Lauri Oksama

University of Turku, Finland

In my presentation, I will address a fundamental question concerning dynamic visual attention, that is, whether tracking of multiple
moving objects is a parallel or serial process. Tracking of multiple moving objects is commonly assumed to be carried out by a fixed-
capacity parallel mechanism. However, we (Oksama & Hyönä, 2008) have recently put forth a serial model (MOMIT) to account for the
tracking performance of multiple moving objects with distinct identities. A serial refresh mechanism is postulated, which makes recourse to
continuous attention switching and a capacity-limited episodic buffer for identity-location bindings. As identity-location bindings are
refreshed serially, a location error is inherent for all other targets except for the focally attended one. The magnitude of this location error is
a key factor in predicting tracking accuracy. MOMIT's predictions were supported by the data of five experiments (Oksama & Hyönä, 2008).
In addition, a mathematical simulation of MOMIT provided a nice fit to the observed data with plausible parameter estimates for the binding
capacity and refresh time.

In terms of eye movements, a serial model of this kind clearly predicts that observers also shift their eyes between targets. Do they?
Or can observers track targets covertly without any overt eye movements (as parallel theories predict)? In our new study, gaze behavior in
a classic multiple object tracking task with identical objects were compared to a tracking task with distinct objects. The results suggest an
“ecumenical” solution to this issue: When the task includes distinct objects (e.g. aircraft on a radar display), targets have to be visited
serially in order to continuously refresh the identity-location bindings (Oksama & Hyönä, 2008). On the other hand, when the target set is
identical (e.g. a flock of white sheep), tracking is carried out using peripheral vision (Pylyshyn, 1989, 2001)..