Final report: Integration of elementary features in texture perception

A long-standing problem in human vision research is how the brain decomposes a visual sce-ne into features and reintegrates them into unambiguous and stable object representations. When objects differ from the surround in several features, they can become more salient than expected from each individual feature. This is known as the "feature synergy" effect. In this project, we investigated the functional role and hypothetical source of feature synergy in the detection and discrimination of 2D texture shapes by (i) combining psychophysical data and modelling of early feature-selective processing, and (ii) recording EEG during simultaneous psychophysical tasks. For textures defined by local modulation of orientation and spatial fre-quency, we found strong summation of both features in detection and localisation tasks, sug-gesting integration by a common mechanism. For different texture modulations (local variability and local feature difference), the summation effect could be explained by local energy-based multiscale models, suggesting feature integration at early processing stages. Psychophysical experiments comparing texture target detection and shape discrimination with highly similar targets showed a decrease of cue summation in difficult shape discriminations compared to simpler ones, indicating only a small cue combination advantage for shape judgements after figure-ground segregation has settled. Psychophysical results thus support that feature syner-gy is limited to detection and discrimination at the figure-ground segregation threshold where more complex shape processing is not yet feasible. While psychophysical and modelling results clearly point to an early integration mechanism, EEG results revealed a strong reduction in P2 and P3 amplitude as a stable correlate of fea-ture synergy, observed in a cluster of 13 adjacent electrodes from left, central and right occipi-tal and central parietal-occipital lobes. Modulations of P2 and especially P3 amplitudes de-pended on the psychophysical task (detection vs. shape discrimination) and were not strongly related to perceptual saliency. These findings suggest that P2 and P3 markers of feature syn-ergy originate from regions involved in feature and shape processing under attentional control, reflecting reduced resource allocation when the target is redundantly defined by two features. Notably, no target/non-target modulation was observed in earlier waveforms than P2 for the lower and middle ranges of target-background feature differences used. Thus, our findings do not rule out the presence of earlier EEG correlates of feature synergy and saliency, particular-ly when stronger feature differences – capable of eliciting segregation-specific responses (tsVEPs, Bach and Meigen, 1992) – are employed. Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 471755627 notReviewed