Academic journal article Perception and Psychophysics

The Role of Location and Motion Information in the Tracking and Recovery of Moving Objects

Academic journal article Perception and Psychophysics

The Role of Location and Motion Information in the Tracking and Recovery of Moving Objects

Article excerpt

Observers in a multiple object tracking task can track about four to five independently moving targets among several moving distractors, even if all of the stimuli disappear for a 300-msec gap. How observers reacquire targets following such a gap reveals what kind of information they can maintain for targets. Previous research has suggested that participants maintain minimal information about a set of moving objects-namely, just their present spatial locations. We report five new experiments that demonstrate retention of location information for at least four objects, and extrapolated motion information for around two objects.

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Much of what we interact with in the world is in motion. Drivers navigating through traffic, parents watching their children on a playground, and air-traffic controllers tracking planes on a runway and in the air; all must constantly track a set of moving things in order to know their current location and anticipate their future motion. The visual system must have mechanisms to track objects of interest as they move around the environment. In this article, we ask what kinds of information about an object may be tracked by these mechanisms.

Previous rxesearch has shown that people can successfully track around 4-5 targets out of 8-10 total moving objects (Pylyshyn & Storm, 1988). This can be demonstrated using the multiple object tracking (MOT) task, in which observers are asked to keep track of a subset of stimuli ("targets") as they move independently around a display. After several seconds of tracking, all of the stimuli stop, and the observer indicates which stimuli were targets. In most cases, the primary dependent variable is the number of targets correctly selected.

In a typical MOT task, the stimuli are identical, so observers can only distinguish targets from distractors by following the trajectory of each stimulus during the trial. Using identical stimuli in the MOT task may seem unrealistically impoverished. After all, parents have multiple cues to distinguish their children from others on a playground, including clothing, hair color, and the sounds of their voices. Removing such cues could, in principle, reduce the effectiveness of tracking mechanisms. However, using stimuli that have no such distinguishing characteristics still allows us to ask fundamental questions about the types of information that may be known about a set of moving objects. The two types of information we will focus on in this article are location information ("Do you know where your child is?") and motion information ("Do you know which way your child is running?").

Research incorporating the MOT task has been used to support visual index theory (e.g., Pylyshyn, 1989, 2001). This theory proposes a psychological mechanism that includes a set of tags ("indexes") that can be associated with a visible object in the environment. Indexes are "sticky," meaning that each index retains its association with an object as that object moves or changes appearance. Visual indexes are supposed to operate relatively early in visual processing, and their role is to assign priority for further processing to certain parts of the visual field (Pylyshyn, 1989). They do not require that an object be identified or that any of its perceptual features be detected. According to the theory, target tracking in an MOT task is mediated by the linking of an index to each target object. Thus, performance in the task depends on the number of individual indexes available.

Under the visual index theory, little information is extracted from each tracked stimulus-indexes mark the current spatial position of each stimulus, and no further information is needed to differentiate targets from distractors. Consistent with this, if the stimuli used in an MOT task vary in color or shape, observers are poor at detecting changes in the features of tracked stimuli (Scholl, Pylyshyn, & Franconeri, 1999; but see Bahrami, 2003). …

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