Electrophysiological correlates of saliency and feature synergy

For spatial frequency and orientation modulated textures, the cue combination advantage in target detection and discrimination is much stronger than expected from independent cue processing, known as "feature synergy". We explored the neural origin of this effect, having 38 observers perform target localization and more difficult shape identification, while simultaneously recording EEG. The results showed a strong feature synergy effect over a wide range of feature contrasts in both tasks, which was accompanied by a strongly reduced posterior P2 amplitude in a time window from 240 to 280 ms after stimulus onset (TOI-1) and to a lesser extent by a reduction of P3 in a later time window from 320 to 380 ms (TOI-2), both in a cluster of 13 adjacent electrodes from the left, central and right occipital and central parietal-occipital lobes. Only a subset of electrodes responded to increased target saliency due to higher feature contrasts, while all electrodes consistently showed the strongest amplitude reduction for the double-cue condition. In TOI-1, ERP amplitudes were lower for localization than for shape identification at the same feature contrasts, but the effect was reversed in TOI-2, where significant double-cue effects mainly concerned shape identification. The task dependence of the P2 reduction and the specificity of the P3 double-cue effects for the more demanding shape identification task indicate that fewer resources need to be allocated to a given task when features are combined. This suggests an origin in sites where features and shapes are processed under attentional control.