The effect of fixation instability on the multifocal VEP.

In multifocal visually evoked potentials (mfVEP), we find reversals in waveform near the horizontal meridian due to convolutions in the cortex. This renders the mfVEP very sensitive to small changes in gaze position. In this study we tested the effects of very small amounts of fixation instability on the mfVEP topography under controlled conditions using four normal subjects. In order to simulate unstable fixation, subjects were instructed to move their fixation point systematically in a clockwise direction between the endpoints of a fixation cross every few seconds (two degree diameter cross = one degree fixation error). Results were compared against a control condition with stable, central fixation. The effects of 0.5 degrees fixation error are small, but 1.0 degrees fixation error can produce a large decrease in root mean square signal amplitude (e.g., 60%) in the central foveal region (i.e., within 1.4 degrees eccentricity). The size of the effect drops off rapidly with eccentricity and varies greatly between areas within a subject, and between the four subjects. Beyond 3.0 degrees eccentricity the effects are minimal. Unstable fixation with relatively small fixation errors caused a dramatic decrease in mfVEP amplitude within three degrees of eccentricity, which can be misinterpreted as loss of macular function. Fixation monitoring is essential to obtain accurate results in the macular area when recording mfVEPs.