In five subjects, we measured visual evoked potentials
(VEPs) elicited by Vernier targets in which the contrast
of the two components of the stimuli were modulated by
sinusoids at distinct frequencies f1
and f2. This approach allows for the
extraction of VEP signatures of spatial interactions, namely,
responses at intermodulation frequencies n1f1 + n2f2,
without the need to introduce motion into the stimulus. The most
prominent interactions were at the sum frequency f1
+ f2, and, for frequency pairs that were sufficiently
separated, the difference frequency f1 −
f2. These responses had a systematic dependence
on the temporal parameters of the stimulus, corresponding to an
effective latency of 145 to 165 ms. Fourth-order interactions were also
detected, particularly at the frequencies 2f1 ±
2f2. These VEP signatures of interaction were similar
to interactions seen for colinear line segments separated by a gap. Thus,
for Vernier stimuli devoid of motion, VEP signatures of interaction are
readily detected but are not specific to hyperacuity displacements. The
distribution of interactions across harmonic orders is consistent with
local rectification preceding the spatial interactions. Their effective
latencies and dependence on spatial parameters are consistent with
interactions within V1 receptive fields or mediated by horizontal
connections between cells with a similar orientation tuning within V1.