Differential effects of optic media opacities on mfERGs and mfVEPs

Short-term delay in neural response with multifocal contact lens might start at the retinal level

INTRODUCTION

Multifocal simultaneous imaging challenges the visual system to process the multiple overlaps of focused and defocused images. Retinal image processing may be an important step in neuroadaptation to multifocal optical images. Our aims are, firstly to evaluate the short-term effect of different multifocal contact lenses (MF) on retinal activity in young healthy subjects (Experiment#1) and secondly, to evaluate any changes in retinal activity in presbyopic patients fitted with MF over a 15-day period (Experiment#2).

METHODS

In Experiment-#1, 10 emmetropic healthy young subjects were included to evaluate the short-term effect of different MFs designs. In Experiment #2, 4 presbyopic subjects were included to wear MF for 15 days. Following the ISCEV Standards, multifocal electroretinograms (mfERGs) were recorded to evaluate different retinal regions under different conditions: with single vision contact lens (SVCL) and with center-distance and center-near MF.

RESULTS

In Exp#1 the peak time of N1, P1 and N2 were found to be delayed with the MF (p ≤ 0.040). There was a significant reduction for N1 amplitude in all retinal regions (p < 0.001), while for P1 and N2 amplitudes this reduction was more significant in the peripheral regions (p < 0.005, ring 5 to 6). With center-near MF the mean response density (nV/deg²) showed a significant decrease in all wave components of the mfERGs response, particularly from Ring 3 to Ring 6 (p < 0.001, all Rings). In Exp#2, the mean mfERG response is similar between SVCL and center-distance MF, while center-near MF showed an increase in implicit time N1 and P1 on day 1 that tends to recover to baseline values after 15 days of MF wear.

CONCLUSIONS

significant changes in the mfERGs responses were found with the MF lens, being most noticeable with the center-near MF lens design. The present results suggest that the observed delay in cortical response described during the adaptation to multifocality may partially begin at the retina level.

Filtering multifocal VEP signals using Prony's method

BACKGROUND

This paper describes use of Prony's method as a filter applied to multifocal visual-evoked-potential (mfVEP) signals. Prony's method can be viewed as an extension of Fourier analysis that allows a signal to be decomposed into a linear combination of functions with different amplitudes, damping factors, frequencies and phase angles.

METHOD

By selecting Prony method parameters, a frequency filter has been developed which improves signal-to-noise ratio (SNR). Three different criteria were applied to data recorded from control subjects to produce three separate datasets: unfiltered raw data, data filtered using the traditional method (fast Fourier transform: FFT), and data filtered using Prony's method.

RESULTS

Filtering using Prony's method improved the signals' original SNR by 44.52%, while the FFT filter improved the SNR by 33.56%. The extent to which signal can be separated from noise was analysed using receiver-operating-characteristic (ROC) curves. The area under the curve (AUC) was greater in the signals filtered using Prony's method than in the original signals or in those filtered using the FFT.

CONCLUSION

filtering using Prony's method improves the quality of mfVEP signal pre-processing when compared with the original signals, or with those filtered using the FFT.

Prediction of higher visual function in macular degeneration with multifocal electroretinogram and multifocal visual evoked potential

OBJECTIVE

Visual search can be guided by past experience of regularities in our visual environment. This search guidance by contextual memory cues is impaired by foveal vision loss. Here we compared retinal and cortical visually evoked responses in their predictive value for contextual cueing impairment and visual acuity.

METHODS

Multifocal electroretinograms to flash stimulation (mfERGs; 103 locations; 55.8° diameter) and visual evoked potentials to pattern-reversal stimulation (mfVEPs; 60 locations; 48.6° diameter) were recorded monocularly in participants with age-related macular degeneration (n = 14 and 16, respectively). Response magnitudes were calculated as the respective signal-to-noise ratios for each eccentricity. Visual acuities (logMAR, range: 0.0-1.2) and contextual cueing effects on visual search (reaction time gain, range: -0.14-0.15) were correlated with the signal-to-noise ratios. A step-wise regression analysis was applied separately to the mfERG- and mfVEP-dataset to determine the eccentricity range and the processing stage that is critical for these visual functions.

RESULTS

Central mfERGs (1.0-3.2°) were the sole predictor of contextual cueing of visual search (p = 0.006), but they were not significant predictors of visual acuity. In contrast, central mfVEPs (1.3-3.2°) were the sole predictor of visual acuity (p < 0.001), but they were not significant predictors of contextual cueing.

CONCLUSIONS

Contextual cueing is more dependent on parafoveal mfERG magnitude while visual acuity is more dependent on parafoveal mfVEP magnitude. The relation of contextual cueing to parafoveal mfERG magnitudes indicates the predictive value of retinal bipolar cell activity for this advanced level of visual function.