The ability to attend to multiple moving objects simultaneously is fundamental to many of the tasks we perform regularly, such as driving, or taking part in team sports.

Numerous studies in which participants are asked to track dots moving around on a screen have led researchers to the  conclusion that 4 is the maximum number of objects that can be tracked. It is therefore widely believed that the "magical number" 4 is the fixed upper limit of visual attention.

This has, therefore, led to the assumption that the visual system has a "fixed architecture" which places a limit on the number of objects that can be tracked simultaneously. But some researchers believe there may be more flexibility in the mechanisms by which attentional resources are allocated during object tracking.

George Alvarez of MIT's Computational Visual Cognition Laboratory and Stephen Franconeri of the Department of Psychology at Northwestern University in Illinois, now provide strong evidence for the so-called flexible resource model. In the Journal of Vision, they report participants in their study were able to track up to 8 objects simultaneously. 

Alvarez and Franconeri recruited 14 students, and presented them with stimuli consisting of 16 green circles on a black background. In each trial, the circles moved at a different, but constant, speed and, unlike  in previous studies, did not come into contact with each other.

There was a strong correlation between the speed at which the circles were moving and the number that could be tracked accurately. It was found 8 circles could be tracked if they were moving at a speed of 1 cm per second. But with each small increase in speed, the number of circles that could be tracked  decreased progressively. (This effect is demonstrated clearly in these demonstrations on Alvarez's website.)

In terms of the flexible resource model, this can be interpreted as meaning that there is a trade-off between the number of objects tracked and the accuracy with which they can be tracked. The greater the speed at which multiple objects are moving, the more attentional resources need to be allocated to each one, and the fewer can be tracked accurately.

Reference:

Alvarez, G. A. & Franconeri, S. L. (2007). How many objects can you track?: Evidence for a resource-limited attentive tracking mechanism. J. Vision 7: 1-10. [Full text] 

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