Distinct rules for perceptual grouping in position-based and velocity-based motion systems

Motion perception relies on at least two distinct systems: a velocity-based motion system driven by early direction-selective cells and a position-based motion system that tracks objects over space and time. However, how these systems interact when operating in parallel remains unclear. We explored their respective contributions to the perceptual organization of motion using a bistable stimulus of eight moving dots, perceived either as rotating in local pairs (local motion percept) or as forming two illusory squares translating around fixation (global motion percept). To disrupt the velocity-based motion system, we varied interstimulus intervals (ISIs) stroboscopically from 0 to 116.6 ms - selectively impairing early direction-selective cells with short temporal integration windows ( $<100$  ms). Additionally, we manipulated contrast polarity to bias perceptual grouping (local-group, global-group, or no-group). We found that the pattern of perceptual bistability shifted markedly at ISIs of 33 ms. For ISIs $\ge$ 33 ms, contrast- and proximity-based grouping strongly influenced perception. For ISIs $<33$  ms, the global motion percept dominated even in the presence of strong static grouping cues (i.e., contrast and proximity), suggesting that the velocity-based motion system introduces a perceptual bias that can override or counteract static grouping cues. These findings reveal distinct, and at times opposing, contributions of velocity- and position-based motion systems to the perceptual organization of motion.