Attention Response Functions During Multiple Object Tracking

Selective attention has a capacity limit and this can be reached during multiple object tracking by, among other things, increasing the number of tracked targets. Interestingly, this effect is hemifield specific (Alvarez & Cavanagh, 2005). When targets are distributed across the visual field, more can be tracked than when these same targets are confined to one hemifield. We used fMRI to investigate whether this effect is reflected in the brain activity associated with multiple object tracking. Increases in BOLD activity that are caused by an increase in the number of tracking targets are known as Attention Response Functions (Culham et al., 2001; Jovicich et al., 2001). This monotonic increase of activity with load is well-known and well-replicated, especially in the parietal lobe. Does this effect also reflect the hemifield independence that is seen behaviorally? Participants tracked between zero and two targets among identical looking distractors separately in each hemifield, leading to a total of between zero and four targets confined to their respective hemifields. This allowed us to manipulate the tracking load in each brain hemisphere separately. If the hemispheres are fully independent, then varying load in one hemifield should not impact the BOLD response to the other. We replicate previous findings that BOLD activity increases with load in the intraparietal sulcus and the superior parietal lobule. With regards to hemisphere independence, the results showed considerable heterogeneity. We discuss our findings with respect to the regions of interest that have been identified for attention response functions in previous literature.