Conventional pattern-reversal VEPs are not equivalent to summed multifocal VEPs

PURPOSE. To compare conventional pattern-reversal visual evoked potentials (cVEPs) with multifocal VEPs (mfVEPs). METHODS. mfVEPs and cVEPs were recorded during the same session in 12 normal subjects with an active electrode at Oz referenced to the inion (Oz-In) or to a midfrontal position, Fz (Oz-Fz). The mfVEP stimulus, a 60-sector dartboard, had a mean luminance of 100 cd/m(2) and a diameter of 42.2degrees. The cVEP checkerboard stimulus subtended 21degrees, had a mean luminance of 75 cd/m(2) and a contrast of 90%. Transient responses (2.5 Hz) were recorded for check sizes ranging from 12 to 50 minutes of arc (minarc). White cardboard masks were used to isolate upper and lower hemifields, within various field windows, for comparison with corresponding parts of the mfVEP. In a second experiment, VEPs were obtained using slowed m-sequences (8 and 16 video frames per m-step), as well as square-wave periodic reversals (2.4 Hz), for both the scaled dartboard display and an unscaled checkerboard display (check size of 50 minarc). RESULTS. The mfVEPs to fast m-sequence stimulation showed a strong polarity reversal between waveforms from the upper versus the lower hemifield. The cVEPs had larger amplitudes (similar to3x) and longer implicit times (similar to15-20 ms) and did not show the polarity reversal. Amplitude asymmetry between upper and lower hemifields was larger for cVEPs than for mfVEPs. As the stimulation rate was slowed, response amplitudes and implicit times of the major features increased, the upper versus lower polarity reversal was generally lost, and asymmetry of hemifield amplitudes grew. The same pattern of results was observed for scaled and unscaled spatial displays and for Oz-Fz and Oz-In signal derivations. CONCLUSIONS. Full-field cVEPs cannot be simply related to the sum of mfVEPs when each are recorded under their typical conditions. The stimulation rate has the largest influence on the differences between the two response types. The findings suggest that contributions from extrastriate sources are greater with the cVEP paradigm or the slowed mfVEP sequence than with the standard mfVEP paradigm.