Academic journal article Attention, Perception and Psychophysics

Can Representational Trajectory Reveal the Nature of an Internal Model of Gravity?

Academic journal article Attention, Perception and Psychophysics

Can Representational Trajectory Reveal the Nature of an Internal Model of Gravity?

Article excerpt

Published online: 28 January 2014

© Psychonomic Society, Inc. 2014

Abstract The memory for the vanishing location of a horizontally moving target is usually displaced forward in the direction of motion (representational momentum) and downward in the direction of gravity (representational gravity). Moreover, this downward displacement has been shown to increase with time (representational trajectory). However, the degree to which different kinematic events change the temporal profile of these displacements remains to be determined. The present article attempts to fill this gap. In the first experiment, we replicate the finding that representational momentum for downward-moving targets is bigger than for upward motions, showing, moreover, that it increases rapidly during the first 300 ms, stabilizing afterward. This temporal profile, but not the increased error for descending targets, is shown to be disrupted when eye movements are not allowed. In the second experiment, we show that the downward drift with time emerges even for static targets. Finally, in the third experiment, we report an increased error for upward-moving targets, as compared with downward movements, when the display is compatible with a downward ego-motion by including vection cues. Thus, the errors in the direction of gravity are compatible with the perceived event and do not merely reflect a retinotopic bias. Overall, these results provide further evidence for an internal model of gravity in the visual representational system.

Keywords Internal model . Representational momentum . Representationalgravity .Motionperception . Eyemovements

Over the last few decades, the role of gravity in structuring our perception of space has come to be a major focus of several research lines. From the classical studies on the subjective visual vertical (see, e.g., Haji-Khamneh & Harris, 2010; Mittelstaedt, 1983, 1986) to the timing of interceptive actions of falling objects (McIntyre, Zago, Berthoz & Lacquaniti, 2001) and time to contact (Baurès & Hecht, 2011), as well as the gravito-inertial force resolution (equivalence principle; all linear accelerometers respond to both linear acceleration and gravity) in the vestibular system (Angelaki, Shaikh, Green &Dickman,2004; Hess & Angelaki, 1999;Merfeld,1995; Merfeld, Zupan & Peterka, 1999), the idea that gravity has been internalized in our perceptual apparatus has been a leading hypothesis upon which a wealth of knowledge has been built. For the most part, these and other approaches share the common assumption that humans possess an internal model of gravity, used to update and complement sensorial inputs in order to maintain reliable estimates of the spatial environment. An internal model is thought to be a general neural process that explicitly mimics the relationship between physical variables in order to provide estimates of physical quantities of both body sensors and environment (Grush, 2005; Poon & Merfeld, 2005; Snyder, 1999; Tin & Poon, 2005). The notion of internal analogues of physical variables has been similarly emphasized by Shepard (2001)inhis concept of second-order isomorphism, on the basis of which spatial mislocalizations have been interpreted, such as repre- sentational momentum and representational gravity (see below; Hubbard, 2005). The aim of the present article is to provide further understanding of the role of an internal model of gravity in these phenomena.

Representational momentum and gravity

When people are instructed to remember and indicate the vanishing location of a moving target, their spatial memory is displaced forward, in the direction of motion, and down- ward, in the direction of gravity (for a review, see Hubbard, 2005). These errors of spatial localization have been taken as evidence for internal analogues of momentum and gravity, respectively, and have been thought to reflect mental extrap- olations that seek to supplant neural delays (see, e. …

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