Processing of one, two or four natural scenes in humans: the limits of parallelism.

The visual processing of objects in natural scenes is fast and efficient, as indexed by behavioral and ERP data [Nature 381 (1996) 520]. The results from a recent experiment suggested that such fast routines work in parallel across the visual field when subjects were presented with two natural scenes simultaneously [Nature Neurosci. 5 (2002) 629]. In the present experiment, the visual system was driven to its limits by presenting one, two or four scenes simultaneously. Behavior and ERP reveal a clear cost in processing an increasing number of scenes. However, a parallel-late selection model can still account for the results. This model is developed and discussed with reference to behavioral, single-unit and ERP data.