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Abstract:
Human observers are amazingly adept at interpreting cluttered natural scenes, whether these
scenes are presented as static photographs or dynamic movies. For example, observers have
little difculty in segmenting a scene into salient objects (e.g., a pedestrian walking through
a park). Of course, the environment is dynamictherefore, we asked whether there is an
advantage for dynamic scenes relative to static ones. To address this question, we devised a dynamic
composite stimulus in which two separate frame sequences were blended into a single
stimulus by averaging the luminance of corresponding frames of the separate sequences. By
varying the relative weight (alpha) of the two original sequences, we can make one sequence
more or less visible in the composite stimulus. Here, we blended frame sequences of pedestrians
walking in a park with various machines in action. Observers were briey presented
with two composite stimuli and they judged whether a human target was present or absent in
one of them. We compared the 50 alpha threshold for detection across three conditions: (1)
coherent dynamic stimuli, (2) static stimuli, and (3) scrambled dynamic stimuli in which we
randomized the frame order of the sequence. Overall, we found that thresholds were lower for
dynamic than for static stimuli. That is, when dynamic information regarding the human target
was available, observers required less static cues (i.e., lower alpha) to detect that target. We
found no difference between coherent and scrambled dynamic composites, suggesting that the
critical component is increased availability of information over time.
