Amplification of the mfVEP Signal-To-Noise Ratio by a Laplacian Analysis

Abstract

Purpose: : The multifocal VEP technique (mfVEP) can be used to test visual pathway function by simultaneously recording cortical potentials from many stimulus locations within the visual field. The aim of this study was to investigate whether the signal-to-noise ratio of pattern-reversal (PR) and pattern-onset (PO) mfVEPs can be amplified by a Laplacian analysis.

Methods: : The mfVEP was recorded from 20 normal subjects using PR and PO dartboard stimuli. The dartboard stimuli consisted of 60 4x4 checkerboard patterns that captured 41° of the central visual field. The mfVEPs were recorded with five occipital electrodes. One electrode was placed at O_Z, the other 4 electrodes were placed 4 cm above, below, to the left and to the right of O_Z. This electrode placement allowed to record and calculate ten bipolar channels and to apply 2 one-dimensional and 1 two-dimensional Laplacian analyses. For each channel the mfVEP traces were squared and averaged across the 60 fields, and normalized to the mean value between 325 and 430 ms. The largest signal-to-noise ratio (SNR_max) of this normalized waveform was used for further analysis.

Results: : The SNR_max of the two-dimensional Laplacian analysis exceeded the largest SNR_max found in any bipolar channel by 35,5% (PR) and 38,6% (PO). When comparing both one-dimensional analyses with the best bipolar channel, corresponding increases in SNR_max were between 4,2% and 13%. The ANOVA showed that the recording channel had a significant effect (p≤0,01) on SNR_max for both PR and PO stimulation. The post-hoc Scheffé test indicated that the SNR_max for the two-dimensional Laplacian analysis was significantly higher than any other calculated channel (p≤0,01), while SNR_max values for both one dimensional analyses were not significantly larger than the best bipolar channel (p>0,999).

Conclusions: : The signal-to-noise ratio of the mfVEP can be amplified significantly by the use of a Laplacian anaylsis. However, an inclusion of both horizontally and vertically aligned electrodes in a two-dimensional Laplacian analysis is required for optimal signal detection. Thus a two-dimensional Laplacian analysis may facilitate the detection of visual pathway function, e.g., in glaucoma patients.

Keywords: electrophysiology: clinical • visual cortex • electrophysiology: non-clinical