Temporal evolution of motion direction judgments

Abstract

Existing models of perceptual decision-making for moving stimuli assume a continuous accumulation of sensory signals for a given direction of motion (Smith & Ratcliff, 2004). This predicts concurrent improvements in estimating motion axis (orientation of motion direction) and motion direction over time. Here, we present a novel finding that the visual system can extract motion axis orientation before detecting motion direction. Methods and results: To estimate the temporal evolution of motion direction judgments, we asked participants to adjust an on-screen arrow to indicate perceived direction of moving stimuli varying in duration (rigid texture motion; randomly chosen direction; 99% contrast, 11° radius, 5°/s speed; stimuli presented at 360Hz). As expected, participants’ responses were completely random for very brief stimulus motions (5ms) and distributed around the actual motion direction for long durations (>90ms). Interestingly, for intermediate durations (30-60ms), participants’ responses exhibited a clear bimodal distribution with equal peaks in the actual motion direction and in the opposite direction. These results indicate that participants extract motion axis information before having a sense of the actual motion direction. In a related experiment, we found that for certain brief stimulus durations, subjects were 100% correct at estimating motion axis while being at chance at perceiving motion direction. Next, we ran several experiments to rule out possible contributions of spatial signals (i.e., the well-known motion streaks). Finally, we explored the contrast and size dependency of this result (5-99% contrast; 1°-11° radius). The results reveled that rapid and early coding of motion axis was mainly associated with large, high-contrast moving stimuli, stimuli associated with center-surround suppression of motion signals (Tadin et al., 2003). Conclusion: We show the visual system can extract motion axis information considerably faster than motion direction information. The specificity to large, high-contrast motion stimuli offers new clues how center-surround suppression affects motion perception. Meeting abstract presented at VSS 2015