Electrophysiological measures of difficulty manipulations and dynamic changes of target load during attentive tracking

Trafton Drew

Visual Attention Lab, Harvard Medical School, BWH

Attention can be divided so that multiple objects can be tracked simultaneously as they move
amongst distractors. Although attentional tracking is known to be highly limited such that most
individuals can track only about four objects simultaneously, relatively little is know of
neurophysiological mechanisms that underlie the processing necessary to perform this task.

Our group has previously shown that the amplitude of a lateralized ERP component (the CDA
for Contralateral Delay Activity) scales with the number of objects being tracked. Following the work of Wolfe and colleagues (2007), in the current study, we asked subjects to dynamically change the number of targets they were tracking during a given trial. CDA amplitude rose when target load increased and declined when target load decreased, allowing us to estimate the time course of adding and dropping items in a dynamic tracking situation. The data suggests that CDA amplitude is sensitive to online changes in the number of items being tracked. We aimed to use the amplitude of this component to better understand a well-known result in the MOT literature. We know that increasing either object speed or the number of distractors decreases tracking performance but it is difficult to understand why this is the case using behavioral data alone. If the decreased capacity estimation is due to the tendency to drop items, then we might expect that CDA amplitude will decrease near the end of difficult trials. On the other hand, if the observed behavioral effect is due to an increased likelihood of inadvertently swapping a target with a distractor, then CDA amplitude should remain constant. Yet another prediction is that more resources will be devoted to each item when difficulty increases, leading to an increase in CDA amplitude as difficulty increases independent of target load.