Abstract
Behavioral studies have shown that statistical properties of object groups are perceived accurately with brief exposure durations. This finding motivated hypotheses that ensemble perception occurs rapidly in vision. However, few neuroimaging studies have explored the timing of ensemble perception. Here we used the superior temporal resolution of electroencephalography to directly compare the timing of ensemble processing to that of individual object processing. The P3b was chosen as a particular component of interest, as it is thought to measure the latency of stimulus evaluation. Participants performed a simple "oddball" task wherein sets of 51 lines with varied orientations sequentially flashed briefly on the display. In these sequences there was a 20% chance of an individual oddball, wherein one marked object tilted clockwise, and a 20% chance of an ensemble oddball, wherein the average orientation of the set tilted 20% clockwise. In counterbalanced blocks, participants were instructed to respond with a keypress to either individual or ensemble oddballs. Event-related potential analysis was performed to test the timing of this processing. At parietal electrodes, P3b components were found for both individual and ensemble oddballs. Ensemble P3b components were found to occur significantly earlier than individual P3b components, as measured with both 50% onset latency and 50% area measures. Using multivariate pattern analysis, ensemble oddball trials were classifiable from standard trials significantly earlier in their timecourse than individual oddball trials. Altogether, these results provide compelling evidence that ensemble perception is an early onset process within vision, and that strikingly these statistical summaries can be available even more rapidly than the properties of individual objects.