VSS 2002 - Vision Sciences Society 2002 Conference Abstracts

Annan, V. and Pylyshyn Z. W. (2002). Can indexes be voluntarily assigned in Multiple Object Tracking? Vision Sciences 2002, Sarasota, FL.

In Multiple Object Tracking (MOT), an observer is able to track 4 - 5 objects in a group of otherwise indistinguishable objects that move independently and unpredictably about a display. According to the Visual Indexing Theory (Pylyshyn, 1989), successful tracking requires that target objects be indexed while they are distinct -- before tracking begins. In the typical MOT task, the target objects are briefly flashed resulting in the automatic assignment of indexes. The question arises whether indexes are only assigned automatically or whether they can be assigned voluntarily in a top-down manner. This study compares several ways of specifying which of 8 items are the targets to be tracked. In the Flash condition the target items were flashed, in the Nonflash condition the targets were the items not flashed, and in the Number condition the targets were specified by number (e.g., items numbered 1-4). The results showed no difference between the three conditions, suggesting that tracking was possible with either voluntary or involuntary indexing. The second experiment tested the hypothesis that voluntary indexing is possible only if the target items are visited serially. The conditions were the same as experiment 1 except that the time available for index assignment was too short to allow targets to be visited serially. In this experiment, targets flashed only once (or, in the Numbers condition, remained visible for about 400 ms). The results showed a decrease in tracking performance for the Number condition, but the Flash and the Nonflash conditions did not differ, suggesting that as long as the designation of targets was done rapidly, the observer did not have to visit each target serially in order to index it. These results suggest that indexing can occur both automatically and voluntarily, and without serially visiting them, so long as the items are successfully specified.

Cohen, E. H. and Pylyshyn Z. W. (2002). Searching through subsets of moving items. Vision Sciences 2002, Sarasota, FL.

We have been exploring the visual search paradigm under conditions where items to be searched move in an unpredictable manner in order to determine if the visual system can reference objects that occupy changing locations. In the present study, the moving search task was combined with a multiple-object-tracking task in which 3 to 5 items were tracked among an equal number of distractors and in which the critical item, when present, occurred in the tracked subset. Subjects tracked a number of placeholders, which, after a few seconds, changed into search items. We showed that under these conditions observers are able to confine their search to the tracked items. For example, when the search subset was a feature set, then even when the nontracked distractor set contained elements with each of the features that defined the critical item (i.e., that made the entire superset of items a conjunction search set), observers were faster at finding a present target. Additionally, subjects were faster for smaller conjunction subsets. This result shows that in the multiple-object tracking paradigm, observers do select the target set as a whole, confirming a finding of Burkell & Pylyshyn (Spatial Vision, 11, 225-258, 1997) that items selected by visual indexes can be accessed directly.

Dennis, J. H. & Pylyshyn, Z.W. (2002). Does Multiple Object Tracking improve when objects are more discriminable? Vision Sciences 2002, Sarasota, FL.

In the Multiple Object Tracking paradigm observers track a designated set of (about 4) objects that move independently and unpredictable among a set of (about 4) identical distractors. It has been shown (Scholl, Pylyshyn & Franconeri, 1999) that, consistent with the Visual Index Theory, changes in the color or shape of tracked objects are not encoded during MOT. Notwithstanding this finding, it might still be useful if some properties distinguished targets from nontargets. Of course if the targets were distinct from nontargets MOT could be accomplished without actually tracking. We developed a paradigm for investigated whether distinctive properties of objects (e.g., color) would improve tracking. In this paradigm objects were always distinct but their properties were not fixed, and therefore targets could not be tracked merely by recalling their properties. The method involved selecting 8 equispaced points on a color circle and rotating all 8 selections around the circle in a continuous manner. Three conditions were compared: (1) Colors of objects were always distinct but continuously changing, (2) colors of all objects were always the same but changed continuously in synchrony, and (3) colors of objects were identical and fixed. Results showed that that performance in these three conditions were significantly different, with (1) being best and (3) being worst. The improvement in tracking arising from maintaining distinct colors is discussed in terms of two possible explanations: Reduced confusability of target-nontarget pairs that came close to one another during a trial, and use of color differences in an "error recovery" stage.

Leonard, C., Pylyshyn, Z. W. Dennis, J.H. & Cohen, E. (2002). The effect of a secondary monitoring task on Multiple Object Tracking. Vision Sciences 2002, Sarasota, FL.

Previous research has shown that observers are able to track at least four moving targets among identical distractors. According to Pylyshyn's account, tracking uses a preattentive mechanism called a Visual Index (or FINST). Several people have challenged the assumption that MOT is preattentive. Treisman (1993) showed that a simultaneous task of monitoring color changes at the screen border results in poorer performance on both tasks. We hypothesize that a task that involves indexing an additional visual object (e.g., the border) may interfere with tracking, but that a task that involves an already-indexed object might not. Subjects performed a multiple object tracking (MOT) task with 4 targets as well as a simultaneous task involving monitoring the occurrence of a specific color change at one of four locations: on a target, non-target, border, and fixation. Both tracking and monitoring performance were recorded. The results showed that tracking was not significantly affected by the monitoring task regardless of its location (although the trend was in the expected direction, with the best performance being when the monitored change was on the target and worst when it was on the border). There were, however, significant differences in reaction times to the secondary monitoring task, as well as differences in the frequency with which the monitored events was missed. The longest RT occurred when the change was at fixation and the shortest when it was on the border. An attempt is made to reconcile these findings with the general assumption that MOT requires attention. Among the relevant factors that are considered is the difficulty of the tracking task and the possible role of inattentional blindness and nontarget inhibition.

Pylyshyn, Z. W. (2002). Tracking multiple identical moving objects: Analysis of recent findings. VisionSciences 2002, Sarasota, FL.

We have been using a Multiple Object Tracking (MOT) paradigm (Pylyshyn & Storm, 1988) to explore people's ability to track a number (usually 4 or 5) of designated visual targets when these are intermixed among a number of nontargets, all of which are identical and move independently in unpredictable ways. This paper will summarize some recent unanticipated results, several of which are being reported at this conference. In this paper I will discuss one such result in greater detail: the finding that observers can track the set of targets accurately while losing track of which one is which (in terms of some unique identifier assigned to them at the start of a trial). In other words target ID performance drops more rapidly with time than tracking performance. This result is puzzling because correct tracking logically requires that observers keep track of each individual item and trace it back in time to a particular item that was made briefly distinct at the start of the trial (in other words, the correspondence problem must be continuously solved for each target). Recent studies have suggested that the more rapid drop in ID performance relative to tracking performance may arise because observers more easily confuse targets they are tracking with other targets that they are also tracking than with nontargets that they are systematically ignoring. We found that targets that come close to other targets during a trial frequently switch identities, whereas targets that come close to nontargets are more readily kept distinct. Possible reasons for this are explored, including the proposal that nontargets are inhibited during tracking. Data are presented showing that under certain conditions nontargets may indeed be inhibited. The implications of such an inhibitory mechanism will be discussed. The various phenomena associated with MOT will be illustrated with animated demonstrations.