Functional MRI and behavioral investigations of capacity limits in human visual attention

Mounting evidence suggests that visual attention may be simultaneously deployed to multiple distinct object locations, but the constraints upon this multi-object attentional system are still debated. Three sets of experiments examined the validity of the fixed slot and flexible resource models of attentional capacity, the role of posterior parietal cortex (PPC) in attentional capacity limitations, and the relationship between capacity limits observed in visual attention and visual short-term memory (VSTM).;Results from multiple object tracking (MOT) experiments have been interpreted as revealing a fixed attentional capacity limit of 4 objects; however, other evidence has suggested that capacity may be more fluid. Here, behavioral MOT experiments demonstrate that the demands placed on attentional resources by distractor suppression and the updating of target locations strongly modulate attentional capacity, even above the fixed 4 item limit, revealing that capacity is limited by both central and local processes.;Prior studies suggest that activity within the intraparietal sulcus (IPS) of human PPC reflects attentional capacity. Here, functional magnetic resonance imaging (fMRI) studies find activation related to attentional capacity and distractor suppression in PPC. Our observations support the flexible capacity theory and are inconsistent with the fixed slot theory. We propose a 'dual indexing' model which supports target enhancement and active suppression of distractors. This model accurately predicts our observations that IPS activation reflects the number of indexed targets and indexed distractors.;Similar capacity limits of approximately 4 objects have also been observed in visual short-term memory (VSTM) tasks. It is unclear whether this reflects shared usage of a single common buffer, shared usage of core processes, or is merely a coincidence. Analysis of within-subject differences in attentional and VSTM capacity revealed that capacity limits are related only when the tasks require the same underlying processes.;These results indicate that attentional capacity is governed by the demands placed on shared processing resources by distractor suppression and target selection/encoding. As these processes become more overloaded, more resources must be devoted to them, causing fewer objects to be maintained. Thus, capacity limits emerge from the demands of various underlying processes common in both VSTM and attentional tasks.