Effect of eccentricity on pattern-pulse multifocal VEP

The sparse pattern-pulse stimulation has been suggested to produce better cortical evoked responses compared to pattern reversal stimulation. This study examines varying pattern-pulse states and the effect of eccentricity of the stimulated visual field on the response amplitude and latency.

The multifocal visual evoked potential (mfVEP) was recorded using Accumap. 58 close-packed checkerboard segments in a dartboard configuration were used. The best configuration for pattern-pulse stimulation was determined. This optimal stimulus condition was then compared to pattern-reversal stimulation at different eccentricities of visual field.in terms of latency and signal/noise ratio (SNR) of mfVEP amplitude.

The maximal response was seen when each element "1" of the binary sequence was represented by two "pattern on" frames followed by two "pattern off" frames while each element "0" of the binary sequence is represented by four "pattern off" frames. There was a significant overall increase of SNR using this pattern-pulse stimulating mode (SNR=15.5+/-3.8) compared with pattern-reversal stimulation (SNR=12.4+/-2.6). However, this was strongly dependant on eccentricity. In rings 1, 2 and 3 SNR improved by 48%, 43% and 26% respectively with ring 4 the effect was marginal and ring 5 was not significantly different. There was also a significant delay (10.1+/-5.3 msec) of the mfVEP response in pattern-pulse stimulation compared to pattern-reversal.

The pattern-pulse method offers some advantages for achieving larger mfVEP signals from the central visual field. However, the more peripheral field where it is the most difficult to obtain signals, does not show any benefit.