Signal to noise ratio considerations in the analysis of sweep visual-evoked potentials

More investigators are recommending rapid sweep displays to estimate visual-evoked potential (VEP) sensory thresholds. Theoretically, phase-sensitive analysis offers a higher signal to noise ratio (SNR) than phase-insensitive techniques and, therefore, a more reliable and equally valid threshold estimate. Phase-sensitive analysis assumes that the VEP phase does not change over the period of one sweep. This study tests the assertion that the VEP phase is sufficiently stable for valid and reliable phase-sensitive detection. Mathematical analysis shows that phase-sensitive detection yields a lower SNR than phase-insensitive analysis if the phase error is < 45 degrees . We recorded the VEP to contrast reversing sinusoidal gratings of sweeping spatial frequency (12.5-0.2 epd) from 26 subjects. In most, phase varied > 180 degrees Over one sweep. Moreover, these large phase shifts could not be diminished by modifying contrast reversal rate, direction of spatial frequency sweep, or sweep time. We conclude that when using spatial frequency sweeps, phase-insensitive detection is superior to phase-sensitive. The filter's bandwidth and the effect of SNR on sensory threshold estimations also are discussed.