Forward displacement, attention and the multiple-object tracking (MOT) task

1. Forward displacement, attention and the multiple-object tracking (MOT) task Roy Allen & Peter McGeorge, School of Psychology, University of Aberdeen

2. Introduction 1 • MOT task (Pylyshyn & Storm, 1988) • Expertise (e.g., Allen et al., 2004; Barker et al., 2010; Green & Bavalier, 2006a, 2006b); • Developmental (e.g., Trick et al., 2005); • What of trajectory information? • Extrapolate target location (Keane & Pylyshyn, 2006): • All items disappeared for 150-900ms; • Performance best when items reappeared at point where they disappeared or back along trajectory • Poorest when items reappeared forward of vanishing point. • But Fencsik et al. (2007): • All items disappeared for 300ms; • As well as successfully tracking 4 items observers also held motion information for two items.

3. Introduction 2 • Like Forward Displacement Bias debate (Finke & Freyd, 1985): • Single object with actual or implied motion, disappears; • Observers misjudge spatial location of disappearance; • Seen as index of ability to extrapolate location from trajectory information; • Subsequently seen as influenced by many factors (e.g., direction of travel, speed, nature of object etc. – see Hubbard, 2005). • Role of attention (Hayes & Freyd, 2002) (implied motion): • Dual task, manipulated probability that one or other of two objects would be probed; • Found when attention had to be divided over both objects (i.e., when probability was 20% or 35%) forward displacement greater compared with when attention more focused (i.e., when probability was 65% or 80%). • But Kerzel (2003) (implied motion); McGeorge et al. (2006): • Both suggested attention was necessary for a forward displacement bias to occur.

4. Introduction 3 • Alvarez and Franconeri (2007) • Proposed a resource-limited model of MOT - as the number of items to be tracked increases, the attentional resources allocated to each item decreases: • This leads to a reduction in tracking accuracy (see also Barker et al., 2010); and, • if we accept the suggestion that attention is necessary to extrapolate target location from trajectory information, as target numbers increase forward displacement bias will decrease. • We investigated this latter prediction in our first experiment

5. Method 1 • 34 participants (10 female) aged from 17 – 29 (M = 20.29, SD = 2.59) • 144 trials

6. Method 2 Perceived target Position Final target Position fd Target trajectory at time of disappearance

7. Results 1 • Discrimination Accuracy:A repeated-measures ANOVA, of the target data, was carried out with Delay (no delay, delay), and Number of targets to be tracked (2, 3, 4, 5) as the within-subjects factors; • There was a main effect of Delay (F(1, 33) = 28.99, p < .001, ηp2 = .47) (No Delay: M = 79.82, SD = 1.60; Delay: M = 70.35, SD = 2.22); • There was also a main effect of Number of targets to be tracked (F(3, 99) = 29.78, p < .001, ηp2 = .47). The interaction was not significant.

8. Results 2 • Forward Displacement • A repeated-measures ANOVA, on the target data, was carried out with Delay (no delay, delay), and Number of targets to be tracked (2, 3, 4, 5) as the within-subjects factors; • There were significant main effects of Type and Number of targets to be tracked. These were moderated by a significant interaction of Type x Number of targets to be tracked (F(1.97, 64.95) = 5.52, p = .006, ηp2 = .14);

9. Results 3 • Paired-samples t-tests (p = .013 for multiple comparisons) showed that performance in the No Delay and Delay conditions varied significantly for 2 and 3 targets (2 targets: t33 = 4.32, p < .001; 3 targets: t33 = 2.68, p = .011); • One-sample t-tests showed that for both 2 and 3 targets, in the No Delay condition, forward displacements were significantly greater than zero (2 targets: t33 = 2.09, p = .044; 3 targets: t33 = 2.33, p = .026).

10. Discussion • Results show forward displacement for small numbers of targets in the No delay condition (supports Fencsik et al., 2007); • Supports idea that attention may be a necessary component of generating a forward displacement bias (Kerzel, 2003: McGeorge et al., 2006) (i.e., in terms of Alvarez and Franconeri (2007) idea of a resource-limited model of MOT, as attention divided amongst more targets so displacement bias disappears); •  Finally, trajectory knowledge appears relatively short-lived (i.e., no forward displacement bias following a1000ms delay); • However, is it just an object’s motion that influences FD, or can what we know about an object also be influential? We investigate this in experiment 2.

11. Experiment 2 • Reed and Vinson (1996, exp 1) contrasted the FD bias shown when participants thought they were viewing either a single rocket or a steeple. • While these two stimuli had the same visual characteristics they have very different typical motions – rockets move, steeples rarely do. • Found that participants, told that the stimulus represented a steeple, showed significantly smaller FD biases than participants told that the same stimulus was a rocket, particularly for implied upward motion. • Greater FD bias for an object that moved, relative to one that did not, is consistent with the use of knowledge of the object’s typical motion to predict its position. • The fact that this bias was found to be greatest when looking at implied upward motion (the typical motion of a rocket), underlines this point. • Knowledge, it appears, influences how we perceive motion (real or implied), and, thus, how easy it is to predict where an object will move, making it easier to track. • Does this effect extend to MOT?

12. Method/Results • 21 participants (11 female), ages ranged from 18 – 25 (M = 20.4, SD = 4.5). • Paradigm was same as experiment 1, but stimuli were either all butterflies or all flowers (pansies). The former are, typically, expected to move in the random way MOT items move. • Discrimination Accuracy:A repeated-measures ANOVA, of the target data, was carried out with Class (butterflies, pansies), and Number of targets to be tracked (2, 3, 4, 5) as the within-subjects factors; • There were main effects of Class and Number of targets to be tracked. These were moderated by a Class x Number of targets to be tracked interaction (F(3, 60) = 10.39, p < .001, ηp2 = .34).

13. Results 2 • Paired-sample t-tests showed that discrimination accuracy for butterflies, on 5-target trials, was significantly greater than that for pansies (t20 = 4.93, p < .001). • Forward displacement: • A repeated-measures ANOVA was carried out, on target data, with Class (Butterfly, Pansy) and Number of targets to be tracked (2, 3, 4, 5) as the within-subjects factors. • Main effects of Class and Number of targets to be tracked were moderated by a significant Class x Number of targets to be tracked interaction (F(3, 60) = 10.22, p < .001, ηp2 = .34).

14. Results 3 • Paired-sample t-tests (p set to .016 for multiple comparisons) showed that butterflies and pansies only differed significantly for 2-target and 4-target trials (2 targets: t20 = 3.53, p = .002; 3 targets: t20 = 0.16, p = .878; 4 targets: t20 = 4.67, p < .001). • For pansies, one-sample t-tests show forward displacement for two and three targets is significantly greater than zero (2 targets: t20 = 5.36, p < .001; 3 targets: t20 = 2.28, p = .033) • For butterflies, forward displacement for three and four targets is significantly greater than zero (2 targets: t20 = 0.22, p = .828; 3 targets: t20 = 6.81, p < .001; 4 targets: t20 = 4.80, p <.001).

15. Conclusions • Experiment 1 confirmed Fenscik et al.’s (2007) claim that observers’ can retain trajectory information about circa 2-3 target objects; • Also showed that such trajectory information decays and is gone by circa 1000ms; • Supports idea that attention may be a necessary component of generating a forward displacement bias (Kerzel, 2003: McGeorge et al., 2006) (i.e., in terms of Alvarez and Franconeri (2007) idea of a resource-limited model of MOT, as attention divided amongst more targets so displacement bias disappears). • Experiment 2 showed differences in tracking (butterflies tracked better than pansies) and trajectory information for butterflies and pansies (2-3 for pansies; 3-4 for butterflies); but, • No trajectory information for 2-target butterfly trials? Suggests the expectation that a type of object moves directs increased attention to them. Suggests attention necessary to prevent fd when objects expected to move.

16. Other analyses • Location Accuracy • Significant main effect of number and class x number interaction that approached significance (F(1.58, 31.71) = 3.46, p = .054, ηp2 = .15) • Paired-sample t-tests showed butterfly accuracy, on 2-target trials, was significantly greater than for pansies (2 targets: t20 = 3.51, p = .002)

17. References 1 • Allen, R., McGeorge, P., Pearson, D. G., & Milne, A. B. (2004). Attention and expertise in multiple target tracking. Applied Cognitive Psychology, 18, 337 - 347. • Alvarez, G. A. & Franconeri, S. L. (2007). How many objects can you track?: Evidence for a resource-limited attentive tracking mechanism. Journal of Vision 7(13):14, 1 – 10. • Barker, K., Allen, R., & McGeorge, P. (2010) Multiple-Object Tracking: Enhanced visuospatial representations as a result of experience. Experimental Psychology, 57, 208 - 214. • Fencsik, D. E., Klieger, S. B. & Horowitz, T. S. (2007). The role of location and motion information in the tracking and recovery of moving objects. Perception & Psychophysics, 69 (4), 567-577. • Finke, R. A. & Freyd, J. J. (1985). Transformations of visual memory induced by implied motions • of pattern elements. Journal of Experimental Psychology: Learning, Memory, & Cognition, 11, 780 - 794. • Green, C. S., & Bavelier, D. (2006a). Enumeration versus multiple object tracking: The case of action video game players. Cognition, 101, 217 - 245. • Green, C. S., & Bavelier, D. (2006b). Effect of action video games on the spatial distribution of visuospatial attention. Journal of Experimental Psychology: Human Perception and Performance, 32 (6), 1465 - 1478. • Hayes, A. E. & Freyd, J. J. (2002). Representational momentum when attention is divided. Visual Cognition, 9 (1/2), 8 - 27.

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