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Comparative Study
. 2006 Jan 18;26(3):821-9.
doi: 10.1523/JNEUROSCI.3542-05.2006.

Mental chronometry of working memory retrieval: a combined functional magnetic resonance imaging and event-related potentials approach

Christoph Bledowski  1 , Kathrin Cohen Kadosh, Michael Wibral, Benny Rahm, Robert A Bittner, Karsten Hoechstetter, Michael Scherg, Konrad Maurer, Rainer Goebel, David E J Linden
Affiliations
Comparative Study

Mental chronometry of working memory retrieval: a combined functional magnetic resonance imaging and event-related potentials approach

Christoph Bledowski et al. J Neurosci. .

Abstract

We used the combination of functional magnetic resonance imaging and event-related potentials to decompose the processing stages (mental chronometry) of working memory retrieval. Our results reveal an early transient activation of inferotemporal cortex, which was accompanied by the onset of a sustained activation of posterior parietal cortex. We furthermore observed late transient responses in ventrolateral prefrontal cortex and late sustained activity in medial frontal and premotor areas. We propose that these neural signatures reflect the cognitive stages of task processing, perceptual evaluation (inferotemporal cortex), storage buffer operations (posterior parietal cortex), active retrieval (ventrolateral prefrontal cortex), and action selection (medial frontal and premotor cortex). This is also supported by their differential temporal contribution to specific subcomponents of the P300 cognitive potential.

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Figures

Figure 1.
Figure 1.
The working memory paradigm. Either one (load 1) or three (load 3) stimuli were presented for 600 ms each, with 400 ms fixation intervals. This was followed by a delay period of 7400 ms, after which the test stimulus was presented for 600 ms. Participants indicated by button press whether the test matched one of the sample stimuli, which was the case on 50% of trials. The test stimulus was followed by an intertrial interval (ITI) of 7400 ms (EEG session) or 9400 ms (fMRI session).
Figure 2.
Figure 2.
ERP responses in the load 1 and load 3 condition. A, Grand average ERP waveforms combined for both conditions (middle) and the corresponding spline-interpolated topographical maps of scalp voltage calculated at the latencies indicated by the yellow lines. B, Separate waveforms for load 1 (blue) and load 3 (red) of electrodes showing prominent ERP components.
Figure 3.
Figure 3.
fMRI activation maps on a surface representation of a template brain. A, Combined map for the retrieval predictor for load 1 and load 3 conditions (yellow) with the areas showing a load effect (significantly higher activation for load 3 than load 1) superimposed in red (overlapping area; orange). B, Evoked hemodynamic responses for load 1 (blue) and load 3 (red) conditions over the entire trial period. The presentation of the test stimulus is indicated in green on the time bar. Because of the hemodynamic delay, the corresponding peak activation would be expected two to three scans (4–6 s) later. Significant differences between load conditions during retrieval phase are indicated by a green asterisk. IFG, Inferior frontal gyrus; pre-SMA, presupplementary motor area; CS, central sulcus.
Figure 4.
Figure 4.
Time courses of the regional source activity. Positions of the regional sources on the fMRI activation maps (middle) surrounded by the source activities and their corresponding topographical voltage maps are shown. Source time courses are shown in blue for load 1 and in red for load 3. The black line represents the difference wave form, with the area shaded in gray denoting the 95% confidence interval as determined by the bootstrap BCa method (see Materials and Methods).
Figure 5.
Figure 5.
Comparison of ERP responses and source activities between encoding and retrieval phase. ERP waveforms (A) of four selected electrodes and source activities (B) for encoding (green) and retrieval (black) averaged across subjects and load conditions are shown. The selected sources showed significant activity differences in the retrieval compared with the encoding source activity in the later part (at ∼450–800 ms) of the source time courses. The dark gray line [diff (95%)] represents the difference waves (retrieval minus encoding), with the area shaded in bright gray denoting the 95% confidence interval as determined by the bootstrap BCa method. nAm, Nanoamperes.
Figure 6.
Figure 6.
Timing of the retrieval processes. Time courses for prominent source activities [>10 nA (nAm)] averaged across subjects and conditions arranged from top (early response) to bottom (late response). Source time courses are shown in black for the right hemisphere (RH), in gray for the left hemisphere (LH), and dashed for the medial frontal cortex.
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