Subitizing, the rapid and accurate enumeration of up to 3-4 items, is often thought to result from parallel access to multiple items in a display. This same limited capacity may mediate the 3-4 item limit in Multiple Object Tracking (MOT). If so, the subitizing limit should remain the same when the enumeration task is made more similar to a tracking task by including item motion and distractors.

In order to test this, we had participants enumerate between 0 and 9 moving target items among distractors, with a fixed number of total elements. Each display was presented for 200ms. We measured sensitivity for each number and found a decreasing linear function, showing no evidence of a distinction between small (up to 3-4 items) or large (>4 items) displays.

This enumeration task required the acquisition of moving items that matched the target color within the limited presentation time. Limits on this selection process could have impeded efficient enumeration, even in the subitizing range. Expt. 2 removed the distractors during the movement phase. Sensitivity was improved for all numbers of items and showed a shallow slope for 0-3 items, declining more sharply thereafter, as in typical enumeration tasks.

Because display duration was not long enough for attentional shifts, Expt. 2 suggest participants are able to efficiently individuate several moving items in a display. However, Expt. 1 shows that a simple feature property cannot be used to segregate several moving target items from distractors for enumeration within 200ms.