Anticipating the Effects of Gravity When Intercepting Moving Objects: Differentiating Up and Down Based on Nonvisual Cues

Abstract

McIntyre. Anticipating the effects of gravity when intercepting moving objects: differentiating up and down based on nonvisual cues. J Neurophysiol 94: [4471][4472][4473][4474][4475][4476][4477][4478][4479][4480] 2005. First published August 24, 2005; doi:10.1152/jn.00527.2005. Intercepting an object requires a precise estimate of its time of arrival at the interception point (time to contact or "TTC"). It has been proposed that knowledge about gravitational acceleration can be combined with first-order, visual-field information to provide a better estimate of TTC when catching falling objects. In this experiment, we investigated the relative role of visual and nonvisual information on motor-response timing in an interceptive task. Subjects were immersed in a stereoscopic virtual environment and asked to intercept with a virtual racket a ball falling from above or rising from below. The ball moved with different initial velocities and could accelerate, decelerate, or move at a constant speed. Depending on the direction of motion, the acceleration or deceleration of the ball could therefore be congruent or not with the acceleration that would be expected due to the force of gravity acting on the ball. Although the best success rate was observed for balls moving at a constant velocity, we systematically found a cross-effect of ball direction and acceleration on success rate and response timing. Racket motion was triggered on average 25 ms earlier when the ball fell from above than when it rose from below, whatever the ball's true acceleration. As visual-flow information was the same in both cases, this shift indicates an influence of the ball's direction relative to gravity on response timing, consistent with the anticipation of the effects of gravity on the flight of the ball. I N T R O D U C T I O NSuccess in intercepting a moving target depends in part on the accuracy with which we estimate the time that remains before the object arrives at the point of interception (time to contact or TTC). A number of visual cues can contribute to TTC estimates, such as dilation of the retinal image, changes in binocular disparity, reduction of the optical gap between the target and interception point, and the evolution of pursuit eye movements (Bootsma and Oudejans 1993;Gray and Regan 1998;Rushton and Wann 1999;Savelsbergh et al. 1993). While it is known that the processing of visual information can provide accurate estimates of an object's distance and velocity, human observers are generally poor at measuring or even detecting acceleration (Werkhoven et al. 1992). This paucity of acceleration and higher-order cues means that available visual information is ambiguous; for a given measurement of position and velocity, there are an infinite number of possible true TTC values that could occur, depending on future changes in velocity. A reasonable way to resolve this ambiguity would be to assume no acceleration. Thus the main hypothesized mechanisms for TTC estimates based on visual information involve first-order approximation...

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

“…This conclusion is in line with previous observations on the interception of targets moving along different orientations (Senot et al 2005). This previous study found that the interception timing for targets accelerating downwards was systematically anticipated relative to when the same targets accelerated upwards.…”

“…However, this difference was not detected in a previous study comparing upward and downward constant speed object motion (Senot et al 2005) with participants in a fixed reference frame aligned with the Earth's vertical and the virtual vertical of the visual stimulation rotating accordingly with conditions as in the present experiment. Senot et al (2005) found an anticipated (delayed) response during the downward (upward) motion only when varying the position of the head coherently with the condition (heading upward/downward), that is, adding the effect of gravity on vestibular and proprioceptive signal (De Saedeleer et al 2013). In our study, we aimed to obtain an equivalent effect by adding realism to the scene.…”

“…This previous study found that the interception timing for targets accelerating downwards was systematically anticipated relative to when the same targets accelerated upwards. This difference was abolished when the target travelled along the horizontal relative to the viewer (Senot et al 2005), and was reversed during the weightless phases of parabolic flight, presumably due to altered influence of vestibular signals (Senot et al 2012). We hypothesize that the high realism of the visual scene in our experiment caused the preponderance of visual over proprioceptive inputs (see Britten 2008).…”

“…In our study, we aimed to obtain an equivalent effect by adding realism to the scene. However, the lack of TTP difference found during constant speed conditions may be related to the better ability to judge constant velocities than constant accelerations as shown by the higher accuracy (success rate) found by Senot et al (2005) during constant speed trials. Nonetheless, in the present study, we found that TTP estimation was more precise during horizontal than vertical condition at constant speed indicating that participants were more confident in timing horizontal constant speed motions.…”

“…On the other hand, studies on interception of moving objects and on estimates of time intervals showed that a priori knowledge about gravitational acceleration, combined with realistic cues about the orientation and size of the visual scene, can influence response timing (Moscatelli and Lacquaniti 2011;Zago et al 2009;Miller et al 2008;Bosco et al 2008;Senot et al 2005;McIntyre et al 2001). This a priori knowledge is thought to be stored in a visual-vestibular 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.…”