Rapid detection of threshold VEPs

Objective: To determine whether a one-dimensional (1D) Laplacian analysis detects steady-state visual evoked potentials (ssVEPs) faster than the standard Oz–Fz montage and to establish the optimum position of Laplacian reference electrodes. Methods: Twenty-two normal adults were shown reversing checks ranging from 1.5′ to 60′. Three electrode montages were investigated: Oz–Fz, LO–Fz and a 1D Laplacian analysis of 3 occipital electrodes (2Oz−(RO+LO)). RO and LO were placed symmetrically and horizontally about Oz. Five different locations for RO and LO were investigated. Recordings were analysed in the frequency domain and the presence (and detection time, DT) or absence of a ssVEP defined statistically. Effects of individual, reference electrode site and check size on DT and phase differences between recording montages were investigated. Results: Laplacian analysis detected ssVEPs to small (3′) checks faster than Oz–Fz, by 12.3 and 4.1 s on average with Laplacian reference electrodes at 15 and 20% of half-head circumference, respectively. The optimum position of reference electrodes was governed by the instantaneous spatial spread of the response and the noise coherence between midline and lateral electrodes. Conclusions: A 1D Laplacian analysis can reduce the time to statistical detection of ssVEPs compared to the traditional Oz–Fz recording for stimuli near the normal acuity threshold of adults. This in turn could be used to minimise the length of a VEP acuity assessment.