Temporal dynamics of perceived motion flow in naturalistic movie sequences

To visualize the temporal dynamics of human visual motion perception under conditions close to everyday life, we measured the time course of the perceived motion vector when the observers viewed naturalistic movie clips that featured large direction changes of target objects. The spatiotemporal position of the target's local motion was probed by a flashing dot that appeared within the range of ± 66.7 ms from the direction change. Observers had to report the perceived local motion using a motion vector matching method. The results show that the deviation of the perceived flow from the physical ground truth increased when the probe was presented near the direction changes. The pattern of errors averaged across multiple trials could be described by a Gaussian temporal smoothing of the local motion vectors, with a window size spanning about 120 ms. The results are consistent with previous reports of the sluggish temporal response of visual motion processing revealed by artificial stimuli and different tasks. However, a detailed examination of the response of each trial of our data indicated that the observers did not report a gradual transition from pre-change to post-change vectors but reported either of the two directions in a bimodal fashion with gradually changing response rates over time. This suggests that even though the motion processing may be sluggish, human observers correctly recognize a sudden direction change as a sudden event, not as a gradual transition.