Tracking multiple independent targets: Evidence for a parallel tracking mechanism*

TLDR

Visual attention is typically focused at a single locus that can shift independently of eye movements, yet evidence suggests that at least one operation can be performed in parallel across multiple independent loci in the visual field. The study investigates whether indexing features and tracking their identity can be carried out in parallel across several loci. Subjects successfully tracked up to five of ten randomly moving objects, achieving 87% accuracy—well above the ~40% predicted by serial scanning—supporting a parallel indexing mechanism that underlies feature binding before pattern recognition.

Abstract

There is considerable evidence that visual attention is concentrated at a single locus in the visual field, and that this locus can be moved independent of eye movements. Two studies are reported which suggest that, while certain aspects of attention require that locations be scanned serially, at least one operation may be carried out in parallel across several independent loci in the visual field. That is the operation of indexing features and tracking their identity. The studies show that: (a) subjects are able to track a subset of up to 5 objects in a field of 10 identical randomly-moving objects in order to distinguish a change in a target from a change in a distractor; and (b) when the speed and distance parameters of the display are designed so that, on the basis of some very conservative assumptions about the speed of attention movement and encoding times, the predicted performance of a serial scanning and updating algorithm would not exceed about 40% accuracy, subjects still manage to do the task with 87% accuracy. These findings are discussed in relation to an earlier, and independently motivated model of feature-binding--called the FINST model--which posits a primitive identity maintenance mechanism that indexes and tracks a limited number of visual objects in parallel. These indexes are hypothesized to serve the function of binding visual features prior to subsequent pattern recognition.