Integration across Time Determines Path Deviation Discrimination for Moving Objects

David Whitaker,Dennis M Levi,Graeme J Kennedy

doi:10.1371/journal.pone.0001930

David Whitaker, Dennis M Levi + Show 1 more

Open Access

https://doi.org/10.1371/journal.pone.0001930

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Abstract

BackgroundHuman vision is vital in determining our interaction with the outside world. In this study we characterize our ability to judge changes in the direction of motion of objects–a common task which can allow us either to intercept moving objects, or else avoid them if they pose a threat.Methodology/Principal FindingsObservers were presented with objects which moved across a computer monitor on a linear path until the midline, at which point they changed their direction of motion, and observers were required to judge the direction of change. In keeping with the variety of objects we encounter in the real world, we varied characteristics of the moving stimuli such as velocity, extent of motion path and the object size. Furthermore, we compared performance for moving objects with the ability of observers to detect a deviation in a line which formed the static trace of the motion path, since it has been suggested that a form of static memory trace may form the basis for these types of judgment. The static line judgments were well described by a ‘scale invariant’ model in which any two stimuli which possess the same two-dimensional geometry (length/width) result in the same level of performance. Performance for the moving objects was entirely different. Irrespective of the path length, object size or velocity of motion, path deviation thresholds depended simply upon the duration of the motion path in seconds.Conclusions/SignificanceHuman vision has long been known to integrate information across space in order to solve spatial tasks such as judgment of orientation or position. Here we demonstrate an intriguing mechanism which integrates direction information across time in order to optimize the judgment of path deviation for moving objects.

Highlights

Newton’s first law of motion states that objects in motion tend to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force
Experiment 1 We begin by presenting data for the static stimuli in order to provide a baseline data set against which path deviation thresholds for the moving blobs can be compared
The data suggest that, provided line length is increased sufficiently, thresholds become independent of blur level

Summary

Introduction

Newton’s first law of motion states that objects in motion tend to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. It is often necessary for humans to use their sense of vision to signal such events and to make revised motor actions in order to either intercept the moving object or else to avoid it, should its new direction pose a threat. Differences of less than 1u could be reliably discriminated, a value which is close to that obtained for the discrimination of orientation for continuous lines This raises the intriguing question of whether judgments about paths of motion are based upon some form of static memory trace (or ‘motion smear’ [2,3,4,5]), upon which subsequent spatial discriminations can be judged. In this study we characterize our ability to judge changes in the direction of motion of objects–a common task which can allow us either to intercept moving objects, or else avoid them if they pose a threat

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