Abstract
Current theories suggest that alpha-frequency oscillations (9–13 Hz) reflect pulsed-inhibitory neural computations that structure visual perception into discrete time frames and sub-serve the individuation of spatiotemporal objects. Here, we investigated the impact of alpha oscillations on object individuation and its capacity limits in dynamic scenes. We recorded whole-head magneto-encephalography (MEG) while 24 participants performed a multiple-object tracking task (MOT). On each trial, the participants viewed 12 randomly moving objects and were required to track a subset of 2, 4 or 8 targets over a period of 2–3 seconds. In different blocks, they switched between different object processing tasks, which were previously reported to differ in their capacity limits (individuation, averaging). For individuation, participants had to indicate the spatial position of one object from the target pool (i.e. partial report). For averaging, they had to indicate the geometrical centroid position of the target pool. Behavioral performance declined from 2 to 4 objects in a similar way for both tasks. At 8 objects, however, averaging was significantly better than individuation performance confirming the previously found between-task differences in object capacity. In the MEG signal, we found a significant alpha power increase from pre-trial baseline in bilateral inferior parietal cortex during MOT, which was equally strong for both tasks. By contrast, we found stronger oscillatory bursting in the alpha band for individuation vs. averaging during MOT. Oscillatory bursting captures single-trial dynamics better compared to across-trial averaged power, because it measures time- and band-limited, high-signal periods above each trial’s respective pre-trial mean. Critically, the alpha bursting effect was set-size specific and only reached significance for 2 and 4 but not for 8 target objects. Presumably, object capacity was exceeded and thus individuation failed at this high set size. Our results suggest that oscillatory alpha bursts underlie object individuation and its capacity limits during MOT.
Acknowledgement: FWF - the Austrian Science Fund