Detection of phi and reverse-phi direction-specific responses using the steady-state VEP
26.433, Saturday, 17-May, 2:45 pm - 6:45 pm, Banyan Breezeway
Keiko Momose1, Alexandra Yakovleva2, Anthony Norcia2; 1Faculty of Human Sciences, Waseda University, 2Department of Psychology, Stanford University
Noninvasive methods for probing direction-selective neuronal activity are of significant value for understanding human motion perception. Previous work using the Visual Evoked Potential has used adaptation to reveal direction-specific activity. Here we present a method that reveals motion mechanisms directly through a signature predicted by the motion energy model: motion energy detectors produce opposite-signed responses to phi and reverse-phi stimuli. We generated apparent motion displays comprised of an array of pairs of 1 deg adjacent square patches that were separated by 2 deg. One patch was flashed on and off at 5.14 Hz, the other at 6.0 Hz. Thirty-three pairs of patches were presented on a hexagonal lattice. Two white squares flashing on a gray background comprised the phi condition. Pairs consisting of one white and one black square comprised the reverse-phi condition. Experienced observers saw apparent motion at the difference-frequency (0.84 Hz). When presented side-by-side, synchronized phi- and reverse-phi displays appeared to move in opposite directions. EEG was recorded in nine normal vision adults using a 128-channel Geodesic Sensor Net with 0.1-50 Hz filtering. Fourier analysis was used to extract the response at the 0.84 Hz difference-frequency as an index of the spatio-temporal nonlinearity underlying the motion energy computation. As predicted by the motion energy model, the phase of difference frequency response for reverse-phi was shifted by 180 deg from that of the phi condition over occipital (O1, Oz and O2) and occipito-temporal cortex. The shift was also found at electrodes over the temporal lobe (near T3 and T4). Because the difference frequency component bears the signature expected of motion energy units, we consider it to be a direct marker of direction-selectivity.