Non-retinotopic feature integration has a neural pace of its own, as revealed by electrical brain imaging

When elements appear in quick succession they can integrate across space and time. Using high-density EEG techniques we show that such non-retinotopic feature integration is not time-locked to stimulus onset, but has a timing of its own. We presented a central line, containing a small vernier offset, quickly followed by pairs of non-offset flanking lines. Although the central line itself stays invisible, observers can discriminate its offset within the two resulting motion streams. Observers attended only one stream while we gave one of the flanking lines an offset opposite to that of the central line. Only when this flank offset was in the attended stream did it integrate with the central offset. This unconscious integration was reflected in voltage topographies time-locked to the behavioural response, between 650 and 250 ms before key press. At these latencies statistical parametric mapping of current densities revealed integration effects in high-level visual, prefrontal and central areas. A separate stimulus-locked analysis revealed no such effects. The results show that endogenously timed processes, rather than stimulus-locked, bottom - up processes, underlie non-retinotopic feature integration. The results furthermore show how high-density EEG can monitor visual information with good spatial and excellent temporal resolution.