Cortical network dynamics during source memory retrieval: current density imaging with individual MRI

Abstract

We investigated the neural correlates of source memory retrieval using low-resolution electromagnetic tomography (LORETA) with 64 channels EEG and individual MRI as a realistic head model. Event-related potentials (ERPs) were recorded while 13 healthy subjects performed the source memory task for the voice of the speaker in spoken words. The source correct condition of old words elicited more positive-going potentials than the correct rejection condition of new words at 400-700 ms post-stimulus and the old/new effects also appeared in the right anterior region between 1,000 and 1,200 ms. We conducted source reconstruction at mean latencies of 311, 604, 793, and 1,100 ms and used statistical parametric mapping for the statistical analysis. The results of source analysis suggest that the activation of the right inferior parietal region may reflect retrieval of source information. The source elicited by the difference ERPs between the source correct and source incorrect conditions exhibited dynamic change of current density activation in the overall cortices with time during source memory retrieval. These results indicate that multiple neural systems may underlie the ability to recollect context.

Figure 1

Grand‐average ERPs elicited by source correct, source incorrect, and correct rejection conditions and topographies of old/new effects and the difference ERP between source correct and source incorrect conditions.

Figure 2

Statistical parametric map of t statistic (SPM{t}) for ERP generator elicited by source correct, source incorrect, and correct rejection conditions at the latency of 604 ms and 1,100 ms. L/A, left anterior; R/P, right posterior. Statistical map of one‐sample t‐test displays significant activation of mean current density in memory conditions. These images were thresholded at t > 3.93 (P < 0.001 uncorrected, extent k = 50). Right bottom images in each comparison displayed the activation region on the rendered image by the two‐surface method of SUMA (http://afni.nimh.nih.gov/afni/suma).

Figure 3

Statistical parametric maps displaying changed current density of source correct ERPs between 604 and 311 ms and between 604 and 793 ms. L/A, left anterior; R/P, right posterior. Significantly activated areas (t = 3.05, P = 0.005 uncorrected, extent k = 30) are shown on the Talairach coordinate. Cortical activities in the right precentral gyrus and supramarginal gyrus (Talairach coordinates 18/−26/66, BA 4; 58/−36/32, BA 40) increased in 604 ms, when compared with those in 311 ms. The activity of right inferior parietal lobule (Talairach coordinates 54/−36/40, BA 40) increased in 604 ms than 793 ms. Right bottom images in each comparison displayed the activation region on the rendered image.

Figure 4

Statistical parametric map of t statistic (SPM{t}) for cortical generator elicited by difference ERP between source correct and source incorrect conditions at the latencies of 604, 793, and 1,100 ms. L/A, left anterior; R/P, right posterior. Statistical map of one‐sample t‐test displays significant activation of mean current density in memory conditions. These images were thresholded at t > 3.93 (P < 0.001 uncorrected, extent k = 50). Right bottom images in each time point displayed the activation region on the rendered image by the two‐surface method of SUMA (http://afni.nimh.nih.gov/afni/suma).