Hemifield Specificity of Attention Response Functions during Multiple-Object Tracking

The difficulty of tracking multiple moving objects among identical distractors increases with the number of tracked targets. Previous research has shown that the number of targets tracked (i.e., load) modulates activity in brain areas related to visuospatial attention, giving rise to so-called "Attention Response Functions" (ARFs). While the hemifield/hemispheric effects of spatial attention (e.g., hemispatial neglect, hemifield capacity limits) are well described, it had not previously been tested whether a hemispheric or hemifield imbalance exists among ARFs. By recording BOLD activity from human brains (n=19, female and male) in a multiple object tracking paradigm, we show that the number of tracked objects modulates activity in a large network of areas bilaterally. A significant effect of contralateral load was found in earlier areas throughout the dorsal and ventral visual streams, while the effects of ipsilateral load emerged in later areas. Both contra- and ipsilateral load significantly influenced activity in the parietal and frontal lobes, specifically the dorsal attention network. In addition, some brain regions in the occipital lobe were significantly more sensitive to contralateral than ipsilateral load. Our results are consistent with findings showing that a diverse set of brain areas contributes to tracking multiple targets. In particular, we extend the canonical view of load-based ARFs to include hemifield bias. Given the hemifield-specific nature of speed and capacity limits to multiple object tracking, we conjecture that areas that show a strong hemifield preference may impose a bottleneck on processing that results in limits on the capacity and speed of tracking.<b>Significance Statement</b> We investigated how attentional effort impacts brain activity. Effort (the number of targets in a multiple object tracking task) parametrically drives activity in the attention system. Our findings reveal brain areas where effort driven increases in activity are dependent on the visual hemifield where targets are tracked. We show that the load-dependent responses differ between earlier visual areas, which prefer targets on the contralateral side, and later areas that respond to targets anywhere in the visual field. This research challenges previous explanations of hemispatial neglect and enhances our understanding of how the brain manages spatial attention and mental effort. Additionally, we identify regions that might be the source of hemifield-specific capacity limits in attentional tracking.