humans12 13 also agreed with a model of the sine-wave flicker ERG that attributed the majority of the linear (or fundamental) component to photoreceptor-related processes, particularly at medium and high frequencies. Some clinical studies15 16 have used the fundamental and second harmonic component of the sine-wave flicker ERG as an indicator of outer and inner retina function, respectively.
Previously, the authors studied the photopic fast flicker ERG using square waves and photostrobe flashes17 and found that the corneal response was virtually eliminated by aspartate or by the combination of DL-2-amino-4-phosphonobutyric acid (APB)18 plus cis-2,3-piperidinedicarboxylic acid (PDA).19 These drugs block transmission to the second-order neurons postsynaptic to cones.20 This implied a significant contribution from the inner retina to the corneally recorded photopic fast flicker ERG. What was curious was the apparent difference on flicker origins reported for sine waves versus findings in this study using brief-flash or square stimuli. The authors’ previous study, however, did not employ sine-wave stimuli and was limited to a 33.3-Hz flicker.
This study revisited the question of the origins of the primate flicker ERG and recorded sine-wave flicker responses from monkeys over a wide range of temporal frequencies on a photopic background before and after applying glutamate analogs. Particular attention was given to the nature of the fundamental frequency component. Photoreceptoral and postreceptoral ON- and OFF-components were obtained by waveform subtractions. The amplitude and phase of these separated components were plotted and characterized by amplitude and phase vectors. The results indicated that postreceptoral neural activity contributes substantially to the fundamental component of the sine-wave flicker ERG. The magnitude of the relative contributions depended on the stimulus frequency. Results of this study also showed that the relative phase between postsynaptic ON- and OFF-components dramatically affects the flicker ERG, and that phase cancellation of these two components creates an amplitude dip near 10 Hz that is normally observed in the primate sine-wave flicker ERG.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionsReferences |
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Drug Application
The drugs and intravitreal injection techniques have been
described previously.21
Drugs were injected into the
vitreous by a 30-gauge needle inserted through the pars plana
approximately 5 mm to 6 mm posterior to the limbus. Drugs (Sigma
Chemical Co., St. Louis, MO) were dissolved in sterile saline and
injected in amounts of 0.05 ml to 0.07 ml of 40 mM APB or 200 mM PDA.
Drug effects were monitored by tracking the photopic b-wave after APB
and the d-wave after PDA. Recordings were started after full drug
effects were seen, normally approximately 60 minutes to 90 minutes
after drug injections, and studies were completed within 5 hours
thereafter. Although these drug effects are mostly reversible after a
recovery period of several weeks, the results shown were recorded from
three eyes not treated previously.
Visual Stimulation
Sine-wave stimuli were produced by a 21/2 inch diameter,
densely packed array of 102 red LEDs (623 nm peak wavelength; 8 nm
half-width) that was positioned 10 cm from the eye. This illuminated a
ping-pong ball hemisphere (40 mm in diameter) that was placed
immediately in front of the eye to give wide-field stimulation.
Sine-wave modulation of the red LEDs was controlled by a digital
function generator (model 39; Wavetek, San Diego, CA), which fed into a
linear power amplifier. Light output was determined as a function of
input voltage and was found to be highly linear at the light levels
used in our study. The sinusoidal light modulation was symmetrical, and
the nonlinearity attributable to the light output under these
conditions was measured as less than 2%. The maximal and minimal
stimulus intensities were 2.90 log cd/m2 and 1.70
log cd/m2 on a constant white background of 40
cd/m2. The net result was 2.66 log
cd/m2 mean luminance and 80% modulation depth.
Recording and Analysis
After an initial 10 minutes of light adaptation at 40
cd/m2, flicker ERGs were recorded using a
Burian–Allen bipolar contact lens electrode (Hansen Ophthalmic
Development Laboratories, Iowa City, IA). A ground electrode was placed
on the ipsilateral ear. Responses were amplified with a band-pass of
0.1 Hz to 1000 Hz at 3 dB, and digitized at a 10 kHz-rate. The system
incorporates a narrow-band, 60-Hz line frequency analog notch filter.
In preliminary studies, it was first confirmed that the analog notch
filter did not affect either amplitude or phase of the flicker ERG
fundamental component for the stimulus frequencies that were used.
Fifty to 100 responses were averaged for each condition.
The amplitude and phase of the fundamental components were analyzed using Fourier coefficients of the response.22 23 The phase lag relative to the stimulus sine wave was presented on polar plots with positive values drawn counterclockwise from the 0° polar axis, as done in earlier studies.23 24 Because Fourier analysis gives phases only within a 0° to 360° range, the authors extrapolated absolute response phases beyond this limit by comparison with phases of adjacent temporal stimulus frequencies.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionsReferences |
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The middle column shows that the waveform after APB is simplified and becomes more sinusoidal; the amplitude becomes larger for stimuli of 10 Hz to 32 Hz. Because APB suppresses transmission from photoreceptors to depolarizing bipolar cells rather selectively, this indicates that the ON-bipolar cells contribute to the control sine-wave flicker ERG response. Subsequent addition of PDA (right-hand column) caused further changes to the response, with a considerable amplitude reduction for stimuli of 10 Hz and above. As the combination of APB + PDA effectively blocks transmission from photoreceptors to all postsynaptic cell types, the responses remaining after APB + PDA represent activity of the cone photoreceptors themselves. This cone cell activity contributes substantially to the responses at 4 Hz but to a progressively lesser degree for higher stimulus rates, and direct cone activity represents less than half of the control waveform for stimuli of 16 Hz and above.
Figure 3 shows a compilation of the amplitude and phase of the fundamental component of these responses. The mean and SE bars are shown for three animals. The fundamental amplitude goes through a minimum at approximately 10 Hz, and is maximum at approximately 40 Hz to 48 Hz. The phase of the fundamental component changes smoothly across frequencies, but it has a small deflection at approximately 10 Hz. For higher frequencies, the phase increases rather linearly with stimulus frequency. These control ERG results agree well with previous reports of human sine-wave flicker ERG.8 11 12 13
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With the further addition of PDA, the amplitude of the fundamental was greatly attenuated (Figure 3 top, crosses). The change was modest compared to controls at frequencies less than 10 Hz but was considerable at frequencies higher than 16 Hz. After APB + PDA, the amplitude of fundamental component decreased monotonically with increasing frequency from 4 Hz to 32 Hz and was quite small at frequencies higher than 24 Hz. Assuming that APB + PDA essentially eliminates signal transmission to cells postsynaptic to the photoreceptors, this result indicates that the control sine-wave ERG contains considerable contributions from postsynaptic neuronal activity, especially at higher stimulus frequencies. Note that even after isolating the photoreceptoral activity by APB + PDA, the frequency–amplitude curve did not show a simple low-pass function, but it had a small local maximum near 50 Hz that was consistently observed for all three monkeys.
Component Analysis
The next step was to determine how the postsynaptic ON- and
OFF-components participated in the sine-wave flicker ERG of the normal
eye. ON- and OFF-components were isolated by subtracting the postdrug
records from the predrug records; the APB-sensitive (ON-) component was
derived by subtracting the post-APB records from the controls, and the
PDA-sensitive (OFF-) component was obtained by subtracting the
post-(APB + PDA) records from the post-APB records. The amplitude and
phase of these components were plotted as a function of stimulus
frequency in Figure 4
. The mean and SEM are shown for three animals. The postsynaptic ON- and
OFF-components had relatively large amplitudes for a wide range of
frequencies of 8 Hz to 40 Hz. These two components behaved somewhat
different from each other and had different frequency-tuning
characteristics. The PDA-sensitive (OFF-) component amplitude had a
maximum around 10 Hz to 30 Hz, yet the APB-sensitive (ON-) component
rose steadily up to 48 Hz and then fell.
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The bold line (Fig. 4 , top) shows the amplitude of net postsynaptic component, obtained by taking the fundamental of the difference of the (APB + PDA) records from the control ERG. For frequencies less than 10 Hz, the photoreceptoral component was larger than the postsynaptic component. However, at stimulus frequencies of 16 Hz and above, the postreceptoral component became significantly larger (P < 0.05) than the photoreceptoral component, and postreceptoral activity essentially dominates the response at frequencies greater than 24 Hz. At 24 Hz and above, amplitude of the photoreceptoral component was less than 20% of the postreceptoral component for all three monkeys.
Origins of the Fundamental Frequency Component Sine-Wave Flicker
ERG Using Constituent Vectors
Figure 5
shows the constituent phase and amplitude vectors for the fundamental
frequency components of 4, 10, and 32 Hz sine-wave flicker responses.
For these vector figures, we used the results from the representative
monkey with waveforms shown in Figure 2
. At 4 Hz, the amplitude of the
ON-component (red arrow) is smaller than and 160° out of phase from
the OFF-component (blue arrow); consequently, the postsynaptic activity
vector (cyan arrow) is dominated by the OFF-component. This
sums with the photoreceptor component (green arrow) that is
shifted approximately 90°. The net resultant ERG (black arrow) is
slightly larger than either the photoreceptor or postsynaptic
components alone and has an intermediate phase. Note that the
photoreceptor activity provides a substantial contribution to the 4 Hz
sine-wave flicker ERG (as was also observed in Fig. 2
), although the
phase is advanced slightly ahead of the photoreceptor vector.
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At 32 Hz, the PDA-sensitive component and the APB-sensitive components both are relatively large, and the phase difference between them is 220° to 225°. They add together to give a large postsynaptic vector that is nearly 180° out of phase with the small photoreceptor vector. Consequently, the flicker ERG at 32-Hz stimulation is dominated nearly entirely by postsynaptic activity.
These examples at three different stimulus frequencies demonstrate the necessity of accounting for both phase and amplitude of the three main ingredient components of flicker ERG. The absolute sizes of the vectors alone are insufficient to describe the results, as illustrated at 10 Hz where the ON- and OFF-components are both large but are diametrically out of phase and nearly cancel each other, resulting in a considerable contribution from the much smaller photoreceptor vector to the resultant ERG.
Vector Model of Flicker ERG for Postsynaptic Diseases
This vector model of the flicker ERG led us to consider how
postsynaptic dysfunction can change the amplitude and phase of the
sine-wave flicker responses. The 32-Hz ERG was selected for this
simulation, because many clinical laboratories use stimuli near 30 Hz
to assess photopic flicker function in patients. Figure 6
shows the effects of altering the ON- or the OFF-component. The
uppermost diagram in Figure 6
shows the normal result at 32-Hz stimulus
frequency. Panels (A) and (B) show a phase delay or amplitude reduction
in the ON-component, respectively. Abnormalities in the ON-pathway
through the depolarizing bipolar cells result in a simulation of some
forms of congenital stationery night blindness (CSNB). A 23° delay
(equivalent to 2 milliseconds delay at 32 Hz stimulation) in the
ON-pathway causes only a 7° delay (i.e., 0.6 millisecond timing
change) of the ERG, but the amplitude increases by nearly 48%. A 50%
reduction in the ON-vector causes a minimal change in the ERG amplitude
but introduces a 35°-phase delay, equivalent to approximately 3
milliseconds.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionsReferences |
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The balance of contributions from the cones versus inner retinal neurons depended on stimulus frequency. At lower frequencies, the magnitude of the photoreceptor component was relatively larger, but for stimuli of approximately 24 Hz and above, the postreceptoral components became the major element in the overall response. These results indicate that postphotoreceptoral neuronal activity is critical for the full-field sine-wave flicker ERG, particularly at higher stimulus frequencies. Thus the functional integrity of the inner retina is essential for developing normal fast flicker responses that are recorded at the cornea.
This study’s results indicate that reductions of the photopic fast flicker ERG should not necessarily be interpreted as indicating cone cell dysfunction, because flicker changes can also arise from postreceptoral dysfunction as shown in our simulation model. These results with sine-wave stimuli, along with previous flicker studies using square-wave and strobe-flash stimuli,17 demonstrate that primate photopic flicker responses are dominated by proximal retinal activity at higher stimulus frequencies, including near 30 Hz, which is used commonly in assessing clinical patients. This holds true even for the fundamental component of the response to sine-wave stimulation and casts doubt on using the fundamental and second harmonic components to deduce photoreceptor versus inner retinal activity in the primate.15 16
Baron and coworkers2 3 4 5 previously studied the origins of the flicker ERG by intraretinal recordings with a microelectrode, and they found similarities between the local ERG (LERG) and the corneal ERG in phase and amplitude as a function of frequency when they recorded between 4 Hz to 12 Hz.2 They concluded that the photoreceptors were the dominant origin of the flicker response. Findings in this study are not in disagreement, as the isolated photoreceptor contribution is greater than the postsynaptic component for frequencies below approximately 10 Hz. Baron et al.2-5 also demonstrated that the response characteristics of LERG to sinusoidal flicker did not change much after isolation of the photoreceptors by application of aspartate.3 However, they did not record simultaneously at the cornea. It has been shown previously that the flicker ERG at the cornea is essentially abolished after application of aspartate,17 leading to the conclusion that the LERG and corneally recorded flicker ERG (at 30 Hz) have different origins. One qualifier is that Bush and Seiving17 used a pseudosine-wave stimulus created by a mechanical shutter. A further example of the disparity between intraretinal and corneal recordings is found in the M-wave,26 which is a large LERG response in the proximal retina but is minimally evident in the corneal ERG.
ON- and OFF-Component Phase Lag Dramatically Affects the Flicker
ERG
The component analysis using waveform subtraction unmasked the
presence of large postsynaptic ON- and OFF-components that were
embedded in the control ERG. These two major components normally
interfere with each other to some extent, due to phase lags of 160°
to 230° that make the net postsynaptic component smaller than either
component alone. This is not surprising, however, because it has been
shown already that blocking either ON- or OFF-components by APB or PDA
caused single-flash photopic ERGs to have larger negative or positive
responses than control ERGs, indicating a "push–pull" interplay
between the ON- and OFF-component.21
The phase differences between ON- and OFF-components depended on stimulus frequency. As stimulus frequency increased, the phase difference increased from 160° to 230° and was approximately 180° out of phase for 10 Hz stimuli. The destructive interference in these postsynaptic components coincides with the amplitude dip at 10 Hz frequency. Clearly, other factors are also necessary for the amplitude dip at 10 Hz, including the relative amplitudes of the ON- and OFF-component, and interactions between photoreceptoral and post-receptoral components. If the amplitudes of the ON- and OFF-components are comparable, their summed vector results in nulling the postsynaptic contribution. However, at 10 Hz, the OFF-component was consistently slightly larger than the ON-component, causing incomplete cancellation. At other frequencies the phase delay is either less than or more than 180°, and incomplete cancellation should occur. Stockman et al.27 observed similar destructive phase cancellation between the fast and slow rod signals in the human scotopic flicker ERG.
One might hope, the cone photoreceptoral activity could be captured in clinical situations by recording the sine-wave ERG at approximately 10 Hz, where the post-receptoral ON- and OFF-components effectively cancel each other and thereby minimize the amplitude contributed by postreceptoral activity (Fig. 4) . However, it should be emphasized that these findings apply only to the normal retina. Results may be quite different for diseased retinas in which pathologic changes may alter the relative amplitudes and/or phases of the ON- and OFF-components.
Photoreceptoral Contributions at Lower Frequencies
In contrast to the fast flicker ERGs, the slow sine-wave ERG had a
relatively larger contribution from the photoreceptors. The 4-Hz
sine-wave ERG was reduced only 25% in fundamental amplitude from the
control after APB + PDA. This indicates that the major portion of this
slow sine-wave response originates at the photoreceptoral level. This
finding may help explain why, in past studies, the conclusions have
been that with relatively slow sine-wave stimuli, the sine-wave flicker
ERG originated predominantly from near the
photoreceptors.2
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Even at low-stimulus frequencies,
however, the postreceptoral ON- and OFF-components participate in the
flicker ERG response.
It can be noted that even after blocking output from the photoreceptors using APB + PDA, the response was not a simple sine wave (Fig. 2) . Further, the frequency-amplitude curve of the photoreceptor component was a low-pass function across 4 Hz to 32 Hz, but above 32 Hz showed a small relative maximum at approximately 50 Hz in all three monkeys (Fig. 3) . These results suggest a complex response of the cone photoreceptors, or that more than the cones alone are contributing even after APB + PDA. One can wonder whether these responses are mediated, in part, through glial cell reaction to cone activity, because both retinal pigment epithelium cells and Müller cells responded to extracellular ionic changes in the subretinal space that result secondarily from photoreceptor light responses.28 However, evidence from the mudpuppy shows that glial cells are unable to follow fast flicker stimuli,29 although this is a cold-blooded amphibian rather than a primate.
Simulation Model for Postreceptoral Disease
Results of our simulation model indicated that retinal pathology
of the postreceptoral components can alter the flicker ERG. Both phase
delays and amplitude reductions of postsynaptic ON- or OFF-components
can change the phase and the amplitude of the final flicker ERGs.
Results showed that the dysfunction of postreceptoral components might
even result in an increased amplitude or faster phase under some
conditions. This suggests that, in clinical recordings of flicker ERG
responses, neither large amplitude nor fast phase necessarily means
that the retina is healthy. Thus care is advised in drawing conclusions
about retinal pathology from flicker changes.
These simulations may eventually aid in understanding the mechanism for an interesting phenomenon in patients with the complete type of congenital stationary night blindness (cCSNB). These subjects are believed to have selective dysfunction of the postsynaptic ON-pathway.30 31 32 Kim et al.24 found that cCSNB patients had a delayed fundamental component without much amplitude change. Although that study used strobe-flash 30-Hz stimuli, this study’s simulation model of the "ON-component reduction" for sine-wave stimuli (Fig. 6B) roughly mimicked the flicker ERG change seen in the cCSNB patients. The present study would predict that, for sine-wave stimuli, ON-pathway "disease" may affect flicker ERG timing in preference to reducing the amplitude. The cCSNB data suggest that the simulation model in Figure 6 may be useful for interpreting flicker ERG change in presumed postsynaptic retinal diseases, although further work is needed to develop models specifically for xenon-pulse flicker, as the Fourier spectrum of the response to a brief-flash, 30-Hz flicker may be considerably different from that of a sine-wave, 30-Hz flicker.
Contribution of Nonlinear or Subharmonic Components
In this study, only the fundamental component was considered.
However, second and higher harmonic components also contribute to the
response for some stimulus rates (data not presented). For example, at
10 Hz, the second harmonic amplitude is as large as the fundamental
amplitude (7.2, 6.5, and 6.2 µV for the second harmonic versus 7.7,
6.2, and 6.0 for the fundamental in the three monkeys studied) and
results in a frequency-doubled appearance for the response to 10 Hz
sine-wave stimulation.
Interestingly, glutamate analogs appeared to have different effects on the different harmonic components. For instance, the application of APB increased the fundamental amplitude across almost all frequencies (Fig. 3) , but APB decreased the second harmonic amplitude to variable degrees (data not shown). This means that the fundamental component becomes dominant after APB with relatively less second harmonic contribution, thus causing the waveforms to appear more sinusoidal after APB (Fig. 2) . Further analysis of the second and higher harmonic components is needed to interpret systematically the origin of the sine-wave flicker ERG.
Crevier and Meister33 found a phenomenon of period-doubling of responses to intense square-wave stimuli of 30 Hz to 70 Hz in the salamander and the human. They attributed this to nonlinear feedback interactions, possibly involving cone photoreceptor and OFF-bipolar cells. Period-doubling causes a one-half harmonic response that could fold back into and contaminate the true fundamental component if responses are averaged out of synchrony. We examined responses elicited at 32 Hz to 64 Hz by single trains of 15 responses without averaging. Period-doubling was evident only for intense square-wave stimuli. No doubling occurred with sine-wave stimuli even at high intensities, and specifically none was found for the stimulus intensities used in the present study.
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Conclusions |
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Acknowledgements |
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Footnotes |
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Submitted for publication May 22, 2000; revised August 31, 2000; accepted September 22, 2000.
Commercial relationships policy: N.
Corresponding author: Paul A. Sieving, Center for Retinal and Macular Degeneration, The W. K. Kellogg Eye Center, University of Michigan, 1000 Wall Street, Ann Arbor, MI 48105. psieving{at}umich.edu
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