An analysis of the VEP to luminance modulation and of its nonlinearity.

Identification of the VEP luminance modulation system was carried out using a relatively long pseudorandom binary m-sequence stimulus. The complete first order cross-correlation function provides a "fingerprint" of the nonlinearity in the time domain. Volterra kernel slices up to third order are recognized in the complete first order cross-correlation function. They all lie close to the major diagonal. Higher order kernel slices are not measurable. A cascade block model (sandwich model) analysis shows the nonlinearity to be an asymmetric rectification. Luminance increase causes a stronger response than does luminance decrease. The pre-filter gain function is low-pass. The post-filter gain function is band-pass tuned near 10 Hz. Coherency spectra are used to examine the channel structure of the system. Two channels operating near 8 and 15 Hz are easily identified. Another broad channel (or possibly multiple channels) is present above 30 Hz. In many subjects, the pseudorandom stimuli allow a complete system identification to be carried out in less than 82 sec.