Abstract
Purpose
To compare measurements of the full-field photopic negative response (PhNR), as well as intra-subject variation in the PhNR, using time and time–frequency domain analyses.
Methods
Full-field ERGs were recorded from 20 normally sighted subjects (aged 24–65 years) elicited by a long-wavelength pulse (3 cd s m−2) presented against a short-wavelength adapting field (12.5 cd m−2). Three to 10 waveforms were obtained from each subject, and each waveform was analyzed using standard time domain analyses of the PhNR, as well as a discrete wavelet transform (DWT) to extract time–frequency components that correspond to the PhNR. Three different measures of the PhNR were derived and compared: (1) amplitude at the PhNR trough; (2) amplitude at 72 ms following stimulus onset; (3) energy in the 11 Hz, 60–120 ms DWT frequency bin that corresponds to the PhNR. In addition, the effect of normalizing the PhNR by the b-wave was evaluated for each of the measures. Coefficients of variation (CVs) were computed for each definition to evaluate intra-subject variation.
Results
PhNR amplitudes measured at the trough and at 72 ms were significantly correlated (r = 0.88, p < 0.001). Additionally, PhNR energy derived by DWT was significantly correlated with the amplitude measured at the trough (r = 0.64, p = 0.002) and at 72 ms (r = 0.60, p = 0.005). Mean (±SD) intra-subject CVs were 26 % (15 %), 49 % (26 %), and 30 % (15 %), for measures at the trough, 72 ms, and DWT, respectively. Normalization by the b-wave amplitude (i.e., PhNR/b) had minimal effect on the intra-subject CVs, whereas normalization by the sum of the b-wave and PhNR amplitudes (i.e., PhNR/[b + PhNR]) substantially reduced the CVs for all three measures (mean CVs were less than 17 % for all conditions).
Conclusions
Although each PhNR definition has advantages and disadvantages, all three metrics provide similar estimates of the PhNR. Intra-subject CVs, however, were relatively high for measurements made at 72 ms, indicating that definitions based on a fixed time point may introduce variability. The substantial decrease in intra-subject variation after normalization by the sum of the PhNR and b-wave amplitudes may be advantageous under some conditions.
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Acknowledgments
This research was supported by National Institutes of Health research Grants R01EY026004 (JM) and P30EY001792 (UIC core grant), an unrestricted departmental grant and a Dolly Green Special Scholar Award (JM) from Research to Prevent Blindness.
Funding
The National Institutes of Health and Research to Prevent Blindness provided financial support in the form of funding. The sponsors had no role in the design or conduct of this research.
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10633_2016_9558_MOESM1_ESM.tif
Supplementary Figure 1: The DWT of the mean waveform is shown in the top panel (replotted from Fig. 1). The middle panel shows the effects on DWT energy of setting the mean response voltage to 0 μV after 60 ms; the PhNR component is absent. The bottom panel shows the effects on DWT energy of setting the mean response voltage to 0 μV from 0 ms to 60 ms; the PhNR component is present, but the early a-wave and b-wave components are strongly attenuated. (TIFF 922 kb)
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Kundra, H., Park, J.C. & McAnany, J.J. Comparison of photopic negative response measurements in the time and time–frequency domains. Doc Ophthalmol 133, 91–98 (2016). https://doi.org/10.1007/s10633-016-9558-6
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DOI: https://doi.org/10.1007/s10633-016-9558-6