Hierarchical integration of individual motions in locally paired-dot stimuli

Recent psychophysical studies suggest that there are two types of motion integration processes in human visual system, i.e., the local and the global integration process. The existence of the local integration process is suggested by the vector-average perception in locally paired-dot (LPD) stimuli. Here, we investigated the relationship between the two motion integration processes by measuring the signal detection thresholds in three corresponding stimuli: (1) standard random-dot kinematograms (RDKs), (2) LPD stimuli the individual dot motions of which were identical to those of RDKs, and (3) pairwise-averaged stimuli the individual dot motions of which corresponded to the vector-averages of locally paired motions in LPD stimuli. We found that the thresholds in LPD stimuli were similar to those in pairwise-averaged stimuli rather than in RDKs. In addition, when dots were paired appropriately, observers could detect coherent motions in LPD stimuli even if the proportions of signal dots were less than the detection thresholds in corresponding RDKs. These results suggest that the local and global integrations of individual motions are carried out hierarchically, and that the global motion perception in LPD stimuli does not depend on individual dot motions directly, but depends on locally integrated motions.