Academic journal article Attention, Perception and Psychophysics

Direction Information in Multiple Object Tracking Is Limited by a Graded Resource

Academic journal article Attention, Perception and Psychophysics

Direction Information in Multiple Object Tracking Is Limited by a Graded Resource

Article excerpt

Is multiple object tracking (MOT) limited by a fixed set of structures (slots), a limited but divisible resource, or both? Here, we answer this question by measuring the precision of the direction representation for tracked targets. The signature of a limited resource is a decrease in precision as the square root of the tracking load. The signature of fixed slots is a fixed precision. Hybrid models predict a rapid decrease to asymptotic precision. In two experiments, observers tracked moving disks and reported target motion direction by adjusting a probe arrow. We derived the precision of representation of correctly tracked targets using a mixture distribution analysis. Precision declined with target load according to the square-root law up to six targets. This finding is inconsistent with both pure and hybrid slot models. Instead, directional information in MOT appears to be limited by a continuously divisible resource.

(ProQuest: ... denotes formulae omitted.)

Since we live in a constantly changing environment, we need to update our representations of the world over time. One somewhat artificial example would be air traffic control, in which the controller needs to update his or her knowledge of aircraft positions and flight plans in order to ensure that takeoffs and landings go smoothly. But some version of this basic cognitive task confronts us in our everyday life, whether we are driving on a busy street, walking through a crowd, or chaperoning a child's birthday party.

The primary laboratory tool for studying updating, particularly spatial updating, is the multiple object tracking (MOT) task (Pylyshyn & Storm, 1988). In the MOT task, observers are presented with an array of identical objects. A subset of these objects is designated as targets. The observer must then track these targets while all of the objects move independently for several seconds (or minutes, as in Wolfe, Place, & Horowitz, 2007) and at the end report which items were targets (for recent reviews, see Cavanagh & Alvarez, 2005; Scholl, 2009).

The typical finding in such experiments is that observers can track three to five objects. What is the nature of this capacity limit? In the visual short-term memory (VSTM) literature, a similar limit of three to five items has been observed. Debate in the VSTM field has contrasted two accounts, which we will term the slot theory and the flexible resource theory. According to the slot theory (Awh, Barton, & Vogel, 2007; Luck & Vogel, 1997), a fixed number of objects can be stored in VSTM, regardless of their complexity. In contrast, the flexible resource theory argues that the number of objects that can be stored is inversely related to the complexity of the objects because more complex objects take up more of the fixed resource (Alvarez & Cavanagh, 2004; Eng, Chen, & Jiang, 2005). This debate has proven quite productive for the study of VSTM.

The MOT literature was originally dominated by the slot approach. The best known model is Pylyshyn's (1989) FINST ("fingers of instantiation") or visual index model, in which a fixed set of pointers can be assigned to objects for tracking, enumeration, or other visual routines; the phenomenon of MOT was predicted on this account. Here, the limit on tracking is the number of pointers.

Alvarez and Franconeri (2007) recently proposed a flexible resource model. In their model, targets are tracked by flexible indexes (FLEXes). The total number of FLEXes is limited by the availability of a finite resource. This resource determines the spatial resolution of each FLEX, such that when fewer items are tracked, resolution is higher. This account predicts that when spatial resolution is at less of a premium, observers could track more objects by allocating the resource more thinly. Kazanovich and Borisyuk (2006) proposed a somewhat similar approach, in which tracking is accomplished by a limited set of central oscillators, and the number of oscillators is limited by a finite resource. …

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