we present a new paradigm to study object discriminability

1. Effect of Object
Discriminability on
Multiple Object Tracking
John L. Dennis and Zenon W. Pylyshyn
Rutgers University
Center for Cognitive Science

2. In the Multiple Object Tracking (MOT)
paradigm, subjects track a subset of objects
that move independently and unpredictably.

3. Scholl, Pylyshyn, & Franconeri, (1999) showed
that a change in the color or shape of a tracked
object is not available for recall.
Blaser, Pylyshyn, Dennis, (in preparation)
showed that tracking is affected by the total time
that objects interacted with each other.
Previous Research:

4. Despite the fact that properties of targets are
not available for recall, it is possible that
tracking might still use distinct properties of
objects to enhance tracking or to recover from
tracking errors when objects interact with
eachother.
Research Question:

5. But if targets consistently differed from non-
targets then targets would not have to be
tracked; subjects could simply recall the
distinguishing property and use that to identify
targets at the end of a trial.
Problem:

6. Solution:
We developed a paradigm in which at each
instant every object is distinct from every other
object, yet the distinguishing property cannot
be used to pick out the targets.
The technique relies on continually changing
objects’ properties so that they are distinct
from one another, yet do not uniquely identify
the objects.

7. Experiment 1:
Asynchronous Color
ChangeEach object was assigned a unique color on a
color circle containing 222 colors, and each
object cycled through this circle once ever 3790
milliseconds.

8. Experiment 1 Results:
Tracking performance is improved when the
objects change their color asynchronously.

9. These findings support the hypothesis that
increased discriminability of target-nontarget
pairs leads to improved tracking.
But it is also possible that the improvement
was due to some other factor related to
changing properties. One should therefore ask
whether there would be a similar improvement
in tracking performance if objects change
synchronously?
Discussion of Experiment
1:

10. Experiment 2:
Synchronous Color
Change

11. Experiment 2 Results:
Tracking performance is improved when the
objects change their color synchronously.

12. Increased discriminabilty cannot explain
why synchronously changing color
improves tracking performance, since pairs
of objects in this case are not more
discriminable.
Discussion of Experiment
2:

13. Further Question:
Are there other features that we could change
on objects, either synchronously or
asynchronously that would similarly improve
tracking performance?

14. Experiment 3:
Feature Change
In Experiment 3, we replicate the
earlier experiments and also examined
whether changes in gray scale,
diameter or shape (the size and
orientation of the axis of an oval),
affect tracking performance.

15. Asynchronous
Diameter
Asynchronous Grey
Scale
Asynchronous
Color
Asynchronous
Shape

16. Experiment 3 Results:

17. Conclusions:
The results show that increased
discriminability improves tracking
performance, but so do similar but task-
irrelevant changes in objects’ properties.
• Observers may use the improved
discriminability to distinguish pairs of
objects that come close together.
• Observers may use the feature
differences when they have temporarily
“lost” objects, as postulated by the Error
Recovery model of Sears & Pylyshyn
(2000).

18. Some speculations for
further research
• Why should tracking improve with
synchronous property changes?
• It has been suggested, (Pylyshyn, 1998),
that the effectiveness of indexes may
decay over time, requiring reactivation by
visits of focal attention. A possibility being
investigated is that salient changes in
some properties of a tracked object may
help to reactivate indexes.

19. References cited
• Blaser, E., Pylyshyn, Z. W., Dennis, J. L. (in preperation).
Effect of density of objects and their interactions on
multiple object tracking.
• Pylyshyn, Z. W. (2002). Tracking multiple identical moving
objects: Analysis of recent findings. Paper presented at
VisionSciences 2002, Sarasota, FL.
• Pylyshyn, Z. W. (1998). Visual indexes in spatial vision and
imagery. In R. D. Wright (Ed.), Visual Attention (pp. 215-
231). New York: Oxford University Press.
• Pylyshyn, Z.W. & Storm, R. W. (1988). Tracking multiple
independent targets: evidence for a parallel tracking
mechanism. Spatial Vision, 3(3), 1-19.
• Scholl, B. J., Pylyshyn, Z. W., & Franconeri, S. (1999). When
are featural and spatiotemporal properties encoded as a
result of attention allocation? Paper presented at the ARVO
Conference, Ft. Lauderdale, FL.
• Sears, C. R., & Pylyshyn, Z. W. (2000). Multiple object
tracking and attentional processes. Canadian Journal of
Experimental Psychology, 54(1), 1-14