Hemifield Effects in Multiple Identity Tracking Charlotte Hudson, Piers D. L. Howe*, Daniel R. Little Melbourne School of Psychological Sciences, The University of Melbourne, Parkville, Victoria, Australia Abstract In everyday life, we often need to attentively track moving objects. A previous study has claimed that this tracking occurs independently in the left and right visual hemifields (Alvarez & Cavanagh, 2005, Psychological Science,16, 637–647). Specifically, it was shown that observers were much more accurate at tracking objects that were spread over both visual hemifields as opposed to when all were confined to a single visual hemifield. In that study, observers were not required to remember the identities of the objects. Conversely, in real life, there is seldom any benefit to tracking an object unless you can also recall its identity. It has been predicted that when observers are required to remember the identities of the tracked objects a bilateral advantage should no longer be observed (Oksama & Hyo¨ na¨, 2008, Cognitive Psychology, 56, 237–283). We tested this prediction and found that a bilateral advantage still occurred, though it was not as strong as when observers were not required to remember the identities of the targets. Even in the later case we found that tracking was not completely independent in the two visual hemifields. We present a combined model of multiple object tracking and multiple identity tracking that can explain our data. Citation: Hudson C, Howe PDL, Little DR (2012) Hemifield Effects in Multiple Identity Tracking. PLoS ONE 7(8): e43796. doi:10.1371/journal.pone.0043796 Editor: Mark Alexander Williams, Macquarie University, Australia Received May 2, 2012; Accepted July 25, 2012; Published August 28, 2012 Copyright: 2012 Hudson et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no support or funding to report. Competing Interests: The authors have declared that no competing interests exist. * E-mail: pdhowe@unimelb.edu.au Introduction Successful interaction with one’s environment requires the ability to pay attention to objects of interest. As objects are frequently in motion, tracking is often required to maintain attention on them as their locations change. Accordingly, object tracking is involved in numerous everyday activities. When waiting to cross the street, for instance, it is necessary to track cars and bicycles to judge a safe time to step out onto the road. To explore the cognitive processes involved in object tracking in a laboratory setting, Pylyshyn and Storm [1] developed the multiple object tracking (MOT) paradigm. The task requires observers to track a subset of identical objects that were briefly cued as targets while they move on a computer monitor. Target tracking accuracy is measured either by asking the observer to click on all the targets at the end of the trial (e.g. [2,3]) or by asking the observer whether a randomly selected object was a target (e.g. [4,5]). The MOT paradigm has been a popular choice among researchers as a way of studying the deployment of attention in dynamic scenes [6]. One of the most salient findings from the MOT literature has been that object tracking is highly capacity limited. People are limited to tracking approximately three to five objects [1,5,7,8]. Individual differences, however, have been reported [9], with expertise evident in people whose occupations rely on advanced spatial cognitive abilities, such as radar operators [10,11]. It has also been suggested that the capacity limit is context-dependent, with factors such as the objects’ movement speed ([12] cf. [13]) and object crowding [14] affecting tracking ability. In a seminal MOT study, Alvarez and Cavanagh [4] directly investigated the source of the object tracking capacity limitation. Given that early visual processing occurs almost exclusively in the cerebral hemisphere contralateral to the visual hemifield of input [15], Alvarez and Cavanagh investigated whether there would be independent constraints for tracking objects in the left and right visual hemifields. To examine this, they modified the typical MOT display by partitioning it into quadrants and adding a central fixation cross. In the baseline condition, observers tracked two targets, both confined to a single hemifield. In the unilateral condition observers tracked four targets, all confined to one visual hemifield. In the bilateral condition, the observers again tracked four targets, but the targets were spread across both hemifields. Their results indicated equal tracking accuracy in the baseline and bilateral conditions, but a dramatic drop in performance in the unilateral condition. Alvarez and Cavanagh’s [4] finding supported the existence of two separate tracking mechanisms, one in the right cerebral hemisphere responsible for tracking objects in the left visual hemifield and another in the left cerebral hemisphere responsible for the right visual hemifield. Both mechanisms could track two, but not four, targets. Thus, observers were highly accurate in the baseline and bilateral conditions because in both conditions there were never more than two targets in a single hemifield. Conversely, observers were overwhelmed by the unilateral condition, as this required them to attempt to track four targets in a single hemifield. Despite the importance of Alvarez and Cavanagh’s [4] results, the MOT paradigm is limited in its applicability because all the objects are identical [9,16]. It is challenging to imagine an everyday scenario in which one is required to track a subset of indistinguishable items. Rather, we tend to experience situations in which the objects we need to track have unique identities that we are required to remember [17]. The advent of multiple identity tracking (MIT), as coined by Oksama and Hyo¨na¨ [9], acknowl- edges this. MIT tasks are a modified version of MOT, typically using easily distinguishable objects that remain visible for the PLOS ONE | www.plosone.org 1 August 2012 | Volume 7 | Issue 8 | e43796