Two-frequency analysis of interactions elicited by Vernier stimuli

In five subjects, we measured visual evoked potentials (VEPs) elicited by V ernier targets in which the contrast of the two components of the stimuli w ere modulated by sinusoids at distinct frequencies f(1) and f(2) This appro ach allows for the extraction of VEP signatures of spatial interactions, na mely, responses at intermodulation frequencies n(1)f(1) + n(2)f(2), without the need to introduce motion into the stimulus. The most prominent interac tions were at the sum frequency f(1) + f(2), and, for frequency pairs that were sufficiently separated, the difference frequency f(1) - f(2). These re sponses had a systematic dependence on the temporal parameters of the stimu lus, corresponding to an effective latency of 145 to 165 ms. Fourth-order i nteractions were also detected, particularly at the frequencies 2f(1) +/- 2 f(2). These VEP signatures of interaction were similar to interactions seen for colinear line segments separated by a gap. Thus, for Vernier stimuli d evoid of motion, VEP signatures of interaction are readily detected but are not specific to hyperacuity displacements. The distribution of interaction s across harmonic orders is consistent with local rectification preceding t he spatial interactions. Their effective latencies and dependence on spatia l parameters are consistent with interactions within V1 receptive fields or mediated by horizontal connections between cells with a similar orientatio n tuning within V1.