Abstract
When solving dynamic visuo-spatial tasks, like multiple-object tracking (MOT), the brain copes with challenges imposed by the number of objects (i.e. load) that has to be tracked, but also by spatial interference from irrelevant objects (i.e. crowding). At present, it is not clear whether load and crowding activate separate cognitive and physiological mechanisms. In particular, cognitive workload recruits cortical systems, like the dorsal and ventral attention networks, and brainstem neuromodulatory systems, such as the locus coeruleus - norepinephrine (LC-NE) system and the ventral tegmental area/substantia nigra dopaminergic (VTA/SN - DA) system. However, it remains unknown what the specific roles of each of these brain systems are in dealing with load versus crowding. We employed a MOT task, where participants had to track simultaneously multiple randomly moving targets. We varied the degree of processing challenge in two ways: first, how many dynamic targets participants had to track (“load”, known at the beginning of every trial); second, the proximity between targets and distractors (i.e., the number of close target-distractor encounters, causing moment-to-moment confusion within the trial). To evaluate the extent to which load and crowding elicits distinct physiological responses, we ran two MOT experiments in separate cohorts. In a first experiment, we measured task-induced saccades and pupil dilations. In a separate fMRI experiment, we measured brain activity elicited by MOT. The behavioral results from both experiments revealed that increased load and crowding led to reduced accuracy in an additive manner, suggesting that they may rely on different mechanisms. Consistent with this, analysis of the ocular data showed that load was associated with pupil dilations, whereas crowding was not. Conversely, both load and crowding were associated with increased frequency of saccades during tracking. The fMRI results showed that dorsal attentional areas were proportionally active with both higher levels of load and crowding. Higher crowding recruited additionally ventral attentional areas that may reflect orienting mechanisms. The activity in the brainstem nuclei VTA/SN and LC showed strongly dissociated patterns. While activations in VTA/SN mainly reflected variations in crowding, activity in LC mainly reflected variations in load. An exploratory individual differences’ approach showed that high-performers were able to recruit dynamically the LC with high difficulty. Thus, distinct brain and brainstem systems deal with spatially versus non-spatially-related effort.