1. INTRODUCTION The working memory (WM) model (Baddeley & Hitch, 1974), has been successfully applied to many visuospatial phenomena. But, as yet, not to the perception and monitoring of moving objects within the external environment. A dual-task paradigm consisting of a computerised multiple-target tracking task (e.g. Pylyshyn & Storm, 1988), together with various secondary tasks believed to tap specific WM resources, may give some insight into the process of tracking multiple moving targets. METHOD Subjects 144 post- and undergraduate students, 73F and 71M took part, aged between 16 and 47 (mean 21.37, SD 4.7). All had normal or corrected-to- normal vision. Each was paid a small honorarium. Participants were allocated to one of four secondary task conditions: visual, auditory, spatial or verbal. For half, these began before target acquisition the remainder after. Materials Six blocks, each of 40 trials, were prepared in advance. Each trial began with a centralised solid white fixation square on a black background. After a delay, ten static crosses (+) appeared, distributed randomly (see fig. 1). A number of these, designated as targets (1-5), were flashed on and off several times [the target acquisition phase (TA)], the remainder were distractors. Figure 1: Screen Display After a further delay, targets ceased flashing and all crosses began to move at random [the target tracking phase (TT)]. After at least 3 seconds, one of the crosses was transformed into a solid white square (the probe) for a period of 100 milliseconds. A random 50% of trials probed targets, the remainder distractors. Design A mixed design with task (single/dual), number of targets (1 - 5) and probe (target/distractor) as within-subjects factors and secondary task (visual/auditory/spatial/verbal) and order (before/after TA) as between. Both task conditions consisted of two blocks of 40 trials, preceded by a practice block of 40. Block use was counterbalanced. Practice trial data were discarded. Roy Allen, Peter M c George & David Pearson, Department of Psychology, University of Aberdeen, Scotland. Tele: +44 (0)1224 – 272387; Fax: +44 (0)1224 - 273426; email: roy.allen@abdn.ac.uk Working Memory: and the visual tracking of multiple moving targets Procedure Participants were tested individually in a darkened room. Each completed the single and dual task conditions. Every trial began by pressing the spacebar. It was emphasised that they should keep their eyes on the fixation square throughout each trial. Further, they had to pay attention to the indicated targets. If, and only if, a target was probed, they had to press the keyboard’s spacebar as quickly as possible. It took 2 hours to complete 200 trials. Accuracy and RTs were recorded. The secondary tasks, carried out during each dual task trial, were labelled: Visual - categorizing single digits, flashed at 1/sec in the fixation square, as high (6-9) or low (1-4); Auditory - categorizing tones, played at 1/sec, as high or low; Spatial – tapping the four corner buttons of a 3 x 3 matrix once/sec; Verbal – repeating “the” once/sec. RESULTS The number of targets that participants could successfully track was obtained by subtracting the number of false alarms (FA)from the number of hits (as per the high-threshold model of signal detection, e.g. Snodgrass & Corwin, 1988). This figure was then compared to the number of misses, using binomial tests. Results suggest that, in the single task, participants can always track 4 targets significantly better than chance, p < 0.05 (see Fig. 2). During the dual task, for the Visual and Auditory conditions, participants can always track 3 targets significantly better than chance, p < 0.01 (see Fig. 3), whilst for the Spatial and Verbal conditions, participants can still track 4 targets significantly better than chance, p < 0.05 (see Fig. 4). Analysis of secondary task data confirmed that participants always allocated adequate and sufficient resources, always performing above 85% accuracy. Other analyses on RTs, response bias and detection sensitivity were carried out but are not reported here. Figure 2: Summary of all tracking task performance in the single task condition. Filled marker indicates number of targets successfully tracked Figure 3: Summary of all tracking task performance in dual task condition, by secondary task; secondary task commencing before TA. Filled markers indicate number of targets successfully tracked in each case. Figure 4: Summary of all tracking task performance in dual task condition, by secondary task; secondary task commencing after TA. Filled markers indicate number of targets successfully tracked in each case. DISCUSSION Whatever the secondary task, performance never significantly varied with secondary task onset. This suggests that the TA phase of the experiment is not associated with WM processes and supports Pylyshyn’s claim that target acquisition is preattentive. TT, on the other hand, does seem to utilise WM. Tracking performance in the Visual condition suggested a role for central executive resources, an idea proposed by Yantis (1992), but decrements might also be due to primary and secondary tasks sharing the same modality. Tracking performance in the Auditory condition, though, refuted this latter interpretation. However, the tracking performance decrement might just be the effect of performing any two tasks simultaneously. Tracking performance in the Verbal condition did not support this notion. Finally, a tracking task might seem to be very spatial in nature, however tracking performance in the Spatial condition did not support this either. An interpretation is, therefore, that the central executive plays a role in the tracking of multiple moving objects. However, a similar pattern might be obtained independently of central executive input if, in fact, visual indexes were amodal. REFERENCES Baddeley, A.D. & Hitch, G.J. (1974). Working Memory. In G.A. Bower (Ed.), Recent advances in learning and motivation, Vol. VIII. (47-90) New York: Academic Press. Pylyshyn, Z.W. & Storm, R.W. (1988). Tracking multiple independent targets: Evidence for a parallel tracking mechanism. Spatial Vision, 3 (3), 179-197. Snodgrass, J.G. & Corwin, J. (1988). Pragmatics of measuring recognition memory - applications to dementia and amnesia. Journal of Experimental Psychology-General 117(1): 34-50. Yantis, S. (1992). Multi-element visual tracking: Attention and perceptual organization. Cognitive Psychology, 24 (3), 295-340.........