Anticipating the effects of visual gravity during simulated self-motion: estimates of time-to-passage along vertical and horizontal paths

Abstract

By simulating self-motion on a virtual rollercoaster, we investigated whether acceleration cued by the optic flow affected the estimate of time-to-passage (TTP) to a target. In particular, we studied the role of a visual acceleration (1 g = 9.8 m/s(2)) simulating the effects of gravity in the scene, by manipulating motion law (accelerated or decelerated at 1 g, constant speed) and motion orientation (vertical, horizontal). Thus, 1-g-accelerated motion in the downward direction or decelerated motion in the upward direction was congruent with the effects of visual gravity. We found that acceleration (positive or negative) is taken into account but is overestimated in module in the calculation of TTP, independently of orientation. In addition, participants signaled TTP earlier when the rollercoaster accelerated downward at 1 g (as during free fall), with respect to when the same acceleration occurred along the horizontal orientation. This time shift indicates an influence of the orientation relative to visual gravity on response timing that could be attributed to the anticipation of the effects of visual gravity on self-motion along the vertical, but not the horizontal orientation. Finally, precision in TTP estimates was higher during vertical fall than when traveling at constant speed along the vertical orientation, consistent with a higher noise in TTP estimates when the motion violates gravity constraints

Sound-evoked vestibular stimulation affects the anticipation of gravity effects during visual self-motion

“…Both in the absence of acoustic stimulation and in the presence of white noise, responses were generally anticipated during downward relative to forward motion irrespective of true kinematics, as predicted by a prior of the visual effects of gravity (Indovina et al 2013a). Strikingly, short-tone bursts generally suppressed the difference in response timing between the two directions.…”

“…In general, TTP estimates for self-motion are based on first-order optic flow and do not make use of retinal acceleration (Festl et al 2012). However, the acceleration due to gravity appears to be taken into account during visually simulated self-motion in the vertical direction: Observers trigger the motor response for passage at a landmark earlier during downward than forward motion, consistent with the a priori that downward but not forward motion is accelerated by gravity (Indovina et al 2013a).…”

Multisensory Integration and Internal Models for Sensing Gravity Effects in Primates

“…The implicit bias toward gravitational motion when viewing an oscillating pendulum is also revealed by the observation that harmonic motion is perceived as uniform [35]. Also, the perceptual judgment of passive egomotion along the vertical direction—simulated by means of immersive visual stimuli—is based on the internal model of gravity [36]. …”

Visual gravitational motion and the vestibular system in humans

“…Visual estimates of time-to-passage during passive self-motion along the cardinal directions have been reported by Indovina et al (2013a). Subjects experienced virtual rides on a roller-coaster in a first-person perspective compatible with forward self-motion (Baumgartner et al, 2008).…”

“…The problem is that the visual system is quite poor at estimating image accelerations (Werkhoven et al, 1992; Dessing and Craig, 2010), as is the oculomotor pursuit system in tracking accelerated targets (Watamaniuk and Heinen, 2003; Bennett and Benguigui, 2013). Nevertheless, visual perception (Moscatelli and Lacquaniti, 2011; Indovina et al, 2013a) and manual interception of targets accelerated by gravity can be very precise (Lacquaniti and Maioli, 1987, 1989; Zago et al, 2004, 2008, 2009; Vishton et al, 2010). It follows that the brain must rely on some trick to supplement on-line visual signals in order to take into account the effects of gravity on object motion or self-motion.…”