This page contains some sample demonstrations and the Online Supplementary Appendix from the following paper:
Alvarez, G. A., & Scholl, B. J. (2005). How does attention select and track spatially extended objects?: New effects of attentional concentration and amplification. Journal of Experimental Psychology: General, 134(4), 461-476.The demonstrations are provided as Quicktime movies, which can be downloaded or viewed directly in most web-browsers. These movies are a bit large and choppy, but they should be sufficient to illustrate the basic conditions. If the movies seem too choppy or if the lines are not drawn smoothly, try downloading the movies and playing them off your local hard drive.
Much recent research has demonstrated that attention can be allocated to discrete objects in addition to spatial locations, but relatively little research has explored the allocation of attention within individual objects which lack structure. While it may be that attention spreads uniformly through relatively small objects, real-world situations (e.g. driving) often involve sustained attention to spatially extended objects, often under conditions of motion and high processing load. Here we explore how attention is used to select and track spatially extended objects in a multiple object tracking (MOT) task. We discover that attention is not distributed uniformly through uniform objects in this situation, and the particular patterns of nonuniformity reveal two novel effects.
Online Supplementary Appendix (PDF file)
The four experiments in our paper provided an extremely rich data set for exploratory analyses, and we have thus analyzed the probe detection data from each experiment in several other ways, both to rule out potential confounds, and to explore the effects of new variables. In this Online Supplementary Appendix we present the data from Experiment 1 in detail along with an explanatory figure, and we then provide the equivalent information for each of the other experiments in a table and associated figures. To foreshadow the results of these auxiliary analyses, we find that the observed effects of attentional concentration and amplification cannot be explained by appeal to either probe-position velocity or probe eccentricity, in any of the experiments. We do, however, discover three auxiliary results that seem theoretically interesting. First, we find that both target and distractor probes are detected better when other targets and distractors are closer. Second, we find that in some cases attentional concentration increases for targets when the probed target lines are intersecting more distractor lines. Finally, we find that recent changes in the length of a line -- i.e. growing or shrinking, beyond the actual line length at the moment of the probe -- can affect probe detection.
Sample Trial #1 (2.2 MB)
Sample Trial #2 (1.9 MB)
Observers were required to keep track of 3 out of 6 moving lines. The 3 target lines are blinked at the beginning of the trial, and then observers must attentionally track them throughout the 20 s motion period, keeping them distinct from the featurally-identical distractors. When the motion ends, observers must use the mouse cursor to click on the three target lines. During the motion period, observers must concurrently monitor for the sporadic appearance of small gray dot probes, and their probe detection performance is used as a measure of the distribution of attention across the lines. By placing the probes locations at various locations along each line, we can thus construct a map of how attention is distributed along lines with various lengths, moving at various speeds, etc. Here the probes are equally likely to occur on the centers or ends of targets or distractors. These experiments yield two surprising findings. First, attention seems to be concentrated at the centers of the lines during tracking, despite their uniformity: probe detection was much more accurate at the centers of the lines than near their endpoints. Second, this 'center advantage' grew as the lines became longer: not only did observers get relatively worse near the endpoints, but they became better at the lines' centers -- as if attention became more concentrated as the objects became more extended. Both of these effects were unusually large (each over 20% in probe detection accuracy). Additional results suggest that these effects reflect automatic visual processing rather than higher-level strategies. Beyond demonstrating that objects can serve as units of attention, these results begin to show how attention is actively allocated to extended objects over time in complex dynamic displays.
Sample Trial with Abnormally Accentuated Probes (2 MB)
The dependent measure used in our studies was probe detection accuracy, which yielded large effects due to the fact that probes were difficult to detect. As a result, you may not notice all of the probes in the first two movies. Here, in contrast, we have accentuated the probes by coloring them bright red rather than dim gray. Our observers never saw such trials, but this may be useful in order to get a sense of the probe frequency and the range of their locations.