Dynamic adjustments in prefrontal, hippocampal, and inferior temporal interactions with increasing visual working memory load - PubMed (original) (raw)
Dynamic adjustments in prefrontal, hippocampal, and inferior temporal interactions with increasing visual working memory load
Jesse Rissman et al. Cereb Cortex. 2008 Jul.
Abstract
The maintenance of visual stimuli across a delay interval in working memory tasks is thought to involve reverberant neural communication between the prefrontal cortex and posterior visual association areas. Recent studies suggest that the hippocampus might also contribute to this retention process, presumably via reciprocal interactions with visual regions. To characterize the nature of these interactions, we performed functional connectivity analysis on an event-related functional magnetic resonance imaging data set in which participants performed a delayed face recognition task. As the number of faces that participants were required to remember was parametrically increased, the right inferior frontal gyrus (IFG) showed a linearly decreasing degree of functional connectivity with the fusiform face area (FFA) during the delay period. In contrast, the hippocampus linearly increased its delay period connectivity with both the FFA and the IFG as the mnemonic load increased. Moreover, the degree to which participants' FFA showed a load-dependent increase in its connectivity with the hippocampus predicted the degree to which its connectivity with the IFG decreased with load. Thus, these neural circuits may dynamically trade off to accommodate the particular mnemonic demands of the task, with IFG-FFA interactions mediating maintenance at lower loads and hippocampal interactions supporting retention at higher loads.
Figures
Fig 1
Structure of the behavioral task. On each trial, four cue stimuli were serially presented for 1 s each. The stimulus set contained one, two, three, or four intact faces, with the remainder of the images composed of scrambled faces. A trial with a memory load of three faces is illustrated here. Participants were instructed to remember all of the intact faces across an 8 s delay period. Following the delay, a probe face appeared for 1 s, and participants made a button press response indicating whether or not the probe face matched any of the cue faces. After responding, participants were instructed to fixate on a crosshair during a 16 s inter-trial interval (ITI).
Fig 2
Mean accuracy and reaction time for each of the four load conditions. Error bars represent standard error of the mean (SEM).
Fig 3
FFA delay period correlations as a function of mnemonic load. The mean correlation (Pearson’s r) of the right FFA’s delay period beta series with that of the right inferior frontal gyrus (IFG) and left hippocampus ROIs is depicted for each face load condition (load is indicated by the numbers below each bar). The FFA shows a linear decrease in its correlation with the right IFG and a linear increase in its correlation with the left hippocampus; the pairwise differences between the “1 Face” and “4 Faces” conditions are also significant for both ROIs (all _p_s<.05). Error bars indicate SEM.
Fig 4
Example of load-dependent functional connectivity changes in an individual participant. In these scatter plots, each data point represents the estimated delay period activity (beta value) from a single trial, taken either from the “1 Face” condition (open diamonds) or “4 Faces” condition (filled circles). The beta values obtained from the right FFA (x-axis; both charts) are plotted against those obtained in the right IFG (y-axis; left-side chart) and left hippocampus (y-axis, right-side chart) across all trials for which this participant made a correct response. The degree to which two regions show correlated fluctuations in their delay period activity across trials is taken as a measure of their functional connectivity.
Fig 5
Load-dependent trade-off between prefrontal and hippocampal connectivity with the FFA. The amount that each participant’s delay period correlation between the right FFA and right IFG changed as a function of load (_y_-axis) is plotted against the corresponding change in their correlation between the right FFA and left hippocampus (_x_-axis). The values of each axis correspond to the difference between the arc hyperbolic tangent-transformed correlation coefficients of the “4 faces” condition and “1 face” condition. A significant negative correlation was observed between these two connectivity change measures.
Fig 6
Schematic summary of inter-regional connectivity at low and high loads. These diagrams depict the mean correlation level between the right FFA, right IFG, and left hippocampus ROIs during the delay period of the “1 Face” (left) and “4 Faces” (right) conditions. The thickness of the each arrow is scaled to reflect the mean correlation between that pair of nodes, the value of which is displayed for each path. All three paths show significant changes in their correlation between these load levels (all _p_s <.05).
References
- Aggleton JP, Shaw C, Gaffan EA. The performance of postencephalitic amnesic subjects on two behavioural tests of memory: concurrent discrimination learning and delayed matching-to-sample. Cortex. 1992;28(3):359–72. -PubMed
- Aguirre GK, Zarahn E, D’Esposito M. The variability of human, BOLD hemodynamic responses. Neuroimage. 1998;8(4):360–9. -PubMed
- Alvarez GA, Cavanagh P. The capacity of visual short-term memory is set both by visual information load and by number of objects. Psychol Sci. 2004;15(2):106–11. -PubMed
- Barcelo F, Suwazono S, Knight RT. Prefrontal modulation of visual processing in humans. Nat Neurosci. 2000;3(4):399–403. -PubMed
- Barde LH, Thompson-Schill SL. Models of functional organization of the lateral prefrontal cortex in verbal working memory: evidence in favor of the process model. J Cogn Neurosci. 2002;14(7):1054–63. -PubMed
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