Further evidence for inhibition of moving nontargets in multiple object tracking

Zenon Pylyshyn
Rutgers University - New Brunswick
Using the Multiple Object Tracking (MOT) task, Pylyshyn & Leonard (VSS03) showed that a small brief probe dot was detected more poorly when it occurred on a nontarget than when it occurred either on a target or in the space between items, suggesting that moving nontarget items were inhibited. Here we generalize this finding by comparing probe detection performance against a baseline condition in which no tracking was required. We examined both a baseline condition in which objects did not move and one in which they moved exactly as in the probe detection task, but without tracking. Detection in the nonmoving control was essentially error-free, but probe detection performance in the moving (non-tracking) control task did depend on the probe location. Nonetheless, the findings reported earlier (worse detection on nontargets than anywhere else) remained after we took account of the baseline performance (using several different baseline comparison methods). Using this new baseline-control method we also showed that inhibition does not spread more than about 1.3 degrees of visual angle from the nontargets as the latter move during a tracking trial. In the present report we also describe some preliminary studies of conditions under which enhancement of targets, as well as inhibition of nontargets, may be obtained. Increasing the number and type of nontargets appeared to improve the detection of probes on targets, relative to baseline. However, we failed to find evidence that potentially distracting nontarget objects were inhibited more than clearly task-irrelevant objects, as would be expected if inhibition were a top-down process applied in order to improve performance on the main tracking task. For example, using the comparison with its matching baseline, we showed that task-irrelevant stationary items (which are never confused with targets in the tracking task) appeared to be inhibited even more than nontargets.
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