The psychophysical lateral interaction paradigm of Polat and Sagi
[30] was adapted to
make it suitable for VEP recording. We focused only on the long range
interactions between non-overlapping test-mask combinations. The amplitude
and phase of the steady-state VEP elicited by small Gabor patches were measured as a
function of stimulus contrast with and without laterally placed patches of similar or
different orientations. VEP recording requires a temporal modulation signal to time-lock the
signal analysis that is necessary to extract the VEP from the EEG background noise, and thus
the Gabors were contrast modulated at 4.1 Hz. The foveal Gabors were either presented alone or in the
presence of two
flanking Gabors of fixed contrast (50%). The flanks, also modulated at 4.1 Hz, were placed 
above and below the foveal Gabor and were oriented either vertically (collinear) or
horizontally (non-collinear). The flank response was also measured separately.
The existence of non-linear lateral interactions was inferred
from failures in a linearity of test and flank response-summation.
Interaction effects similar to these seen psychophysically were observed in the evoked potential
experiments
[29]. VEP responses were larger and faster for collinear targets and masks than was predicted by
the linear sum of separate target and mask responses. Responses were smaller and slower for orthogonal orientations of
target and mask. The range of the interaction for the collinear configuration
observed in the VEP was up to 
or 9-12 wavelengths --- similar to Polat & Sagi's
limit of 12 wavelengths.
Facilitation was maximum at 
and decreased as a function of distance and was
non-significance than the isolated target response level at 
.