title: Cortical capacity constraints for visual working memory: dissociation of fMRI load effects in a fronto-parietal network.
abstract: Working memory (WM) capacity limitations and their neurophysiological correlates are of special relevance for the understanding of higher cognitive functions. Evidence from behavioral studies suggests that restricted attentional resources contribute to these capacity limitations. In an event-related functional magnetic resonance imaging (fMRI) study, we probed the capacity of the human visual WM system for up to four complex nonnatural objects using a delayed discrimination task. A number of prefrontal and parietal areas bilaterally showed increased blood oxygen level-dependent activity, relative to baseline, throughout the task when more than one object had to be held in memory. Monotonic increases in response to memory load were observed bilaterally in the dorsolateral prefrontal cortex (DLPFC) and the presupplementary motor area (pre-SMA). Conversely, activity in the frontal eye fields (FEFs) and in areas along the intraparietal sulcus (IPS) peaked when subjects had to maintain only two or three objects and decreased in the highest load condition. This dissociation of memory load effects on cortical activity suggests that the cognitive operations subserved by the IPS and FEF, which are most likely related to attention, fail to support visual WM when the capacity limit is approached. The correlation of brain activity with performance implies that only the operations performed by the DLPFC and pre-SMA, which support an integrated representation of visual information, helped subjects to maintain a reasonable level of performance in the highest load condition. These results indicate that at least two distinct cortical subsystems are recruited for visual WM, and that their interplay changes when the capacity limit is reached.
authors.affiliation: Max-Planck-Institut für Himforschung, Deutschordenstrasse 46, D-60528 Frankfurt, Germany. linden@mpih-frankfurt.mpg.de
authors.forename: David E J
authors.initials: DE
authors.lastname: Linden
authors.forename: Robert A
authors.initials: RA
authors.lastname: Bittner
authors.forename: Lars
authors.initials: L
authors.lastname: Muckli
authors.forename: James A
authors.initials: JA
authors.lastname: Waltz
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Kriegeskorte
authors.forename: Rainer
authors.initials: R
authors.lastname: Goebel
authors.forename: Wolf
authors.initials: W
authors.lastname: Singer
authors.forename: Matthias H J
authors.initials: MH
authors.lastname: Munk
mesh_terms: D000328:Adult; D000465:Algorithms; D001288:Attention / Q000502:physiology; D002540:Cerebral Cortex / Q000502:physiology*; D004192:Discrimination, Psychological / Q000502:physiology; D005133:Eye Movements / Q000502:physiology; D005260:Female; D005625:Frontal Lobe / Q000502:physiology*; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008570:Memory, Short-Term / Q000502:physiology*; D009415:Nerve Net / Q000502:physiology*; D010100:Oxygen / Q000097:blood; D010296:Parietal Lobe / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D014794:Visual Fields / Q000502:physiology; D014796:Visual Perception / Q000502:physiology*
chemical_list: D010100:Oxygen