Academic journal article Perception and Psychophysics

The Perception of Visually Presented Yaw and Pitch Turns: Assessing the Contribution of Motion, Static, and Cognitive Cues

Academic journal article Perception and Psychophysics

The Perception of Visually Presented Yaw and Pitch Turns: Assessing the Contribution of Motion, Static, and Cognitive Cues

Article excerpt

Terrestrial gravity restricts human locomotion to surfaces in which turns involve rotations around the body axis. Because observers are usually upright, one might expect the effects of gravity to induce differences in the processing of vertical versus horizontal turns. Subjects observed visual scenes of bending tunnels, either statically or dynamically, as if they were moving passively through the visual scene and were then asked to reproduce the turn deviation of the tunnel with a trackball. In order to disentangle inertia-related (earth-centered) from vision-related (body-centered) factors, the subjects were either upright or lying on their right side during the observations. Furthermore, the availability of continuous optic flow, geometrical cues, and eye movement were manipulated in three experiments. The results allowed us to characterize the factors' contributions as follows. Forward turns (pitch down) with all cues were largely overestimated, as compared with backward turns (pitch up). First, eye movements known to be irregular for vertical stimulation were largely responsible for this asymmetry. Second, geometry-based estimations are, to some extent, asymmetrical. Third, a cognitive effect corresponding to the evaluation of navigability for upward and downward turns was found (i.e., topdown influences, such as the fear of falling often reported), which tended to increase the estimation of turns in the direction of gravity.

The updating of one's perceived orientation when one is turning around the body axis in the horizontal plane (yaw rotations) can be performed either by extracting one's heading from optic flow when visual cues are present (Bertin, Israël, & Lappe, 2000; Lappe, Bremmer, & van den Berg, 1999; Warren & Hannon, 1988; Warren, Kay, Zosh, Duchon, & Sahuc, 2001) or by integrating vestibular acceleration (Israël, Bronstein, Kanayama, Faldon, & Gresty, 1996; Israël, Sievering, & Koenig, 1995; Ivanenko, Grasso, Israël, & Berthoz, 1997) and proprioceptive cues (Amorim, Glasauer, Corpinot, & Berthoz, 1997; Mittelstaedt, 1999). All these studies showed a symmetrical perception of turnings, whether the subjects were going leftward or rightward.

Less is known about the perception of pitch rotations performed around a horizontal axis. Cohen and Larson (1974) asked subjects to adjust the pitch of their bodies in complete darkness to 13 different goal orientations between prone and supine positions. They found that the subjects underestimated their pitch orientation when they were tilted less than 60° backward or forward from the vertical and overestimated their pitch orientation when they were nearly prone, indicating an asymmetry in the processing of vestibular and somatosensory cues in order to estimate their body pitch orientation. A strong asymmetry in pitch-induced sensations was also reported when a pitch-rotating optic flow subtending a large field of view was observed (Young, Oman, & Dichgans, 1975). Downward pitch stimuli generated a significantly stronger pitch sensation than did upward pitch stimuli. This asymmetry in the vertical plane turns (pitch) was absent in horizontal plane turns (yaw).

It is likely that the difference in gravity's effects on pitch versus yaw rotations can explain the differences in behavior between these two actions, at both a physiological and a cognitive level. Indeed, there are several anatomical and physiological characteristics of the central nervous system relating to gravity that might come into play. First of all, pitch and yaw rotations stimulate physiological sensors in a fundamentally different fashion. In natural conditions, humans stabilize their heads in an upright posture (Pozzo, Berthoz, & Lefort, 1990); therefore, the gravity axis matches the rotation axis for yaw rotations. Under these circumstances, the receptors sensing gravity are essentially unperturbed, whether one is turning rightward or leftward. …

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