Electrophysiological correlates of feature synergy

Modulating textures jointly in orientation and spatial frequency makes them easily distinguishable from the surround. The performance benefit of double-cue targets in detection and discrimination tasks is stronger than expected from independent feature processing, known as "feature synergy". To explore the neural origin of this effect, we had 38 observers perform a texture figure localization task and a more demanding shape identification task, while simultaneously recording EEG. The results showed a strong feature synergy effect in both tasks, which was accompanied by significantly reduced posterior ERP amplitudes in a cluster of 13 adjacent electrodes from the left, central and right occipital and central parieto-occipital lobes. The double-cue specific amplitude reduction occurred within a time window ranging from 200 to 290 ms around the P2 (TOI-1) and, to a lesser extent, at later times ranging from 290 to 380 ms, including the P3 peak (TOI-2). In TOI-1, but not in TOI-2, the cluster electrodes responded to enhanced figure-ground segregation and also encoded the perceptual summation of this effect for double-cue targets. Moreover, ERP reduction was stronger for localization than for shape identification in TOI-1, but the effect was reversed in TOI-2, where significant double-cue effects mainly concerned shape identification. Different task influences on the EEG correlate of feature synergy during earlier and later time periods indicate that fewer resources are necessary for a given task when targets are redundantly defined. This suggests an origin in sites where features and shapes are processed under attentional control.