Visual cues in triggering the rotational maneuvers

To analyze how the rotational maneuvers in inverted landings are visually triggered, we calculated the time traces of the CV of three putative visual cues (RREV, ωy, and ωx) prior to landing. The CV is a measurement of the dispersion of a variable defined by the ratio of its SD and mean. Assuming that rotational maneuvers are triggered after some time delay when the triggering visual cue reached a threshold at a particular time instant, then the CV of this visual cue at this time instant should be lower than those at other time instants; in addition, the CV of the triggering visual should be lower than those of other nontriggering visual cues. In short, the CV of the triggering visual cue should reach a minimum prior to the triggering moment and be the lowest when compared with the CV of other nontriggering visual cues.

In this analysis, we investigated the triggering of pitch and roll separately. Take the pitch triggering as an example. First, the time traces of the visual cues from different landing trials were aligned at the instant of the start of the pitch. Next, the CV was calculated for each of the visual cues. To identify the triggering visual cue, we first defined a low-value region for the CV of each visual cue (between the two breakpoints that sandwich the low-value valley region of a CV curve), and the visual cue with the lowest minimal CV was identified as the triggering cue. The mean value of this visual cue over the minimum CV range was identified as the triggering threshold. In addition, the time delay in the triggering process due to the sensorimotor transduction can be also identified. Identifying the triggering mechanism for the roll was similar to those for the pitch, except that the time traces of the visual cues were now aligned at the instant of the start of roll.

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